Determination of the set pressure for safety valves and deviations from the set pressure

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Set pressure refers to the predetermined pressure at which the valve disc of a safety valve begins to open under operating conditions; it is the gauge pressure measured at the inlet of the valve. At this pressure, under the specified operating conditions, the force generated by the medium pressure to open the valve is balanced by the force that keeps the valve disc in place on the valve. At this pressure, a measurable opening height begins to appear, and the medium is discharged in a continuous manner that can be perceived visually or audibly. 1) Safety valves for steam boilers: a. The setting pressures of the safety valves on the boiler drum (shell) and superheater shall be in accordance with Table 1. b. The set pressure of the safety valves for economizers, reheaters, and startup separators of once-through boilers shall be 1.1 times the operating pressure at the installation site. c. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa; it is ±3% of the set pressure when the set pressure is between 0.5 and 2.3 MPa; it is ±0.07 MPa when the set pressure is between 2.3 and 7.0 MPa; and it is ±1% of the set pressure when the set pressure is greater than 7.0 MPa. Table 1: Setting pressures of safety valves on the boiler shell and superheater
Rated operating pressure (MPa) | Setting pressure of safety valve (MPa)
≤0.8 | Operating pressure + 0.03 | Operating pressure + 0.05
0.8 < P ≤ 5.9 | 1.04 times the operating pressure | 1.06 times the operating pressure
>5.9 | 1.05 times the operating pressure | 1.08 times the operating pressure
Note: A safety valve must be installed on the boiler, and its setting pressure should be adjusted according to the lower value specified in the table. For boilers equipped with superheaters, the safety valve adjusted to a lower pressure must be the safety valve on the superheater, so as to ensure that it opens first. The working pressure in the table refers, for pulse-type safety valves, to the working pressure at the point where the impulse is generated, and for other types of safety valves, to the working pressure at the location where the safety valve is installed. (2) Safety valves for hot water boilers a. The set pressure of the safety valves shall be in accordance with Table 2. b. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa, and it is ±3% of the set pressure when the set pressure is greater than 0.5 MPa. Table 2 Setting pressure of safety valves for hot water boilers (MPa): 1.12 times the operating pressure, but not less than the operating pressure + 0.07 MPa; 1.14 times the operating pressure, but not less than the operating pressure + 0.10 MPa. Note: A safety valve on the boiler must be adjusted to the lower setting pressure specified in the table. The working pressure here refers to the working pressure of the components directly connected to the safety valve. (3) Safety valves for fixed pressure vessels a. When only one safety valve is installed on a fixed pressure vessel, the set pressure of the safety valve shall not be greater than the design pressure of the pressure vessel ； When multiple safety valves are installed on a stationary pressure vessel, the set pressure of one of the safety valves shall not exceed the design pressure of the pressure vessel. The set pressures of the remaining safety valves may be appropriately increased, but they must not exceed 1.05 times the design pressure. b. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa, and it is ±3% of the set pressure when the set pressure is greater than 0.5 MPa. (4) Safety valves for mobile pressure vessels: a. The setting pressure of the safety valve shall be 1.05 to 1.10 times the design pressure of the tank, and the reseating pressure shall not be less than 0.8 times the opening pressure. The setting pressure of safety valves for low-temperature cryogenic tank trucks shall not exceed the design pressure of the tank. b. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa, and it is ±3% of the set pressure when the set pressure is greater than 0.5 MPa. (5) Safety valves for pressure pipelines a. The set pressure of safety valves for industrial metal pipelines is 1.05 to 1.1 times the maximum operating pressure ； The set pressure of the safety valves used in pipelines at oil and gas stations is 1.05 to 1.15 times the maximum operating pressure. b. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa, and it is ±3% of the set pressure when the set pressure is greater than 0.5 MPa. (6) Safety valve for medical hyperbaric oxygen chambers a. The set pressure of the safety valve is the maximum operating pressure + 0.02 MPa. b. Adjust the pressure deviation to ±0.015 MPa. (7) Safety valve for compressor a. The set pressure of the safety valve is the maximum operating pressure + 0.02 MPa. b. Set pressure deviation: It is ±0.015 MPa when the set pressure is less than or equal to 0.5 MPa, and it is ±3% of the set pressure when the set pressure is greater than 0.5 MPa.