The Selection of Safety Valves
Safety valves are categorized into various types based on their structure, emission capacity, and operational characteristics. Structurally, they can be classified as spring-loaded or lever-operated. For applications requiring high flow rates, pulse-type (pilot-operated) safety valves are utilized. According to the discharge volume, they are divided into full-lift and low-lift categories. Based on structural and loading mechanisms, safety valves are further classified into three types: lever-operated, spring-loaded, and pulse-type.
In terms of medium emission methods, safety valves are categorized into fully enclosed, semi-enclosed, and open types. Depending on the valve disc opening size, they are divided into spring-loaded low-lift closed high-pressure safety valves and spring-loaded full-lift safety valves. Understanding these classifications is crucial for selecting appropriate safety valves. Additionally, it is essential to consider factors such as the type of medium, corrosion resistance, temperature, pressure (working and set pressures), and connection methods (e.g., threaded or flanged).
Guidelines for Selecting Safety Valves
1. The nominal diameter of the safety valve must ensure its discharge capacity meets or exceeds the required safe discharge volume of the pressure vessel.
2. Determine the pressure rating of the safety valve based on the design pressure and temperature of the pressure vessel.
3. Steam boilers and steam pipelines typically use unsealed full-lift safety valves with wrenches. For steam applications exceeding 3 MPa or gas applications with medium temperatures above 320°C, a radiator-equipped (finned) type should be selected.
4. Hot water boilers generally use unsealed low-lift safety valves with wrenches.
5. For non-hazardous media like air, hot water, or steam above 60°C, a plate-handled safety valve is recommended.
6. Incompressible media such as water usually require closed low-lift safety valves or safety relief valves.
7. High-pressure water systems, such as those in high-pressure water heaters or heat exchangers, typically use closed full-lift safety valves.
8. Compressible media like gases generally require closed full-lift safety valves, suitable for applications like gas storage tanks and pipelines.
9. Large-diameter, high-flow, and high-pressure systems often employ pulse-type safety valves, such as those used in temperature and pressure reduction devices or power station boilers.
10. Flammable, toxic, or highly hazardous media must use closed safety valves. If a lifting mechanism is required, a closed safety valve with a plate handle should be chosen.
11. When backpressure may fluctuate by more than 10% of the set pressure or in systems handling toxic or flammable materials, a bellows-equipped safety valve should be selected.
12. Systems that may experience negative pressure during operation should use vacuum or negative pressure safety valves.
13. Low-freezing-point media systems should use insulated jacketed safety valves.
14. Liquefied gas transport vehicles, such as train tankers and automobile tankers, should use built-in safety valves.
15. Oil tank tops typically use hydraulic safety valves, which should be used in conjunction with breather valves.
16. Pilot-operated safety valves are commonly used in underground drainage or natural gas pipelines.
17. A safety return valve is typically installed on the liquid phase return pipeline at the outlet of the tank pump in liquefied petroleum gas stations.
18. Material selection for safety valves should align with the characteristics of the medium. For example, ammonia media cannot use copper or copper-containing safety valves; acetylene cannot use safety valves with 70% copper content.
19. For large leakage conditions, full-lift safety valves are preferred; for stable working pressure and small discharge requirements, low-lift safety valves are suitable; for high-pressure and large-discharge applications, non-direct acting types like pulse-type safety valves are appropriate. Containers longer than 6 meters should have two or more safety valves.
20. E-class steam boilers or fixed containers with low working pressure can use deadweight or lever-weight safety valves. Spring-loaded safety valves are suitable for mobile equipment.
21. For media prone to clogging, a series-combined relief valve and bursting disc relief device is recommended.
22. The spring pressure rating should be selected based on the nominal pressure of the safety valve.
