CMC · California Mechanical Code
When are fusible plugs, rupture members, or dual relief valves required for refrigeration pressure vessels?
Short homeowner summary: Refrigeration vessels under the California Mechanical Code are protected based on their size: small vessels (≤ 3 ft³) may use a fusible plug or relief valve; medium vessels (>3 and <10 ft³) must use relief valves (no fusible plugs); and large vessels (≥10 ft³) that vent to the atmosphere require rupture disks or two relief valves with a three‑way valve unless a narrow exception applies. All devices must be sized and piped per the code.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
Pressure vessels are protected according to their internal gross volume. Small vessels (≤ 3 cubic feet) may be protected by a fusible plug or pressure-relief device, vessels > 3 to < 10 cubic feet must use pressure-relief devices (fusible plugs prohibited), and vessels ≥ 10 cubic feet discharging to the atmosphere must use one or more rupture member(s) or dual pressure‑relief valves (with a three‑way valve for testing/repair). See § 1113.2, § 1113.3, and § 1113.6 for the controlling rules.
The single most important rule: If a refrigeration pressure vessel is 10 cubic feet or larger and it will discharge to the atmosphere, you must provide rupture member(s) or dual pressure‑relief valves with a three‑way valve for testing — you cannot rely on a single valve unless very specific exceptions are met. § 1113.6
Requirements in detail
Key thresholds (decision table)
| Internal gross volume | Required protection | Fusible plugs allowed? | Code Reference |
|---|---|---|---|
| ≤ 3 cu ft (0.1 m³) | One or more pressure‑relief devices OR a fusible plug | Yes (either option permitted) | § 1113.2 |
| > 3 and < 10 cu ft (0.1–0.28 m³) | One or more pressure‑relief devices (fusible plugs prohibited) | No (fusible plugs not permitted) | § 1113.2 |
| ≥ 10 cu ft (0.28 m³) discharging to atmosphere | One or more rupture member(s) OR dual pressure‑relief valves; if dual valves are used they must be installed with a three‑way valve for testing/repair | N/A — fusible plugs are not the specified option for this size | § 1113.6 |
Notes:
- Minimum discharge capacity and sizing are governed by § 1113.5, which references ASHRAE 15 for sizing. The CMC text requires sizing per ASHRAE 15 but the detailed sizing formulas are found in ASHRAE 15 / referenced sections. § 1113.5
- For vessels that discharge into the lowside of the system, a single relief valve (not a rupture member) is generally prohibited on vessels ≥ 10 cu ft except where specific conditions apply (see Exceptions & special cases). § 1113.3
Installation / ancillary requirements that affect choice
- Where dual pressure‑relief valves are used, they must be paired with a three‑way valve so one valve can be taken out for testing/repair while the other remains providing protection. § 1113.6
- Rupture members (rupture disks) must be sized and marked in accordance with the code and ASME BPVC requirements; their rated rupture pressure must not exceed the design pressure of the protected part. § 1112.10.1 and § 1115.4.1
- Discharge piping from a fusible plug or rupture member must include provisions to prevent plugging when the device operates, must be at least the device outlet size, and must meet maximum length / back‑pressure limits calculated per the code. § 1112.12.1, § 1112.12.2, § 1112.12.3, § 1112.12.4
Where fusible plugs may be located
- Pressure‑relief devices are required to be connected above the liquid level, but the code makes a specific exception for fusible plugs used on the high side — they are permitted to be located above or below the liquid refrigerant level. § 1112.8 (exception)
Exceptions & special cases
- Single relief valve permitted on vessels ≥ 10 cu ft only when all these conditions are met:
- The relief valves discharge into the lowside of the system.
- The vessel has shutoff valves that permit pumpdown of the refrigerant charge from the vessel.
- Other pressure vessels in the system are separately protected per § 1113.1.
This exception is explicit in § 1113.6.
- Vessels with inside dimensions ≤ 6 inches have specific marking and protection rules: they must be listed or marked per ASME BPVC or tested equivalently, and they must be protected by either a pressure‑relief device or a fusible plug. See § 1117.1.
- Sizing and rated discharge capacity: the code delegates discharge capacity calculations to ASHRAE 15 and ASME BPVC procedures; the CMC references these for the minimum required capacities (see § 1113.5 and § 1112.14 for rupture/fusible rating formulas). If you need numerical sizing you must consult ASHRAE 15 and the referenced equations. § 1113.5, § 1112.14
Common mistakes
- Assuming a single relief valve is acceptable on a receiver ≥ 10 cu ft that discharges to atmosphere (not allowed except under narrow exceptions). § 1113.6
- Installing a fusible plug on a vessel > 3 cu ft and < 10 cu ft — fusible plugs are explicitly prohibited for that size range. § 1113.2
- Forgetting three‑way valve provisions when using dual relief valves — dual valves without the three‑way arrangement do not meet the accessibility/testing requirement. § 1113.6
- Failing to size the device per § 1113.5 (which points to ASHRAE 15 and ASME BPVC) — undersized devices or undersized discharge piping can create unsafe back pressure. § 1113.5, § 1112.12.4
- Omitting marking / nameplate data: rupture members and fusible plugs require manufacturer markings and melt‑temperature markings respectively as required by § 1115.4.1 and § 1115.4.2. § 1115.4.1, § 1115.4.2
Worked example — concrete scenario
Scenario: A receiver has an internal gross volume of 12 cubic feet and will discharge to the atmosphere.
Step 1 — Volume check:
- 12 cu ft ≥ 10 cu ft, so § 1113.6 applies. § 1113.2 volume thresholds confirm larger‑vessel rules.
Step 2 — Choose protection:
- Because it discharges to the atmosphere, the vessel must use one or more rupture member(s) OR dual pressure‑relief valves. If you choose dual valves, you must install them with a three‑way valve to allow testing/repair without leaving the vessel unprotected. § 1113.6
Step 3 — Consider exception (single valve):
- A single relief valve would be allowed only if (a) the relief valve discharges into the lowside of the system, (b) the receiver has shutoff valves to permit pumpdown, and (c) other vessels are separately protected per § 1113.1. If these are not all met, the single‑valve option is not permitted. § 1113.6
Step 4 — Sizing & piping:
- Size the rupture member or relief valves per § 1113.5 (ASHRAE 15). Size discharge piping at least to the outlet size of the device, and verify equivalent length/back‑pressure limits per § 1112.12.2–.4. Also ensure marking/identification per § 1115.4. § 1113.5, § 1112.12.2–.4, § 1115.4
Outcome (practical): Install dual pressure‑relief valves sized per ASHRAE 15, connect through a three‑way valve for testing/repair, use full‑size discharge piping with measures to prevent plugging, and ensure required markings are present.
Related provisions (CMC sections)
- § 1113.1 — General overpressure protection requirements (ASME BPVC reference).
- § 1113.2 — Type of protection by internal gross volume (fusible plug allowance/ prohibition).
- § 1113.3 — Discharging into the lowside of the system; single‑valve restrictions.
- § 1113.5 — Discharge capacity; sizing per ASHRAE 15.
- § 1112.12.1–.4 — Discharge piping, outlet size, back‑pressure / maximum length rules (affects fusible/rupture installations).
- § 1112.14 — Rating formulas for rupture members and fusible plugs (used for atmospheric discharge sizing).
- § 1115.4.1 and § 1115.4.2 — Marking requirements for rupture members and fusible plugs.
- § 1117.1 — Special rules for pressure vessels with inside dimensions 6 inches or less.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 304.8 High relevance — show source text
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
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REFRIGERATION
1113.2 Type of Protection. Pressure vessels with an internal gross volume of 3 cubic feet (0.1 m [3] ) or less shall use one or more pressure relief devices or a fusible plug. Pressure vessels of more than 3 cubic feet (0.1 m [3] ) but less than 10 cubic feet (0.28 m [3] ) internal gross volume shall use one or more pressure relief devices. Fusible plugs shall not be used.
[ASHRAE 15:9.7.2.1, 9.7.2.2]
1113.3 Discharging into Lowside of System. For pressure-relief valves discharging into the lowside of the system, a single relief valve (not rupture member) of the required relieving capacity shall not be used on vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume except under the conditions permitted in Section 1112.11.3. [ASHRAE 15:9.7.3]
1113.4 Parallel Pressure-Relief Devices. Two or more
pressure-relief devices in parallel to obtain the required capacity shall be considered as one pressure-relief device. The discharge capacity shall be the sum of the capacities required for each pressure vessel being protected.
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 ASHRAE 15.
1113.6 Three-Way Valve. Pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume shall use one or more rupture member(s) or dual pressure-relief valves where discharging to the atmosphere. Dual pressure-relief valves shall be installed with a three-way valve to allow testing or repair. Where dual relief valves are used, the valve shall comply with Section 1113.5.
Exception: A single relief valve shall be permitted on pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume where in accordance with the following conditions:
(1) The relief valves are located on the lowside of the sys tem.
(2) The vessel is provided with shutoff valves designed to allow pumpdown of the refrigerant charge of the pressure vessel.
(3) Other pressure vessels in the system are separately protected in accordance with Section 1113.1. [ASHRAE 15:9.7.2.3]
1114.0 Special Discharge Requirements.
CMC § 1112.14 High relevance — show source text
Where:
C = Rated discharge capacity expressed as mass flow of air, pounds per minute.
d = Smallest of the internal diameter of the inlet pipe, retaining flanges, fusible plug, and rupture member , inches.
For rupture members:
[Equation 1112.14(3)]
P1 = (rated pressure in psig x 1.1) + 14.7 psia
For fusible plugs:
P1 = 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 pound per minute = 0.00756 kg/s
1113.0 Overpressure Protection.
1113.1 General. 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]
»
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]
CMC § 1112.12.4 High relevance — show source text
»
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,
FEETPOUNDS PER SQUARE INCH,
ABSOLUTE (Pa)
014.7
50014.4
100014.2
150013.9
200013.7
250013.4
300013.2
350012.9
400012.7
450012.5
500012.2
600011.8
700011.3
800010.9
900010.5
100010.1 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
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CMC § 1113.4 High relevance — show source text
1113.4 Parallel Pressure-Relief Devices. Two or more
pressure-relief devices in parallel to obtain the required capacity shall be considered as one pressure-relief device. The discharge capacity shall be the sum of the capacities required for each pressure vessel being protected.
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 ASHRAE 15.
1113.6 Three-Way Valve. Pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume shall use one or more rupture member(s) or dual pressure-relief valves where discharging to the atmosphere. Dual pressure-relief valves shall be installed with a three-way valve to allow testing or repair. Where dual relief valves are used, the valve shall comply with Section 1113.5.
Exception: A single relief valve shall be permitted on pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume where in accordance with the following conditions:
(1) The relief valves are located on the lowside of the sys tem.
(2) The vessel is provided with shutoff valves designed to allow pumpdown of the refrigerant charge of the pressure vessel.
(3) Other pressure vessels in the system are separately protected in accordance with Section 1113.1. [ASHRAE 15:9.7.2.3]
1114.0 Special Discharge Requirements.
1114.1 General. Systems containing other than Group A1 or B1 refrigerants shall discharge to atmosphere through an approved flaring device.
Exceptions:
(1) Where the Authority Having Jurisdiction determines upon review of a rational engineering analysis that fire, health, or environmental hazards will not result from the proposed atmospheric release.
(2) Lithium bromide absorption system using water as the refrigerant.
1114.2 Design Requirements. Flaring devices shall be designed to incinerate the entire discharge. The products of refrigerant incineration shall not pose health or environmental hazards. Incineration shall be automatic upon initiation of discharge, shall be designed to prevent blow-back, and shall not expose structures or materials to the threat of fire. Standby fuel, such as LP-Gas, and standby power shall have the capacity to operate for one and a half times the required time for complete incineration of the charge.
1114.3 Testing. Flaring systems shall be tested to demonstrate their safety and effectiveness. A report from an approved agency shall be submitted detailing the emission products from the system as installed.
1115.0 Labeling and Identification.
1115.1 General. In addition to labels required elsewhere in this chapter, a refrigeration system shall be provided with identification labels in accordance with Section 1115.2
through Section 1115.5.
1115.2 Volume and Type. A condenser, receiver, absorber, accumulator and similar equipment having an internal volume of more than 3 cubic feet (0.1 m [3] ) and containing refrigerant shall be equipped with a permanent label setting forth the type of refrigerant in such vessel.
1115.3 Permanent Sign. In a refrigeration machinery room and for a direct refrigerating system of more than 10 horsepower (7.5 kW), there shall be a permanent sign at an approved location giving the following information:
(1) Name of contractor installing the equipment.
CMC § 1112.12.4. High relevance — show source text
( P [2] 0 P 22 ) d - ln
(P0P2 )f - C [2] r 6 - f
2025 CALIFORNIA MECHANICAL CODE 243
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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]
1112.13 Rating of Pressure-Relief Device. The rated discharge capacity of a pressure-relief device expressed in pounds of air per minute (kg/s), shall be determined in accordance with ASME BPVC Section VIII.1. Pipe and fittings between the pressure-relief valve and the parts of the system it protects shall have not less than the area of the pressurerelief valve inlet area. [ASHRAE 15:9.7.6]
1112.14 Rating of Rupture Members and Fusible Plugs. The rated discharge capacity of a rupture member or fusible plug discharging to the atmosphere under critical flow conditions, in pounds of air per minute (kg/s), shall be determined in accordance with the following formulas:
C = 0.64 P l d [2] [Equation 1112.14(1)]
d = 1.25
√C / P l [Equation 1112.14(2)]Where:
C = Rated discharge capacity expressed as mass flow of air, pounds per minute.
d = Smallest of the internal diameter of the inlet pipe, retaining flanges, fusible plug, and rupture member , inches.
For rupture members:
[Equation 1112.14(3)]
P1 = (rated pressure in psig x 1.1) + 14.7 psia
For fusible plugs:
P1 = 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 pound per minute = 0.00756 kg/s
1113.0 Overpressure Protection.
1113.1 General. 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]
»
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):
CMC § 1114.2 High relevance — show source text
(2) Lithium bromide absorption system using water as the refrigerant.
1114.2 Design Requirements. Flaring devices shall be designed to incinerate the entire discharge. The products of refrigerant incineration shall not pose health or environmental hazards. Incineration shall be automatic upon initiation of discharge, shall be designed to prevent blow-back, and shall not expose structures or materials to the threat of fire. Standby fuel, such as LP-Gas, and standby power shall have the capacity to operate for one and a half times the required time for complete incineration of the charge.
1114.3 Testing. Flaring systems shall be tested to demonstrate their safety and effectiveness. A report from an approved agency shall be submitted detailing the emission products from the system as installed.
1115.0 Labeling and Identification.
1115.1 General. In addition to labels required elsewhere in this chapter, a refrigeration system shall be provided with identification labels in accordance with Section 1115.2
through Section 1115.5.
1115.2 Volume and Type. A condenser, receiver, absorber, accumulator and similar equipment having an internal volume of more than 3 cubic feet (0.1 m [3] ) and containing refrigerant shall be equipped with a permanent label setting forth the type of refrigerant in such vessel.
1115.3 Permanent Sign. In a refrigeration machinery room and for a direct refrigerating system of more than 10 horsepower (7.5 kW), there shall be a permanent sign at an approved location giving the following information:
(1) Name of contractor installing the equipment.
(2) Name and number designation of refrigerant in system.
(3) Pounds of refrigerant in system.
1115.4 Marking of Pressure-Relief Devices. Pressurerelief valves for refrigerant containing components shall be set and sealed by the manufacturer or an assembler as defined in ASME BPVC Section VIII.1. Each pressure relief valve shall be marked by the manufacturer or assembler with the data required in ASME BPVC Section VIII.1.
Exception: Relief valves for systems with design pressures of 15 pounds-force per square inch gauge (psig) (103 kPa gauge) or less shall be marked by the manufacturer with the pressure setting capacity. [ASHRAE 15:9.6.1]
1115.4.1 Rupture Members. Rupture members for refrigerant pressure vessels shall be marked with the data required in accordance with ASME BPVC Section VIII.1. [ASHRAE 15:9.6.2]
1115.4.2 Fusible Plugs. Fusible plugs shall be marked with the melting temperatures in °F (°C).
[ASHRAE 15:9.6.3]
1115.5 Nameplate. Each self-contained system and each separate condensing unit, compressor, or compressor unit sold for field assembly in a refrigerating system shall carry a nameplate marked with the manufacturer’s name, nationally registered trademark or trade name, identification num
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CMC § 9.4.5 High relevance — show source text
[ASHRAE 15:9.4.5]
1112.7 Stop Valves Prohibited. Stop valves shall not be located between a pressure-relief device and parts of the system protected thereby. A three-way valve, used in conjunction with the dual relief valve in accordance with Section
1113.6, shall not be considered a stop valve. [ASHRAE 15:9.4.6]
1112.8 Location. Pressure-relief devices shall be connected
directly to the pressure vessel or other parts of the system protected thereby. These devices shall be connected above the liquid refrigerant level and installed so that they are accessible for inspection and repair, and so that they are not capable of being readily rendered inoperative.
Exception: Where fusible plugs are used on the highside, they shall be located above or below the liquid refrigerant level. [ASHRAE 15:9.4.8]
1112.9 Materials. The seats and discs of pressure-relief devices shall be constructed of compatible material to resist refrigerant corrosion or other chemical action caused by the refrigerant. Seats or discs of cast iron shall not be used. Seats and discs shall be limited in distortion, by pressure or other cause, to a set pressure change of not more than 5 percent in a span of five years. [ASHRAE 15:9.4.9]
1112.10 Pressure-Relief Valve Setting. Pressure-relief valves shall start to function at a pressure not exceeding the design pressure of the parts of the system protected.
Exception: Relief valves that discharge into other parts of the system shall comply with Section 1112.11.3. [ASHRAE 15:9.5.1]
1112.10.1 Rupture Member Setting. Rupture members used in lieu of, or in series with, a relief valve shall have a nominal rated rupture pressure not exceeding the design pressure of the parts of the system protected. The conditions of application shall comply with ASME BPVC Section VIII.1. The size of rupture members installed ahead of relief valves shall not be less than the
relief-valve inlet. [ASHRAE 15:9.5.2]
1112.11 Discharge from Pressure-Relief Devices. Pressure-relief systems designed for vapor shall comply with Section 1112.11.1 through Section 1112.11.4.1.
1112.11.1 Discharging Location Interior to Build- ing. Pressure-relief devices, including fusible plugs, serving refrigeration systems shall be permitted to discharge to the interior of a building where in accordance with all of the following:
(1) The system contains less than 110 pounds (49.9 kg) of a Group A1 or A2L refrigerant.
(2) The system contains less than 6.6 pounds (2.99 kg) of a Group A2, B1, B2 or B2L refrigerant.
(3) The system does not contain any quantity of a Group A3 or B3 refrigerant.
(4) The system is to be installed in a machinery room in accordance with Section 1106.0.
(5) The refrigerant concentration limits in Section 1104.2 are not exceeded. Refrigeration systems that do not comply with the above requirements shall comply with the requirements of Section 1112.11.2 through Section 1112.11.4. [ASHRAE 15:9.7.8.1]
CMC § 0.62 High relevance — show source text
(4) Systems erected on the premises using Group A1 refrigerant and with copper tubing not exceeding 0.62 of an inch (15.7 mm) outside diameter shall be tested by means of the refrigerant charged into the system at the saturated vapor pressure of the refrigerant at not less than 68°F (20°C). [ASHRAE 15:10.1.2]
1116.4 Declaration. A dated declaration of test shall be
provided for systems containing more than 55 pounds (24.9 kg) of refrigerant. The declaration shall give the name of the refrigerant and the field test pressure applied to the highside and the lowside of the system. The declaration of test shall be signed by the installer and, where an inspector is present at the tests, the inspector shall also sign the declaration. Where requested, copies of this declaration shall be furnished to the Authority Having Jurisdiction. [ASHRAE 15:10.2]
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1116.5 Brine Systems. Brine-containing portions of a system shall be tested at one and a half times the design pressure of the system using brine as the test fluid.
1117.0 Refrigerant-Containing Pressure Vessels.
1117.1 Inside Dimensions 6 Inches or Less. Pressure vessels having inside dimensions of 6 inches (152 mm) or less shall be:
(1) Listed either individually or as part of an assembly by an approved, nationally recognized testing laboratory, or
(2) Marked directly on the vessel or on a nameplate attached to the vessel with a “U” or “UM” symbol signifying compliance with ASME BPVC Section VIII.1, or
(3) When requested by the Authority Having Jurisdiction, the manufacturer shall provide documentation to confirm that the vessel meets the design, fabrication, and testing requirements of ASME BPVC Section VIII.1.
Exception: Vessels having an internal or external design pressure of 15 psig (103 kPag) or less.
Pressure vessels having inside dimensions of 6 inches (152 mm) or less shall be protected by either a pressure-relief device or a fusible plug. [ASHRAE 15:9.3.1.1]
1117.1.1 Pressure-Relief Device. Where a pressurerelief device is used to protect a pressure vessel having an inside dimension of 6 inches (152 mm) or less, the ultimate strength of the pressure vessel so protected shall withstand a pressure of not less than 3.0 times the design pressure. [ASHRAE 15:9.3.1.2]
1117.1.2 Fusible Plug. Where a fusible plug is used to protect a pressure vessel having an inside diameter of 6 inches (152 mm) or less, the ultimate strength of the pressure vessel so protected shall withstand a pressure 2.5 times the saturation pressure of the refrigerant used at the temperature stamped on the fusible plug or 2.5 times the critical pressure of the refrigerant used, whichever is less.
[ASHRAE 15:9.3.1.3]
CMC § 1112.13 High relevance — show source text
1112.13 Rating of Pressure-Relief Device. . . . . . . . . . . . . . . . . . . . . . 244
1112.14 Rating of Rupture Members and Fusible Plugs . . . . . . . . . . . . . . . . 244
1113.0 Overpressure Protection. . . . . . . . 244
1113.1 General. . . . . . . . . . . . . . . . . . . . . 244
1113.2 Type of Protection . . . . . . . . . . . . 245
1113.3 Discharging into Lowside of System . . . . . . . . . . . . . . . . . . . . . 245
1113.4 Parallel Pressure-Relief Devices . . 245
1113.5 Discharge Capacity . . . . . . . . . . . 245
1113.6 Three-Way Valve . . . . . . . . . . . . . 245
1114.0 Special Discharge Requirements. . 245
1114.1 General. . . . . . . . . . . . . . . . . . . . . 245
1114.2 Design Requirements . . . . . . . . . . 245
1114.3 Testing . . . . . . . . . . . . . . . . . . . . . 245
1115.0 Labeling and Identification . . . . . . 245
1115.1 General. . . . . . . . . . . . . . . . . . . . . 245
1115.2 Volume and Type . . . . . . . . . . . . . 245
1115.3 Permanent Sign . . . . . . . . . . . . . . 245
1115.4 Marking of Pressure-Relief Devices. . . . . . . . . . . . . . . . . . . . . 245
1115.5 Nameplate . . . . . . . . . . . . . . . . . . 245
1116.0 Testing of Refrigeration Equipment . . . . . . . . . . . . . . . . . . 246
1116.1 Factory Tests . . . . . . . . . . . . . . . . 246
1116.2 Field Tests . . . . . . . . . . . . . . . . . . 246
1116.3 Test Gases . . . . . . . . . . . . . . . . . . 246
1116.4 Declaration . . . . . . . . . . . . . . . . . . 246
1116.5 Brine Systems . . . . . . . . . . . . . . . 247
1117.0 Refrigerant-Containing Pressure Vessels . . . . . . . . . . . . . . . . . . . . . 247
1117.1 Inside Dimensions 6 Inches
or Less . . . . . . . . . . . . . . . . . . . . . 247
1117.2 Inside Dimensions More than
6 Inches . . . . . . . . . . . . . . . . . . . . 247
1117.3 Pressure Vessels for 15 psig or Less . . . . . . . . . . . . . . . . . . . . . 247
CMC § 1115.4 High relevance — show source text
(1) Name of contractor installing the equipment.
(2) Name and number designation of refrigerant in system.
(3) Pounds of refrigerant in system.
1115.4 Marking of Pressure-Relief Devices. Pressurerelief valves for refrigerant containing components shall be set and sealed by the manufacturer or an assembler as defined in ASME BPVC Section VIII.1. Each pressure relief valve shall be marked by the manufacturer or assembler with the data required in ASME BPVC Section VIII.1.
Exception: Relief valves for systems with design pressures of 15 pounds-force per square inch gauge (psig) (103 kPa gauge) or less shall be marked by the manufacturer with the pressure setting capacity. [ASHRAE 15:9.6.1]
1115.4.1 Rupture Members. Rupture members for refrigerant pressure vessels shall be marked with the data required in accordance with ASME BPVC Section VIII.1. [ASHRAE 15:9.6.2]
1115.4.2 Fusible Plugs. Fusible plugs shall be marked with the melting temperatures in °F (°C).
[ASHRAE 15:9.6.3]
1115.5 Nameplate. Each self-contained system and each separate condensing unit, compressor, or compressor unit sold for field assembly in a refrigerating system shall carry a nameplate marked with the manufacturer’s name, nationally registered trademark or trade name, identification num
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ber, design pressures, and refrigerant for which it is designed. The refrigerant shall be designated by the refrigerant number (“R-” number) as shown in Table 1102.3. {ASHRAE 15:9.15}
Heat pumps and electric cooling appliances shall bear a factory-applied nameplate in accordance with Section 307.3.
1116.0 Testing of Refrigeration Equipment.
1116.1 Factory Tests. Refrigerant-containing parts of unit systems shall be tested and proved tight by the manufacturer at not less than the design pressure for which they are rated. Pressure vessels shall be tested in accordance with Section
1117.0. [ASHRAE 15:9.14.1]
1116.1.1 Testing Procedure. Tests shall be performed with dry nitrogen or another nonflammable, nonreactive, dried gas. Oxygen, air, or mixtures containing them shall not be used. The means used to build up the test pressure shall have either a pressure limiting device or a pressure reducing device and a gauge on the outlet side. The pressure relief device shall be set above the test pressure but low enough to prevent permanent deformation of the system’s components.
Exceptions:
(1) Mixtures of dry nitrogen, inert gases, and Class 1 refrigerants shall be permitted for factory tests.
(2) Mixtures of dry nitrogen, inert gases, or a combination thereof with Class 2L, Class 2, or Class 3 refrigerants in concentrations not exceeding the lesser of a refrigerant weight fraction (mass fraction) of 5 percent or 25 percent of the LFL shall be permitted for factory tests.
CMC § 1112.12.4 High relevance — show source text
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
2025 CALIFORNIA MECHANICAL CODE 243
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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]
1112.13 Rating of Pressure-Relief Device. The rated discharge capacity of a pressure-relief device expressed in pounds of air per minute (kg/s), shall be determined in accordance with ASME BPVC Section VIII.1. Pipe and fittings between the pressure-relief valve and the parts of the system it protects shall have not less than the area of the pressurerelief valve inlet area. [ASHRAE 15:9.7.6]
1112.14 Rating of Rupture Members and Fusible Plugs. The rated discharge capacity of a rupture member or fusible plug discharging to the atmosphere under critical flow conditions, in pounds of air per minute (kg/s), shall be determined in accordance with the following formulas:
C = 0.64 P l d [2] [Equation 1112.14(1)]
CMC § 1112.5 High relevance — show source text
1112.5 Hydrostatic Expansion. Pressure rise resulting from hydrostatic expansion due to temperature rise of liquid refrigerant trapped in or between closed valves shall be addressed in accordance with Section 1112.5.1 and Section 1112.5.2. [ASHRAE 15:9.4.3] 1112.5.1 Hydrostatic Expansion During Normal Operation. Where trapping of liquid with subsequent hydrostatic expansion is capable of occurring automatically during normal operation or during standby, shipping, or power failure, engineering controls shall be used that are capable of preventing the pressure from exceeding the design pressure. Acceptable engineering controls include but are not limited to the following:
(1) Pressure relief device to relieve hydrostatic pressure to another part of the system.
(2) Reseating pressure relief valve to relieve the hydrostatic pressure to an approved treatment system.
[ASHRAE 15:9.4.3.1]
1112.5.2 Hydrostatic Expansion During Mainte- nance. Where trapping of liquid with subsequent hydrostatic expansion is capable of occurring only during maintenance—i.e., when personnel are performing main
2025 CALIFORNIA MECHANICAL CODE 241
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tenance tasks—either engineering or administrative controls shall be used to relieve or prevent the hydrostatic overpressure. [ASHRAE 15:9.4.3.2]
1112.6 Actuation. Pressure-relief devices shall be direct pressure actuated or pilot operated. Pilot-operated pressurerelief valves shall be self-actuated, and the main valve shall open automatically at the set pressure and, where an essential part of the pilot fails, shall discharge its full rated capacity.
[ASHRAE 15:9.4.5]
1112.7 Stop Valves Prohibited. Stop valves shall not be located between a pressure-relief device and parts of the system protected thereby. A three-way valve, used in conjunction with the dual relief valve in accordance with Section
1113.6, shall not be considered a stop valve. [ASHRAE 15:9.4.6]
1112.8 Location. Pressure-relief devices shall be connected
directly to the pressure vessel or other parts of the system protected thereby. These devices shall be connected above the liquid refrigerant level and installed so that they are accessible for inspection and repair, and so that they are not capable of being readily rendered inoperative.
Exception: Where fusible plugs are used on the highside, they shall be located above or below the liquid refrigerant level. [ASHRAE 15:9.4.8]
1112.9 Materials. The seats and discs of pressure-relief devices shall be constructed of compatible material to resist refrigerant corrosion or other chemical action caused by the refrigerant. Seats or discs of cast iron shall not be used. Seats and discs shall be limited in distortion, by pressure or other cause, to a set pressure change of not more than 5 percent in a span of five years. [ASHRAE 15:9.4.9]
1112.10 Pressure-Relief Valve Setting. Pressure-relief valves shall start to function at a pressure not exceeding the design pressure of the parts of the system protected.
Frequently asked questions
Can I use a fusible plug on a 5 cubic‑foot receiver?
No. For vessels > 3 cu ft and < 10 cu ft, the CMC requires pressure‑relief devices and prohibits fusible plugs. § 1113.2
If my 12 cu ft receiver vents into the lowside, can I use a single valve?
Only if all conditions of the exception are met: the valve discharges to the lowside, the vessel has shutoff valves permitting pumpdown, and other vessels are separately protected per § 1113.1. Otherwise you must use rupture members or dual valves with a three‑way valve. § 1113.3, § 1113.6
When is a three‑way valve required?
A three‑way valve is required whenever dual pressure‑relief valves are installed on vessels ≥ 10 cu ft discharging to atmosphere, to permit testing or repair of one valve while maintaining protection. § 1113.6
What do I use to size a rupture member or relief valve?
The CMC requires sizing in accordance with § 1113.5, which references ASHRAE 15 and ASME BPVC procedures; the CMC also provides rupture/fusible rating formulas (see § 1112.14) for atmospheric discharge. Consult ASHRAE 15 and ASME BPVC for numeric procedures. § 1113.5, § 1112.14
Are fusible plugs required to be marked?
Yes — fusible plugs must be marked with their melting temperatures. § 1115.4.2
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