1112.12 Discharge Piping — 2025 California Mechanical Code
California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Mechanical Code (Title 24, Part 4) § 1112.12 · Effective January 1, 2026 · Official source: Building Standards Commission
The piping used for pressure-relief device discharge shall be in accordance with Section 1112.12.1 through Section 1112.12.5. [ASHRAE 15:9.7.9]
1112.12.1 Piping Connection. Piping connected to the discharge side of a fusible plug or rupture member shall have provisions to prevent plugging of the pipe upon operation of a fusible plug or rupture member.
[ASHRAE 15:9.7.9.1]
1112.12.2 Pipe Size. The size of the discharge pipe from the pressure-relief device or fusible plug shall be not less than the outlet size of the pressure-relief device or fusible plug. [ASHRAE 15:9.7.9.2]
1112.12.3 Maximum Length. The maximum length of the discharge piping installed on the outlet of pressure-relief devices and fusible plugs discharging to the atmosphere shall be determined in accordance with Section 1112.12.4 and Section 1112.12.5. {ASHRAE 15:9.7.9.3}
1112.12.4 Design Back Pressure. The design back pressure due to flow in the discharge piping at the outlet of pressure relief devices and fusible plugs, discharging to atmosphere, shall be limited by the allowable equivalent length of piping determined in accordance with Equation 1112.12.4(1).
[Equation 1112.12.4(1)]
L= 0.2146 - d [5] ( P [2] 0 P 22 ) d - ln ( P0P2 )
f - C [2] r 6 - f
Where:
L = Equivalent length of discharge piping, feet.
Cr = Rated capacity as stamped on the pressure relief device in pounds per minute (lb/min), or in standard cubic feet per minute (SCFM) multiplied by 0.0764, or as calculated in Section 1112.14 for a rupture member or fusible plug, or as adjusted for reduced capacity due to piping in accordance with the manufacturer of the device, or as adjusted for reduced capacity due to piping as estimated by an approved method.
f = Moody friction factor in fully turbulent flow.
d = Inside diameter of pipe or tube, inches.
ln = Natural logarithm.
P2 = Absolute pressure at outlet of discharge piping, psia.
P0 = Allowed back pressure (absolute) at the outlet of pressure relief device, psia.
22 ) d - ln (
L=
0.2146 - d [5] ( P [2] 0 P
( P [2] 0 P 22 ) d - ln ( P0P2 )
f - C [2] r 6 - f
REFRIGERATION
expected to operate simultaneously, connect to a common discharge pipe, the common pipe shall be sized large enough to prevent the back pressure at each pressure-relief device from exceeding the maximum allowable back pressure in accordance with Section 1112.12.4.
[ASHRAE 15:9.7.9.3.3]
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