# Simscape fixed displacement pump pressure gain wrong?

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Sandro G le 27 Mar 2024
When working with the fixed displacement pump block in Matlab Simscape I noticed that the nominal pressure gain parameter is not working as expected. From my understanding, the wanted pressure difference between port B and A of the pump is set by the nominal pressure gain parameter. So if I want a pressure of 210 bar at port B i should insert this pressure minus the atmospheric pressure at port A: 210-1.01325bar=208.987bar. This however results in the pressure at B being wrong by a factor of 100. If I insert a nominal pressure gain of 2.0899 i get 210bar at B.
What am I missing here?
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Sandro G le 3 Avr 2024
Alright I figured it out. I misunderstood the nominal pressure gain parameter. I thought that it describes the delta p of the pump. Thats not the case however. I had a closer look on the equations in the MathWorks Block description https://ch.mathworks.com/help/hydro/ref/fixeddisplacementpumpil.html
The nominal pressure gain is used to calculate the pump leakage. 2.0899 bar nominal pressure gain results in a pA of 210 bar because the leakage of the pump serves as a pressure relief. Since I was using ideal models with a efficiency of 0.99, the results just happened to be different by a factor of 100, hence the confusion.
Thanks to Yifeng Tang for the support
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Yifeng Tang le 27 Mar 2024
Hi Sandro, one quick thing to fix: you need another reservoir at port B, otherwise the fluid has nowhere to go. The reservoir at the outlet can be set to be at the higher pressure (by the nominal gain probably) than the inlet, and you should observe the mdot_A at the pump.
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Sandro G le 2 Avr 2024
Hello, thanks for your answer. I have been working with other systems as well, where the fluid has actually somewhere to go. The result is the same. A nominal pressure gain of 2.0899 bar results in 210 bar at the pump outlet.
Yifeng Tang le 2 Avr 2024
Hi Sandro,
Thanks for sharing the model.
It looks like you are trying to pump fluids into the A side of the hydraulic cylinder to move the piston. For the first 0.2 seconds of the simulation, it's indeed doing that, as you can see from the following figure (from Results Explorer), where the mdot through the pump is non-zero, and the pressure isn't crazy. Then the piston hit the end of the stroke (0.01 m, as set in the actuator block) and the hard stop is engaged. This leads to the pressure to raise until the pressure is high enough to halt the flow through the pump (except for the valve leakage I suppose), and the cylinder force balances the hard stop force.
So I would say the model is doing what it's designed to do at this point. Typically these systems have a relief valve at the pump outlet to protect for too high a pressure. In such case, you'll see most of fluids leaves through relief while the pressure is bounded by the relief pressure when the piston hits the hardstop.
The nominal pressure gain is there to define ONE operating condition the pump is designed for. When the conditions around it changes, it will respond with different flow rate for different pressure difference.
Hope this makes sense.

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