How to select, repair and calibrate safety valves in coal chemical system
Coal is chemically processed into gas, liquid and solid fuels and other chemical products. During the production process, the pressure of the medium in the equipment or pipeline rises due to operational errors, mechanical failures, and abnormal chemical reactions. As a safety protection valve, the safety valve is normally closed in the coal chemical system. When the medium exceeds a certain pressure, it is automatically opened. By discharging the medium outside the system to prevent the pressure of the medium in the pipeline or equipment from exceeding the specified value. A safety valve plays an important protective role for personal safety and equipment operation, and is closely related to energy conservation and environmental protection.
Safety valve selection
According to the pressure and medium diversity and complexity of the coal chemical system, the influence of temperature, pressure, medium and other factors in the equipment should be taken into consideration when selecting the safety valve. According to the design requirements, the appropriate safety valve type is configured according to the circulation diameter and diameter of the safety valve. When determining the safety valve, the nominal pressure must be greater than the setting pressure. When the full opening is reached, the pressure should not exceed the nominal pressure of the safety valve. The smaller the spring working pressure range is, the better the safety valve performance will be.
- (1) When discharging non-toxic and non-polluting medium, use open-type safety valve with open bonnet if it is allowed to leak to the atmosphere; if it is required to perform regular opening test on safety valve, use safety valve with lifting wrench. When the medium pressure reaches more than 75% of the set pressure, the lift wrench is used to check the flexibility of the valve opening.
- (2) Closed safety valves must be used for toxic or flammable media. The bonnet and protective cover gaskets should have a good seal to ensure that the discharged media does not leak. After the safety valve starts to jump, the medium should be discharged according to the specified outlet. At the same time, a gas tightness test is required.
- (3) For high back pressure, back pressure balanced safety valve or pilot type safety valve should be used. If necessary, install two safety valves that are mutually standby. The inlet and outlet shut-off valves are preferably mechanical interlocking devices, which can meet the required amount of medium discharge during maintenance.
- (4) Corrugated pipe safety valve should be used for medium with high toxicity, strong corrosion and extreme danger. Bellows safety valve uses bellows to isolate spring and guiding mechanism from the medium, and can eliminate the influence of back pressure fluctuation on valve performance. It also protects springs and other internals from media corrosion.
- (5) Safety valves with radiators are often used in applications where the medium temperature is high. ① When the operating temperature of the enclosed safety valve exceeds 250 °C and the operating temperature of the open safety valve exceeds 350 °C, a safety valve with a radiator is used; ② When the pressure of the gas medium in the equipment is greater than 3.0 MP and the temperature exceeds 235 °C, a safety valve with a cooling fin is used.
Safety valve repair
When the coal chemical system is unstable, the medium in the equipment is affected by system pressure and temperature changes. Particles and impurities in the medium will rush into the valve body, and some solid impurities will fall on the sealing surface, causing leakage between the valve core and the valve seat, and forming an indentation on the sealing surface, and the medium stored in the sealing surface will corrode. The pit marks caused by corrosion cause continuous leakage or adhesion of the medium and cannot meet the production requirements. At this time, the safety valve should be repaired.
The repair of the safety valve is mainly for the repair of the sealing surface of the safety valve. Most of the safety valves in chemical equipment have residual pungent chemicals after use, so workers should pay attention to their own protection when disassembling. The valve body and the spring of some safety valve are adhered to the medium coagulation. After disintegration, high temperature steam or other non-corrosive cleaning agent should be used to remove the residue. Non-metallic sealing surfaces are generally replaced or turned after being damaged. The metal sealing surface is ground in the case of less severe damage. When grinding, the seat and spool must be ground separately with a flat abrasive. When the sealing surface is damaged seriously or there is obvious damage such as crushing, the grinding paste with a particle size of 300-400 is used for preliminary grinding, and then the grinding paste with a particle size of 600-800 for fine grinding, and finally abrasive paste with a particle size of 1000-1200 is used for polishing. Every time you change the paste, you must use acetone to scrub. When polishing, grind it with a small amount of abrasive paste until it is bright. After grinding, clean the paste on the valve core and sealing surface. Grind several times until the sealing requirements are met.
Safety valve calibration
On-line calibration and checker calibration are the two most commonly used calibration methods. According to the equipment conditions of the coal chemical plant, the type of medium, the working pressure, etc., select the appropriate calibration method. The advantage of on-line calibration is that it is carried out while the equipment is running. The safety valve does not need to be removed from the equipment. The verification speed is faster, it is convenient for the verification of the welded safety valve, and the return pressure can be measured. The verification result is closer to the actual work. The disadvantage is that the calibration is dangerous and the leak test cannot be done. The calibration table calibration is to open the safety valve through medium such as nitrogen or water, and record the opening pressure. The advantage is to solve the safety valve setting and leak detection, for safety valves with normal temperature medium and working temperature below 250 °C, saving adjustment time of newly installed safety valve, reducing energy consumption and reducing work risk.he disadvantage is that there is an error between the operating temperature and the normal temperature (the spring becomes soft at high temperature), and only the opening pressure setting and the sealing test can be performed, and the return pressure cannot be verified. During calibration, it should be noted that for safety valves that cannot consider back pressure relief valves, high temperature or low temperature media, the calibration table calibration should be used. The high-pressure safety valve spring adjustment can only be fine-tuned, and the verified airflow and water flow must be filtered.
When checking the safety valve, pay attention to the adjustment of the return pressure. The return pressure refers to the static pressure at the inlet of the safety valve when the valve flap is in re-contact with the valve seat, the valve opening height is zero, and the medium stops continuously flowing out as the system pressure decreases. If the return pressure is too low, the medium outflow will be large, causing loss of medium and energy. If the return pressure is too high for the system medium to discharge, the valve will have a frequency jump. Therefore, as far as the emission can be achieved, the return pressure should be increased as much as possible to reduce the loss of medium and energy. The adjustment method is to adjust the gap by adjusting the ring. The smaller the gap is, the larger the resistance is. The greater the force that holds the spool is, the more difficult it is to return to the seat. The larger the gap, the easier the spool will fall back and the higher the return pressure. Since the normal temperature calibration table cannot test the opening height and the discharge amount, the adjustment ring should be kept in the factory state when the calibration table verifies the safety valve.
Let me make a brief description of the selection, maintenance and verification of safety valves for coal chemical systems. Before the equipment is put into operation, there should be enough purging to prevent the welding debris, iron pin and other debris in the pipeline from being carried out with the medium, and to prevent harder debris from eroding the sealing surface and causing damage to the sealing surface when the safety valvejumps. After purging according to the specifications, most safety valves can maintain a good seal after the jump, and some faults can be avoided, which is of great significance for ensuring the safe operation and energy saving of the equipment.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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