If it is a big line, then the operators will hate you, if it is a small line, then why bother (small ball valves and small gate valves are about the same price).
you are very funny. it is an 8 inch line and is the outlet from a oil-gas separator. the isolating valves are part of the PSV's control set. i dont know whether a gate valve can be tightly sealed or not.
Actually, I wasn't trying to be funny. I once installed a 12-inch gate valve in a valve-can on a gathering system. The valve takes almost 100 turns to operate, and the linkage from the handwheel to the top of the can is not as well aligned as I would have hoped. For a while we were cycling it once a week to run pigs - I got hate mail.
If an 8-inch gate valve ever completely seals, it is a coincidence. They are not made for a bubble-tight seal and I would never consider one for an isolation under a PSV.
We use gates for PSV, PCV, control valve loops, pumps, compressors...isolation period, all the time to great success. Location is 30,000 BPD crude unit and 10,000 BPD high pressure hydrotreater.
Use a reputable manufacture such as Crane, Velan, Powell, Bonne Forge and you'll be fine as long as the metalurgy matches the service.
If the service is high pressure and you really want to get as close to 100% isolation as reasonably feasible, then spec out an Orbit Valve.
