Cv vs Position in a Gate Valve

[Eduardo Rodrigues]

thread408-211223

Can anybody help me on how to estimate Cv vs %open in a gate valve?

The mate of the closed thread has done that, and I´d like to know how...

 

[SPDL310]

As stated in the thread you refenced this is not a good idea as you are likely to damage your valve as well as downstream equipment by using it to control flow for an extended period of time. A gate valve should be used as an on/off valve. If you need to control flow or pressure it is highly recommended that you use a control valve of one variety or another. Why do you need a Cv for incremental opening of a gate valve?

The responses in your linked thread were pretty clear to me. There is no single empirical formula to do this as it is situational. The documents most conductive to you learning about calculating Cv and flow in general are Crane TP410 and the Fisher Control valve handbook. Those two books will explain more than any individual in this forum can. That being said here are my suggestions on how to go about this. The Cv will be roughly proportional to the cross sectional area exposed to flow. If half of the circular area is exposed the Cv would be roughly equivalent to 50% of the full open Cv. You won't get a ton of position/Cv accuracy, but it should give you a rough estimate of how it is changing. Another method if you have a characteristic curve for a similar valve is to scale it to the full Cv of your specific vale. The most accurate way as previously mentioned is to test your valve.

 

[MortenA]

@eduardo the old tread gave you refernces i googled the one mentioned and found a scanned copy with the resistance curve mentioned. Then theres only some footwork left...

Handbook of Hydraulic resistance by Idelchik

Best regards, Morten

--- Best regards, Morten Andersen

 

[mk3223]

You might do the calculation to get a number of Cv per valve disc opening as an "information".
But, IMO, it won't be able to apply it for the real flow pattern on the gate valve similar to that of the globe valve.

 

[Nick_Gray_88]

Just workout the open area of the disc against the seat then plug those figures into the calculations from Crane?? The area will vary depending on what type of gate valve you’re thinking of, I.e. a circle closing over a circular area will create a crescent shaped area.

Just need to be a bit smart about how you calculate the Cv from the crane equations…

As previously mentioned in the thread this should only be for information not as a means of using a gate valv to control the fluid.

 

[HitokiriAoshi]

you can't calculate it. closest is empirical testing / ANSYS simulation, you are not taking into consideration of any of the turbulence which comes with non fully opened gate valves. which wouldn't matter after the valve breaks.

Luke | Valve Hax |

https://valvehax.com/

 

[moltenmetal]

Assuming you're using a gate to take any significant pressure drop for an extended period, it soon will have a significantly nonzero leakage when fully closed due to gate chatter against the seat.

(

http://www.spitfireresearch.com)

 

[gerhardl]

Summary of all posts above reflects my experience through many years:

1. You should under all circumstances select a regulating valve for regulating.
2. You might use a gate valve to regulate a flow, but always under very limited conditions regarding flow speed and pressure differences.
3. You might try to calculate a result, but have to check the result 'in the field'.
4. The valve will be damaged (most likely by cavitation) if calculated or operated outside possible limits.

 

[horacio Torres]

it is really difficult to do.... it better installed a control valve to do the job.

horacio

https://www.linkedin.com/company/lagotuy/

 

[Compositepro]

https://www.industrialcontrolsonline.com/training/online/controlling-flow-ball-valves

Gate valves are fairly linear.