Page 269 - Valve Selection Handbook

7

SIZING PRESSURE

RELIEF DEVICES

Flow-through pressure relief devices may be of either one of the fol-
lowing categories. Two of these have subcategories:

• Gas, or vapor, flow
• Subsonic or non-choked flow
• Sonic or choked flow
• Liquid flow
• Mixed-phase flow (vapor plus liquid)
• Gas containing liquid droplets
• Liquid containing gas bubbles
• Liquid flashing to vapor through valve

Each of these has an appropriate formula or procedure for determining
the required nozzle size or flow rate. In the case of mixed-phase flow,
however, firm recommendations from statutory bodies and standard
organizations on appropriate sizing equations are not yet available. 1
The sizing equations are presented in SI and imperial units. For
nomenclature refer to Table 7-1 and Figure 7-1.

1
Compilation of draft document ISO xxxx "Design of safety valves and connected vent
lines for gas/liquid two-phase flow" in preparation at time of publication of this book.

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