Safety valve is the safety accessory of boiler, pressure vessel and other pressure equipment, which can effectively prevent explosion accidents caused by overpressure of production devices.
Therefore, safety valve is very important in chemical plant.
To thoroughly understand the safety valve, we must start with its classification, characteristics, type selection and installation requirements.
Classification and characteristics of safety valves
According to the difference of overall structure and loading mechanism
Safety valves can be divided into heavy hammer lever type, spring type and pulse type according to their overall structure and loading mechanism.
Among the three forms of safety valves, the spring-loaded safety valve is widely used.
1. Heavy hammer lever safety valve
The heavy hammer lever safety valve uses the heavy hammer and lever to balance the force acting on the valve disc.
According to the lever principle, it can use the weight with smaller mass to obtain a larger force through the increase of 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 safety valve has the advantages of simple structure, easy and accurate adjustment, and the added load will not increase greatly due to the rise of the valve disc.
It is suitable for occasions with high temperature.
It was widely used in the past, especially in boilers and pressure vessels with high temperature.
However, the structure of heavy hammer lever safety valve is bulky, the loading mechanism is easy to vibrate, and often leaks due to vibration;
The reseating pressure is low, so it is difficult to close and keep tight after opening.
2. Spring type safety valve
The spring-loaded safety valve uses the force of the 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 needed.
The spring-loaded safety valve has the advantages of light and compact structure, high sensitivity, unlimited installation position, and low sensitivity to vibration, so it can be used on mobile pressure vessels.
The applied load will change with the opening of the valve, that is, with the rise of the valve disc, the compression of the spring increases and the force acting on the valve disc also increases.
This is unfavorable to the rapid opening of the 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 used on containers with high temperature, the thermal insulation or heat dissipation of the spring is often considered, which makes the structure more complex.
3. Pulse safety valve
Pulse type safety valve is composed of main valve and auxiliary valve.
It drives the action of main valve through the pulse action of auxiliary valve.
Its structure is complex.
It is usually only suitable for boilers and pressure vessels with large safety discharge capacity.
According to different medium discharge modes
According to the different ways of medium discharge, the safety valve can be divided into three types: fully closed, semi closed and open.
1. Fully closed safety valve
When the fully enclosed safety valve exhausts, all the gas is discharged through the exhaust pipe, and the medium cannot leak outside.
It is mainly used for containers with toxic and flammable gas as the medium.
2. Semi closed safety valve
Part of the gas discharged by the semi closed safety valve passes through the exhaust pipe, and part of it leaks from the gap between the valve cover and the valve stem.
It is mostly used in containers whose medium is gas that will not pollute the environment.
3. Open safety valve
The valve cover of the open safety valve is open to connect the spring chamber with the atmosphere, which is conducive to reducing the temperature of the spring.
It is mainly applicable to containers with steam as the medium and high-temperature gas that does not pollute the atmosphere.
According to the ratio of the maximum opening height of the valve disc to the flow channel
According to the ratio of the maximum opening height of the valve disc to the flow path diameter of the safety valve, the safety valve is mainly divided into micro open safety valve and full open safety valve.
1. Micro lift safety valve
The opening height of the micro opening safety valve is less than 1 / 4 of the channel diameter, usually 1 / 40-1 / 20 of the channel diameter.
The action process of micro opening safety valve is proportional action type, which is mainly used in liquid occasions and sometimes in gas occasions with small emission.
2. Full lift safety valve
The opening height of full lift safety valve is greater than or equal to 1 / 4 of the diameter of flow channel.
The discharge area of full lift safety valve is the minimum cross-sectional area of valve seat throat.
Its action process belongs to the 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.
According to the principle of action
According to the classification of action principle, it can be divided into direct action safety valve and indirect action safety valve.
1. Direct acting safety valve
The direct acting safety valve is opened under the direct action of the working medium, that is, it overcomes the mechanical load imposed by the loading mechanism on the valve disc by relying on the pressure of the working medium to open the valve.
The safety valve has the advantages of simple structure, rapid action and good reliability. However, due to structural loading, its load is limited and can not be used in high-pressure and large-diameter occasions.
2. Non direct acting safety valve
This kind of safety valve can be divided into pilot safety valve and safety valve with power auxiliary device.
Safety valve selection instructions
Determination of various parameters of safety valve:
1. Determine the nominal pressure of the safety valve and select the nominal pressure according to the valve material, working temperature and maximum working pressure.
2. Determine the working pressure level of the safety valve
The working pressure level is selected according to the design pressure and design temperature of the pressure vessel.
The working pressure of the safety valve has different meanings from that of the spring.
The working pressure of the safety valve 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 system or equipment.
The working pressure level of the spring refers to the allowable working pressure range of a spring.
Within this pressure range, the opening pressure (i.e. setting pressure) of the safety valve can be adjusted by changing the preload compression of the spring.
The safety valve with the same nominal pressure can be divided into many different working pressure levels according to the spring design requirements.
When selecting the safety valve, the working pressure level of the valve shall be determined according to the required opening pressure value.
3. Determine the discharge pressure PD of the safety valve
The discharge pressure of the safety valve is generally 1.1 times of the set pressure (opening pressure), and the discharge pressure of the steam boiler safety valve is 1.03 times of the set pressure.
4. Determine the size of the safety valve
The discharge capacity of the safety valve shall be determined according to the necessary discharge, and the discharge capacity of the safety valve shall be ≥ the necessary discharge.
The necessary discharge of the protected system refers to the amount that must be excluded to prevent overpressure in case of abnormal overpressure of the system.
It is determined by the working conditions, capacity and possible overpressure of the system or equipment.
5. Determination of material selection the material of safety valve shall consider the working temperature and pressure of the medium, the performance of the medium, as well as the manufacturability and economy of the material.
Determination of special structure of safety valve
① The safety valve with radiator (fin) shall be selected for the safety valve for steam with opening pressure greater than 3Mpa or the safety valve for gas with medium temperature greater than 320 ℃.
② For the safety valve bearing additional back pressure, and the change of back pressure exceeds 10% of the set pressure, the bellows safety valve shall be selected.
In addition, for the safety valve with corrosive medium, bellows safety valve shall also be selected to prevent the spring and guide mechanism from being corroded by the medium.
③ For flammable, extremely toxic or highly hazardous media, closed safety valve must be used. If it is necessary to use a safety valve with lifting mechanism, closed safety valve with wrench shall be used.
④ For non hazardous media such as air, hot water or steam above 60 ℃, safety valve with wrench shall be adopted.
⑤ The built-in safety valve shall be adopted for the liquefaction tank (tank) truck.
⑥ For the working condition with large discharge capacity, the full open type shall be selected;
For the working conditions with stable working pressure and small discharge capacity, the micro start type should be selected;
For working conditions with high pressure and large discharge capacity, indirect starting type, such as pulse safety valve, should be selected;
For vessels longer than 6m, two or more safety valves shall be set.
⑦ For fixed vessels with low working pressure, static weight type (pressure cooker) or lever weight type safety valve can be used.
Spring type safety valve shall be adopted for mobile equipment.
⑧ If the medium is thick and easy to be blocked, the series combined relief device of safety valve and bursting disc should be selected.
Installation position and requirements of safety valve
1. Installation position
① Install vertically upward.
② The installation position shall be as close to the protected equipment or pipeline as possible.
③ It shall be installed at a place easy for maintenance and adjustment, with enough space around.
④ The safety valve of the pressure vessel is installed in the gas phase space above the liquid level of the vessel body, or on the pipeline with the connection point located in the gas phase space of the pressure vessel.
⑤ For containers and equipment containing flammable, toxic and viscous media, a stop valve can be installed in front of the safety valve, but the flow area of the stop valve shall not be less than the minimum flow area of the safety valve, and a lead seal shall be installed to ensure that the stop valve is fully open and normally open.
⑥ The safety valve shall be installed reversely with the stop valve to reduce the influence of self gravity of the valve and avoid stress fatigue and discharge vibration fatigue.
⑦ For the safety valve that may be blocked or corroded by materials, a bursting disc shall be set in front of its inlet, an inspection valve shall be installed between the safety valve and the bursting disc, and anti blocking measures such as back purging, heat tracing or thermal insulation shall be taken on its inlet pipeline.
⑧ The safety valve installed on the pipeline shall be set at the place where the fluid pressure is relatively stable and a certain distance from the fluctuation source, and shall not be installed at the dead corner of the horizontal pipeline.
⑨ For pipelines, heat exchangers or pressure vessels with liquid medium, when the valve is closed, thermal expansion may occur, resulting in high pressure, the safety valve can be installed horizontally to discharge the liquid directly downward.
⑩ For air emergency discharge, the pipe orifice shall be flat, non sharp and free of burrs to prevent electrostatic discharge.
The installation point of safety valve shall not make the safety valve bear excessive back pressure, which shall be within the specified allowable range.
The valve body of the safety valve shall be supported stably.
For containers with extremely and highly hazardous or flammable and explosive media, the outlet of the safety valve shall be led to a safe place and properly handled.
If more than two safety valves share a discharge pipe, the cross-sectional area of the discharge pipe shall not be less than the sum of the cross-sectional areas of the outlet of all safety valves, but oxygen or combustible gas and other two gases that can produce chemical reaction with each other cannot share a discharge pipe.
2. Installation requirements
① For corrosive media, the safety valve shall be installed in combination with the bursting disc.
② For highly toxic media, the type of safety valve with good sealing shall be selected.
③ For the high-temperature medium safety valve, the high temperature has a great impact on the spring, and the spring-loaded safety valve should be avoided.
④ For important safety valves or safety valves vulnerable to fire, spray protection shall be set.
⑤ For the safety valve for spherical tank, double safety valves shall be installed, and the pressure relief capacity of any safety valve can meet the safety pressure relief requirements of spherical tank.
⑥ The safety valve shall be provided with electrostatic jumper.
⑦ In order to prevent the safety valve from opening and closing repeatedly, resulting in flutter and damaging the valve, reduce the pressure drop of the inlet pipeline of the safety valve, that is, increase the inlet pipe diameter and shorten the inlet pipe section.