Pumps, flow and restrictions.....

[mmurphy]

Jerry, a centrifical water pump will only lift water to a "Head" (height) based on 2 things. Diameter of impellar, and RPM. (Your arm throwing something straight up in the air to apoint it stops and comes back down at you.)
Most big flowing impellar type pumps are started "Dead Headed" against a valve. That does a couple of things also. Reduces shock on system from a major change in flow, and takes less amperage to start motor.
Some now might be thinking, How can dead heading a pump (centrifical) make it less amps to start? The easiest way for me to tell you is if pump is pumping 100 galons/second (small pump) 8.34 lbs/gal would= 834 lbs/sec. Deadhead a pump will just turn the water in the impellar causing no lift. Do that with a positive displacement pump, things will fly apart every where.
I think where some of this is confusing some people at is, they put a restriction in and they see it slow a cylinder down. Now they say AKKAMAAN has got to have this wrong.
Most operations with a cylinder is feathered in or does not see the full pressure build up on both sides of orifice. (untill end of travel) So there for it is hard to understand the whole concept of the thread if they are thinking of how they are running a hydraulic cylinder with orifice in place.

 

[Egon]

Diameter of impellar, and RPM.

Wouldn't this just affect the mass and velocity of fluid moved [discharged]?

 

[mmurphy]

Wouldn't this just affect the mass and velocity of fluid moved [discharged]?

They are interdependant of each other. And yes size of pump comes in to the picture also. I was thinking of a one pump size at the time of posting.

 

[Jerry/MT]

Jerry, a centrifical water pump will only lift water to a "Head" (height) based on 2 things. Diameter of impellar, and RPM. (Your arm throwing something straight up in the air to apoint it stops and comes back down at you.)
Most big flowing impellar type pumps are started "Dead Headed" against a valve. That does a couple of things also. Reduces shock on system from a major change in flow, and takes less amperage to start motor.
Some now might be thinking, How can dead heading a pump (centrifical) make it less amps to start? The easiest way for me to tell you is if pump is pumping 100 galons/second (small pump) 8.34 lbs/gal would= 834 lbs/sec. Deadhead a pump will just turn the water in the impellar causing no lift. Do that with a positive displacement pump, things will fly apart every where.
I think where some of this is confusing some people at is, they put a restriction in and they see it slow a cylinder down. Now they say AKKAMAAN has got to have this wrong.
Most operations with a cylinder is feathered in or does not see the full pressure build up on both sides of orifice. (untill end of travel) So there for it is hard to understand the whole concept of the thread if they are thinking of how they are running a hydraulic cylinder with orifice in place.

Thanks for your response. I guess I should have read the whole thread. I was responding to the first post.

Respectfully, I understand how a centrifugal pumps starts as I start them everyday on my irrigation system and I'm familiar with the performance aspects of centrifugal pumps.(When deadheaded between line changes these pumps produce about 65psi. Depending on the irrigation configuration they run up to 500+ gpm and even higher when filling lines.)

I take exception to your statement that only the centrifugal pump impeller diameter and the rpm (basically tip speed) determine the pump head. The other requirement is the discharge restriction. As an example in my irrigation system when fillling lines, the disharge pressure as measured in the discharge manifold is zero and the line flow is basically open channel since the lines are not running full. As the system fills, the sprinkler nozzles and drains set the pressure and it rises above zero psi. When the drains close, the nozzles are controlling the pump head ( absent a leak that occurs occaisonally) and it rises to it steady state value that's determined by the line losses and the aggregate nozzle area.

I have to admit that I don't have much practical experience with gear pumps or other positive displacements pumps so I guess I'm just showing my ignorance in that area. None the less I enjoyed the discussion and most importantly I learned something new.

 

[Jerry/MT]

I think it confuses more than help, to refer to the waterpump system, which actually acts more like a constant pressure system....this is actually the reason why people misunderstand a constant flow system.....

If you read the entire thread, you can see that we already have discussed the impact of internal leakage. My point is to proof, that a constant flow can not be reduced without out diversion. Internal leakage is a form of diversion too. wdchyd's video clearly shows that flow is constant through restrictor, until PRV opens.

A constant pressure system will react different on a flow restrictor. No flow diversion needed, but indeed "pressure diversion"....but thats another thread......more to come about that.....



You are welcome!

I guess I should have read the entire thread but I didn't.

I have to admit that I don't have much practical experience with gear pumps or other positive displacements pumps so I guess I'm just showing my ignorance in that area. None the less I enjoyed the discussion and most importantly, I learned something new.

 

[Egon]

I take exception to your statement that only the centrifugal pump impeller diameter and the rpm (basically tip speed) determine the pump head.

Don't forget the number of stages in determining head.