What are the principles for selecting safety valves for tower equipment?

SH/T 3121-2000 Specification for Process Design of Refining Units 7.1 Safety Valves; HG/T 20570-95 Technical Requirements for Process System Engineering Design, specifically HG/T 20570.1-95 regarding the installation and selection of safety valves.

(1) When selecting a safety valve, the most critical factor is its discharge capacity; that is, the discharge capacity of the safety valve used in tower-type equipment must be greater than its safe discharge volume. Only in this way can it ensure that gas is discharged promptly once the safety valve opens when the container is under overpressure, thereby preventing the pressure inside the container from rising further. (2) Attention should be paid to the pressure range of the safety valve, as each type of safety valve has a specific operating pressure range. Spring-loaded safety valves designed for high pressures should not be used in low-pressure vessels with excessive relief, nor should safety valves designed for low pressures be used in towers operating at higher pressures with excessive loading; when selecting such valves, springs of the same rating appropriate for the operating pressure of the tower equipment should be used. (3) The process conditions of tower equipment and the characteristics of the working medium should be taken into consideration; generally, spring-type safety valves are suitable for tower equipment. For towers with lower pressures and no vibration effects, lever-type safety valves can be used. If the working medium of tower-type equipment contains toxic, flammable, explosive gases, or other gases that pollute the atmosphere, a closed-type safety valve should be used.

Principles for selecting safety valves: 1. For steam boiler safety valves, open-type, fully-opening spring safety valves are generally chosen. 2. For safety valves used in liquid media, micro-opening spring safety valves are generally selected. 3. For air or other gaseous media, or safety valves, closed full-open spring safety valves are generally selected. 4. For safety valves used in liquefied petroleum gas road tankers or liquefied petroleum gas railway tank cars, combined-type internal safety valves are generally selected. 5. Safety valves are used at the outlet of oil production wells, with pilot-operated safety valves being commonly selected. 6. For the high-pressure bypass safety valves in steam power generation equipment, pilot-operated safety valves with both safety and control functions are generally selected. 7. If regular opening tests of the safety valve are required, a safety valve equipped with a lifting wrench should be used. When the medium pressure reaches 75% or more of the opening pressure, the lifting wrench can be utilized to slightly lift the valve disc away from the valve seat, in order to check the flexibility of the safety valve’s opening mechanism. 8. When the temperature of the medium is high, in order to reduce the temperature in the spring chamber, safety valves equipped with radiators should be used generally when the operating temperature for closed-type safety valves exceeds 300°C and for open-type safety valves exceeds 350°C. 9. If the back pressure at the outlet of the safety valve is variable, and the degree of variation exceeds 10% of the opening pressure, then a diaphragm safety valve should be used. 10. When the medium is corrosive, a bellows safety valve should be used to prevent important components from failing due to corrosion by the medium.

1) When selecting a safety valve, the most crucial factor is its discharge capacity; that is, the discharge capacity of the safety valve used in tower-type equipment must be greater than its safe discharge volume. Only in this way can it ensure that gas is discharged promptly once the safety valve opens when the container is under overpressure, thereby preventing the pressure inside the container from rising further. 2) Attention should be paid to the pressure range of the safety valve, as each safety valve has a specific operating pressure range. Spring-loaded safety valves designed for high pressures should not be used in low-pressure vessels with excessive relief, nor should safety valves designed for low pressures be used in towers operating at higher pressures with excessive loading; when selecting such valves, springs of the same rating appropriate for the operating pressure of the tower equipment should be used. 3) The process conditions of tower equipment and the characteristics of the working medium should be taken into consideration; generally, spring-type safety valves are suitable for tower equipment. For towers with lower pressures and no vibration effects, lever-type safety valves can be used. If the working medium of tower-type equipment contains toxic, flammable, explosive gases, or other gases that pollute the atmosphere, a closed-type safety valve should be used.

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