The extended bonnet construction is one option to dissipate heat and to lower the heat at the stem packing similar to the reduction of cryogenic valve temp propagation to the stem packing and to avoid subsequent failure of the packing and operation of the valve. In all, the subject of concern is the stem packing failure. Other options are finned bonnet constr, stem packing with separate cooling system.
Protection for valve handwheel operation is not a major concern as it can be chosen as Motor/Pneumatic operated valve. Most of the hand operated valves requires once in a year operation in such high temp service.
Coming to the stem packing,
1)The usual ones are in one or more combination of graphite, metal rings, Elastomer rings, Lip seals, simple TEFLON rings etc.
2) In the moderately higher temp. range, only Graphite+alloy is used in diff. arrangements.
3) For very high temp such as your case, cooling system or the extended bonnets and same as 2) packings are used. Also many new proprietary packing arrangements are available in the market. Evaluating them is an important part of a material engineer's role.
Additional one to be noted is, the valve basic design is also to be reviewed. Process engr. is not a valve specialist. So the material engineer is supposed to evaluate such cases. In this case a rising stem valve type design to be avoided as it has vertical and rotation wearing on the packing. For controlling a butterfly valve could be evaluated. For Gate types, a knife gate valve could be thought of. Pl. take note that such evaluation has many things to be accounted for such as leakage allowance, emission, hazops, ATEX concerns, operation difficulty, trim erosion and not the least process parameters like pressure drops, zigma allowance, opening/closing period, service fluid etc.
Coming to your question,
1) Parameters controlling the bonnet height ...depends on the stem packing design and heat propagation to the packing. As a thumb rule for alloy constr. 250mm shall be the min. height.
2) On tests....apart from the usual tests, service condition test is required to be performed. But if the manufacturer has a prototype test certificate, it could be accepted. Again it depends on the application of the valve. In your case prototype should be OK, in my opinion.
3) Create these valves as special parts and specify the individual condition of operation and design to ensure correct valve/packing design is offered by manuf.
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Important note is the temp you specified and the valve material specified does not go well together. Being a HP steam and at 700 deg. c(which itself is very unusual), SS is no good and it has to high alloy grade. Or your design condition is to be verified. B16.34 defined the temp. limits in the notes of every rating table.
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Regards,
Kannan.
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
Snamprogetti
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