A safety valve is a special valve that is normally closed under external force. When the pressure of the medium in the equipment or pipeline rises above a specified value, it discharges the medium to the outside of the system to prevent the pressure of the medium in the pipeline or equipment from exceeding the specified value. Safety valves belong to the category of automatic valves and are mainly used in boilers, pressure vessels, and pipelines to control the pressure to not exceed the specified value, playing an important role in protecting personal safety and equipment operation.
Selection of safety valves
The operating temperature determines the operating temperature range of the safety valve, and the operating pressure determines its nominal pressure. The calculated set pressure value of the safety valve then determines the set pressure range of the spring or lever. The material and structural type of the safety valve are determined by the medium being used, and the throat diameter is calculated based on the valve's discharge capacity. The following are general rules for selecting safety valves.
1. Hot water boilers generally use non-sealed, lever-operated, slightly open safety valves.
2. Steam boilers or steam pipelines generally use open-type safety valves with wrenches or safety relief valves.
3. For incompressible liquid media such as water, closed-loop micro-opening safety valves are generally used. Examples include high-pressure feedwater heaters and heat exchangers.
4. High-pressure water supply generally uses a closed full-opening safety valve, such as for gas storage tanks and gas pipelines.
5. For compressible media such as gases, closed full-opening safety valves are generally used.
6. Class E steam boilers generally use static weight safety valves. Large displacement and high-pressure systems generally use pulse-type safety valves.
7. Large diameter. Such as desuperheating and pressure reducing devices, power plant boilers, etc.
8. Tank cars, truck tank cars, and storage tanks transporting liquefied gas generally use internal safety valves. They must be used in conjunction with breather valves.
9. Hydraulic safety valves are generally used on the top of oil tanks.
10. Pilot-operated safety valves are generally used for downhole drainage or natural gas pipelines.
11. Safety reflux valves are generally used on the liquid phase reflux pipeline at the outlet of the tank pump of a liquefied petroleum gas station.
12. Vacuum negative pressure safety valves are generally used in systems where negative pressure may be generated during operation.
13. Bellows safety valves are generally used for containers or piping systems with large back pressure fluctuations and those containing toxic or flammable substances.
14. For systems where the medium has a low freezing point, a jacketed safety valve is generally selected.
Safety valve installation precautions
Safety valves are important safety accessories on special equipment, and their installation has corresponding requirements. Some common points are listed below:
1. Boilers with a rated evaporation capacity greater than 0.5 t/h shall be equipped with at least two safety valves; boilers with a rated evaporation capacity less than or equal to 0.5 t/h shall be equipped with at least one safety valve. Safety valves must be installed at the outlet of the separable economizer and the outlet of the steam superheater.
2. Safety valves should be installed vertically at the highest point of the boiler drum or header. No steam outlet pipes or valves should be installed between the safety valve and the boiler drum or header.
3. Lever-type safety valves must have a device to prevent the counterweight from moving on its own and a guide to limit the lever from going off track. Spring-type safety valves must have a lifting handle and a device to prevent the adjusting screw from being turned arbitrarily.
4. For boilers with a rated steam pressure less than or equal to 3.82 MPa, the throat diameter of the safety valve shall not be less than 25 mm; for boilers with a rated steam pressure greater than 3.82 MPa, the throat diameter of the safety valve shall not be less than 20 mm.
5. The cross-sectional area of the connecting pipe between the safety valve and the boiler should not be less than the inlet cross-sectional area of the safety valve. If several safety valves are installed together on a short pipe directly connected to the boiler drum, the passage cross-sectional area of the short pipe should not be less than 1.25 times the exhaust area of all safety valves.
6. Safety valves should generally be equipped with a vent pipe, which should lead directly to a safe location and have sufficient cross-sectional area to ensure unobstructed venting. A drain pipe connecting to the safe location should be installed at the bottom of the safety valve's vent pipe. Valves are not permitted to be installed on either the vent pipe or the drain pipe.
