Page 288 - Valve Selection Handbook

Sizing Pressure Relief Devices 275

Coefficient of discharge method. The coefficient of discharge method,
using a coefficient of discharge K equal to 0.62, may be used within the
following limitations:

1. The rupture disc discharges directly to the atmosphere, is installed
within eight pipe diameters from the vessel nozzle entry, and has a
length of discharge pipe not greater than five pipe diameters from
the rupture disc device.
2. The nominal diameters of the inlet and discharge piping are equal to
or greater than the stamped NFS (or NB) designator of the device.
3. The area, A, to be used in the theoretical flow equation is the mini-
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mum net flow area as specified by the rupture disc manufacturer.

Within these limitations, the following sizing equations similar to
those for pressure relief valves may be applied:

Sizing Equations for Gas or Vapor Other than Steam

Customary Mixed Imperial Units, Sonic Nozzle Flow

Weight flow—Ib/h

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The minimum flow area is the calculated net area after a complete burst of the disc with
appropriate allowance for any structural members which may reduce the net flow area
through the rupture disc device. The net flow area for sizing purposes shall not exceed
the nominal pipe size area of the rupture disc device.