Learning Forum

[Frane Ćalušić]

Hi, I have been trying to calculate a Kv value of a valve by this formula:

Kv = Q/dP, where Q is flow and dP is pressure drop.

My input is (shown in image): 5bar at inlet and 4 bar at outlet. With these values I would get dP = 1, so the formulas comes to this:

Kv = Q.

The Kv value that I should get should be around 230 m3/h (0.0638 m3/s). This has been confirmed with real tests. I have already did a simulation in SolidWorks Flow and it worked. But with Fluent I keep getting a big volume flow rate 7300800 m3/h (2.023 m3/s).

What could be the problem?

[Rohan Gulavani]

 

Please check Inlet pressure. In fluent, pressure inlet BC takes Total pressure while Pressure outlet takes guage pressure. So in kv formula dp would be less than 1.

Also, Please remember to use dp unit in SI, i.e. in pa not in bar. 

So first check BC’s are correct, then check for static pressure at inlet and then take difference (static pr difference between inlet and outlet) to get correct kv for your set up

 

[Frane Ćalušić]

You have pictures of the BC for inlet and outlet in the original post (first picture). Can you tell me what exactly should I change in them because I still don't understand completely? 

I want the dP to be equal to 1.

[Rohan Gulavani]

I think you want to constraint your results i.e. dp=1 bar. I guess you have to increase pressure at inlet till you get dp =1. Remember here pressures are in bar. You have to take into pa while calculating Q and kv

[Frane Ćalušić]

How do you mean pressures are in [bar]? You mean in Ansys or in the formula? Because if you mean in Ansys they are in [Pa].

[Rob]

Can you check the model scale? Ie is the domain the size you're expecting? From there, have you used the correct material: try deleting air if it's not being used. 

Finally, set the inlet velocity and outlet pressure and find Kv from that if it's constant density.