In certain Low temperature and cryogenic applications ,shell specifies valve bonnet to be non extended.To be more specific it states the valves to be non operable valves (ref shell MESC 77/200). Basically, the Shell Specification 77/200 covers valves in service between -20 deg c and -196Deg C and specifies these as "operable" valves. Valves with operating temps between 0 and -20 Deg c are deemed non operable. Basically a valve can be deemed non-operable by Shell, if they for example specify a valve with a design temperature below -20Deg C, however they state that the valve will not meet these low temperature conditions and thus is not required to operate at low temperature.
Thus SHELL could deem the valve "NON-OPERABLE".
Depending on your valve type, the length of a standard bonnet and the packing type used in your valve, it may not be neccassary to offer an extended bonnet at -40 Deg C.
For example, a globe control valve, may be working on a service where the design temperature is -46 Deg C and may be operating below -20 Deg C.... however the packing type offered is PTFE Chevrons and is suitable up to -50Deg C... so an extended bonnet is not required to protect the packing. Because it is not offered, and to decrease the overall cost (thus making the valve more competitive) it is offered with a standard bonnet.
To be very general (which i do not like been) the valve is only going to be "operable" if it is made from a grade of 300 stainless (which is suitable to -196 Deg C)... operable testing would subject the valve to cryogenic temperatures, which other materials may not be suitable for

MORE NEWS