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- To
demonstrate compliance with the safety criteria.
- To
provide information required to complete a safety analysis of the system or
component in final, installed configuration, and to estimate interaction with
any connected systems.
4.2
Relief Valve Sizing
Pressure relief valves must be installed on all vessels and
piping which contain cryogenic fluids or might under some failure conditions
contain cryogenic fluids (e.g. cryostat vacuum vessels). Note that
current standards require pressure relief devices to be ASME code-certified.
Several steps are involved in determining flow requirements
for relief valves:
- Establish
the maximum safe working pressure (MSWP) for all piping and vessels that may
contain cryogenic fluids. Note that current standards require that any
cryogenic storage vessel be designed by the rules of Section VIII of the ASME
Pressure Vessel Code
- Determine
the maximum rate of efflux of the contained cryogenic fluid required to
maintain pressure below the MSWP in a worst-case failure scenario.
Failure scenarios might include:
- Failure
of a cryostat insulating vacuum to atmosphere.
- Failure
of a cryostat insulating vacuum to the contained cryogen.
- Flow
of cryogen from a connected system due to a valve failure or operator error.
- Trapping
of cryogenic fluid due to valve failure or operator error.
- Show
that the relief valves, as actually installed, provide sufficient relief
capacity. For high flow rates, the pressure drop in any plumbing leading
to the relief valves may need to be considered. Flow capacity should be
estimated for the particular gas involved, at the appropriate temperature and
pressure
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