Throttle a pump harmful?

[Ross Bagley]

I've read here and on Marc's tank website that you shouldn't throttle
centrifugal pumps with a ball valve but that you should divert some of
the flow back to the intake side.

I hope I'm not being obtuse when I ask, "Why?"

Speaking specifically about Mag pumps, the manufacturer doesn't warn
about or otherwise recommend any minimum flow rates through the pump,
and the backpressure from a constriction in the line should be
identical to the backpressure from a higher head application.

At least that's what my math comes out to. I don't think the pump
can tell if the reason it's not pumping as much is because there's
a 5' head or an almost closed gate valve in the line.

If I'm wrong about this, could someone please take the time to
explain how the two cases (partially closed valve vs. increased
head) are different from the pump's perspective?

Or, alternatively, something else (not necessarily pump damage) may be
the reason to keep the flow through rate at a maximum. Perhaps
something like plankton mortality rates (just stabbing in the dark
here) that get much worse if the backpressure goes beyond a certain
point...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Marc Levenson]

Ross, my whole reasoning is to avoid putting any undue stress on the pump, and
that is my only reason. Why add head pressure to a pump that doesn't need it?
I believe that the extra effort may lead to the pump running at a higher
temperature, possibly warming the tank water more.

Marc


Ross Bagley wrote:

> I've read here and on Marc's tank website that you shouldn't throttle
> centrifugal pumps with a ball valve but that you should divert some of
> the flow back to the intake side.
>
> I hope I'm not being obtuse when I ask, "Why?"
>
> Speaking specifically about Mag pumps, the manufacturer doesn't warn
> about or otherwise recommend any minimum flow rates through the pump,
> and the backpressure from a constriction in the line should be
> identical to the backpressure from a higher head application.
>
> At least that's what my math comes out to. I don't think the pump
> can tell if the reason it's not pumping as much is because there's
> a 5' head or an almost closed gate valve in the line.
>
> If I'm wrong about this, could someone please take the time to
> explain how the two cases (partially closed valve vs. increased
> head) are different from the pump's perspective?
>
> Or, alternatively, something else (not necessarily pump damage) may be
> the reason to keep the flow through rate at a maximum. Perhaps
> something like plankton mortality rates (just stabbing in the dark
> here) that get much worse if the backpressure goes beyond a certain
> point...
>
> Regards,
> Ross
>
> -- Ross Bagley http://rossbagley.com/rba
> "Security is mostly a superstition. It does not exist in nature...
> Life is either a daring adventure or nothing." -- Helen Keller

--
Personal Page: http://www.sparklingfloorservice.com/oanda/index.html
Business Page: http://www.sparklingfloorservice.com
Marine Hobbyist: http://www.melevsreef.com

[Phil Krasnostein]

Ross / Marc

Do you have a pump curve? It's a long time since I was doing chem. eng. but
my memory says the pump curve gives flow / head / efficiency and power
consumption. Until you have a look at that you don't really know which
condition will generate more heat. I think stress on the pump should not be
an issue -- they should be designed to run within their stated range.

Phil


"Marc Levenson" > wrote in message
...
> Ross, my whole reasoning is to avoid putting any undue stress on the pump,
and
> that is my only reason. Why add head pressure to a pump that doesn't need
it?
> I believe that the extra effort may lead to the pump running at a higher
> temperature, possibly warming the tank water more.
>
> Marc
>
>
> Ross Bagley wrote:
>
> > I've read here and on Marc's tank website that you shouldn't throttle
> > centrifugal pumps with a ball valve but that you should divert some of
> > the flow back to the intake side.
> >
> > I hope I'm not being obtuse when I ask, "Why?"
> >
> > Speaking specifically about Mag pumps, the manufacturer doesn't warn
> > about or otherwise recommend any minimum flow rates through the pump,
> > and the backpressure from a constriction in the line should be
> > identical to the backpressure from a higher head application.
> >
> > At least that's what my math comes out to. I don't think the pump
> > can tell if the reason it's not pumping as much is because there's
> > a 5' head or an almost closed gate valve in the line.
> >
> > If I'm wrong about this, could someone please take the time to
> > explain how the two cases (partially closed valve vs. increased
> > head) are different from the pump's perspective?
> >
> > Or, alternatively, something else (not necessarily pump damage) may be
> > the reason to keep the flow through rate at a maximum. Perhaps
> > something like plankton mortality rates (just stabbing in the dark
> > here) that get much worse if the backpressure goes beyond a certain
> > point...
> >
> > Regards,
> > Ross
> >
> > -- Ross Bagley http://rossbagley.com/rba
> > "Security is mostly a superstition. It does not exist in nature...
> > Life is either a daring adventure or nothing." -- Helen Keller
>
> --
> Personal Page: http://www.sparklingfloorservice.com/oanda/index.html
> Business Page: http://www.sparklingfloorservice.com
> Marine Hobbyist: http://www.melevsreef.com
>
>

[Gary V. Deutschmann, Sr.]

Hi Ross

For most impeller pumps and centrifugal pumps it wouldn't matter at
all, because they are designed to operate within a wide range.

But there is a better way than clamping down the output feed line.
Install a T-Fitting in the output line and a line connected to the
T-Fitting as a return line to your sump, you can install a valve or
clamp this line to increase output from the feed line.
This method works well on all pumps, keeps heat buildup lower and
places less stress on the pump.

We have similar set-ups on all of our bottle filling equipment, except
instead of a manual valve it has a spring loaded ball valve that can
be set at various pressures. When the filling head solenoid opens,
you have the desired head pressure. When the filling head valve
closes, the spring loaded valve is forced open with the excess head
pressure and allows the product to recycle back to its own carboy
(sump). Some systems use a split solenoid so that when one side is
open the other side is closed, but you get unequal head pressure for a
split second as the solenoid switches, which can cause a splash of the
product, so these split solenoids are rarely used. In fact, most
small bottlers and repackagers use gravity feed rather than pump feed
to save costs on electric and equipment replacement costs.

TTUL
Gary

[Ross Bagley]

(Gary V. Deutschmann, Sr.) writes:

[...snip...]

> But there is a better way than clamping down the output feed line.
> Install a T-Fitting in the output line and a line connected to the
> T-Fitting as a return line to your sump, you can install a valve or
> clamp this line to increase output from the feed line.
> This method works well on all pumps, keeps heat buildup lower and
> places less stress on the pump.

Thanks for the answer. This does respond to the core of the
question that I was asking. So what you're saying is that operating a
pump at a higher head does a few things:

1) increases wear/stress on the impeller/motor
2) increases heat production/reduces efficiency

Both sound reasonable and plausible, but I have heard that lower flow
rates can make some pumps work less (that they can work more
efficiently at heads greater than 0ft than they do at 0ft). This has
been asserted for the Rainbow Lifegard Quiet One pump on this very
newsgroup. This assertion is also plausible if the efficiency of
a pump is nonlinear (goes up at lower pressures, then drops again
at higher pressures, going back to zero at the pump's max head).

Now, what I really wonder is: does anyone have any actual numbers to
support either set of assertions. These numbers might only apply to a
particular make/model of pump, but any empirically gathered numbers
would help to satisfy my curiousity.

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Phil Krasnostein]

Ross

You need the pump curve for the particular pump to answer those questions.

Phil


"Ross Bagley" > wrote in message
...
> (Gary V. Deutschmann, Sr.) writes:
>
> [...snip...]
>
> > But there is a better way than clamping down the output feed line.
> > Install a T-Fitting in the output line and a line connected to the
> > T-Fitting as a return line to your sump, you can install a valve or
> > clamp this line to increase output from the feed line.
> > This method works well on all pumps, keeps heat buildup lower and
> > places less stress on the pump.
>
> Thanks for the answer. This does respond to the core of the
> question that I was asking. So what you're saying is that operating a
> pump at a higher head does a few things:
>
> 1) increases wear/stress on the impeller/motor
> 2) increases heat production/reduces efficiency
>
> Both sound reasonable and plausible, but I have heard that lower flow
> rates can make some pumps work less (that they can work more
> efficiently at heads greater than 0ft than they do at 0ft). This has
> been asserted for the Rainbow Lifegard Quiet One pump on this very
> newsgroup. This assertion is also plausible if the efficiency of
> a pump is nonlinear (goes up at lower pressures, then drops again
> at higher pressures, going back to zero at the pump's max head).
>
> Now, what I really wonder is: does anyone have any actual numbers to
> support either set of assertions. These numbers might only apply to a
> particular make/model of pump, but any empirically gathered numbers
> would help to satisfy my curiousity.
>
> Regards,
> Ross
>
> -- Ross Bagley http://rossbagley.com/rba
> "Security is mostly a superstition. It does not exist in nature...
> Life is either a daring adventure or nothing." -- Helen Keller

[Ross Bagley]

"Phil Krasnostein" > writes:

> Ross
>
> You need the pump curve for the particular pump to answer those questions.

Given these pump curves, what can you tell me? Or is there another curve
(more/different data) that would be needed to answer my question?

http://www.marinedepot.com/aquarium_pumps_pentair_aquatics_rainbow_lifegard_q uiet_one_information.asp#qone800

There's definitely a "belly" to the curves and it seems that if you
chose a point where the area within the rectangle of the flow and
height is maximized, you may have found some sort of a sweet spot.

But is that sweet spot likely to have the lowest wear on the pump? Is
the pump likely to run most efficiently and produce the least waste
heat at that point on the curve? What might that sweet spot mean
to us as aquarists?

Thanks for all the help,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Richard Reynolds]

> Given these pump curves, what can you tell me?

for what you are after, no one can tell you anything from those graphs

> Or is there another curve
> (more/different data) that would be needed to answer my question?

very much so, you need one that has that curve and some kind of electrical consumption
curve(you could get the answer from almost any electrical curve), it would be nice to have
a MTBF curve added to that, another might be heat transfer.

when water moves slower thru a hot object it picks up more heat some of that is set, some
of it depends on what the water is moving thru.



> There's definitely a "belly" to the curves and it seems that if you
> chose a point where the area within the rectangle of the flow and
> height is maximized, you may have found some sort of a sweet spot.

> But is that sweet spot likely to have the lowest wear on the pump? Is
> the pump likely to run most efficiently and produce the least waste
> heat at that point on the curve? What might that sweet spot mean
> to us as aquarists?

that sweet spot probibly doesnt mean anything, except where the most gph is. most
aquarium pumps dont list the data you want. some larger like 1/4+ hp pumps do.


--
Richard Reynolds

[Phil Krasnostein]

Ross
These curves are simple and don't tell you much. You also need efficiency
and power consumption data. For some general info, have a look at

http://www.mcnallyinstitute.com/06-html/6-01.html

In my experience, centrifugal pumps are generally designed to be throttled
by valves, and won't wear out because of it -- certainly energy loss across
the valve restriction will generate some heat.

Phil

"Ross Bagley" > wrote in message
...
> "Phil Krasnostein" > writes:
>
> > Ross
> >
> > You need the pump curve for the particular pump to answer those
questions.
>
> Given these pump curves, what can you tell me? Or is there another curve
> (more/different data) that would be needed to answer my question?
>
>
http://www.marinedepot.com/aquarium_pumps_pentair_aquatics_rainbow_lifegard_
quiet_one_information.asp#qone800
>
> There's definitely a "belly" to the curves and it seems that if you
> chose a point where the area within the rectangle of the flow and
> height is maximized, you may have found some sort of a sweet spot.
>
> But is that sweet spot likely to have the lowest wear on the pump? Is
> the pump likely to run most efficiently and produce the least waste
> heat at that point on the curve? What might that sweet spot mean
> to us as aquarists?
>
> Thanks for all the help,
> Ross
>
> -- Ross Bagley http://rossbagley.com/rba
> "Security is mostly a superstition. It does not exist in nature...
> Life is either a daring adventure or nothing." -- Helen Keller

[Gary V. Deutschmann, Sr.]

Hi Ross

It's very simple to hook up an ammeter to check the current draw as
well as a thermometer to check motor temperature.

It only makes sense, that to do MORE work, will require MORE energy
and produce MORE heat.

We use MaxiJet1000's to pump heavy viscous liquid, they are the
coolest running of all submersibles we have tried, but do run right at
their automatic shut-off pre-set temperature when the backpressure is
idling high for a long period of time.
The benefit to using MaxiJet's is that they DO HAVE internal thermal
sensors to shut them down, rather than burn them up as some pumps we
have tried.

Higher heat, higher resistance, higher electrical consumption!

You are correct about SOME pumps that REQUIRE Head Pressure to run
more efficiently because of their design.
Although not seen much in aquaria usage, worm drive and screw drive
pumps often need a 'load' on them to function efficiently. Some worm
drive pumps without a 'load' will self-destruct from backlashing of
the gears.

Magnetic driven pumps one would think would not be affected at all by
head pressure, because there is no interaction between the impeller
and the engine, which is just an electromagnet being pulsed to drive
the core which spins the impeller.

But under a heavy load, the core heats up, which in turn causes the
winding driving it to heat up and under enough load, it will get hot
enough to trip the thermal sensor (if your pump has one).

If the load (backpressure) is too great, the magnetics breaks down and
the engine cannot overcome the load on the core.
Most magnetic driven pumps have floating impellers, you will often
hear them chatter as you start up the pump. This is to prevent the
magnetics breakdown as the pump starts and keep start up heat to a
minimum. If you rigidly affix the impeller to the core, you will find
that most magnetic pumps will keep losing their magnetics hold on the
core as they try to start and in some cases may not start at all.

In essence, an alternator works the same way, load it down and it will
heat up.

TTUL
Gary


(Ross Bagley) verbositized:

(Gary V. Deutschmann, Sr.) writes:
>
>[...snip...]
>
>> But there is a better way than clamping down the output feed line.
>> Install a T-Fitting in the output line and a line connected to the
>> T-Fitting as a return line to your sump, you can install a valve or
>> clamp this line to increase output from the feed line.
>> This method works well on all pumps, keeps heat buildup lower and
>> places less stress on the pump.
>
>Thanks for the answer. This does respond to the core of the
>question that I was asking. So what you're saying is that operating a
>pump at a higher head does a few things:
>
>1) increases wear/stress on the impeller/motor
>2) increases heat production/reduces efficiency
>
>Both sound reasonable and plausible, but I have heard that lower flow
>rates can make some pumps work less (that they can work more
>efficiently at heads greater than 0ft than they do at 0ft). This has
>been asserted for the Rainbow Lifegard Quiet One pump on this very
>newsgroup. This assertion is also plausible if the efficiency of
>a pump is nonlinear (goes up at lower pressures, then drops again
>at higher pressures, going back to zero at the pump's max head).
>
>Now, what I really wonder is: does anyone have any actual numbers to
>support either set of assertions. These numbers might only apply to a
>particular make/model of pump, but any empirically gathered numbers
>would help to satisfy my curiousity.
>
>Regards,
>Ross
>
>-- Ross Bagley http://rossbagley.com/rba
>"Security is mostly a superstition. It does not exist in nature...
>Life is either a daring adventure or nothing." -- Helen Keller

[Ross Bagley]

"Richard Reynolds" > writes:

> > Given these pump curves, what can you tell me?
>
> for what you are after, no one can tell you anything from those graphs

That's kinda what I figured. I'm getting the distinct impression that
I would have to buy several different pumps and do some testing myself
to get real answers to my questions.

Possible test chassis configuration:
1) A temperature controlled sump.
2) Demand flow from the sump into a test chamber with the pump,
or a line directly from the sump to the pump input through a
bulkhead (depending on submerged/external).
3) A temperature sensor on the side of the pump casing.
4) A pressure sensor at the pump output connection.
5) An ammeter on the pump's power supply.
6) An output tank incorporating a simple flow meter.
7) A rack so the output tank can be moved up and down (this will be
impractical in my garage for heads over 8' but that may be enough).
8) A temperature sensor in the output tank.
9) Return line to the temperature control systems of the sump.
10) A data-aquisition system to record all of this data (so I
don't have to do manual data recording).

The experiment variables would be the pump model selected, the
temperature of the sump, and the height of the head.

The experiment could then be varied to include reduced pump flow
strategies (like the throttle valve vs. shunt approaches which are the
topic of this thread).

> it would be nice to have
> a MTBF curve added to that, another might be heat transfer.

Well, the MTBF curve will have to be sourced from the manufacturers,
even if I do conduct this experiment. When I worked at Texas
Instruments in the early 90's I helped them summarize the device
qualification data to determine MTBF and there's simply no way I could
afford to buy and run enough pumps to get significant accuracy on an
MTBF number (hundreds of test units minimum).

What's really too bad is that MTBF doesn't appear to be available for
any of the smaller powerheads that are so popular among aquarists.

Now to see how much this experiment would cost...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ross Bagley]

"Phil Krasnostein" > writes:

> Ross
> These curves are simple and don't tell you much. You also need efficiency
> and power consumption data. For some general info, have a look at
>
> http://www.mcnallyinstitute.com/06-html/6-01.html

That's an interesting read. Thanks for the link.

> In my experience, centrifugal pumps are generally designed to be throttled
> by valves, and won't wear out because of it -- certainly energy loss across
> the valve restriction will generate some heat.

To be sure, but is the heat generated an inevitable "feature" of the
selected pump, and if not, does either throttling or shunting minimize
the generated heat?

In another response, I have proposed an experimental harness to
test the question, though I won't be able to test for increased
wear and tear any more precisely than "signs of wear" (damaged
impellers, etc.).

I'm actually thinking about doing this experiment. I am a crazy man...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Marc Levenson]

Ross, I replied to your email, but it came back "delivery failure" so I'll just
post my comments here:

As you know, I'm just a hobbyist and go on reasoning more than anything
usually. I do keep an open mind and am willing to adjust my own systems if the
advice turns out ot be different than my own configuration.

Please do let me know what you end up finding out. So I understand your plan,
do you plan to put a pump in a bucket and another bucket up at 5' (for example)
and pump it full throttle and then try it again with a ball valve restricting
flow?

Keep in mind that if you do the Tee system as I recommend, you need to isolate
that
so the return pump isn't pulling in air bubbles. Matter of fact, you'll need to
isolate the pump anyway to avoid airbubbles from the water draining down.

Then you plan to test water temperature? Better keep the ambient room temp. the
same if possible so that the test takes place under the same conditions.

Marc


Ross Bagley wrote:

> Marc Levenson > writes:
>
> > Ross, my whole reasoning is to avoid putting any undue stress on the
> > pump, and that is my only reason. Why add head pressure to a pump
> > that doesn't need it? I believe that the extra effort may lead to
> > the pump running at a higher temperature, possibly warming the tank
> > water more.
>
> I figured that that was your reasoning, and it makes a lot of sense.
> What I'm more interested in knowing is if the counter-proposals (that
> some head can make certain centrifugal pumps run cooler and/or more
> efficiently) hold any water.
>
> So to speak :)
>
> Based on the discussion in other parts of this thread and the lack of
> any conclusive data on the subject, I'm seriously considering actually
> setting up a test to find out. Once I find out how much it will cost,
> I'm going to take the price info to the one with the purse strings and
> see if I can't get some funding :)
>
> Regards,
> Ross



--
Personal Page: http://www.sparklingfloorservice.com/oanda/index.html
Business Page: http://www.sparklingfloorservice.com
Marine Hobbyist: http://www.melevsreef.com

[Richard Reynolds]

> That's kinda what I figured. I'm getting the distinct impression that
> I would have to buy several different pumps and do some testing myself
> to get real answers to my questions.

maybe.

> Possible test chassis configuration:
> 1) A temperature controlled sump.
maybe but probibly not.

> 2) Demand flow from the sump into a test chamber with the pump,
> or a line directly from the sump to the pump input through a
> bulkhead (depending on submerged/external).

ok ish

> 3) A temperature sensor on the side of the pump casing.

but
if your gona do it you have to have 2 one at the input one at the output and compare the
difference, just because heat is generated doesnt mean its xfered to water. though a 3rd
on the casing might be interesting it isnt the heat you need.

> 4) A pressure sensor at the pump output connection.

and a gate valve youd wanna adjust to simulate a different head.

> 5) An ammeter on the pump's power supply.

yep

> 6) An output tank incorporating a simple flow meter.

sounds too complex to me, just attach flow meter on output end of gate valve.

> 7) A rack so the output tank can be moved up and down (this will be
> impractical in my garage for heads over 8' but that may be enough).

nah to impractical head height and PSI are different measurements of the same thing you
can simulate "head" by increasing the PSI load on the pump using a gate valve.

> 8) A temperature sensor in the output tank.
to far from the pump

> 9) Return line to the temperature control systems of the sump.
that part really doesnt work in my brain, but dont have a good reason why yet.

> 10) A data-aquisition system to record all of this data (so I
> don't have to do manual data recording).

YEP!!!!

>
> The experiment variables would be the pump model selected, the
> temperature of the sump, and the height of the head.
>
> The experiment could then be varied to include reduced pump flow
> strategies (like the throttle valve vs. shunt approaches which are the
> topic of this thread).

:D

> > it would be nice to have
> > a MTBF curve added to that, another might be heat transfer.
>
> Well, the MTBF curve will have to be sourced from the manufacturers,
> even if I do conduct this experiment. When I worked at Texas
> Instruments in the early 90's I helped them summarize the device
> qualification data to determine MTBF and there's simply no way I could
> afford to buy and run enough pumps to get significant accuracy on an
> MTBF number (hundreds of test units minimum).

more like thousands you would have to pick common head heights and test hundreds of each,
common are like 0, 1, 4 ... but yea.

> What's really too bad is that MTBF doesn't appear to be available for
> any of the smaller powerheads that are so popular among aquarists.

I know you can get them for rio, and mag drives, you can actually find some of this
already for different pumps in different conifgurations.

> Now to see how much this experiment would cost...

$$$$$$$$$

--
Richard Reynolds

[Phil Krasnostein]

"Ross Bagley" > wrote in message ...
> "Phil Krasnostein" > writes:
>
> > Ross
> > These curves are simple and don't tell you much. You also need efficiency
> > and power consumption data. For some general info, have a look at
> >
> > http://www.mcnallyinstitute.com/06-html/6-01.html
>
> That's an interesting read. Thanks for the link.
>
> > In my experience, centrifugal pumps are generally designed to be throttled
> > by valves, and won't wear out because of it -- certainly energy loss across
> > the valve restriction will generate some heat.
>
> To be sure, but is the heat generated an inevitable "feature" of the
> selected pump, and if not, does either throttling or shunting minimize
> the generated heat?
>
> In another response, I have proposed an experimental harness to
> test the question, though I won't be able to test for increased
> wear and tear any more precisely than "signs of wear" (damaged
> impellers, etc.).
>
> I'm actually thinking about doing this experiment. I am a crazy man...

YEP -- SOUNDS THAT WAY!!!!!!!!!!

Phil

>
> Regards,
> Ross
>
> -- Ross Bagley http://rossbagley.com/rba
> "Security is mostly a superstition. It does not exist in nature...
> Life is either a daring adventure or nothing." -- Helen Keller

[Ct Midnite]

On 06 Mar 2004 09:34:10 EST, (Gary V.
Deutschmann, Sr.) wrote:

>Hi Ross
>
>It's very simple to hook up an ammeter to check the current draw as
>well as a thermometer to check motor temperature.
>
>It only makes sense, that to do MORE work, will require MORE energy
>and produce MORE heat.

This is exactly what you should try. You will find that the more you
restrict the flow the less current you will draw.

Did this experiment in physics myself much to the surprise of the
teacher. Both with air and with water pumps. Gradually reducing the
flow of either dropped the amps to about 1/3 the original when stopped
completely.

Work is equal to force times distance. Close the valve you still have
the force but not the distance.

This goes contrary to the way we think of work because when you put a
resistance on us we work harder but not really our internal work is
greater and we sweat and heat up but the work being done on the object
is actually less.

If you doubt me, just restrict the flow to your pump and listen to the
motor. It will speed up. How can a electric motor with out any
governor be working harder but speed up. It can't.

If you still doubt me, get and amp meter. I can still see the
teachers face. :)

Ct Midnite

PS. The only exception to this would be a positive displacement pump.
If you restrict the flow of these they will either stop turning or
blow up your lines. They have to have an escape valve like Marcs tee
but centrifugal pumps do not.

http://www.geocities.com/ctmidnite53/

[Gary V. Deutschmann, Sr.]

Hi CT

Your were obtaining correct measurements but inherintly FALSE readings
do to an effect called CAVITATION!
Where CAVITATION is present, WORK ceases(or is reduced), the LOAD is
reduced due to Cavitation, ergo current drop is eminent.

It's akin to going uphill in your car, if you restrict the vehicles
upward movement to the point the drive wheels lose traction and begin
to spin on the pavement, the horsepower consumed will decrease due to
less friction between the tires and road, but the WORK will also be
creatly reduced if not ceased entirely as on ice, and you could
actually begin to slide backwards down the hill for loss of traction.

I can guarantee you that increasing the load on a pump increases its
power consumption. If you do not show an increase in power
consumption, then you have not increased the load the pump is doing.

If increasing the load the engine must do, decreased the current or
horsepower needed, then we would need a 450 Cummings engine to drive a
wris****ch and a hearing aid battery to power an aircraft carrier.

TTUL
Gary

[Ct Midnite]

On 07 Mar 2004 10:01:04 EST, (Gary V.
Deutschmann, Sr.) wrote:

I certainly don't want to get into a flame war. Please believe me
when I say that your take on this is the take the vast majority of
peoples have. In my class it was 24 to 2 in your favor. But it is
wrong.

>I can guarantee you that increasing the load on a pump increases its
>power consumption. If you do not show an increase in power
>consumption, then you have not increased the load the pump is doing.

You are exactly right here. This is the misunderstanding. By
restricting the water flow you do not increase the load on the pump.
You decrease it. Restricting flow does not increase load, more water
flow increases load. Work really is equal to force times distance.
The more you move a given distance the more work is done. The less
you move a given distance the less work is done. More water, more
work. Less water, less work.

Please before you really start calling me names and questioning my
intelligence put the amp meter on the pump and gradually decrease the
flow. You will be quite surprised.

It becomes easier to see with a gas pump. I have a 3 hp cent pump to
fill my 1000 gal field sprayer. With the valve fully opened and max
water through it the motor is pulled down to a fraction of it's no
load speed and obviously working very hard. You start shutting the
valve and the motor just keeps easing up until it's just running like
it's hooked up to nothing. It's pretty dramatic the difference.

If you shut off the water flow out of a pump the work it's doing is
close to zero. Only heating the water in side the pump a little from
moving the water around and pressure but no work is being done. Your
pump may burn up because it was designed to run at a lower rpm or it
uses the water as a coolant but not because it's being over worked.

Honest. I wouldn't kid my fellow friends on the group. :)

Ct Midnite

If you want a easy test of what I'm saying go get out your canister
vacuum cleaner. Put you hand over the tube. The motor will speed up.
And not because it's doing more work. Because it's doing less.


>Hi CT
>
>Your were obtaining correct measurements but inherintly FALSE readings
>do to an effect called CAVITATION!
>Where CAVITATION is present, WORK ceases(or is reduced), the LOAD is
>reduced due to Cavitation, ergo current drop is eminent.
>
>It's akin to going uphill in your car, if you restrict the vehicles
>upward movement to the point the drive wheels lose traction and begin
>to spin on the pavement, the horsepower consumed will decrease due to
>less friction between the tires and road, but the WORK will also be
>creatly reduced if not ceased entirely as on ice, and you could
>actually begin to slide backwards down the hill for loss of traction.
>
>I can guarantee you that increasing the load on a pump increases its
>power consumption. If you do not show an increase in power
>consumption, then you have not increased the load the pump is doing.
>
>If increasing the load the engine must do, decreased the current or
>horsepower needed, then we would need a 450 Cummings engine to drive a
>wris****ch and a hearing aid battery to power an aircraft carrier.
>
>TTUL
>Gary


http://www.geocities.com/ctmidnite53/

[Ct Midnite]

For anyone else reading these posts don't think that anything I'm
saying about energy used negates Marc's design with his tee and
running it back into the sump.

It's probably the way to go. Much less pressure in the system I would
assume means much less damage to the organisms in the water. The less
the pressure differences across cell membranes, the less likely they
will rupture. And the tee system would give you the lowest pressure
possible for any one point in the system.

But it's not to reduce work on the pump.

Ct Midnite

http://www.geocities.com/ctmidnite53/

[JCBlueEyes]

Unless your system is providing the necessary amount of back pressure from head
alone, to make the pump run within its performance curve, then the pump is not
operating efficiently and you will not get the flowrates the pump is rated to
provide. In this case throttling the pump somewhat will increase its
efficiency.

I agree with the previous comments ... you need a proper pump curve.

JCB

[Ross Bagley]

Marc Levenson > writes:

> Ross, I replied to your email, but it came back "delivery failure"
> so I'll just post my comments here:

Hmmm. That's frustrating. should work. I just
sent a test message on a quick loop no problems...

> As you know, I'm just a hobbyist and go on reasoning more than
> anything usually. I do keep an open mind and am willing to adjust
> my own systems if the advice turns out ot be different than my own
> configuration.

That's the attitude that I have as well. Though it's often easier
to plan ahead than to adjust later. On a new project that I'll be
starting next year, I'm trying to plan, plan, plan as far ahead as
is practical. Understanding how the sump plumbing will go together
is the current topic, which is why I started this thread.

> Please do let me know what you end up finding out. So I understand
> your plan, do you plan to put a pump in a bucket and another bucket
> up at 5' (for example) and pump it full throttle and then try it
> again with a ball valve restricting flow?

A little more sophisticated than that, but that's the initial plan.
Basically, the goals will be to measure pump efficiency, water heating
and impeller wear (qualititive only) for different pumps at different
moderate heads. Then I intend to modify the experiment to include
various means of reducing pump flow and obtain equivalent data under
those circumstances.

> Keep in mind that if you do the Tee system as I recommend, you need
> to isolate that so the return pump isn't pulling in air bubbles.
> Matter of fact, you'll need to isolate the pump anyway to avoid
> airbubbles from the water draining down.

This was the plan, but it's good to be reminded that the return circuit
could introduce more bubbles.

> Then you plan to test water temperature? Better keep the ambient
> room temp. the same if possible so that the test takes place under
> the same conditions.

The ambient temperature will be recorded and controlled as much as
practical. Water temperatures will be controlled within very tight
parameters (and will be treated as one of the experimental variables).
Living in SoCal without A/C means that I'm going to need a decent
chiller for the next big project so if I get approval, I'm just going
to go ahead and get it now to use it for the experiment.

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ross Bagley]

Ct Midnite <mreef2.10.muffin@spamgourmet.(nospam)com> writes:

> For anyone else reading these posts don't think that anything I'm
> saying about energy used negates Marc's design with his tee and
> running it back into the sump.
>
> It's probably the way to go. Much less pressure in the system I would
> assume means much less damage to the organisms in the water. The less
> the pressure differences across cell membranes, the less likely they
> will rupture. And the tee system would give you the lowest pressure
> possible for any one point in the system.
>
> But it's not to reduce work on the pump.

Now you're talking my kind of talk. This was my "first alternate" in
the list of possible reasons to tee off the return line back to the
sump. Reducing the pressure inside the impeller chamber seems likely
to reduce the mortality of phytoplankton passing through the pump.

Which is a very good thing IMHO.

This may also really simplify the experimental apparatus. I'm not
really all that concerned about imparted heat. Though I will need to
buy/borrow a decent microscope to count phytoplankton populations in
the water...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ct Midnite]

On 07 Mar 2004 14:41:58 -0500, (Ross Bagley)
wrote:

>
>Now you're talking my kind of talk. This was my "first alternate" in
>the list of possible reasons to tee off the return line back to the
>sump. Reducing the pressure inside the impeller chamber seems likely
>to reduce the mortality of phytoplankton passing through the pump.
>
>Which is a very good thing IMHO.

I was so concerned with this when I set up a refugium for a tank I
keep in the basement that I just syphon water from my ref to the main
tank with a big 1 1/4 inch pvc. I keep seahorses in the main tank and
wanted all the bugs to make it through to them alive, big bugs
included.

Works like a charm.

Ct Midnite

http://www.geocities.com/ctmidnite53/

[Gary V. Deutschmann, Sr.]

Hi CT

I agree with you on the work = force x distance.

Restricting a flow downstream does not generate more work upstream.
However, we are talking about a closed system.
Within this system we have friction from the piping, which increases
work and/or slows down the flowrate, while at the same time increasing
head pressure.

When you restrict the flow, you increase head pressure (backpressure),
this in turn increases the psi (force) of the pump. If your pump
pumps 5 gallons per minute at 1 psi and you clamp it down to 1 gallon
per minute, the backpressure would jump up to 5 psi. Provided there
is no backwash or centrifugal loss, which for this purpose is the same
as cavitation. 5x1=5 or 1x5=5

Your example of the vacuum cleaner hose being blocked is a perfect
example of cavitation, the movement of air is stopped and the blade
freewheels spinning the air around it as if it is a part of the blade
itself, no work is being performed so the motor speeds up, it has no
load, therefore less current is drawn.

Ohm's Law is pretty cut and dried!

I think the test you conducted was on a pump that allows plenty of
backwash, which means it does not have the ability to generate higher
psi by restricting the output. It's output psi is basically a fixed
pressure. You can read that as an impeller with plenty of spacing
between the sidewall (housing) and the impeller blades.

On a pump like this, reducing the flowrate does decrease the load on
the motor. But only because the backwash area is designed to allow
for a partial cavitation so the pump may be throttled without
increasing the load on the motor. The reason you don't see an
increase in current draw is because it has a fixed psi output rating.
In affect, the inside of the pump head is acting exactly like a
bleedoff line for the excess, but because of additional partial
cavitation within the head by using this method, current draw could
actually decrease.

So, with the particular pump you are using in your test, the output
psi is a fixed number. Therefore the FORCE is a fixed number, and YES
the load on the pump is decreased with less water flow. But it is
still due to cavitation or whirlpool affect inside the pump head,
which was designed especially and on purpose for this to occur.
In simple terms, using a fixed FORCE, such as 1 horsepower, means that
pumping 1 gallon per minute would require LESS WORK and less
horsepower than pumping 5 gallons per minute. BUT, the psi (other
than that caused by friction on the lines) will remain the same at the
output.

What you are seeing and hearing and testing is NOT the whole picture
of what is going on inside that pump!

TTUL
Gary







Ct Midnite <mreef2.10.muffin@spamgourmet.(nospam)com> verbositized:

>On 07 Mar 2004 10:01:04 EST, (Gary V.
>Deutschmann, Sr.) wrote:
>
>I certainly don't want to get into a flame war. Please believe me
>when I say that your take on this is the take the vast majority of
>peoples have. In my class it was 24 to 2 in your favor. But it is
>wrong.
>
>>I can guarantee you that increasing the load on a pump increases its
>>power consumption. If you do not show an increase in power
>>consumption, then you have not increased the load the pump is doing.
>
>You are exactly right here. This is the misunderstanding. By
>restricting the water flow you do not increase the load on the pump.
>You decrease it. Restricting flow does not increase load, more water
>flow increases load. Work really is equal to force times distance.
>The more you move a given distance the more work is done. The less
>you move a given distance the less work is done. More water, more
>work. Less water, less work.
>
>Please before you really start calling me names and questioning my
>intelligence put the amp meter on the pump and gradually decrease the
>flow. You will be quite surprised.
>
>It becomes easier to see with a gas pump. I have a 3 hp cent pump to
>fill my 1000 gal field sprayer. With the valve fully opened and max
>water through it the motor is pulled down to a fraction of it's no
>load speed and obviously working very hard. You start shutting the
>valve and the motor just keeps easing up until it's just running like
>it's hooked up to nothing. It's pretty dramatic the difference.
>
>If you shut off the water flow out of a pump the work it's doing is
>close to zero. Only heating the water in side the pump a little from
>moving the water around and pressure but no work is being done. Your
>pump may burn up because it was designed to run at a lower rpm or it
>uses the water as a coolant but not because it's being over worked.
>
>Honest. I wouldn't kid my fellow friends on the group. :)
>
>Ct Midnite
>
>If you want a easy test of what I'm saying go get out your canister
>vacuum cleaner. Put you hand over the tube. The motor will speed up.
>And not because it's doing more work. Because it's doing less.
>
>
>>Hi CT
>>
>>Your were obtaining correct measurements but inherintly FALSE readings
>>do to an effect called CAVITATION!
>>Where CAVITATION is present, WORK ceases(or is reduced), the LOAD is
>>reduced due to Cavitation, ergo current drop is eminent.
>>
>>It's akin to going uphill in your car, if you restrict the vehicles
>>upward movement to the point the drive wheels lose traction and begin
>>to spin on the pavement, the horsepower consumed will decrease due to
>>less friction between the tires and road, but the WORK will also be
>>creatly reduced if not ceased entirely as on ice, and you could
>>actually begin to slide backwards down the hill for loss of traction.
>>
>>I can guarantee you that increasing the load on a pump increases its
>>power consumption. If you do not show an increase in power
>>consumption, then you have not increased the load the pump is doing.
>>
>>If increasing the load the engine must do, decreased the current or
>>horsepower needed, then we would need a 450 Cummings engine to drive a
>>wris****ch and a hearing aid battery to power an aircraft carrier.
>>
>>TTUL
>>Gary
>
>
>http://www.geocities.com/ctmidnite53/

[Ct Midnite]

Hey Gary,

Ok I just looked up the cut out view on a mag pump that seems to be
very popular with this group. It's just a common, though maybe high
quality, centrifugal pump. Same design as other little propeller
pumps. Do you think some how that aquarium pumps are a close
tolerance special type of pump? It's just a little water wheel that
kicks water to the outside and out a hole. If you restrict the flow
water just goes on around and hopes to get out the next time around.

If you know the term for "going on around" and it's cavitation then
ok, cavitation goes on in all centrifugal pumps. Positive
displacement pumps don't work that way but we usually don't use that
type of pump for aquariums.

I don't know what type of pumps you work with in your line of work but
these aquarium pumps are no big deal.

Let's try a different approach. You used the term "load" when you
talk about a restriction in the line. You said "I can guarantee you
that increasing the load on a pump increases its
power consumption." This is true and where the misunderstanding comes
in.

In an electrical circuit when you increase the "load" you introduce
something with less resistance than you had before. Such as the load
is increased when you jump from a 10 watt bulb to a 100 watt bulb.
The 10 w had more resistance than the 100 w bulb. You decrease the
resistance and increase the load.

The exact same thing is involved in your water circuit. You open the
valve (decrease the resistance), you increase the load. Not the other
way around. Closing a valve or restricting flow does not increase the
load, it lessens it. Water flow is the load. The more of it, the
more the load.

One more thing. If you agree that work = force x distance and you are
moving less water but consuming more amps then where is all that
energy going? Motor heat? I think they would burn up very quickly.

Did you do a test and see or just know I'm up to my eyebrows? :)

Love talking physics. :)

Ct Midnite

On 08 Mar 2004 09:23:28 EST, (Gary V.
Deutschmann, Sr.) wrote:

>Hi CT
>
>I agree with you on the work = force x distance.
>
>Restricting a flow downstream does not generate more work upstream.
>However, we are talking about a closed system.

Doesn't matter.

>Within this system we have friction from the piping, which increases
>work and/or slows down the flowrate, while at the same time increasing
>head pressure.

All true but not sure the point.

>When you restrict the flow, you increase head pressure (backpressure),
>this in turn increases the psi (force) of the pump. If your pump
>pumps 5 gallons per minute at 1 psi and you clamp it down to 1 gallon
>per minute, the backpressure would jump up to 5 psi. Provided there
>is no backwash or centrifugal loss, which for this purpose is the same
>as cavitation. 5x1=5 or 1x5=5

That's the point. The pumps we use have lots of backwash and cent
loss. Otherwise you wouldn't ever have a max head. It would pump as
high as you wanted. There would be no max head.
>
>Your example of the vacuum cleaner hose being blocked is a perfect
>example of cavitation, the movement of air is stopped and the blade
>freewheels spinning the air around it as if it is a part of the blade
>itself, no work is being performed so the motor speeds up, it has no
>load, therefore less current is drawn.

Vacuum cleaner pumps and cent pumps we use are very similar.
>
>Ohm's Law is pretty cut and dried!
>
>I think the test you conducted was on a pump that allows plenty of
>backwash, which means it does not have the ability to generate higher
>psi by restricting the output. It's output psi is basically a fixed
>pressure. You can read that as an impeller with plenty of spacing
>between the sidewall (housing) and the impeller blades.

I think the pumps we use do too.

>On a pump like this, reducing the flowrate does decrease the load on
>the motor. But only because the backwash area is designed to allow
>for a partial cavitation so the pump may be throttled without
>increasing the load on the motor. The reason you don't see an
>increase in current draw is because it has a fixed psi output rating.
>In affect, the inside of the pump head is acting exactly like a
>bleedoff line for the excess, but because of additional partial
>cavitation within the head by using this method, current draw could
>actually decrease.

All of the mag I look up today said specifically that they could be
throttled. So what are we arguing about. :)


>So, with the particular pump you are using in your test, the output
>psi is a fixed number. Therefore the FORCE is a fixed number, and YES
>the load on the pump is decreased with less water flow. But it is
>still due to cavitation or whirlpool affect inside the pump head,
>which was designed especially and on purpose for this to occur.
>In simple terms, using a fixed FORCE, such as 1 horsepower, means that
>pumping 1 gallon per minute would require LESS WORK and less
>horsepower than pumping 5 gallons per minute. BUT, the psi (other
>than that caused by friction on the lines) will remain the same at the
>output.

You know I don't have a mag pump or anything like it. I've been using
power heads to move my water around and I have no reason to restrict
them. I wish I did have one because I would test it out immediately.

You know I'm sure I've been wrong a couple of times in my life but I
still don't think this is one of them. :)

Wish you would and let us know what the amp meter said and when.


>What you are seeing and hearing and testing is NOT the whole picture
>of what is going on inside that pump!
>
>TTUL
>Gary
>
>
>
>
>
>
>
>Ct Midnite <mreef2.10.muffin@spamgourmet.(nospam)com> verbositized:
>
>>On 07 Mar 2004 10:01:04 EST, (Gary V.
>>Deutschmann, Sr.) wrote:
>>
>>I certainly don't want to get into a flame war. Please believe me
>>when I say that your take on this is the take the vast majority of
>>peoples have. In my class it was 24 to 2 in your favor. But it is
>>wrong.
>>
>>>I can guarantee you that increasing the load on a pump increases its
>>>power consumption. If you do not show an increase in power
>>>consumption, then you have not increased the load the pump is doing.
>>
>>You are exactly right here. This is the misunderstanding. By
>>restricting the water flow you do not increase the load on the pump.
>>You decrease it. Restricting flow does not increase load, more water
>>flow increases load. Work really is equal to force times distance.
>>The more you move a given distance the more work is done. The less
>>you move a given distance the less work is done. More water, more
>>work. Less water, less work.
>>
>>Please before you really start calling me names and questioning my
>>intelligence put the amp meter on the pump and gradually decrease the
>>flow. You will be quite surprised.
>>
>>It becomes easier to see with a gas pump. I have a 3 hp cent pump to
>>fill my 1000 gal field sprayer. With the valve fully opened and max
>>water through it the motor is pulled down to a fraction of it's no
>>load speed and obviously working very hard. You start shutting the
>>valve and the motor just keeps easing up until it's just running like
>>it's hooked up to nothing. It's pretty dramatic the difference.
>>
>>If you shut off the water flow out of a pump the work it's doing is
>>close to zero. Only heating the water in side the pump a little from
>>moving the water around and pressure but no work is being done. Your
>>pump may burn up because it was designed to run at a lower rpm or it
>>uses the water as a coolant but not because it's being over worked.
>>
>>Honest. I wouldn't kid my fellow friends on the group. :)
>>
>>Ct Midnite
>>
>>If you want a easy test of what I'm saying go get out your canister
>>vacuum cleaner. Put you hand over the tube. The motor will speed up.
>>And not because it's doing more work. Because it's doing less.
>>
>>
>>>Hi CT
>>>
>>>Your were obtaining correct measurements but inherintly FALSE readings
>>>do to an effect called CAVITATION!
>>>Where CAVITATION is present, WORK ceases(or is reduced), the LOAD is
>>>reduced due to Cavitation, ergo current drop is eminent.
>>>
>>>It's akin to going uphill in your car, if you restrict the vehicles
>>>upward movement to the point the drive wheels lose traction and begin
>>>to spin on the pavement, the horsepower consumed will decrease due to
>>>less friction between the tires and road, but the WORK will also be
>>>creatly reduced if not ceased entirely as on ice, and you could
>>>actually begin to slide backwards down the hill for loss of traction.
>>>
>>>I can guarantee you that increasing the load on a pump increases its
>>>power consumption. If you do not show an increase in power
>>>consumption, then you have not increased the load the pump is doing.
>>>
>>>If increasing the load the engine must do, decreased the current or
>>>horsepower needed, then we would need a 450 Cummings engine to drive a
>>>wris****ch and a hearing aid battery to power an aircraft carrier.
>>>
>>>TTUL
>>>Gary
>>
>>
>>http://www.geocities.com/ctmidnite53/


http://www.geocities.com/ctmidnite53/

[Ross Bagley]

Ct Midnite <mreef2.10.muffin@spamgourmet.(nospam)com> writes:

> If you know the term for "going on around" and it's cavitation then
> ok, cavitation goes on in all centrifugal pumps.

Cavitation is when the pressure behind/around the impeller blades
causes gas to bubble out of solution. In our tanks, this gas is made
up of dissolved atmospheric gases as well as water vapor.

On a boat propellor, cavitation (normally from overpowering an
aggressively pitched propellor) results in very fast wear on the
following surfaces down to the trailing edge largely due to the shock
waves that accompany the expansion and collapse of the cavitated
gas bubbles.

I don't really know how an aquarium pump can cavitate, given that pump
designers are certainly aware of the damage that cavitation can do to
the impeller surfaces and would match the impeller and impeller cavity
with the motor power and magnetic coupling (I'd design the magnetic
coupling to slip before the impeller cavitated).

I wonder if people are using the word cavitation to mean that there's
entrained bubbles in the input flow. Bubbles in the intake flow will
certainly reduce pump efficiency, though how much the efficiency is
reduced is probably related to bubble size and percentage of gas in
the gas/water mix. But that's not cavitation, at least not strictly
speaking...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Gary V. Deutschmann, Sr.]

Hi Ross

The gas bubbles are only secondary to what cavitation really is!
Cavitation is the formation of a partial vacuum in a liquid by a fast
moving solid object, such as impellers, ceasing momentum or flow
through the area of the vacuum.
A partial vacuum will cause an outgasing of certain gasses and enough
vacuum will even outgas the liquid itself leaving behind only the
solids that were suspended in the liquid.

The event of Cavitation is not at all harmful to the screw or impeller
and actually reduces the friction across the face of the screw or
impeller blades. However, the bearings and engine driving it, running
under no load condition may be severely damaged.

TTUL
Gary

[Gary V. Deutschmann, Sr.]

Ct Midnite <mreef2.10.muffin@spamgourmet.(nospam)com> verbositized:

>
>Hey Gary,
>
>Ok I just looked up the cut out view on a mag pump that seems to be
>very popular with this group. It's just a common, though maybe high
>quality, centrifugal pump.
In a very diluted sort of way perhaps. Aquarium pumps are normally
impeller driven pumps that use the centrifugal action of the water
itself to attain head pressure and flow rates.
True centrifugal pumps use a massive spinning pump head and can
develop extremely high head pressures and normally have no blades of
any type associated with them.

> Same design as other little propeller
>pumps. Do you think some how that aquarium pumps are a close
>tolerance special type of pump?
Not at all, the clearance inside the pump head, between the impellers
and the housing is often very sloppy with plenty of space.

>It's just a little water wheel that
>kicks water to the outside and out a hole.
EXACTLY - It spins the water into centrifugal action, which water that
does not exit the hole is in a state of cavitation.
To PROVE cavitation is present in this type of pump design, and that
you will find that the lossy space around the impeller is in
cavitation, you can (if the pump is primed) use it above the water
level to pump water out of a container.
In other words, a lossy method of pumping by centrifugal action of the
water by an impeller, should not be able to develop a vacuum as a true
tight pump would do naturally. Therefore a vacuum must be created by
some action that overcomes the spacial gap between the housing and
impeller. The only action that can do this, as far as I understand,
is that of cavitation, the action of creating a vacuum in a
pressureless body of water.

>If you restrict the flow
>water just goes on around and hopes to get out the next time around.
The PERFECT definition of Cavitation!

>If you know the term for "going on around" and it's cavitation then
>ok, cavitation goes on in all centrifugal pumps.
Cavitation goes on in all impeller headed pumps, in most screw drive
pumps. But in true centrifugal pumps, cavitation is nearly impossible
in the true sense of cavitation. The pump head is always moving
faster than the water through it.
Think of it this way, if you stir a glass of tea, and moving your
spoon faster than the tea is spinning in the glass, cavitation is
present. But no matter how fast you spin the glass itself, the tea
inside is not in a state of cavitation.

I can see why impeller driven pumps are erroneously called centrifugal
pumps, because the pump causes centrifugal action of the liquid the
pump is pumping. But the pump itself is not operating as a
centrifuge. Centrifugal pumps are most often utilized where solids
need to be extracted from the water while maintaining very high head
pressures or where extremely high head pressures alone are needed.

>Positive
>displacement pumps don't work that way but we usually don't use that
>type of pump for aquariums.
Well, unless you have a Doser!

>I don't know what type of pumps you work with in your line of work but
>these aquarium pumps are no big deal.
We use little MaxiJet1000's for most of our packaging operations.
They are cheap, relatively efficient (meaning a MaxiJet900 uses much
less current for roughly the same output), and they seem to hold up
forever with no problems.

But I have had many years of experience with pumps of all types!
I was virtually raised with my feet wet all the time. Our family
operated 12 acres under glass, most hydroponically, plus we had
several outdoor irrigation and sprinkling systems. We had some
extremely high pressure pumps to run numerous water hydraulic powered
Skinner lines, etc.
Plus for a short period of time, I was into commercial fountain design
and water show applications.

>Let's try a different approach. You used the term "load" when you
>talk about a restriction in the line. You said "I can guarantee you
>that increasing the load on a pump increases its
>power consumption." This is true and where the misunderstanding comes
>in.
In a true lossless pump, restricting the flow will cause an increase
in the head pressure as it tries to maintain its design flowrate.
Take a simple garden hose! If your water supply is at 60psi at the
tap, because of losses going through the tap, your line pressure is
more than likely cut in half outside the tap at full flow rate.
Ok, add a hose to the tap and there is resistance in the hose, by the
time you get to the end of the hose, the resistance of the hose has
also decreased the psi at the output of this hose. Let's assume you
have only 20psi at the unrestricted output end of the hose. Let us
also assume that the output flowrate is 5 gallons per minute at 20psi.

You then add a large bore sweeper nozzle to the hose. Two things will
occur after you do this. The smaller opening would dictate that you
will be moving less water, but, backpressure will be created that will
increase the psi at the nozzle probably back up to 30psi as the hose
tries to deliver it's full flow rate. With the increased psi at the
nozzle, you are back up to 5 gallons per minute, but with the smaller
nozzle you are gaining more DISTANCE from the stream, or if you
measure the FORCE at the same distance the hose stream fell without
the nozzle, you will find the FORCE to be greater.

Switch to a smaller sweeper nozzle and you definately will decrease
the gallonage output because there will not be enough psi to maintain
the flowrate of 5 gallons per minute through this small orifice. But
if you placed a pressure guage on the hose itself, you will find that
you also overcame the psi loss through the tap and the hose psi could
very well be up to 60 psi at the nozzle because the flow rate is
reduced. Allowing the source to keep up with the demand!

>In an electrical circuit when you increase the "load" you introduce
>something with less resistance than you had before. Such as the load
>is increased when you jump from a 10 watt bulb to a 100 watt bulb.
>The 10 w had more resistance than the 100 w bulb. You decrease the
>resistance and increase the load.
Precisely! Ohm's law, which can also be applied to fluid dynamics.
A 10 watt bulb at 120 volts has 1,440 OHM's of Resistance. The higher
the resistance (in Ohm's) the least amount of Current will flow. BUT,
you are overlooking the Amperage which is also Current. A 10 watt
lamp will pull .083 Amps, and a 100 watt lamp will pull .833 Amp,
roughly 1/10th Amp versus a full Amp in round figures.

When you increase head pressure by decreasing the flow rate, you are
also lowering one type of resistance Ohms, not increasing it, while
increasing the resistance as Amperage.
In the example above 10 watts is 1,440 Ohms and 100 watts is only 144
Ohms of resistance. In resistance as Ohms. The higher the resistance
the LEAST amount of Current will flow.
BUT, the reverse is true when looking at Current as Flow rate.
In the example above 10 watts is .083 Amps and 100 watts is .833 Amps.

We are looking at this from a constant preset voltage or head
pressure. Voltage is what determines the push or pressure behind
current flow.
If we changed our figures to show a varying voltage based on a fixed
flowrate at different psi levels, I think you would be surprised at
the answers.

>The exact same thing is involved in your water circuit. You open the
>valve (decrease the resistance), you increase the load. Not the other
>way around. Closing a valve or restricting flow does not increase the
>load, it lessens it. Water flow is the load. The more of it, the
>more the load.
Nope! A closed water circuit will maintain a steady psi at whatever
the pump is designed to maintain. You open this circuit with a valve
allowing the water to flow (current) the psi will (theoretically drop
equally in the whole circuit), but in real life, you will have
differing psi levels along the length of the circuit based on the
various resistances it encounters along that route. The psi at the
head is constant, but the psi at a 5 foot head is exponentially
reduced.
Comparing Flowrate (amperage) to Load (watts) is the same measurement
only using different numbers from different ends of the scale, when
not compared with Pressure (voltage) or psi (power).

It's like saying my car has more horsepower than yours because my
tires are smaller. Although it is true that a smaller tire will
produce the greater horsepower on the friction plane (where the rubber
meets the road) if driven by the same engine and gear ratio to a
larger tire on that same friction plane.
But it is not true in the sense of what horsepower is sitting under
the hood.
It is possible to make a tire so large that the engine cannot develop
enough horsepower to turn it. Or Restrict the forward movement of the
car so that the small tire will lose it friction plane more easily
than the larger tire would. In either case, the tires will lose their
friction and begin spinning on the pavement. In such a case, the
RESTRICTION has caused Cavitation, and the engine is not working as
hard, thus using less current, but it still has the same horsepower.

>One more thing. If you agree that work = force x distance and you are
>moving less water but consuming more amps then where is all that
>energy going? Motor heat? I think they would burn up very quickly.
P (power) = E (volts) x I (amperage)
Work (power) = Force (PSI) x Distance (amperage)
Moving LESS Water over the SAME distance but at a higher PSI.
If the POWER of the pump is equal, All of that energy is going to
increase the FORCE or PSI at the head. Unless the head begins to
cavitate.

>Did you do a test and see or just know I'm up to my eyebrows? :)
When working with irrigation, and or fountains for that matter, if I'm
using a 5 horsepower pump where a 20 horsepower pump is needed, I will
not get the desired FORCE to cover the Distance the water must travel.
All of the horsepower is consumed in trying to overcome resistance in
the lines and head height.
The more resistance in the lines, the harder the pump has to work to
force that water through them. Increasing the pipe diameter reduces
the resistance against the pump head, psi at the pump head goes down,
while current, flowrate increases, but it is still using 20 horsepower
to drive the head. Resistance in this case is equivalent to current
not ohms. Resistance does control both voltage (horsepower) and
current flow. But it does not control the physical horsepower of the
engine itself, it only controls the available horsepower used by the
pump head.
I'm sure you have used a gasoline push mower before. What happens
when the blades hit the unmowed grass? The engine slows down because
of the resistance of the grass against the blades of the mower. The
governer applies more gas to the engine to bring the rpms (psi) back
up. The engine is working HARDER due to the increased resistance
against the blades.
Taller grass is more restrictive because it needs to be cut many more
times before exiting the discharge chute. It is more dense and
therefore harder to cut and requires more horsepower.

But what if you had a lawnmower with NO governer, one set at a fixed
horsepower. If the grass was a certain low height, you could zip
through the lawn cutting it fairly rapidly. You are covering MORE
Distance using Less FORCE PSI because the resistance is low.
But if you hit taller grass, MORE resistance, your Distance will
decrease in order to maintain the same FORCE.
If you continue at the same Distance, flowrate, against this higher
resistance, it will require more FORCE psi, up to the limits of the
preset horsepower of your mower. If you cross that point, the mower
will die from overload beyond it's available horsepower.
Now, if it were possible for the mower to get choked with grass in
such a way that it clung to the blades and quit cutting, as you passed
over uncut grass it would just be fanned downward and not cut, the
engine would build back up to speed RPMs PSI, but no work is being
performed, therefore the engine is for all purposes just freewheeling
at idle.

It takes MORE juice to run a 100 watt light bulb than a 10 watt.
By the same token, it takes more juice to pump 5 gallons per minute,
than it does 1 gallon per minute.
Which I know you are going to say, that is what I was saying all
along!
But it's not really. If you take a 20 horsepower motor to do the work
of a 5 horsepower motor, you are wasting 15 horsepower of juice.
If the pumphead was near lossless, by restricting the pumpheads
output, like hitting tall grass with a mower, the pumps motors
governer will kick in and speed up the fuel to the motor to try to
overcome the added load on the head. In other words, trying to keep
the psi up to where it was established by design.

Aquarium pumps are much different as they allow and actually rely on a
certain amount of cavitation within their design. Even though you may
stop the flow exiting the pump, the impeller is still spinning against
it's highest rated psi rating, unless of course it is in full
cavitation. But set cavitation aside for a moment. If the pump
cannot go into cavitation, at full output the psi is very low.
As you restrict the output, the flowrate goes down, but the psi at the
head goes up. Maintaining psi is where the energy is consumed, psi is
what moves the water. Unless restricted, the higher the psi rating,
the higher the flowrate will be. But in a sense, the reverse is also
true. A 4 cylinder car can go 80 miles an hour (flowrate) just as an
8 cylinder car can go 80 miles an hour. Now we hit a hill, which car
needs to use all it's got to climb that hill. The hill is a
restriction to the Distance unless more FORCE psi is applied to the
drive train (impeller). Albeit, the 8 cylinder has more horsepower
and may consume more gasoline than the 4 cylinder. But by the same
token, it will not have to work as hard to climb the hill.

Placing a restriction in the path of something will increase the LOAD
on the device (engine) pushing it. In aquarium pumps, the engine has
a fixed voltage, therefore the only thing that can happen is the
current consumption go up to do the work. Now lets assume that both
the voltage and current are fixed amounts, like the lawnmower with a
fixed throttle and horsepower. As you clamp down on the output hose,
restricting the flow, yes you are decreasing the flowrate, but the psi
at the pump head has gone up, and if you restrict the output side
enough the pump head will reach it's maximum psi potential. Restrict
it further and the pump will go into partial or even full cavitation.

>Love talking physics. :)

Not me, it gives me headaches, hi hi.....

I think what you are experiencing by testing the current draw on your
pump is like what is experienced with a high pressure tank with a set
of control valves. The tank may be at 3000 psi, but the valves
control that pressure to maintain only 30 psi on the lines.
The nozzle of the torch is set to use X number of cu ft of gas per
hour, flowrate. By restricting this flow to lets say 1/2 of the
flowrate, you have not increased the load on the regulating valves.

Aquarium pumps have enough internal backwash between the impeller and
the housing that restricting the flowrate does not appreciably affect
the psi rating of the pump to any measurable degree.

You might think of it like a gearbox. If you gear the output shaft
low enough, even a AA battery powered motor could create enormous
horsepower and force at the output shaft, but at such a decreased
Distance and Torque that it is virtually useless for anything. Any
load applied to this output shaft would probably not be seen at the
drive end.

Think of those little wall transformers. They are a closed circuit,
consuming electricity whether you use them or not. The psi is
available at the output side, whether you use it or not. They can
only supply up to the amperage of their design, without affecting the
input side at all. Aquarium pumps are much like that. The run
merrily along at their pre-designed PSI, whether they are at full flow
or at near full restriction. At full restriction, they would also
have to be in full cavitation and psi will drop to zero.
Have you ever tried to start the flow in an aquarium pump that went
into cavitation? About the only way is to shut it down and restart
it, allowing the cavitation to cease, or the air extracted by the
vacuum to escape.

TTUL
Gary

[Ct Midnite]

Ok, you've succeeded. You've given me a headache too. :)

Too much to answer quickly, but I'll get back to you.

Ct Midnite

http://www.geocities.com/ctmidnite53/

[Ross Bagley]

(Gary V. Deutschmann, Sr.) writes:

> Hi Ross
>
> The gas bubbles are only secondary to what cavitation really is!

Well, as I stated in my post, there are at least two kinds of
bubbles we're talking about: bubbles due to cavitation and gas bubbles
mixed into the pump intake.

The following section shows that you understand what cavitation is.

> Cavitation is the formation of a partial vacuum in a liquid by a fast
> moving solid object, such as impellers, ceasing momentum or flow
> through the area of the vacuum.
> A partial vacuum will cause an outgasing of certain gasses and enough
> vacuum will even outgas the liquid itself leaving behind only the
> solids that were suspended in the liquid.

Which is a decent restatement of what I said in my post. But what
still hasn't been explained is how a properly designed pump would
overdrive it's impeller and result in cavitation.

Simply closing off the output won't cause cavitation in a pump as the
water in the impeller chamber will simply "spin up" with the impeller
and join it going around and around, with some turbulence coming off
of the now blocked chamber output port and a little less turbulence
than that rolling off of the unmoving outside surface of the
centrifugal cavity.

> The event of Cavitation is not at all harmful to the screw or impeller
> and actually reduces the friction across the face of the screw or
> impeller blades.

Erm. Cavitation leaves a visibly characteristic erosion on propellor
blades. For propellors that are overdrives, cavitation damage is the
primary culprit in early replacement. Call up any boat store and ask
them if you should worry if you think your prop is cavitating and why.

If you can provide evidence that cavitation doesn't effect centrifugal
impellers, I'll accept it, but I strongly suspect that cavitation
simply isn't happening in the pumps. Cavitation is and extremely
violent transition from liquid to gaseous and back again. It would
be very noisy if there was a steady-state cavitation process occuring.

> However, the bearings and engine driving it, running
> under no load condition may be severely damaged.

Cavitation doesn't eliminate or even reduce the load on a propellor.
During cavitation, the leading edge and the back of the blade
pressures are still quite high. On the front of the blade, however,
is a bubble, which is, as you say, made up of water vapor in the
vacuum created by the propellor.

Now, when you say that cavitation is happening in an aquarium pump,
exactly what circumstances can cause it to happen? As I previously
explained, I highly doubt that simply blocking the output would cause
cavitation. What common conditions are you thinking cause cavitation
in an aquarium pump?

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ross Bagley]

(Gary V. Deutschmann, Sr.) writes:

> EXACTLY - It spins the water into centrifugal action, which water that
> does not exit the hole is in a state of cavitation.

This statement is not true.

> To PROVE cavitation is present in this type of pump design, and that
> you will find that the lossy space around the impeller is in
> cavitation, you can (if the pump is primed) use it above the water
> level to pump water out of a container.

You've demonstrated that a centrifugal pump generates a vacuum on
it's input, but you haven't demonstrated that that vacuum is enough
to pull gasses out of solution.

The vacuum that causes 100% vaporization of water is rather strong.
As in right about 32 feet of head. None of the centrifugal pumps we
can purchase can pull 32 feet of head.

A vacuum is not cavitation. Cavitation is the creation of bubbles
by creating enough vacuum that the fluid vaporizes. Cavitation is
extremely violent and there's no evidence that it ever happens in
an aquarium pump.

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ct Midnite]

Hey Gary,

I tend to agree with Ross. I'm not sure cavitation is going on in the
pumps we use. When you block or restrict the output water just goes
around in the pump. I'm not sure it produces bubbles of any kind.
And if it is it's not continually creating work for the motor. If the
bubbles are formed they would give back the work when the disappeared.
And if they stayed they also won't create continual work. But I
didn't introduce that term so enough of that.

Ok I still have a cent well pump here on the farm hooked to a pressure
tank at one location. (Used to have them everywhere. Now mostly we
have rural water. ) Last night I went to the pit and listened as the
pump turned on and went through a cycle from low pressure that turns
it on to high pressure that turns it off. It started out and as it
progressed the frequency of the motor climbed higher and higher until
it turned off. This is the exact same pump as a little giant or most
other of the really big pumps used in aquaria. Very simple cent pump
designed to give a fairly high pressure at high rpms. Close to the
same principle as a little mag pump.

Now this is exactly what I thought would happen but wanted to make
sure I wasn't up to my eyebrows before answering you. I didn't have
an amp meter on this but I know that the only way that a motor can
increase in rpm's is for it to be doing less work. You don't increase
the amps being consumed if the motor speeds up. I think if any of you
would listen to your pumps and slowly close a valve you would hear the
same thing, increased rpms. If we can't agree that higher rpm's equal
less current draw then we have a problem here only an amp meter would
solve. But I would hope that most would see that an increased motor
speed would mean the motor is doing less work. If your motor doesn't
do this then you have a non thottleable pump which most of you would
not have.

There are two types of pumps that I have ran into over the years that
cannot be throttled. One was a piston pump that took in liquid
through one valve and pushed it out another. This type of pump has to
have a bypass or it will just stop turning when restricted. You can't
compress a liquid.

The other pump is a vain pump used in gas pumps. In these the
impeller is offset in the pump housing. The vain instead of fixed
like in cent or impeller pumps are fitted into slots on the rotor and
would move in and out freely to make water tight cells in the pump
head as the rotor turns. They actually come in contact with the
inside surface of the housing. Since the rotor is offset as it turns
the cells actually change volume. They take liquid in at the largest
size cell and it exits at the smallest size. These pumps also have to
have a bypass valve or they stop turning. Again you can't compress a
liquid.

Both of these pumps could be restricted before the intake but I have a
feeling they would not react favorably to that, but they wouldn't stop
turning, they would form little pockets of vapor inside the pump that
would expand and contract.

There are probably other types of pumps that are of the positive
displacement type (liquid in equals liquid out, no ifs, ands, or buts)
but I don't know what they are. I've used of a roller pump years ago
but don't know how it worked. Never had it apart.

These impeller pumps we use are neither type. If flow is restricted
water just goes on around inside the pump with little work being done.
It's much easier to circulate inside the pump head than come out the
hole and proceed down the line. The motor doesn't know what's going
on in the pump, it just knows that it doesn't have to work as hard and
speeds up. I have never seen it otherwise with a type like these.

If you restrict the output or input and you think that the motor is
doing more work (drawing more current) and the fact is externally it's
obviously not (water is not being moved), then the only place it can
go is into heat. Energy can't be destroyed. If you're drawing more
energy in and getting less energy out the only explanation is heat
generation, and lots of it. And this doesn't happen. In a fully
blocked pump the water may warm up inside the housing over a few
minutes time but this is mainly from the motor. And the motor may
warm up a little but this is because the water flow is not carrying it
away.

I know it's a difficult concept to grasp. If it's creating more
pressure, it has to be working harder. But force isn't work. And as
long as the water can just circulate inside the head creating force
does not equal more work.

It's kind of like winding a spring. It takes some energy to wind the
spring (create pressure at the output of the pump) but once it's done
(which is almost instantaneous in a blocked pump) there is no work
being done after you wind the spring until you use that force (let
water flow). People get force and work mixed up all the time. The
pump can have the pressure at the output for ever available but no
work is being done until it is used. It's just spinning around inside
the head.

Now I think (could be wrong) you might have granted that this may be
true if you shut it off entirely but it is true from the moment you
start shutting the valve until it is finally closed. The amperage
will drop. A fairly straight line graph.

If some one would do the test I will guarantee you that the amperage
used on a pump fully throttled would be very close to the amperage
used on a dry pump, no water present.

That last statement has to be complete madness from your point of
view. :)

Thanks for the mental exercise.

Ct Midnite





http://www.geocities.com/ctmidnite53/

[Gary V. Deutschmann, Sr.]

Hi Ross

Most of your questions and responses are looking at cavitation in it's
Shear instance. Whereas most of my responses have been in referrence
to cavitation in its gyrational or Centrifugal instance.

A boat in an open body of water when the propeller, prop, screw, is
overdriven results in Shear Cavitation. And I agee, yes it is very
harmful to the prop.

But in a closed system, such as inside a pump housing, when the output
is restricted, or the input for that matter, throughput is reduced or
ceases alltogether. The fluid inside the pumphead goes into
gyrational motion and keeps pace with the speed of the impellers
rather than being driven out of the pumphead by centrifugal force.

A driven screw or impeller that looses it's ability to perform the
work due to gyrational movement of the fluid within it's blades is
also called Cavitation, in this case Centrifugal Cavitation. The
centrifuge action creates a vacuum near the axle and causes outgassing
from the fluid. And that is why the term Cavitation still applies to
flat bladed centrifugal action pumps.

I have not studied the effects of cavitation on open screws such as a
boat props. So my statement concerning them is more than likely in
error. Although I have had cavitation occur many times while trying
to get skiers up and out of the water, the only thing I worried about
at the time was blowing the motor, which would naturally rev up due to
loss of friction with the water passing through the blades.
Not much unlike a car tire losing friction on the roadway.

TTUL
Gary


(Ross Bagley) verbositized:

(Gary V. Deutschmann, Sr.) writes:
>
>> Hi Ross
>>
>> The gas bubbles are only secondary to what cavitation really is!
>
>Well, as I stated in my post, there are at least two kinds of
>bubbles we're talking about: bubbles due to cavitation and gas bubbles
>mixed into the pump intake.
>
>The following section shows that you understand what cavitation is.
>
>> Cavitation is the formation of a partial vacuum in a liquid by a fast
>> moving solid object, such as impellers, ceasing momentum or flow
>> through the area of the vacuum.
>> A partial vacuum will cause an outgasing of certain gasses and enough
>> vacuum will even outgas the liquid itself leaving behind only the
>> solids that were suspended in the liquid.
>
>Which is a decent restatement of what I said in my post. But what
>still hasn't been explained is how a properly designed pump would
>overdrive it's impeller and result in cavitation.
>
>Simply closing off the output won't cause cavitation in a pump as the
>water in the impeller chamber will simply "spin up" with the impeller
>and join it going around and around, with some turbulence coming off
>of the now blocked chamber output port and a little less turbulence
>than that rolling off of the unmoving outside surface of the
>centrifugal cavity.
>
>> The event of Cavitation is not at all harmful to the screw or impeller
>> and actually reduces the friction across the face of the screw or
>> impeller blades.
>
>Erm. Cavitation leaves a visibly characteristic erosion on propellor
>blades. For propellors that are overdrives, cavitation damage is the
>primary culprit in early replacement. Call up any boat store and ask
>them if you should worry if you think your prop is cavitating and why.
>
>If you can provide evidence that cavitation doesn't effect centrifugal
>impellers, I'll accept it, but I strongly suspect that cavitation
>simply isn't happening in the pumps. Cavitation is and extremely
>violent transition from liquid to gaseous and back again. It would
>be very noisy if there was a steady-state cavitation process occuring.
>
>> However, the bearings and engine driving it, running
>> under no load condition may be severely damaged.
>
>Cavitation doesn't eliminate or even reduce the load on a propellor.
>During cavitation, the leading edge and the back of the blade
>pressures are still quite high. On the front of the blade, however,
>is a bubble, which is, as you say, made up of water vapor in the
>vacuum created by the propellor.
>
>Now, when you say that cavitation is happening in an aquarium pump,
>exactly what circumstances can cause it to happen? As I previously
>explained, I highly doubt that simply blocking the output would cause
>cavitation. What common conditions are you thinking cause cavitation
>in an aquarium pump?
>
>Regards,
>Ross
>
>-- Ross Bagley http://rossbagley.com/rba
>"Security is mostly a superstition. It does not exist in nature...
>Life is either a daring adventure or nothing." -- Helen Keller
>
>

[Gary V. Deutschmann, Sr.]

Hi Ross

Look up the other definitions of Cavitation!

Such as Cavity Cavitation caused by a gyrational or centrifugal
actions.

I do agree with you that the term Cavitation is primarily meant to
mean the effect that causes separation of liquid parts often into a
gasseous state.

But Cavitation is also applied to almost anything wherein a cavity is
formed. A hole in the ground is also a cavity, but that also is
something entirely different.

How about if we take a totally different approach!

You tell us what TERM should be used when, in the instance of a closed
pump head, the fluid quits to flow and just spins around in it's own
housing driven by the impeller?

The ONLY term that I know of that applies to this phenomenon is
Cavitation. And cavitation applies also to even low inches of vacuum.

TTUL
Gary

[Gary V. Deutschmann, Sr.]

Hi CT

All of the observations you have made, and your take on them in your
correspondence below is accurate!

Reducing or restricting the flow is physically causing less work for
the motor, ergo less current draw, less juice being used.

But the real question is WHY?

You already noted that pumps with no backwash or relief valve will
work harder or stall if they are restricted! If partially restricted
they will draw more current trying to overcome the restriction, with
an increase in head pressure as well.

So the question is still WHY, on some pumps, does the current draw go
down, the work on the motor become less and/or the engine speeds up.

It is simply because the pumphead has a built-in backwash system or to
please Ross, rather than say go into Cavitation, I will simply say the
water inside the pumphead begins to spin around with the impeller
blades and loses its friction. It turned gyrational!

In a properly operating pump, the water enters near the center of the
impellers, these impellers PUSH the water around and outward due to
centrifugal force of the water being slung around and outward at the
same time. If the pump is efficient, there is very little backwash
and very little loss.

But as you increase the resistance against these actions, they lose
friction and begin to spin within the impellers without being forced
outward. Centrifugal action decreases.
There being LESS friction, the motor uses less current and does less
work.

So you see, your observations are correct! And your measurements are
correct. But your original statement is flawed only in that it does
not contain the CAUSE of the motor using less energy.

It is NOT the reduced flowrate that results in the lower current draw.
Reducing the flowrate by restriction should cause an increase in the
work the motor is doing and an increase in the current draw of the
motor. Therefore, something ELSE must be happening!

For lack of a better word, the pumphead has gone into Cavitation, has
lost its friction with the fluid passing across it's impeller blades,
and is just sitting there spinning, like a car tire on ice.

If you place a lead ball and a feather into a round glass container,
and drop them at the same time, they will both hit the ground at the
same time. Their speed will accelerate as the objects fall, up to the
point of boyancy, weight to friction drag imposed on the glass
containers. I know, somebody will come along and say they will still
continue to increase in speed. I'm not getting that deep into the
topic.

Now, if you take the lead ball and the feather and drop each of them
as the object they are. The lead ball will hit the ground long before
the feather will. Why, because the friction and drag on the feather
are much greater than on the lead ball. If dropped from high enough,
the lead ball could easily burn up and become a gas from the heat the
friction will cause, unless it attains boyancy. The feather attains
boyancy fairly rapidly and will slowly float to earth, riding on the
molecules of the air. It will never speed up beyond the point of
boyancy, it won't heat up and it won't burn up from speed of falling.
In the case of the feather, friction is its friend, but in the case of
the lead ball, friction can become its enemy.

Now lets look at the pumps again.
If you reduce FRICTION on the impeller, the engine will not work as
hard, it will not draw as much current. But it is also doing less
work.

It is moving less liquid per minute, but this is NOT the reason why it
is consuming less energy and doing less work. The reason why is
simply due to less Friction. Again, the same as a car tire spinning
on ice.

You brought up a piston pump, which is a good example. If you
restrict the output, the engine will do more work and use more energy
by raising the psi of the outflowing liquid.
Now, lets add a check valve that allows the pressure to escape and not
allow the pump to run over a certain psi. The pump, running at
maxiumum psi will do the same amount of work and move the same amount
of fluid, regardless of how much you are letting flow out the other
end. But if you open the valve wide open, allow the most fluid
possible to flow with no restriction, the pump will actually run
slower and consume less energy. Why, because there are no
restrictions on it, the psi it must produce is less.

Now, take out the check valve completely or set it to zero psi, and
completely block the output, the pump will hum merrily away using very
little energy and consuming very little current.
But it is NOT because it is moving less liquid, it is still moving
exactly the same amount of liquid, but with zero head pressure, zero
psi.

I think we have beat this dead horse long enough!

I know you understand how pumps operate and what will happen with the
different kinds of pumps.

But cause and effect are not always the same thing and not always in
relation to each other without another outside effect being introduced
to gum up the works.

I can dump paint on somebodys car. Some of the paint will run down
the car and onto the ground. It doesn't matter if the car is sitting
still or going 70 miles per hour, some paint is going to get on the
ground, just not in the same place.

But PAINT could still get on the ground if no car was involved!
So the car itself is NOT the cause that produced paint on the ground.
It was the act of turning a paint can upside down that caused the
paint to get on the ground.

It's NOT the restriction that caused the MOTOR to speed up.
It's NOT the decreased flow rate that caused the MOTOR to speed up.
What it is, is the decreased FRICTION on the impeller, causing the
water to spin within the impeller in its housing, rather than pass
through to the output, that caused the Motor to speed up.

Restricting the output line CAUSED a decreased flow rate!
Inadvertently the decreased flow rate caused the motor to speed up,
but was NOT the true reason or CAUSE for the motor speed up.

I hope I am finally making some sense?

TTUL
Gary

[Ross Bagley]

(Gary V. Deutschmann, Sr.) writes:

> How about if we take a totally different approach!
>
> You tell us what TERM should be used when, in the instance of a closed
> pump head, the fluid quits to flow and just spins around in it's own
> housing driven by the impeller?

Well, I don't mean to imply that I'm an expert in pumps, though I
am still familiar with fluid dynamics and some of the nonlinear
boundary conditions that happen in certain systems (like the boat
propellor cases that I've used).

Given that caveat, I would probably characterize the state of a pump
in the condition you describe as "having backpressure" until the
backpressure equalled the pump's output pressure at which point the
pump would be "stalled". You could also say that in this case, the
impeller was stalled (to distinguish this case from where the
electrical motor was loaded to the point that it was no longer
rotating -- i.e. the motor was stalled).

Your mental model appears to be that the impeller is spinning in gases
cavitated out of solution while my mental model is that the impeller
is spinning in water with no/few bubbles but that the backpressure
from the output port prevents it from escaping the housing.

> The ONLY term that I know of that applies to this phenomenon is
> Cavitation. And cavitation applies also to even low inches of
> vacuum.

To apply cavitation to the case you're describing, I think you would
have to demonstrate that gas pockets were actually appearing out of
the pumped fluid within the pump housing. Given the low input
pressures these pumps are generating, I highly doubt that this is
happening, even when the pump isn't driving it's full capacity.

We could probably modify an impeller housing with a polycarbonate
window by cutting off one of the sides and bolting a polycarbonate
plate on the side (with an opening in the middle for the pump's input
port). That would give us direct observation of the impeller chamber,
we'd still need a high speed camera with strobe to "stop" the impeller
long enough to observe the wedge of fluid around a blade to see if
there were bubbles present that were not present in the input stream
(and therefore being drawn out of solution).

Now, before I start sounding too confident in my own conclusions, I'm
wondering if some nearly saturated gases may come out of solution in
the lower pressure environment of the pump housing and accumulate on
the wall until getting too large and being forced into the output
flow. This could be a plausible explanation for the bubbles that
periodically appear from some fully submerged powerheads in some of
my friend's aquariums.

I don't know if that's actually cavitation or not, but it's a lot
closer in concept than the reduced output scenario we're currently
using as an example. In fact, it probably wouldn't take all that much
to make me believe that that really is a low volume cavitation... For
the moment, I'll simply say that I don't know.

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller

[Ct Midnite]

On 10 Mar 2004 16:21:54 EST, (Gary V.
Deutschmann, Sr.) wrote:

>Hi CT
>
>All of the observations you have made, and your take on them in your
>correspondence below is accurate!

"This is exactly what you should try. You will find that the more you
restrict the flow the less current you will draw." This is the first
statement I made in my first reply.

>Reducing or restricting the flow is physically causing less work for
>the motor, ergo less current draw, less juice being used.

And for the group then you agree that restricting flow will not make
the motor work harder and burn up. That's what they need to know. It
doesn't hurt a thing to turn down the flow rate if it will help in
their particular application. It will even take less juice.

>But the real question is WHY?
>
>You already noted that pumps with no backwash or relief valve will
>work harder or stall if they are restricted! If partially restricted
>they will draw more current trying to overcome the restriction, with
>an increase in head pressure as well.

They don't actually work harder, they don't work at all. But this is
because you have two completely different, not even similar ways of
moving water.

With a positive displacement pump you are actually pushing water
mechanically with a solid object. Piston pushing or a cavity trying
to get smaller. The laws of physics make it impossible. They don't
relate to centrifugal pumps. Apples and oranges.

>So the question is still WHY, on some pumps, does the current draw go
>down, the work on the motor become less and/or the engine speeds up.

>It is simply because the pumphead has a built-in backwash system or to
>please Ross, rather than say go into Cavitation, I will simply say the
>water inside the pumphead begins to spin around with the impeller
>blades and loses its friction. It turned gyrational!

My point is that most pumps that we will use in aquaria will be like
this. If your pump says to not throttle you had better not. But most
will not say this. There is absolutely no difference between
restricting flow by a valve or restricting flow by height. Effect is
the same. So if they say not to throttle it's probably because they
use the water as a coolant and they don't want you restricting flow to
the point your motor over heats. And they probably also have a max
height that they want you to attempt to pump the water somewhat below
what it's capable of.

>In a properly operating pump, the water enters near the center of the
>impellers, these impellers PUSH the water around and outward due to
>centrifugal force of the water being slung around and outward at the
>same time. If the pump is efficient, there is very little backwash
>and very little loss.

The higher the efficiency, the higher the max pressure it can produce.

>But as you increase the resistance against these actions, they lose
>friction and begin to spin within the impellers without being forced
>outward. Centrifugal action decreases.
>There being LESS friction, the motor uses less current and does less
>work.

I'm not comfortable with the word friction here. Centrifugal action
would not decrease at all. I don't think the word friction applies
here.

>So you see, your observations are correct! And your measurements are
>correct. But your original statement is flawed only in that it does
>not contain the CAUSE of the motor using less energy.

All I know and I think all I ever meant to say was that it simply has
to be working less or it would burn up very quickly. The net effect
of the system can only be measured buy two things. The amount of
water that is pumped and the heat it produces. Electrical energy is
used and has to be converted to something. Either movement of water
or heat. If you would stop or slow the movement of water (do less
work) with the same energy used (or more was what was suggested by an
earlier writer) all the extra energy would have to be converted to
heat and that doesn't happen.

>It is NOT the reduced flowrate that results in the lower current draw.
>Reducing the flowrate by restriction should cause an increase in the
>work the motor is doing and an increase in the current draw of the
>motor. Therefore, something ELSE must be happening!

This is where the problem lies. You think it should and I say it
shouldn't. You think blocking a cent pump should make it work harder
and it shouldn't. The water would like to be thrown way away from the
center but the housing keeps it from doing that. The work is done
when the water is able to escape from the outlet and new water is
introduced from the intake and sped up to the speed of the rotor.
Work is not done until the water is able to escape. The only work
until then is speeding up the water and giving it the potential energy
(pressure) to escape outwardly when the outlet comes around because of
it's speed.

>For lack of a better word, the pumphead has gone into Cavitation, has
>lost its friction with the fluid passing across it's impeller blades,
>and is just sitting there spinning, like a car tire on ice.

I don't think this is what's happening at all. Don't see the
connection with the car tire.

>If you place a lead ball and a feather into a round glass container,
>and drop them at the same time, they will both hit the ground at the
>same time. Their speed will accelerate as the objects fall, up to the
>point of boyancy, weight to friction drag imposed on the glass
>containers. I know, somebody will come along and say they will still
>continue to increase in speed. I'm not getting that deep into the
>topic.
>
>Now, if you take the lead ball and the feather and drop each of them
>as the object they are. The lead ball will hit the ground long before
>the feather will. Why, because the friction and drag on the feather
>are much greater than on the lead ball. If dropped from high enough,
>the lead ball could easily burn up and become a gas from the heat the
>friction will cause, unless it attains boyancy. The feather attains
>boyancy fairly rapidly and will slowly float to earth, riding on the
>molecules of the air. It will never speed up beyond the point of
>boyancy, it won't heat up and it won't burn up from speed of falling.
>In the case of the feather, friction is its friend, but in the case of
>the lead ball, friction can become its enemy.

>Now lets look at the pumps again.
>If you reduce FRICTION on the impeller, the engine will not work as
>hard, it will not draw as much current. But it is also doing less
>work.

All true but I don't think friction is the right way of looking at
this. Not friction the way most people think of friction. Resistance
to turning the impeller is reduced because of what I've said above and
if you want to call that friction ok.

>It is moving less liquid per minute, but this is NOT the reason why it
>is consuming less energy and doing less work. The reason why is
>simply due to less Friction. Again, the same as a car tire spinning
>on ice.

Sorry, I haven't grasped the car thing yet.

>You brought up a piston pump, which is a good example. If you
>restrict the output, the engine will do more work and use more energy
>by raising the psi of the outflowing liquid.

Again I don't think it will work at all.

>Now, lets add a check valve that allows the pressure to escape and not
>allow the pump to run over a certain psi. The pump, running at
>maxiumum psi will do the same amount of work and move the same amount
>of fluid, regardless of how much you are letting flow out the other
>end. But if you open the valve wide open, allow the most fluid
>possible to flow with no restriction, the pump will actually run
>slower and consume less energy. Why, because there are no
>restrictions on it, the psi it must produce is less.

I'm not sure this is right. It would run slower but it wouldn't
consume less energy.

I'm going to really through you with this but higher psi doesn't take
more energy for a given pump. The max pressure produced by a pump is
as you restrict it's flow. You probably say "That's obvious." But
not for the reason you think. It's because the motor is turning
faster and creating more cent force. A pump under full flow is
producing less pressure than a restricted pump because the impeller is
turning slower. Pressure out of a pump is a function of the speed of
the impeller. The more you restrict the faster the motor turns and
the more pressure you have available.

Creating force (pressure) doesn't take energy, it's using it that
does. Force (pressure) doesn't equal work, Force(pressure) times
distance (moving it) equals work.



>Now, take out the check valve completely or set it to zero psi, and
>completely block the output, the pump will hum merrily away using very
>little energy and consuming very little current.
>But it is NOT because it is moving less liquid, it is still moving
>exactly the same amount of liquid, but with zero head pressure, zero
>psi.

If you are talking about a closed system then the dynamics are
different. The water entering the pump would be under pressure from
the output side and the pump would just be helping it along. Water
already under pressure. But I could completely be missing your point
here. Not to sure what you mean.

>I think we have beat this dead horse long enough!

Yeah but it was great. Physics is phun!

>I know you understand how pumps operate and what will happen with the
>different kinds of pumps.
>
>But cause and effect are not always the same thing and not always in
>relation to each other without another outside effect being introduced
>to gum up the works.
>
>I can dump paint on somebodys car. Some of the paint will run down
>the car and onto the ground. It doesn't matter if the car is sitting
>still or going 70 miles per hour, some paint is going to get on the
>ground, just not in the same place.
>
>But PAINT could still get on the ground if no car was involved!
>So the car itself is NOT the cause that produced paint on the ground.
>It was the act of turning a paint can upside down that caused the
>paint to get on the ground.
>
>It's NOT the restriction that caused the MOTOR to speed up.
>It's NOT the decreased flow rate that caused the MOTOR to speed up.

I think it is.

>What it is, is the decreased FRICTION on the impeller, causing the
>water to spin within the impeller in its housing, rather than pass
>through to the output, that caused the Motor to speed up.
>
>Restricting the output line CAUSED a decreased flow rate!
>Inadvertently the decreased flow rate caused the motor to speed up,
>but was NOT the true reason or CAUSE for the motor speed up.

We might have to agree to disagree on this one. :) To me it's pretty
cut and dried.

But it's made me think about things and in ways I have thought about
for quite awhile. I think we agree whole heartedly on what happens.
Just not why.

I was phun.

>I hope I am finally making some sense?
>
>TTUL
>Gary


Ct Midnite

http://www.geocities.com/ctmidnite53/

[Ct Midnite]

It was phun. :)

To much to proofread. :)


Ct Midnite


http://www.geocities.com/ctmidnite53/

[Gary V. Deutschmann, Sr.]

Hi CT

As long as we agree to disagree on the WHY, Yes it has been PHUN!, hi
hi....

It has also given me something else to study up on a little more
deeply than I have in the past.

I helped design the Eddy Current Brake used at the Alton, Illinois
lock and dam on the Mississippi River when I was working for Sverdrup
& Parcel many eons ago. And that was deeper into fluid dynamics than
I ever wanted to be again. Some really CRAZY things happen in fluids
that are not at all obvious or even understandable until it's too late
to do anything about it. Unfortunately, some of these things should
have been as obvious as the nose your face, but nobody thought of them
at the time the design was being assembled. It was only after the
fact that somebody goofed and forgot to figure it in.

When I was in the fountain industry, a major designer and contractor
became the laughing stock of the whole fountain industry.
He wrongfully assumed that the surface of water was perfectly flat!
After building an extremely LONG fountain tier using laser transoms to
insure it was perfectly level and that the water was supposed to flow
over the entire perimeter of the structured tier, he couldn't
understand why it only flowed over the center of the two longitudinal
walls and not even equally across from each other.

It was simple! He did not take into consideration the curvature of
the earth nor it's rotation, in his design.

I often wonder how many people know that water boils twice or more
times when placed in a microwave oven. Or anything that heats rapidly
or slowly for that matter, but it is just not as obvious.

If you don't believe me, place a cup of water in your microwave, you
will find that it comes to a boil that slows down or sometimes stops
completely and then begins to boil again.

The reason is simple! Chemicals in the water that have a lower
boiling point than water, boil first until they are converted to gas
and escape the water, but the temperature is still not high enough for
the water itself to boil, so it stops boiling (or slows down) and then
goes into full boil as the temperature reaches the boiling point of
water.

Without knowing about the chemical additives in water, it would be
considered a weird phenomenon of water to boil twice.

Perhaps neither of us fully realize what causes a restriction on a
pump with internal backwash to run more efficiently.
You have your ideas which to you seem theoretically correct and I have
my ideas which I feel are theoretically correct. If things follow
Murphy's Laws the way they should, neither of us are completely wrong
and neither of us are completely right.

We both know just enough to get ourselves into trouble, hi hi.....

I'm sure there is some 'WEIRD' & 'SCIENTIFIC' reason for the rule, 'I'
before 'E' except after 'C'! Which has no ruling basis on my
statement concerning it!

TTUL
Gary