# What causes the pressure behavior in valves?

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Michael Littmann el 24 de Ag. de 2021
Respondida: Yifeng Tang el 29 de Jul. de 2022
The pressure in the system should be built up, maintained and reduced. The model is created according to this logic (picture).
Logic:
Valve1 open and valve2 closed - pressure is built up
Valve1 and valve2 closed - the pressure currently prevailing in the system is maintained
Valve1 closed and valve2 open - pressure is reduced
On the one hand, I cannot understand the pressure at the beginning of the simulation - why is it not zero when the valves are closed but exactly half the value of the pressure source?
On the other hand, the pressure drops precisely to this mean value after the first valve is closed and is not maintained.
What else has to be considered with the hydraulics in Simscape in order to implement the desired behavior?
I hope my concern has become clear and someone can help me.
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### Respuestas (1)

Yifeng Tang el 29 de Jul. de 2022
Hi Michael,
I think there are a couple things at play here:
First, there is currently no volume at all between the two reservoirs. The hydraulic line connections do not imply any fluid volume, and the valves have no volume either.
And the valves can never be fully "closed". Instead, the valve passage area drops to leakage specified in the block. This is done both because it's physical (no zero-leakage valve exists) and numerically much more efficient and stable. "Shutting" a valve changes the system of equations and that's a crazy thing to allow to happen in the middle of a simulation.
Since there is no volume, there is not any fluid to "hold" the initial pressure. The system of equations is instead solving for the pressure drop under this tiny leakage flow. Adding some realistic volume between the valves should give you results that are more intuitive.
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