on/off contol valve

[cheme2016]

I need to spec. out an on/off full port valve on the 150# steam line that feeds the turbine. This valve is a 2" ball valve. I do not have Cv of the valve to come up with delta p thru the valve. How should I spec this valve when Cv is not known yet? Thanks

 

[Latexman]

Do you have Crane Technical Paper No. 410? If not, you should get it. Buy it online for $60 here

https://store.flowoffluids.com/shop/publications/TP410.html

In the mean time,
For 2" f_T = 0.019
K₁ = 3f_T
C_V = 29.9d²/(K₁)^0.5

Otherwise, contact the valve vendor or manufacturer.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[moltenmetal]

A truly full port ball valve has a Cv which is almost indistinguishable from a piece of pipe of the same length as the face to face dimension of the valve. To be conservative, add 50% extra length for some disturbance around the seats- but make sure it's truly full port- some valves which claim to be full port have a bore which is still a fair bit smaller than sch40 pipe of that size. But if you want to call something a "control valve", it has to be the controlling resistance to flow in the line- that doesn't sound like what you're doing. An on/off valve which doesn't even try to regulate flow is referred to as an automated block valve (ABV) or emergency block valve (EBV), not as a control valve.

 

[georgeverghese]

You can reverse engineer the required Cv for this trip valve by searching for the range of Cvs' for full port ball valves on the net and computing the dp ( this approach will work if you have the right line for the feed stream to the turbine). Then set the max dp calculated to be the dp permissible for this valve.

More important would be to spec out (a) the max permissible closing time for this steam turbine service (b) whether you need a double trip valve configured to work as an automated double block and bleed to ensure there is no steam leakage in to the turbine body that could otherwise cause temperature bow.