Selection of pressure vessel safety valve

Unless the pressure source comes from outside the container and can be reliably controlled, a safety relief device should be installed. This device is a special device to prevent the pressure vessel from overpressure during use. It ensures that the pressure vessel is under normal working pressure. , and when the pressure in the container exceeds the specified value for some reason, the medium in the container can be automatically and quickly released, so that the pressure container is always kept within the allowable pressure range. For this reason, the safety relief device simultaneously satisfies the performance that the relief capacity is greater than the safe relief amount of the container and that it acts accurately under the specified pressure.

The widely used safety pressure relief device is an automatic pressure relief valve (referred to as safety valve) opened by the inlet static pressure. It relies on the pressure of the medium itself to discharge a certain amount of fluid to prevent the pressure in the container or system from exceeding a predetermined safety value. ; When the pressure in the container returns to normal, the valve closes by itself and prevents the medium from continuing to discharge. The lack of safety valve or improper selection of safety valve in the pressure vessel body or system is an unsafe factor that cannot be ignored. This paper introduces the selection and installation of safety valves for a wide range of safety pressure relief devices in pressure vessels based on the experience in practical work. the main points in .

1. Selection principle of safety valve

There are many types of safety valves, and the reliability and performance of their actions are directly related to equipment and personal safety, and are closely related to energy conservation and environmental protection. The selection of safety valve should pay attention to the following issues:

(1) The safety valve is produced by a unit with a manufacturing license, and the quality certificate and nameplate data are complete.

(2) The rated discharge capacity of the safety valve is greater than or equal to the safe discharge capacity of the container.

(3) The safety valve generally adopts the direct spring load type. If the safety valve controlled by the pilot valve is used, it is required that when the auxiliary device such as the pilot valve fails, the main valve can still be opened automatically when the relief pressure is not exceeded, and all the Rated discharge volume.

(4) For toxic, flammable, explosive or refrigerant and other media that will pollute the atmosphere, a closed safety valve is used. If a lifting mechanism is required, a closed safety valve with a wrench must be used.

(5) For the safety valve for steam with the opening pressure greater than 3MPa or the safety valve for gas with the medium temperature exceeding 350℃, a safety valve with a radiator should be used to prevent the discharge medium from directly eroding the spring.

(6) When the safety valve is likely to withstand back pressure or the medium is highly corrosive, the safety valve with bellows should be selected.

(7) When there are regular opening inspection requirements for the safety valve, a reliable lifting mechanism should be provided; for toxic, flammable and explosive media, unless the user has special requirements, no lifting mechanism is required.

(8) In the occasions where the safety relief is large, the full-opening safety valve should be selected. On the contrary, in the occasions where the safety relief is small or the stress is required to be stable, the micro-opening safety valve should be selected.

(9) The sealing surface type of the safety valve should meet the performance requirements of the safety valve.

2 Selection conditions of safety valve

(1) Determination of nominal diameter

The determination of the nominal diameter of the safety valve should first be based on standard calculation or actual use experience to determine the small safety discharge amount required to ensure the safety of the equipment; then calculate the small discharge area of the safety valve, and then according to the opening height of the safety valve (full opening or micro-opening). Kai) Calculate the throat diameter of the valve seat of the safety valve, and determine the nominal diameter of the safety valve according to the throat diameter of the standard series of safety valves that is slightly larger but closer to the calculated valve seat throat diameter.

(2) Determination of pressure level

The nominal pressure of the safety valve and the working pressure and the working pressure level of the spring have different meanings and cannot be confused. Working pressure refers to the static pressure in front of the safety valve during normal operation, which is the same as the working pressure of the protected equipment; and the working pressure level of the spring refers to the working pressure range allowed by the spring, within this range. Inside, the setting pressure of the safety valve can be adjusted by changing the preload compression amount of the spring. The safety valve with the same nominal pressure can be divided into various working pressure levels according to the design requirements of the spring. The nominal pressure of the safety valve is generally not lower than the design pressure of the container. When it is used at high temperature, the reduction of the allowable stress of the valve material should be considered.

When selecting a safety valve, the nominal pressure of the valve and the working pressure level of the spring should be determined according to the design pressure and the required set pressure.

(3) Determination of valve material

When selecting the material of the safety valve, various factors such as the working temperature and working pressure of the medium, the performance of the medium, and the manufacturability and economy of the material should be considered. In general, if there are no special requirements, when the nominal pressure of the valve is not more than 16MPa, choose gray cast iron; when it is not more than 25MPa, choose carbon steel; choose stainless steel for corrosive media; choose chrome molybdenum steel when the temperature is high.