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flow rate and pressure

flow rate and pressure

flow rate and pressure

I am an electrical engineer and have a question reagarding a hydraulic generator. The hydraulic system of this unit is tied into the rest of the hydraulic system so the flow to the generator varies quite a bit. There is a built in regulator for maintaining some nominal flow rate(8gpm). This regulator doesnt seem to work as described or is not very well made. So the generator will put out voltages as high as 190 volts when rotating at 4300rpm(frequency is arounf 70 Hz). My question is if we put in a ball valve to restrict the flow to some nominal (8gpm) then if the pressure of the system increases, will this increase the flow rate? What relationship is there between pressure and flow rate in a hydraulic system? I have submersible pumping experience(oil and water). I cant understand why this would not help maintain a more costant flow rate? Can anyone help. I was thinking the pressure of the system would only increase if a hydraulic load is turned off or something. Excuse my ignorance on hydraulic systems. Thanks to all in advance.

RE: flow rate and pressure

Buzz, I believe you have to start at the source, i.e. the pump. Just as troubleshooting an electrical circuit.Hydraulics are VERY closely related to electrical theory.Just a different medium,electrons vs oil.Pressure affects flow in the same way as voltage affects current.Is pump pressure-compensated or positive displacement?If pressure comp the pump maintains a set pressure with no flow until a valve/actuator is activated.Positive displacement has a fixed output with system flow in an open loop.Pressure initiates when a valve/actuator is activated.I believe either system will be affected by other actuators in the system.You may not have proper pump capacity to operate gen and other machinery.Proper temperature and/or viscosity can affect reaction time of regulation as well.Hope this steers you in the right direction.Scott  

RE: flow rate and pressure

Scott, Thanks for the reply. The pumps are positive displacement pumps so I guess the flow would be constant and the pressure would vary. Do I remember my physics right that the pressure will vary with pipe diameter(assuming constant flow rate) or do I have it backwards? So putting in a flow restrictor will only affect the system a little bit depending on the flow and pressures the other systems are calling for. So if the hydraulic load is basically constant then the ball valve would work for a static system but not likely to work in a dynamic environment very well. Do you agree? Thanks again!!

RE: flow rate and pressure


Hopefully I am not over-simplifying things, but flowrate, Q, is simply determined by the product of density, rho, velocity, V, and area, A (Q = rho*V*A).  With incompressible, subsonic flow (such as hydraulic oil), decreasing area increases velocity and decreases pressure.  Conversely, increasing area decreases velocity and increases pressure.  Hope this helps.


RE: flow rate and pressure

In a hydraulic system system pressure is determined by resistance to flow and the low resistance section of the circuit usually gets the flow and determines the pressure.  RPM of a motor is determined by displacement and flow.  If the displacement is fixed, ie not a variable displacement motor or pump then rpm is directly proportional to flow rate, and the pressure is proportional to the torque.  If your pump is fixed displacement putting a flow resistive valve in the line might not help at all. It might just increase system pressure. What you need to do is equate flow to electrical current.  If you have a constant current device and add a resistor the voltage just goes up.  First thing to do is to draw the hydraulic circuit.  Next identify each type of component in the circuit. Identify all Pistons, pumps, motors, etc  determine wether each item is constant displacement or variable and what controls the displacement. It's possible that other components in the system have varied needs for flow and pressure and I'm guessing you would like to maintain a constant rpm at generator, and let the pressure rise or fall with electrical load => torque.  It's possible a displacement controller is not functioning properly, on the pump or the motor.  If you can't identify what the components are get a part number.  If you would like I'll help you with the schematic directly let me know.

RE: flow rate and pressure

Buzz!  Hydraulic systems usually include unloaders which divert flow back to the reservoir to maintain system supply pressure/flow somewhat stable.  The regulator you mentioned probably may not be funtioning properly.  The problem most often found in hydraulic systems is dirt which interferes with valve pilots, or sliding valve spools each of which is a part of the unloader.  My guess is the unloader is stuck and not functioning.  Putting a restriction device like a ball valve to restrict flow would only work to limit max speed.  Unloaders usually have adjustment screws which work to increase spring tension on the hydraulic pilot so you should be able to check to see if this adjustment will vary the generator speed.  If making an adjustment to the pilot has no effect, I'll bet the valve is stuck - a common problem.  Hope this helps.  Leo

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