Selection and verification of pressure vessel safety accessories

Selection and verification of pressure vessel safety accessories

1 Pressure vessel and its safety accessories

　　 Pressure vessel is a closed device, not only carries a certain pressure, but usually contains gas or liquid inside, and metal and non-metal materials are common. There is a large pressure difference between the inside and outside, which is potentially dangerous. In the process of manufacturing and use, due to improper design, operation and use behavior, it is easy to cause safety accidents, so there are strict requirements in standard control. The pressure vessel has its own characteristics of use, coupled with the unique chemical process of the internal medium, it is necessary to install supporting safety accessories to ensure the safe use of the pressure vessel and the normal progress of the process. In order to ensure the safe operation of pressure vessels, some safety devices are often installed on the equipment, such as safety valves, pressure gauges, temperature measuring instruments, liquid level gauges, emergency shut-off devices, bursting disc devices, etc. Divided according to performance and purpose, commonly used safety accessories can be divided into 4 types, namely pressure relief device, metering device, alarm device and interlocking device. Among them, the most critical safety accessories include safety relief devices and pressure gauges.

　　2 Selection and verification of common safety accessories

　　2.1 safety valve

　　 In pressure equipment, the safety valve is an overpressure protection device. After the equipment pressure exceeds a predetermined value, the safety valve will automatically open, reducing the pressure of the equipment by means of pressure relief. When the pressure is lower than the specified value, the safety valve will automatically close to avoid the loss of medium in the equipment. Safety valves are widely used in pressure vessels, pressure pipelines and boilers, and belong to the category of automatic valves. Under normal circumstances, the body of the container is a suitable installation position for the safety valve. When installing liquefied gas in the gas phase, the safety issue of discharge cannot be ignored.

　　 2.1.1 Selection of safety valve

The selection of safety valve should not only combine the process conditions of the container, but also consider the characteristics of the medium, and comprehensively analyze the working pressure range, safe discharge volume, discharge method, loading and closing mechanism and other factors. Most of the safety valves. According to the different types of disc loading, safety valves can be divided into spring type and static weight type. Among them, the spring-loaded safety valve is relatively compact in structure, has the advantages of small valve body volume, sensitive action, etc., and the impact of vibration is relatively small; the disadvantage is that its elasticity is reduced under high temperature conditions. The deadweight safety valve has high accuracy and is easy to adjust, which is suitable for high temperature working conditions; but the structure is relatively bulky and is not suitable for high pressure vessels. According to the different ways of gas emission, it can be divided into three types: fully enclosed, semi-enclosed, and open. Among them, the medium discharged by the fully enclosed safety valve will not leak, and it is suitable for toxic and flammable medium, and medium that is easy to pollute the environment. Semi-closed safety valve, the exhaust pipe is the main discharge channel of the medium, for gas medium with little or no pollution to the environment. For open safety valves, the discharged medium is air or non-polluting high-temperature gas. According to the different opening heights, there are 3 types of full lift, middle lift and micro lift. Among them, gas and steam media should use full-lift safety valves, and liquid media are suitable for micro-lift safety valves. When selecting a safety valve, the most critical factor is the discharge volume of the safety valve. With reference to the safety discharge standard of the pressure vessel, the value should be greater than or equal to the former. At the same time, consider whether the range of working pressure can match the corresponding pressure vessel.

　　 2.1.2 Check of safety valve

　　Before installation work, hydraulic test and air tightness test are indispensable links, which need to be operated by professionals, and calibration and adjustment can be made after the test is qualified. The set pressure of the safety valve should be less than or equal to the design pressure of the pressure vessel. The maximum allowable working pressure is usually marked on the design drawings and nameplate, but it can also be used as the maximum allowable working pressure. The safety valve should not be opened too frequently, and leakage of toxic and flammable media should be avoided. After the calibration adjustment is completed, the safety valve needs to be sealed. The periodic calibration cycle of the safety valve shall not be less than once a year. When special circumstances occur and certain conditions are met, the verification period must be appropriately extended after the approval of the technical person in charge of the user unit.

　　2.2 pressure gauge

　　 Among the safety accessories of pressure vessels, the pressure gauge is an extremely important safety device for measuring the pressure of the medium inside the vessel. In industrial process control, technical measurement and other processes, pressure gauges are extremely widely used. The sensitive components in the pressure gauge will undergo elastic deformation under the action of external pressure fluctuations. Through the conversion mechanism of the movement in the watch, the generated pressure deformation is transmitted to the pointer, and the pointer rotates to display the pressure. There are many types of pressure gauges, including digital type, conventional type, special type, vibration-resistant type, corrosion-resistant type, seismic type, contact type, remote transmission type, etc., forming a complete series.

　　 2.2.1 Selection of pressure gauge

　　 First of all, the range of the pressure gauge must be considered. The selected pressure gauge must not only be suitable for the medium in the pressure vessel, but also be suitable for the working pressure. Compared with the design pressure value, the scale limit value of the pressure dial is preferably 1.5 to 3.0 times. The range of the pressure used should be less than or equal to 2/3 of the scale limit; secondly, the accuracy of the pressure gauge should be taken into account, and the accuracy grade is marked on the dial. Need to combine the pressure level of the equipment and actual work needs to determine the accuracy level. The range and accuracy of the pressure gauge will affect the actual error of the pressure gauge; finally, the dial diameter of the pressure gauge must be considered, generally not less than 100mm. When the installation position of the pressure gauge is higher or farther, the diameter of the dial should be increased appropriately.

　　2.2.2 Calibration of pressure gauge

　　 The selection and installation of pressure gauges have strict standards, and their calibration and maintenance cannot be ignored. It is directly related to whether the pressure gauge can maintain sensitivity and accuracy. The calibration of pressure gauges shall strictly abide by the relevant regulations of the national metrology department. Before installation, the pressure gauge must be checked. On the dial, mark the red line indicating the working pressure. The marked position cannot be on the dial glass. When the glass rotates, it is easy for the operator to have an illusion and cause an accident. The calibration cycle of the pressure gauge is once every six months. When the calibration result is qualified, the lead seal can be added to indicate the next calibration date. The installation and use of pressure gauges that are unqualified and have no lead seals are strictly prohibited.

　　3 Conclusion

　　 For the safe and stable operation of pressure vessels, safety accessories play a vital role. Not only must the safety accessories be selected correctly, but also effective verification must be done. This is also one of the contents of the regular inspection of pressure vessels. When the safety accessories are found to be inaccurate or insensitive, they must be overhauled immediately, and replaced if necessary, so as to ensure the function of the safety accessories. It is forbidden to disassemble, close and use safety accessories arbitrarily. Pressure vessels must be equipped with safety accessories in accordance with regulations before they can be put into use normally to ensure the safe and stable operation of pressure vessels.