The optimum sizing of pressure relief devices is essential to safely protect a piping system from overpressurization. An undersized system would not be in a position to relieve the system effectively and an oversized system will result in unnecessary material and labor costs.
Flow-through pressure relief devices may be of either one of the following 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.
| Mixed SI Units | Mixed U.S. Units | ||
|---|---|---|---|
| A | Flow area | mm 2 | in. 2 |
| C | Gas flow constant for sonic nozzle flow, based on isentropic coefficient k at standard conditions (See Table 7-2. If k is unknown, use C = 315 for a conservative result) | ||
| d | Internal pipe diameter | mm | in. |
| f | Pipe friction factor = number of velocity heads lost in length of pipe equal to diameter | ||
| F | Gas flow constant for subsonic nozzle flow, based on isentropic coefficient k at standard conditions and the absolute... |
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