Page 286 - Valve Selection Handbook

Sizing Pressure Relief Devices 273

Introducing the above pressure ratio into Equation 7-19 and rearrang-
ing the value of the resistance coefficient of the discharge pipeline for
10% built-up back pressure may be obtained from Equation 7-20:

For gas or vapor:

This is the proposed sizing procedure:

1. Calculate the value of the pressure ratio (Pi/P T), from Equation 7-21

where the pressures P are absolute pressures.
2. Calculate from the pressure ratio (PI/PT) the value of P T. If the pres-
sure P T is higher than the atmospheric pressure P 3, the pressure P T is
critical. If the calculated value of P T is lower than the atmospheric
pressure P3, the pressure at the discharge pipe outlet is the pressure P 3.
3. Calculate the value of ?2/Px and determine from Figure 7-9 the
resistance coefficient of the discharge pipeline for the permissible
built-up back pressure in gas or vapor service.

Sizing of Rupture Discs

Rupture discs may be employed in pressure systems subject not only
to nonviolent pressure excursions, as in regimes in which pressure relief
valves are employed, but also in systems subject to violent pressure
excursion resulting from the deflagration of flammable gases or dusts in
low-strength containers. Each pressure excursion regime requires a dif-
ferent approach to rupture disc sizing.