What is a safety valve? What are the types of safety valves

Safety valve is a special valve that the opening and closing parts are normally closed under the action of external force When the medium pressure in the equipment or pipeline increases beyond the specified value, the medium pressure in the pipeline or equipment is prevented from exceeding the specified value by discharging the medium outside the system Safety valve belongs to automatic valve category, which is mainly used in boiler, pressure vessel and pipeline The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation Note the safety valve can only be used after pressure test The safety valve is classified according to its overall structure and loading mechanism 1 The heavy hammer lever safety valve uses the heavy hammer and lever to balance the force acting on the valve disc According to the principle of lever, it can use a weight with smaller mass to obtain a larger force by increasing the lever, and adjust the opening pressure of the safety valve by moving the position of the weight (or changing the mass of the weight) The heavy hammer lever type safety valve is simple in structure, easy to adjust and more accurate The added load will not increase greatly due to the rise of the valve disc It is suitable for the occasion with high temperature It was widely used in the past, especially in the boiler and pressure vessel with high temperature However, the structure of the heavy hammer lever safety valve is relatively heavy, the loading mechanism is easy to vibrate, and often causes leakage due to vibration; its reseating pressure is low, and it is not easy to close and keep tight after opening 2 Spring micro open safety valve spring micro open safety valve uses the force of compression spring to balance the force acting on the valve disc The compression of the coil spring can be adjusted by turning the adjusting nut on it With this structure, the opening (setting) pressure of the safety valve can be corrected as required The spring micro lift safety valve has the advantages of light and compact structure, high sensitivity, unrestricted installation position, and small sensitivity to vibration, so it can be used on mobile pressure vessels The disadvantage of this safety valve is that the load applied will change with the opening of the valve, that is, with the increase of the valve disc, the compression amount of the spring increases, and the force acting on the valve disc also increases This is not good for quick opening of safety valve In addition, the spring on the valve will reduce its elasticity due to the influence of high temperature for a long time When it is used on the container with high temperature, it is often necessary to consider the heat insulation or heat dissipation of the spring, so that the structure becomes complex 3 The pulse type safety valve is composed of main valve and auxiliary valve The action of main valve is driven by the pulse function of auxiliary valve Its structure is complex It is usually only suitable for boilers and pressure vessels with large safety discharge capacity According to the different types of medium discharge 1 When the fully closed safety valve exhausts, all the gas is discharged through the exhaust pipe, and the medium can't leak outside It is mainly used for the container where the medium is gas 2 Part of the gas discharged by the semi closed safety valve passes through the exhaust pipe, and part of the gas also leaks out from the gap between the valve cover and the valve rod, which is mostly used for the container with the medium of gas that will not pollute the environment 3 The valve cover of the open safety valve is open, so that the spring chamber is connected with the atmosphere, which is conducive to reducing the temperature of the spring It is mainly suitable for the container with medium of steam and high temperature gas that does not pollute the atmosphere According to the ratio of the z-height of the valve disc opening to the diameter of the flow passage of the safety valve, the opening height of the spring micro opening closed high-pressure safety valve micro opening safety valve is less than 1 / 4 of the flow passage diameter, usually 1 / 40-1 / 20 of the flow passage diameter The action process of the micro opening safety valve is proportional action type, which is mainly used in liquid occasions and sometimes in gas occasions with small discharge 2 The opening height of the spring full open safety valve full open safety valve is greater than or equal to 1 / 4 of the flow passage diameter The discharge area of the full open safety valve is the small section Z of the valve seat throat Its action process is a two-stage action type, which can be fully opened only with the help of a lifting mechanism The fully opened safety valve is mainly used in the occasion of gas medium 3 The opening height of medium lift safety valve is between micro lift and full lift That is to say, it can be made into two-stage action or proportional action The safety valve is classified according to the principle of action 1 The direct acting safety valve is opened under the direct action of the working medium, that is, the mechanical load imposed on the valve disc by the loading mechanism is overcome by the action of the working medium pressure to open the valve The safety valve has the advantages of simple structure, quick action and good reliability However, due to the structural loading, its load size is limited and can not be used in high-pressure, large-diameter occasions 2 This kind of safety valve can be divided into pilot safety valve and safety valve with power assisted device The pilot safety valve is driven or controlled by the medium discharged from the pilot valve The pilot valve itself is a direct acting safety valve, sometimes other forms of valves are also used The pilot type safety valve is suitable for high pressure and large caliber occasions The main valve of the pilot safety valve can also be designed to be sealed by the working medium, or the mechanical load on the valve disc is much larger than that of the direct acting safety valve, so it has good sealing performance At the same time, its movement is rarely affected by back pressure The disadvantage of the safety valve is that its reliability is related to the main valve and the pilot valve Its action is not as fast and reliable as the direct acting safety valve, and its structure is more complex The above is the type of safety valve and safety valve According to the difference of its overall structure and loading mechanism, it can be divided into three types: heavy hammer lever type, spring type and pulse type According to the different ways of medium discharge, the safety valve can be divided into three types: fully closed, semi closed and open According to the principle of action, it can be divided into direct acting safety valve and indirect acting safety valve.