Which Valve First

I've seen a globe/ball or gate combination used alot here in Canadian gas plants. I dunno if it really matters who comes first. The globe is for throttling when the CV is out of service and the ball/gate is used to open/close. I dunno the reasoning for both. Maybe its simply a double block.

I agree with Jop. Normally the Cntrl Valve stations that I have seen is a Control valve with two gate valves on either side and a Globe valve on the bypass. The Gate valves are to isolate the system when the Control valve is not workng while the Globe valve has to throttle the flow.

Dear jop and all,

Finally I got answer from one of my boss,

Actually the ball valve has to come first, then globe valve.
Reason 1. Globe valve is suspect for leakage so it come second,
Reason 2. Ball valve easy to operate even in high pressure when we compare with globe valve, so the ball it come first.

I’m guessing I’m NOT seeing the big picture on this arrangement!!? …”IF” you put a
”block valve” in front of the by-pass valve (globe vlv.) HOW do you determine the by-pass valve was leaking? Do you have a drain between the two valves? IF that was the idea then how do you know it’s not the “block valve” that was leaking? BUT why would you go to the expense of doing so for a by-pass valve?
I too have not seen this type of valving arrangement in all the years I’ve done this type of work. BUT I’m open to possiblies; please give me a good reason for it!

Hi,
I went through my recent projects, and found out there are 7 projects using by-pass on gas inlet or outlet. 4 of them using gate valve or ball valve then followed by a globe valve. main line size from 4nps to 8nps, but all the bypass are 2nps. i don't know the reason, but it seems that process engineer like to do this way.