The sum of the 2 branches flow is different than pump flow

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Flávio Luiz on 20 May 2024 at 18:28
Commented: Flávio Luiz on 21 May 2024 at 17:41
Hi there,
I am trying a very simple hydraulic circuit, consisting of a Fixed-Displacement Pump and two hydraulic branches, each one with a pressure relief valve. When I measure the flow after the pressure relief valves I found that the sum of both is different than the pump flow.
My conceptual idea is that the sum of both flows after relife valve should be equal (or closely) to the pump flow. Where is my mistake?
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Accepted Answer

Yifeng Tang on 21 May 2024 at 15:15
Hi Flavio,
I looked at the model and below are my observations.
1. the parameters and inputs for the pump look ridiculous: at 1m^3/rad displacement and 100rad/s speed input, it's trying to move 100m^3/s of liquid, and over about 9000MPa pressure gain (!!!). Because of the huge pressure, the density of the liquid changed so much and the more leakage occurs, leading to the actual volumetric flow being ~50m^3/s
2. The ~9000MPa pressure gain is a result of trying to forcing 50m^3/s of flow through the opening of ~0.1m^2 at the two valves. That's about 500m/s flow speed, if not higher. This doesn't sound right.
3. Now the volumetric flowrate measurements: the sensor measures the volumetric flow rate, which is not the conserved quantity in an isothermal liquid network. Mass flow rate is conserved. In your simulation, the density before and after the pressure relief valves are VERY different. You may place a "Liquid Properties Sensor (IL)" to measure the density and you'll see they are ~60 times different! Again, this is because of the unreasonably high pressure, which is a result of the unreasonable parameters and inputs for the pump.
Suggestion:
1. Try to parametrize and drive the pump with more reasonable parameters.
2. Make sure the pump matches the valves, meaning the flow generated by the pump should generate some but not huge pressure over the pressure relief valve.
3. Use Simscape Result Explorer often, and more often.
4. If haven't already, take Simscape Onramp (free and online).
5. If possible, consider Simscape training and/or Simscape Fluids training.
Happy modeling!
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Flávio Luiz on 21 May 2024 at 17:41
Hi Yifeng Tang! Thanks a lot for your responses. After properly parametrizing the pump and valves the results are much more realistics.
Thanks a lot!

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