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.

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