Pumps, flow and restrictions.....

[AKKAMAAN]

Pumps, flow and restrictions.....

The pumps we use in hydrostatic systems are so called POSITIVE PUMPS, the are designed to force almost the entire flow FORWARD. A NEGATIVE PUMP are designed to allow flow going BACKWARD as well as forward. From now I only talk positive pumps.

There are pumps with FIXED displacement and VARIABLE displacement. A variable displacement pump can, internally, change its displacement per revolution. That means flow can be changed with out changing the pump shaft RPM. Of course it still takes RPM's to turn the pump around..
From now I only talk fixed displacement pumps.

A fixed displacement pump needs a protection from extreme high pressure. WE use a Pressure Relief Valve, PRV, to BY-PASS flow when pressure exceed the settings on the PRV. Pump flow will still stay intact, the same. Oil just take a different path back to reservoir.

Can some one now tell me how a smaller fitting, QC, bent hose, smaller valve etc (any kind of restriction), on the PRESSURE SIDE of the pump, can decrease flow?????:laughing::thumbsup:

 

[Egon]

Can some one now tell me how a smaller fitting, QC, bent hose, smaller valve etc (any kind of restriction), on the PRESSURE SIDE of the pump, can decrease flow?????:laughing::thumbsup:

Not hard at all;

you get fluid coming out of non designed aperture and the reservoir depletes till the suction is above fluid level!:thumbsup::thumbsup:

The prime mover runs out of power or cannot transfer power properly to the pump!!:thumbsup::thumbsup:

The internal pump seals allow fluid back to the suction side!:thumbsup::thumbsup:

Heat generated at restriction allows the fluid to transform into a fluid of completely different characteristics!:thumbsup::thumbsup:

 

[J_J]

Is it not true that on a constant volume pump, any tube, hose, fitting smaller than the pump outlet, will add resistance, impede flow, and raise pressure. Fluid velocity has to increase to move that same amount of fluid.

 

[Egon]

[QUOTEFluid velocity has to increase to move that same amount of fluid.][/quote]

Yes, till pressure upstream of the restriction increases beyond what the system can handle or the heat generated at the restriction is sufficient to change the characteristics of the suction fluid.:thumbsup:

 

[Sweden-Art]

It wont. Restriction=Increased velocity > decreased pressure

 

[SPYDERLK]

Using your parameters, it does not decrease flow from the pump, but if the prv is before the restriction the flow may divide, some going to the sump instead of to the load. In the real world the flow will decrease a little due pressure driven fluid bypass thru clearance w/i the pump, - and usually also a small rpm loss due greater than normal loading [PxV]. Some forgivenes here, but lots of power is being wasted = heat= avalanching bypass situation. Now if the prv is after the restriction there is virtually no forgiveness. Pressure goes above relief according to the degree of restriction. If severe, something gives quickly. If its a kink in the hose heat may pop it quick enuf to save the system.
larry

 

[mmurphy]

If fluid has to go somewhere against restriction, I would say internal leakage.
Velocity=pressure drop, so high pressure would=low flow.

 

[Egon]

It wont. Restriction=Increased velocity > decreased pressure

As the restriction decreases in size the flow through the restriction increases in speed and has a lower pressure, yes!

But when the restriction gets smaller the pressure upstream increases, yes.

When the restriction is small enough and the upstream pressure is large enough something has to change as the PD pump just keeps building up the pressure, yes

Either there will be an mechanical equipment failure or a designed valve of some type must open, yes.

 

[SPYDERLK]

Is it xnotx true that on a constant volume pump, any tube, hose, fitting smaller than the pump outlet, will add resistance, impede flow, and raise pressure. Fluid velocity has to increase to move that same amount of fluid.

Yes?
larry

 

[mmurphy]

AKKAMAAN is still out camping:laughing:. I believe actually out fishing!!

 

[Ford tractor]

Easily.

Pick up a soda straw and a coffee stir straw. Blow as hard as you can thru the soda straw until you are out of air. Now blow as hard as you can thru the stir straw until you are out of air. Which one resulted in more flow? Which one resulted in you getting more tired / red in the face?

If flow is restricted too much, either the relief valve will pop open, the pump will internally leak back, the pump will cavitate or the pump will grenade. The best result is that the relief will open. If you have the restriction before the relief, the other 3 are in play. None of them results in a healthy pump or a fat wallet, just the time frame to pay varies.

 

[AKKAMAAN]

OK everybody....Maybe I wasn't clear enough....my point is very simple......

Here is a detailed example, you guys only have to answer Increase, Decrease or Stay the same?? Just give an "back it up" explanation for your chosen answer.

The system consists of a reservoir, suction line, pressure line, a variable restriction (orifice), and a return line.
We assume no back pressure in return line.

Pump and the rest of the system, is 100% efficient, which means no internal or external leakage.

A FIXED, 10 GPM pump flow (a regular gear pump), is going through an orifice which cross section area is 1/2 square inch, at 500PSI supply pressure.

We also assume a fixed viscosity and temperature. No restrictions (full atmospheric pressure 14.7psi) on suction side.

Prime mover keeps a FIXED rpm.

Now we decrease the orifice area to 1/4 square inch.

Will the flow Increase, Decrease or Stay the same??

 

[wdchyd]

Pumps, flow and restrictions..

Can some one now tell me how a smaller fitting, QC, bent hose, smaller valve etc (any kind of restriction), on the PRESSURE SIDE of the pump, can decrease flow?????:laughing::thumbsup:

Theoretically in a perfect world of 100% pump efficiency there should not be any flow decrease ( velocity through the restrictions increase ), but as pressure and heat increase so does volumetric efficiency through pump slippage, wear and mechanical tolerances...thus a decrease in pump flow....Never seen 100% efficiency:confused2:

 

[wdchyd]

OK everybody....Maybe I wasn't clear enough....my point is very simple......

Here is a detailed example, you guys only have to answer Increase, Decrease or Stay the same?? Just give an "back it up" explanation for your chosen answer.

The system consists of a reservoir, suction line, pressure line, a variable restriction (orifice), and a return line.
We assume no back pressure in return line.

Pump and the rest of the system, is 100% efficient, which means no internal or external leakage.

A FIXED, 10 GPM pump flow (a regular gear pump), is going through an orifice which cross section area is 1/2 square inch, at 500PSI supply pressure.

We also assume a fixed viscosity and temperature. No restrictions (full atmospheric pressure 14.7psi) on suction side.

Prime mover keeps a FIXED rpm.

Now we decrease the orifice area to 1/4 square inch.

Will the flow Increase, Decrease or Stay the same??

Sorry you posted this while I'm still learning how to type with one finger:laughing:

 

[AKKAMAAN]

Sorry you posted this while I'm still learning how to type with one finger:laughing:

thats ok:laughing:....you can chk your PM box too.....:thumbsup:

 

[Egon]

Now we decrease the orifice area to 1/4 square inch.

Will the flow Increase, Decrease or Stay the same??[/QUOTE]

Oh; now you asked a question that has no answerer. The temperature and viscosity of the fluid are very critical in calculating the flows. Without those no calculations possible. The type of pipe and it's layout will also be needed.

And don't ask me to do the calculations, my math skills are limited to addition. One plus anuder one plus anuder and so on.:laughing:

And before all else it has to be determined if the original flow rate, pressure and orifice size all agree with each other which will involve more of them calculations!


With a PD pump running in a stable mode the flow rate stays the same. It cannot increase or decrease.:thumbsup:

 

[SPYDERLK]

OK everybody....Maybe I wasn't clear enough....my point is very simple......

Here is a detailed example, you guys only have to answer Increase, Decrease or Stay the same?? Just give an "back it up" explanation for your chosen answer.

You dont list the pressure capacity of the pump so an absolute answer cannot be given. The flow will either stay the same or decrease to zero as the pump blows up due the added pressure needed to maintain the positive displacement 10 gpm flow. At 500psi to drive 10gpm thru a 1/2sqin orifice we are talking some pretty viscous fluid. ... thats like -- 3/4" dia
larry

 

[mmurphy]

If we are assuming pressure will be the same, flow would have to decrease cause restriction would either have to cause a pressure increase to get through the same amount of flow to do the same amount of work.
Pressure does not do work at a stalled position. Flow does work up to stalled position. Now I am fishing.:confused2:

 

[AKKAMAAN]

Oh; now you asked a question that has no answerer. The temperature and viscosity of the fluid are very critical in calculating the flows. Without those no calculations possible. The type of pipe and it's layout will also be needed.

And don't ask me to do the calculations, my math skills are limited to addition. One plus anuder one plus anuder and so on.:laughing:

This don't need calculations....it is much more simple than that.....
btw....how do you know temperature and viscosity is very critical at calculations of flow, when you do not know how to make the calculations?? LOL
Actually, temperature and viscosity, will have more impact on pressure than flow....it just takes more force/torque/pressure to push thick fluid through a restriction than it take to push thin fluid through the same restriction....flow is
fixed/constant remember....:thumbsup:

Viscosity indeed have impact on internal leakage....but that factor is disabled in my example.......HAHAHAHA:laughing::laughing:

 

[AKKAMAAN]

You dont list the pressure capacity of the pump so an absolute answer cannot be given. The flow will either stay the same or decrease to zero as the pump blows up due the added pressure needed to maintain the positive displacement 10 gpm flow. At 500psi to drive 10gpm thru a 1/2sqin orifice we are talking some pretty viscous fluid. ... thats like -- 3/4" dia
larry

Sorry larry, you are making this more complicated than it is....you do not even have to think about fitting sizes....OK...any pressure can be considered way below your "pump capacity"....