Pumps, flow and restrictions.....

[David Devuono]

Good to know who the experts are. Not sure to start a new thread or hyjack this one? I am making a tracked vehicle. I am not interested in using a dual variable pump (Zero turn pump) because I don't want to build a separate system for the rest of tractor hydraulics so I am going with a constant output pump. Splitting the flow equally to two motor spools. Ahead of one of the spools I will either have a priority flow control or a diverter valve to a 3rd 3 spool cylinder valve. The motor spools are 180 degree rotating spools and not push\pull spools. As such the work ports, I believe are blocked when in the (non-detent) center position. There will be an external pressure relief across the work ports in each motor circuit. I am planning to start out with a main system relief at about 2000 psi. My question is, what should i set the motor circuit relief vales at? Should they also be 2000 psi or can I go to 2450 psi (max constant motor pressure?) The main purpose of these valves is to protect the system from any shock loads the motors might put on the system from too sudden stops, steep down hill etc.

thanks

 

[wdchyd]

AKKAMAN,

Here's the video I promised, sorry for the delay.....:ashamed:
.Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More....

Hope this keeps the discussion in the right direction of "Pumps, flow & restrictions"......(Man, I Hate restrictions!!!)

 

[IceStationZebra]

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:

AKKAMAAN, do I need to get Peter Nachtwey over here?

ISZ

 

[AKKAMAAN]

AKKAMAAN, do I need to get Peter Nachtwey over here?

ISZ

PN is one of the most brilliant minds in motion control..
But I think he prefere to put most of his effots into the other end of the motion control science....
Dont even think I could earn a seat in his class room....LOL....

 

[AKKAMAAN]

AKKAMAN,

Here's the video I promised, sorry for the delay.....:ashamed:
.Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More....

Hope this keeps the discussion in the right direction of "Pumps, flow & restrictions"......(Man, I Hate restrictions!!!)

Thank you wdchyd! Great video shot, showing exact what we wanted to see....no reduced flow when we restrict with the "valve"....only when PRV start by passing we get reduced flow through restriction....

 

[wdchyd]

Thank you wdchyd! Great video shot, showing exact what we wanted to see....no reduced flow when we restrict with the "valve"....only when PRV start by passing we get reduced flow through restriction....

If I plumbed directly downstream of the pump you would have seen the effect of direct flow from the pump....GPM's would have been right up there with no PRV reduction....

Glad you liked it....A video first for me....

 

[AKKAMAAN]

If I plumbed directly downstream of the pump you would have seen the effect of direct flow from the pump....GPM's would have been right up there with no PRV reduction....

Glad you liked it....A video first for me....

Does not matter where you make the test, it is still the pump flow.....

You do not want to do this test without a PRV.........believe me!! on a Webster flow meter, there is a brass washer that breaks when you get to 6000psi, VERY DANGEROUS...that valve is not "proportional" and will build 2000psi just by a few degrees, when valve is 75 % closed....:shocked:

 

[wdchyd]

Does not matter where you make the test, it is still the pump flow.....

You do not want to do this test without a PRV.........believe me!! on a Webster flow meter, there is a brass washer that breaks when you get to 6000psi, VERY DANGEROUS...that valve is not "proportional" and will build 2000psi just by a few degrees, when valve is 75 % closed....:shocked:

You got that right!!!....we're using one tomorrow to test 80 Gpm, 2100 psi on a Dennison Vane Pump.....

 

[Jerry/MT]

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:

Anything that increases the pressure loss downstream of the pump will cause the pressure of the pump to rise. Consider a water pump with a gate valve attached to the outlet. If the valve is completely closed, the pump is deadheaded and achieves its maximum presure. (Of course that's a useless situation since with no flow, why have a pump?) As you open the gate valve the pressure drops and the flow increases. Conversely as you close the valve the flow will decrease and the pressure will rise. Downstream pressure losses act like that gate valve; they reduce the effective area that the pump discharges into. A bend, an orifice, a fitting, the friction of the hose or tube, etc all cause pressure losses, i.e. they are like closing down the gate valve and they force the pump to operate at a higher pressure and less delivered flow.

Even a so called constant displacement pump will show some flow change with downstream restriction, due to internal leakage as the pressure rises. Is it zero? In general No. So the discussion centers on where is the pump designed to operate. If we operate it at a fixed speed and at some range away from zero flow, where the pressure-flow characteristc is absolutely vertical ( i.e. dW/dp=0] n=constant) than we can say that downstream restrictions will not affect the flow rate.

By the way I have seen high presure ratio fans that at high speed exhibit vertical charactersitics similar to constant displacement pumps so this phenomenon is not limited to gear pumps etc. These fans are, however, unstable above certain pressure ratios due to certain aerodynamic considerations.

thanks for the thought provoking discussion.

 

[AKKAMAAN]

.
......Consider a water pump with a gate valve attached to the outlet. If the valve is completely closed, the pump is deadheaded and achieves its maximum presure.......

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.....

Even a so called constant displacement pump will show some flow change with downstream restriction, due to internal leakage as the pressure rises. Is it zero? In general No. So the discussion centers on where is the pump designed to operate. If we operate it at a fixed speed and at some range away from zero flow, where the pressure-flow characteristc is absolutely vertical ( i.e. dW/dp=0] n=constant) than we can say that downstream restrictions will not affect the flow rate.

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.....

thanks for the thought provoking discussion.

You are welcome!