1113.1 General — 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) § 1113.1 · Effective January 1, 2026 · Official source: Building Standards Commission
Pressure vessels shall be provided with overpressure protection in accordance with ASME BPVC Section VIII.1. Pressure vessels containing liquid refrigerant that are capable of being isolated by stop valves from other parts of a refrigerating system shall be provided with overpressure protection. Pressure relief devices or fusible plugs shall be sized in accordance with Section 1113.5. [ASHRAE 15:9.7.1, 9.7.2]
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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 1112.12.4):
(1) For conventional relief valves: 15 percent of set
pressure:
P0 = (0.15• P )+ Pa [Equation 1112.12.4(2)] (2) For balanced relief valves: 25 percent of set pres sure:
P0 = (0.25• P )+ Pa [Equation 1112.12.4(3)]
(3) For rupture disks alone, fusible plugs, or 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]
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
| ELEVATION ABOVE SEA LEVEL, FEET |
POUNDS PER SQUARE INCH, ABSOLUTE (Pa) |
|---|---|
0 |
14.7 |
500 |
14.4 |
1000 |
14.2 |
1500 |
13.9 |
2000 |
13.7 |
2500 |
13.4 |
3000 |
13.2 |
3500 |
12.9 |
4000 |
12.7 |
4500 |
12.5 |
5000 |
12.2 |
6000 |
11.8 |
7000 |
11.3 |
8000 |
10.9 |
9000 |
10.5 |
1000 |
10.1 |
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
1112.12.5 Simultaneous Operation. When outlets of two or more relief devices or fusible plugs, which are
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
244 2025 CALIFORNIA MECHANICAL CODE
REFRIGERATION
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