Control Valve CFD Analysis

[neroverdi41]

Hello;

the parameters required from a control valve are inlet and outlet pressures, flow rate and temperature. when analyzing I can only specify 2 boundary conditions: either inlet pressure+flow rate or inlet pressure+outlet pressure. which one is more correct? because I cannot give flow rate while giving inlet and outlet pressure and the program calculates flow rate according to the geometry of the valve and inlet and outlet pressures.

 

[SPDL310]

This is question best posed in the CFD forum. You will be fighting convergence with the pressure inlet/pressure outlet configuration. You will see better convergence with mass flow inlet/pressure outlet.

 

[Doug Hunter]

The answer to your question depends on the purpose of your CFD analysis and the specifications to which you are trying to demonstrate compliance. What are you seeking to demonstrate with your analysis?

 

[neroverdi41]

Bu, CFD forumunda en iyi sorulan sorudur. Basınç girişi/basınç çıkışı yapılandırmasıyla yakınsamayla mücadele edeceksiniz. Kütle akış girişi/basınç çıkışıyla daha iyi yakınsama göreceksiniz.

When I give it as volumetric inlet and pressure output, the inlet pressure starts to rise to very utopian values and it gets ridiculous. for this reason, I give pressure to the inlet and flow rate to the outlet, which is correct according to both formulas. my understanding; when I give inlet and outlet pressures, I see the maximum flow rate of the valve, and when I give inlet pressure and outlet flow rate, I see how much pressure drop the valve provides. but I don't know which one I should choose.

 

[neroverdi41]

The answer to your question depends on the purpose of your CFD analysis and the specifications to which you are trying to demonstrate compliance. What are you seeking to demonstrate with your analysis?

I want the valve to provide the maximum flow rate by entering the specified inlet and outlet pressures. at worst I want it to be close to the maximum flow rate.

 

[SPDL310]

I have used pressure inlet/outlet configuration to get Cv values for valves that have been tested. I generally try to allow my modeled geometry to incorporate greater spacing than shown in ANSI/ISA 75.02.01. That way I can place a monitoring plane at the specified pressure tap distances to take averaged pressure readings at that section. The issue with that method and the turbulence models that I have employed is that the convergence values generated are low ie. 10^-1 or -2. To get to 10^3 or lower I generally need to take the solution transient. This complicates the process by requiring additional computational time and resources as well as requiring additional data processing for averaging the flow over longer periods of time. I have seen good correlation with flow test results generally with error < +- 5%.

 

[LittleInch]

I suspect the easiest solution is to input inlet pressure and flow rate and then modify one or the other to achieve your desired outlet pressure.

As this seems to be a continuation of your other thread, how exactly are you modelling the inside of the valve?

 

[neroverdi41]

I suspect the easiest solution is to input inlet pressure and flow rate and then modify one or the other to achieve your desired outlet pressure.

As this seems to be a continuation of your other thread, how exactly are you modelling the inside of the valve?

this is a continuation of my other work, you think right. but I'm not sure how to give the flow rate. this is the first time I'm hearing from you. what I can't understand is, from 37-36 bar pressure, should I get a maximum flow rate of 100,000 sm3/h to pass through? or should I get 36 bar pressure at the outlet by entering 37 bar and giving 100,000 sm3/h outlet flow rate?

 

[Doug Hunter]

I want the valve to provide the maximum flow rate by entering the specified inlet and outlet pressures. at worst I want it to be close to the maximum flow rate.

If your design goal is maximum flow rate, then I agree with specifying the pressure at both ends as boundary conditions. That way you can compare different valve design iterations under the same conditions. As part of the boundary conditions, you'll also want the high-pressure side to act as an unlimited flow source, and the low-pressure side as an unlimited flow sink.

 

[neroverdi41]

If your design goal is maximum flow rate, then I agree with specifying the pressure at both ends as boundary conditions. That way you can compare different valve design iterations under the same conditions. As part of the boundary conditions, you'll also want the high-pressure side to act as an unlimited flow source, and the low-pressure side as an unlimited flow sink.

I have never taken the minimum as a basis until now because my aim is to see the rated Cv value. for this reason, either by giving the inlet-outlet and seeing the flow rate or by giving the inlet pressure-outlet flow rate and seeing how much pressure drop the cage provides. I was not sure which would be more accurate.

 

[Doug Hunter]

I think you can still calculate Cv from the simulation results that you get with the pressure boundary conditions.

 

[neroverdi41]

I think you can still calculate Cv from the simulation results that you get with the pressure boundary conditions.

Thank you for your information. I will definitely take this into consideration.