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Safety valve selection principle

Jul 18, 2018

Safety valve selection principle


1. Form and materials


The form of the safety valve is chosen according to the loading method. Most of the containers with low working pressure and high temperature choose the lever type, while most of the high-pressure vessels choose the spring type.According to the method of gas emission, if the medium in the container is poisonous, inflammable, explosive gas or refrigerant and other gases that will pollute the atmosphere, the closed type should be adopted. Semi-closed or open type can be adopted for air or other harmless gases.According to the degree of opening, both high pressure vessels and medium and low pressure vessels with large safety discharge and small strength margin can adopt fully opened safety valves to reduce the opening area of the vessels.When the medium is air, water vapor and safety valves are required to be opened regularly, spanner should be selected. For toxic, inflammable and explosive media, unless there are special requirements, no spanner should be selected.


When the nominal pressure of the valve is less than or equal to 1.6MPa, cast iron shall be selected if the material of the valve body of the safety valve has no special requirements.Carbon steel with nominal pressure less than or equal to 2.5mpa;For corrosive medium, choose stainless steel, high temperature of choice chrome molybdenum steel.


2. Rated discharge amount


The rated discharge of the safety valve must be greater than or equal to the safe discharge of the container.The rated discharge amount of the safety valve can be checked by its nameplate, and the safe discharge amount of the container can be calculated according to the relevant provisions of gb150-1998.


When the safety valve is opened, the valve disc is opened quickly and stably in the state of discharge. The maximum pressure in the container is always within the allowable pressure range of the container.If the rated discharge is too small, the pressure in the container will exceed the allowable pressure, causing danger, especially for the liquid medium with small compressibility.But if the rated discharge capacity is too big, due to the discharge area is too large, the disc opens rapid discharge medium, make the container near the relief valve pressure fell sharply to the back seat, just rising quickly back to the seat, disc produce severe impact and closed, away from the valve of high-pressure has not eliminated, however, when the relief valve pressure and near to the opening pressure, valve to open again, so, cause damage to disc and sealing surface, when used in liquid medium can also cause water hammer phenomena of the system.


3. Working pressure range


The maximum working pressure of the safety valve shall not be higher than its own nominal pressure; otherwise, the valve body shall not be strong enough and the valve with too high nominal pressure shall not be selected. In this way, not only high investment but also the opening precision will be affected.Relief valve nameplate nominal pressure is shown on the body when working temperature does not exceed 200 ℃ allowed under the highest pressure, more than 200 ℃ after allowing pressure will be lower than the nominal pressure.The nominal pressure of the safety valve and the maximum allowable pressure at different temperatures are shown in table 7-1.


Table 1-1 nominal pressure and allowable pressure of a spring-loaded safety valve

Body material nominal pressure the maximum allowable pressure /MPa at the following temperatures

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