Gate valve throttling 1

[arbitrader]

I know that gate valves should never be used for throttling flow. I had someone ask me if a gate valve is used to throttle flow say by opening 50% would it damage the seats, seals, etc on the valve?

I wasn't sure of the answer. Can anyone offer any insight into the problems that are associated with throttling with a gate valve?

Thanks

 

[scalleke]

Yes, you will probably damage the wedge and the seats.

The valve guide tollerances about half way down are large enough to make that the edge of the wedge and the seats can make contact. The moving liquid will make sure that this happens repeatedly.

In most cases seats are stellite and wedges are softer SS. The wedge will be damaged and this damage will be wiped over the wedge face at the following valve closure?

Best Regards.

Scalleke

 

[arbitrader]

Thanks!

 

[BobPE]

dileod:

the problem with gate valves is that you will not get the valve to throttle until it is approximately 90 percent closed. It is at this point that point you get a rather steep headloss curve progressing to valve closure. Closing it 50 percent will only induce a small minor loss to the system, not enough for throttling anything significant.

scalleke is right at the point the valve can throttle, it will suffer damage if its used at that point for any period of time.

BobPE

 

[TBP]

If you throttle with a gate valve, a swirl is set up that scrubs the back side of the gate. The gate can be eroded. How much damage and how fast it occurs depends on the fluid and pressure drop across the valve. If nothing else, you have really terrible control with a gate valve.

I see throttled gate valves most often ahead of control valves to prevent hunting. Many control valves are seriously oversized, and hunt even at what is a high process load. Throttling the upstream isolation valve provides a pressure drop ahead of the control valve, and the reduced inlet pressure has the effect of "making the control valve smaller", often eliminating the hunting. However, when the time comes to work on the control valve, the upstream gate valve typically won't hold, because it's been damaged due to throttling.