# Inlet pressure boundary condition for incompressible analysis

Hi everybody.
I’m trying to simulate a piping system with pumps and outlets (e.g. a fire-protection system). The technical data of the pumps are known (flow rate and pressure head), so i think to set a constant pressure inlet value as boundary condition to check the pressure drop and flow rate at the outlet. I’m in doubt with the inlet pressure definition in case of incompressible simulations. For instance, if a 90 m pressure head is applied have I to set about 882,372 Pa / 1000 kg/cm = 882.4 for the total pressure value?

Hi @rosson,

Can you go into detail with the relevant sources on how you obtained the final value?

Also I believe you can set a mass flow rate as one of the inlet conditions. Is it not possible to set that as the input instead of recalculating to some pressure value?

Cheers.

Regards,
Barry

Hi Barry,

``````      I also think I can set a mass/volume flow rate but I was asked to test the plant setting a constant pressure at inlet. So I was in doubt with the boundary condition because I read a note about the specific pressure (divided by density) in case of incompressible flows. That means 882,372 ---> 882.4 in case of fresh water, am I correct?
``````

I’m not expert in pumps operating, I’m afraid that the total pressure in OpenFOAM is not the the pressure head from the pump technical data but a combination of head and velocity (flow rate).

Hi @rosson,

Let me call on my fellow @PowerUsers_CFD to see if they can help you with this issue.

Cheers.

Regards,
Barry

Hi @rosson,

This is no longer the case in SimScale, pressure for incompressible cases is now in (Pa) to make setup and interpretation easier.

If you know the flow rate you can provide a 0Pa condition at the outlet, the pressure drop across the system will be the same at any reference pressure for incompressible simulations.

Most pump data use a pressure vs volume flow rate datasheet, I don’t think this kind of input is currently implemented, so please make a request for a vote for features firstly. For now, you could approximate the volume flow rate from this plot and adjust it when you have a better idea of the pressure at the pump exit.

Hope this helps,
Darren

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Thank you very much, Barry and Darren.

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