1113.5 Discharge Capacity.

The minimum required discharge capacity of the pressure-relief device or fusible plug for a pressure vessel shall be determined in accordance with Equation 1113.5: C = ƒDL (Equation 1113.5)

Where: C = Minimum required discharge capacity of the relief

device expressed as mass flow of air, pounds per minute (kg/s).

Absolute saturation pressure, corresponding to the

stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller, pound-force per square inch atmosphere, psia. [ASHRAE 15:9.7.7]

For SI units: 1 inch = 25.4 mm, 1 pound-force per square inch = 6.8947 kPa, 1 pound per minute = 0.00756 kg/s

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For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa, 1 pound per minute = 0.00756 kg/s Unless the maximum allowable back pressure ( P0 ) is specified by the relief valve manufacturer, the following maximum allowable back pressure values shall be used for P0, where P is the set pressure and Pa is atmospheric pressure at the nominal elevation of the installation (see Table 1112.12.3): For conventional relief valves: 15 percent of set pressure: P0 = (0.15• P )+ Pa [Equation 1112.12.4(2)] For balanced relief valves: 25 percent of set pressure: P0 = (0.25• P )+ Pa [Equation 1112.12.4(3)] For rupture disks alone: fusible plugs, and pilot operated relief devices, 50 percent of set pressure: P0 = (0.50• P )+ Pa [Equation 1112.12.4(4)] For fusible plugs, P shall be the saturated absolute pressure for the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller. [ASHRAE 15:9.7.9.3.1, 9.7.9.3.2] 1112.12.5 Simultaneous Operation. Where outlets of two or more relief devices or fusible plugs, which are expected to operate simultaneously, connect to a common discharge pipe, the common pipe shall be sized large enough to prevent the outlet pressure at each relief device from exceeding the maximum allowable outlet pressure in accordance with Section 1112.12.4.

[ASHRAE 15:9.7.9.3.3]