CV by FEA simulation
To give an example, I get approximately a CV of 200 for a 2 inches ball valve, but I should have about 500. And I have numerous cases like this. The models I use for simulations are based on the standards ISA S75-01 and -02, so I add pipes at the inlet and outlet of my CAD model. Then I tried with a variety of limit conditions (pressure at inlet and/or outlet, flow at inlet and/or outlet,...) but nothing seems coherent. For starters, you need at least 10 pipe diameters downstream of the valve in order for the flow to fully develop in the solution. You might as well put the same upstream as well. By having the "ends" of you model so close to the valve, the boundary conditions you are setting at the inlet and outlet are influencing the flow calculations inside the valve. When you run the solution with much longer pipes, you then take the 4 pressure measurements at the wall of the pipe 2 diameters upstream and 6 downstream just where the pressure tap would be. Average the numbers at each location and calculate the Cv. You should be much closer. Keep in mind that the test results are allowed to vary by as much as +/- 5%. Realistically, unless you are using a very high-end CFD package, you're doing great if you get within 10%.
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