Perhaps for some people a special valve is one that is not available from stock or not in a catalog, etc. For me, a three-way butterfly valve, 10-stage noise control valve, titanium or Hastelloy B-2 valve or a 100 NPS valve is special.
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B16.34 forward "A key and basic issue of mutual concern in this coordination was the matter of pressure temperature ratings. It was necessary to incorporate the SP 66-type ratings in the new standard, but at the same time also to provide ratings equivalent to those in B16.5 covering the buttwelding equivalents of flanged-end valves. Subcommittee 4 had made definitive recommendations for revisions in the flange ratings and it was obviously desirable to rationalize the two types of ratings as they would appear side-by-side in the new standard.
The results of these efforts appear herein in the form of pressure-temperature ratings tables. The method of computing the ratings is detailed in Nonmandatory Appendix B. The ratings differ from the pre-1968 B16.5 ratings because they are now calculated as a function of the mechanical properties of the pressure boundary materials, in contrast to the empirical basis used previously. A change in the SP 66-type rating (herein designated Special Class) discontinues the application of a plasticity factor at elevated temperatures which, in the opinion of the committee, could not be justified in dimension-sensitive valves."
As JLSeagull mentioned, a valve may be considered special due to its material composition, for example if it is made of an alloy, such as duplex, super duplex, alloy 20, monel, inconel, incoloy, hastelloy 254 SMO (6MO / 6Moly) or titanium.
Other special characteristics may be extreme size, very high pressure rating, an unusual or a combined end connection type, application, location (subsea).
Anything that doesn't adhere to API or other norms would also be special; I see custom made valve requirements daily.
We could all help more if you could specify.
Heather Smith Alloy valves from The Alloy Valve Stockist
