Selection of safety valves

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Parameters: The selection of a safety valve is determined by the operating pressure, which determines the nominal pressure of the valve; the operating temperature determines the temperature range in which the valve can be used. The set pressure value of the valve, as calculated, determines the set pressure range for the spring or lever. The material and structural design of the valve are determined by the medium being handled, while the throat diameter of the valve is calculated based on its discharge capacity. Therefore, the following conditions must be clearly defined when selecting a safety valve: 1) The operating conditions under which the safety valve will be used: type of medium, actual operating pressure, temperature ； 2) The opening pressure (set pressure), discharge pressure, and return pressure of the safety valve, so that the manufacturer can adjust them according to the user’s requirements ； or the emission volume and valve opening height. Selection principle: When discharging gases or vapors, a full-open safety valve should be used. When discharging liquids, full-open or slightly-open safety valves should be used; when discharging water vapor or air, safety valves equipped with levers can be employed. For safety valves used with gases at a set pressure greater than 3 MPa and a temperature exceeding 235°C, safety valves equipped with radiators are selected to prevent the discharged medium from directly eroding the spring. When the discharge medium is allowed to be released into the atmosphere, an open-type valve cap safety valve should be used ； For applications where leakage to the atmosphere is not allowed, a safety valve with a closed valve cap should be used. For discharging highly toxic, highly corrosive, and extremely dangerous media, a bellows safety valve should be used. In applications with high back pressure, a back pressure balanced safety valve or a pilot-operated safety valve should be used. On some important occasions, it is sometimes necessary to install two safety valves that serve as backups for each other. Two safe inlet and outlet isolation valves should be equipped with mechanical interlock devices to ensure that the required venting area for the container is met at all times, including during maintenance. Specific application examples: 1) For incompressible media such as water, closed micro-tilt safety valves are generally used, with the A41H type being a representative example. 2) Compressible media such as gases generally use closed full-opening safety valves, such as in gas storage tanks and gas pipelines. Typically represented by the A42Y type. 3) Steam boilers or steam pipes generally use unenclosed, wrench-operated full-opening safety valves. It is usually represented by the A48Y type ； 4) Hot water boilers generally use unsealed, wrench-operated slightly-open safety valves. That is, it is usually represented by A47Y ； 5) Closed slightly opening safety valves are generally used for high-pressure feed water, such as in high-pressure feed water heaters and heat exchangers. Typically, the A41H type is used as a representative. According to the standards specified in JB/T308-2004 \"Method for Coding Valve Models\", the letter A is generally used as the code for the valve type. The connection methods are the same as those for other valves: flange connection 4, threaded connection 2, welded connection 6, etc., with flange connection being the most common. The structural type is as shown in the table below (taken from JB/T308-2004 \"Method for Determining Valve Model Numbers\"). As can be seen from this table, in terms of structural type, all values from 0 to 8 except 6 are spring-type safety valves ； Closed type 0, 1, 2, 4 ； Open-type 3, 7, 8 ； 1,3,7 Slightly Open ； 0, 2, 4, 8 – fully open type; 3, 7, 8, 4 – with wrench. For example: A44H is a lever-type safety valve with flange connection ； A49H is a flanged pulse safety valve ； A41 is a flange-connected, spring-actuated, slightly open safety valve ； A42 is a flange-connected, spring-actuated, fully open safety valve ； A47 is a flange-connected, spring-operated, non-enclosed safety valve with a wrench for slight opening ； A48 is a flange-connected, spring-loaded, non-enclosed safety valve with a wrench for full opening.