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Analysis and Solution of the Fault Causes of Safety Valves

Time : 2023-06-26 Hits : 13

Safety valve is a very important protective valve, widely used in various pressure vessels and pipeline systems. When the pressure in the pressure system exceeds the specified value, it can automatically open and discharge excess medium into the atmosphere to ensure the safe operation of the pressure vessel and pipeline system, prevent accidents, and automatically close when the pressure in the system drops back to or slightly below the working pressure. The reliability of safety valve operation is directly related to the safety of equipment and personnel, so it must be given attention. Common faults of safety valves include leakage of the safety valve, leakage of the joint surface of the safety valve body, failure of the main safety valve after the action of the impulse safety valve, failure of the valve disc to reseat after discharge, failure of the safety valve to open when the specified pressure is reached, continued increase in pressure after discharge, frequent jumping or vibration of the valve disc, and failure to open when the specified pressure is reached.

1. Leakage of safety valve

Under normal working pressure of the equipment, leakage beyond the allowable level occurs at the sealing surface of the valve disc and valve seat, and the leakage of the safety valve not only causes medium loss. In addition, the continuous leakage of the medium can also damage the hard sealing material. However, the sealing surfaces of commonly used safety valves are made of metal materials. Although efforts are made to make them smooth and flat, it is also very difficult to achieve absolute non leakage under pressure of the medium.

The reasons for leakage between the sealing surface of the valve disc and valve seat are:

1) There is dirt between the sealing surface of the valve disc and valve seat. The solution is to use a lifting wrench to open and close the valve several times, flushing away dirt and dirt.

2) Damage to the sealing surface. According to the degree of damage, repair it by grinding or grinding after turning. After repair, the sealing surface should be ensured to be smooth, and its roughness should be maintained no higher than Ra=0.2 μ M.

3) Due to improper assembly or pipeline loads, the concentricity of the parts is damaged. Additional loads on the pipeline should be reassembled or eliminated.

4) The opening pressure of the valve is too close to the normal working pressure of the equipment, resulting in the sealing surface being too low compared to the pressure. When the valve is subjected to vibration or medium pressure fluctuations, leakage is more likely to occur. The opening pressure should be adjusted appropriately based on the strength conditions of the equipment.

5) The spring relaxes, causing a decrease in set pressure and causing valve leakage. It may be caused by high temperature or corrosion, and measures such as replacing the spring or even replacing the valve (if it is improper selection) should be taken according to the reason. If it is only caused by improper adjustment, simply tighten the adjusting screw appropriately.

6) The elasticity of the spring decreases or loses its elasticity. Measures such as replacing the spring and readjusting the opening pressure should be taken.

7) The valve stem is bent, tilted, or the lever is misaligned with the fulcrum, causing the valve core to be misaligned with the valve disc. Should be reassembled or replaced;

2. Leakage of the joint surface of the safety valve body

The leakage phenomenon at the joint surface between the upper and lower valve bodies is mainly caused by the following factors:

1)Insufficient or biased tightening of the bolts on the joint surface results in poor sealing of the joint surface. The elimination method is to adjust the tightening force of the bolts. When tightening the bolts, it is necessary to tighten them diagonally. It is best to measure the gaps at all points while tightening, and tighten the bolts until they are still tight, making the gaps at all points on the joint surface consistent

2) The toothed sealing gasket on the joint surface of the safety valve body does not meet the standard. For example, defects such as slight grooves in the radial direction of the toothed sealing gasket, poor parallelism, and excessively pointed or sloping teeth can all cause sealing failure. This causes leakage on the valve body mating surface. By ensuring the quality of spare parts during maintenance and using standard toothed sealing gaskets, this phenomenon can be avoided.

3) The flatness of the joint surface of the safety valve body is too poor or it is blocked by hard impurities, causing sealing failure. The method to eliminate leakage on the valve body joint surface caused by poor flatness is to disassemble the valve and regrind the joint surface until it meets the quality standards. If the seal fails due to impurities blocking, carefully clean the joint surface during valve assembly to prevent impurities from falling into it.

3. The main safety valve does not operate after the impulse safety valve is activated

This phenomenon is often referred to as the refusal to move the main safety door. The failure to operate the main safety door poses a significant threat to the operating boiler and poses a significant equipment hazard, seriously affecting the safe operation of the equipment. Once the medium pressure in the operating pressure vessel and pipeline exceeds the rated value, the main safety door will not operate, which can easily cause equipment damage and major accidents during overpressure operation. The main safety valve of the main safety valve is mainly related to the following factors: the moving parts of the safety valve are stuck, which may be due to improper assembly, mixing of dirt and impurities, or corrosion of parts; Poor surface finish of the piston chamber, surface damage, and defects such as grooves and hard spots.

4. The valve opening and closing are not flexible and crisp.

The main reasons for the inflexibility and crispness of valve opening and closing may be:

1) Improper adjustment of the regulating ring results in a prolonged opening process or delayed reseating of the valve. It should be readjusted.

2) The internal moving parts have jamming phenomenon, which may be caused by improper assembly, mixing of dirt or corrosion of parts. The cause should be identified and eliminated.

3) The discharge pipeline has excessive resistance, which creates a large back pressure during discharge, resulting in insufficient valve opening. The resistance of the discharge pipeline should be reduced.

5. The valve disc does not reseat after discharge.

The valve disc does not reseat after the safety valve is discharged. This is mainly caused by the spring bending of the valve stem, improper installation position of the valve disc, or jamming. It should be reassembled.

6. The safety valve does not open when the specified pressure is reached.

The reason for this situation is inaccurate constant pressure. The compression amount of the spring or the position of the heavy hammer should be readjusted; The valve disc adheres to the valve seat. Regular manual air release or water discharge tests should be conducted on the safety valve; The lever of the lever type safety valve is stuck or the heavy hammer is moved. The position of the heavy hammer should be readjusted and the lever should move freely.

7. Changes in account opening pressure rate.

After the safety valve is adjusted, the actual opening pressure is allowed to have a certain deviation from the set value. According to GB12243-89, the allowable deviation value is ± 0.02MPa when the set pressure is ≤ 1.0MPa; When the set pressure is>1.0MPa, it is ± 2% of the set pressure value. Exceeding the above tolerance range is considered abnormal.

The reasons for the change in opening pressure value may include:

1) Caused by changes in operating temperature. For example, when a valve is adjusted at room temperature for use at high temperatures, the opening pressure often decreases. This can be adjusted by appropriately tightening the screw. But if the temperature of the spring chamber is too high due to the regression of the selection, it should be replaced with an appropriate model (such as a valve with a radiator).

2) Caused by spring corrosion. The spring should be replaced. In situations where the medium is highly corrosive, springs coated with fluoroplastics on the surface or safety valves with corrugated pipe isolation mechanisms should be selected.

3) Caused by changes in back pressure. When there is a significant change in back pressure, a stable back pressure bellows safety valve should be selected.

4) Internal moving parts are stuck. It should be checked and eliminated.

8. After exhaust, the pressure continues to rise.

This is mainly because the selected safety valve has a smaller displacement than the safety discharge capacity of the equipment, and a suitable safety valve should be selected again; If the centerline of the valve stem is not aligned or the spring is rusted, causing the valve disc to not open to the expected height, the valve stem should be reassembled or the spring replaced; If the exhaust pipe section is not sufficient, exhaust pipes that meet the safe emission area should be used.

9. Valve disc frequency jump or vibration.

Mainly due to the high spring stiffness. Springs with appropriate stiffness should be used instead; Improper adjustment of the adjustment ring results in excessively high reseating pressure. The position of the adjustment ring should be readjusted; The discharge pipeline has excessive resistance, resulting in excessive discharge back pressure. The resistance of the discharge pipeline should be reduced.

10. Valve oscillation.

Valve oscillation refers to frequent opening and closing of the valve disc. The possible reasons for this are as follows:

1) The valve has excessive discharge capacity (relative to the required displacement). The rated displacement of the selected valve should be as close as possible to the necessary discharge capacity of the equipment.

2) The diameter of the inlet pipeline is too small or the resistance is too high. The inner diameter of the inlet pipe should not be less than the inlet diameter of the valve, or the resistance of the inlet pipe should be reduced.

3) The discharge pipeline has excessive resistance, resulting in excessive back pressure during discharge. The resistance of the discharge pipeline should be reduced.

4) The spring stiffness is too high. Springs with lower stiffness should be used instead.

5) Improper adjustment of the adjustment ring results in excessively high reseating pressure. The position of the adjustment ring should be readjusted.

11. Opening below the specified pressure.

When the specified pressure is reached, it is mainly due to inaccurate constant pressure; The spring's elasticity decreases due to aging. Adjust the screw appropriately or replace the spring.

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