Boundary Conditions for a Valve

Thamarai · December 9, 2019, 20:23

Hi All,

Iam working on a fire hydrant valve where I have to calculate
1.the outlet pressure( Max Limit is 4.5 bar) for a given Inlet pressure varying from 5 bar to 20 bar. In this case how I can give Boundary conditions? I have tried with Velocity Inlet and pressure outlet but I want to calculate for Varying Inlet pressure.?

2. Have to calculate the force actingin the piston( Top & Bottom). How CAn I calculate? I have attached a picture for your reference.

Thank You

LuckyTran · December 10, 2019, 10:47

2) There's a force report/monitor.
1) You should be able to use a pressure inlet and pressure outlet (with targeted mass flow rate settings). You need to specify the stagnation pressure at a pressure inlet (and not the static pressure). Actually, you can't impose a static pressure at any inlet, it's non-physical, unless the flow is choked.

The inlet pressure of 5 bar is known how? I.e. is that the mains pressure or running pressure?

Let's say you have a barometer at the inlet. If the pressure is 5 bar when the valve is closed and there is no flow, then when the valve is opened, the pressure will drop slightly to less than 5 bar. However, the stagnation pressure still is 5 bar and you can apply that as your BC.

But if you've already opened the valve and then you take a peek at the barometer and you see it reading 5 bar with flow, then that's the static pressure obviously. But now you don't know the stagnation pressure.

Unfortunately, there's no way to impose an inlet static pressure because it's non-physical unless the flow is supersonic.

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December 10, 2019, 10:47		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	2) There's a force report/monitor. 1) You should be able to use a pressure inlet and pressure outlet (with targeted mass flow rate settings). You need to specify the stagnation pressure at a pressure inlet (and not the static pressure). Actually, you can't impose a static pressure at any inlet, it's non-physical, unless the flow is choked. The inlet pressure of 5 bar is known how? I.e. is that the mains pressure or running pressure? Let's say you have a barometer at the inlet. If the pressure is 5 bar when the valve is closed and there is no flow, then when the valve is opened, the pressure will drop slightly to less than 5 bar. However, the stagnation pressure still is 5 bar and you can apply that as your BC. But if you've already opened the valve and then you take a peek at the barometer and you see it reading 5 bar with flow, then that's the static pressure obviously. But now you don't know the stagnation pressure. Unfortunately, there's no way to impose an inlet static pressure because it's non-physical unless the flow is supersonic.