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!!