What you do depends on whether the valves are welded or bolted bonnet: 
1.  For bolted bonnet valves we disassemble the valves, match all parts so they can be reassembled correctly (proper orientation and all...), heat treat the piping including the ball valve bodies (either in an oven or with local stress relief heating), and reassemble the valves after cool down.  Use new bonnet gaskets and hydrotest for leak tightness. 
2.  For welded bonnet valves, bonnet disassembly is not an option without destroying the valve.  Therefore, remove the packing from the valve's stuffing box (it will be destroyed during the PWHT operation) and back the gate or plug off of the seat so the valve is about 1/4 to 1/3 open.  Run through the PWHT cycle, cool down, repack the valve, and hydrotest. 
We have used these procedures successfully for many years in wet H2S or other sulfide service.

MORE NEWS