The safety valve is an automatic pressure relief protection device opened by the inlet static pressure. It is one of the most important safety accessories of the pressure vessel. Its function is: when the pressure in the vessel exceeds a certain value, it automatically opens the valve by relying on the pressure of the medium itself to quickly discharge a certain amount of medium. When the pressure in the container drops to the allowable value, the valve closes automatically, so that the pressure in the container is always lower than the upper limit of the allowable pressure, and automatically prevents possible accidents due to overpressure. Therefore, the safety valve is also known as the final protection device of the pressure container.
The safety valve can be opened and closed automatically according to the working pressure. It is generally installed on the equipment or pipeline of the closed system to protect the safety of the system. Safety valves are automatic valves, which are mainly used in boilers, pressure vessels and pipelines. The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. Safety valve is a valve for safety protection. Its opening and closing parts are normally closed under the action of external force. When the medium pressure in the equipment or pipeline rises and exceeds the specified value, it will open automatically. By discharging medium outside the system, the medium pressure in the pipeline or equipment will be prevented from exceeding the specified value. When the pressure in the equipment or pipeline exceeds the specified value, it will automatically start pressure relief to ensure that the medium pressure in the equipment and pipeline is within the specified value and prevent accidents. The valve opens automatically, and the medium is discharged and depressurized; When the pressure returns to the normal value, the spring pressure pushes the valve core to the valve seat, and the valve closes automatically. The function of safety valves is to prevent the medium pressure in the pipeline or device from exceeding the specified value, so as to achieve the purpose of safety protection.
Classification of safety valves
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 a small weight to obtain a large 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 weight 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.
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 small sensitivity to vibration, so it can be used for mobile pressure vessels. The disadvantage of this safety valve is that the added 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.
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.
Selection of safety valve
The selection of safety valve shall be considered from the following aspects:
1. The structural form mainly depends on the process conditions, working medium and characteristics of the equipment. General boilers and pressure vessels mostly use spring-loaded safety valves. If the equipment medium is toxic, flammable and explosive, the closed safety valve shall be selected.
2. Boilers, high-pressure vessels and medium and low-pressure vessels with large safety discharge capacity and small wall thickness corrosion allowance should choose full open safety valves.
3. Pressure range, each safety valve has a certain working pressure range. Appropriate safety valves shall be selected according to the maximum allowable working pressure of the equipment.
4. The discharge capacity must be greater than the safety discharge capacity of the equipment, so as to ensure that in case of overpressure, the safety valve is opened to discharge part of the medium in time to avoid the continuous rise of pressure. For boilers, the total displacement of safety valves must be greater than the maximum continuous evaporation. For pressure vessels, the discharge capacity of the safety valve must be greater than or equal to the safety discharge capacity of the pressure vessel. Common faults of safety valve.
During the normal use and calibration of the safety valve, various faults will often occur and be found. The main cause of the fault is improper design, manufacturing, selection or use. If these faults are not eliminated in time, they will affect the efficiency and service life of the valve, and even can not play a role in safety protection. Common faults and elimination methods are as follows:
1. Leak. Under the normal working pressure of the equipment, leakage exceeding the allowable degree occurs between the valve disc and the sealing surface of the valve seat. The reason is: there is dirt between the valve disc and the sealing surface of the valve seat. The lifting wrench can be used to open the valve several times to wash away the dirt; The sealing surface is damaged. It shall be repaired by grinding or grinding after turning according to the degree of damage; The valve rod is bent, tilted or the lever and fulcrum are deflected, so that the valve core and valve disc are misaligned. Reassemble or replace; Spring elasticity decreases or loses elasticity. Measures such as replacing the spring and readjusting the opening pressure shall be taken.
2. Do not open when the specified pressure is reached. The reason for this is the inaccurate constant pressure. The compression of the spring or the position of the heavy hammer shall be readjusted; The disc is stuck to the valve seat. The safety valve shall be subject to manual air release or water discharge test regularly; The lever of the lever type safety valve is stuck or the heavy hammer is moved. The position of the weight shall be readjusted and the lever shall move freely.
3. Open when the specified pressure is not reached. The main reason is that the constant pressure is inaccurate; The spring is aging and the elasticity decreases. Properly tighten the adjusting screw or replace the spring.
4. The pressure continues to rise after venting. This is mainly because the displacement of the selected safety valve is smaller than that of the equipment, and the appropriate safety valve should be re selected; If the center line of the valve rod is not aligned or the spring is rusted, so that the valve disc cannot be opened to the due height, the valve rod shall be reassembled or the spring shall be replaced; If the section of the exhaust pipe is not enough, the exhaust pipe conforming to the safe discharge area shall be adopted.
5. Disc frequency hopping or vibration. The main reason is that the spring stiffness is too large. Springs with appropriate stiffness shall be used; Improper adjustment of the adjusting ring makes the reseating pressure too high. Readjust the position of the adjusting ring; The resistance of discharge pipeline is too large, resulting in excessive discharge back pressure. Discharge pipe resistance shall be reduced.
6. The valve disc does not return to its seat after discharge. This is mainly caused by spring bending, improper installation position of valve stem and valve disc or being stuck. Reassemble.
Precautions for safety valve installation
(1) The newly installed safety valve shall be attached with the product certificate. Before installation, it shall be re calibrated, lead sealed and issued with the safety valve calibration certificate.
(2) The safety valve shall be installed vertically and at the gas phase interface of the vessel or pipeline.
(3) The outlet of the safety valve shall be free of resistance to avoid back pressure. If a discharge pipe is installed, its inner diameter shall be greater than the outlet diameter of the safety valve. Attention shall be paid to antifreeze at the discharge port of the safety valve. For containers containing flammable, toxic and highly toxic media, the discharge pipe shall be directly connected to an outdoor safe place or have facilities for proper treatment. No valve is allowed to be installed in the discharge pipe.
(4) No valve shall be installed between pressure equipment and safety valve. For containers containing flammable, explosive, toxic or viscous media, stop valves can be installed for easy replacement and cleaning, and their structure and diameter shall not hinder the normal operation of safety valve. During normal operation, the stop valve must be fully opened and sealed with lead.
(5) For pressure vessels with flammable, explosive or toxic media, the media discharged by the safety valve must have safety devices and recovery systems. The lever safety valve must be installed in a vertical position, and the spring safety valve should also be installed vertically to avoid affecting its action. During installation, attention shall also be paid to the fit, coaxiality of parts and uniform stress of each bolt. The cross-sectional area of the connecting short pipe between the safety valve and the boiler pressure vessel shall not be less than the flow cross-section of the safety valve. The whole safety valve shall be on the same connecting pipe, and the cross-sectional area of the connecting pipe shall not be less than 1.25 times the total flow cross-sectional area of the safety valve.
China safety valve manufacturer www.sfutube.com supplies safety valve for industrial use, in kinds of materials such as carbon steel, stainless steel, alloy steel, etc.