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[MW:1015] extended bonnet valves for high temperature service

By Materials & Welding
Dear friends
I have a query related to valves used at very high temperature for the refinery in Russia Im working on. The service are high temperature hydrocarbon, process with hydrogen or hot air and high pressure steam with ratings of 300# and 2500#. Design temperatures(stream list not yet issued by process) in FEED contractor piping class are 700 to 750 degC. Valve material(CF8, C>0.04)The FEED contactor has put a note against the valves(gate and globe) in his pipng class ' valves shall be supplied with extended bonnet and heat dissipating elements to obtain a packing temperature of 450degC maximum'. We have come across extended bonnet valves for cryogenic service.I would like to know if members have experience with extended bonnet valves for high temperature service. What are the parametres to be considered by the vendor in deciding the height of the bonnet? What are the tests/set up to be performed by the vendor to demonstrate that temperature at packing is <450°C.Is it feasible to have a extroordinarily long bonnet  in order to maintain T<=450 deg C?? What are the other packing materials other than graphite that can be used to compensate for the height of the bonnet. line gaskets are spiral wound with SS304H windings and mica/graphite filler.
 
Regards
Shyam

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