Failure modes of safety valves and countermeasures

Thread Content

It does not activate under normal conditions; it activates at an over-set pressure, that is, only when the pressure exceeds the device’s maximum allowable operating pressure. This eliminates the overload protection function of the safety valve. The reasons for this situation and the corresponding countermeasures are as follows: (1) The set pressure is incorrect; it has been set too high. Countermeasure: Readjust the compression amount of the spring. (2) The valve disc is stuck to the valve seat. The safety valve failed to operate during prolonged operation, and rust formed due to a lack of regular inspection and maintenance, which caused the valve disc to stick to the valve seat. Countermeasures: Regularly perform manual venting or water discharge tests on the safety valves, and be careful to prevent moisture and humidity during use. (3) Internal foreign objects get stuck, preventing the valve disc from rising over the obstacle, which results in an increased opening pressure or an inability to open the valve. Countermeasure: Check and remove foreign objects. (4) External foreign object obstruction: Human-induced additional weight on the static or lever-type safety valve, or other factors that prevent the valve disc from opening properly. Countermeasure: Check and remove foreign objects. (5) A decrease in operating temperature leads to an increase in the opening pressure. Countermeasure: When adjusted for normal temperature use in low-temperature conditions, the set pressure value at normal temperature should be slightly lower than the required opening pressure value. It opens below the set pressure, that is, it activates before reaching the set pressure. Mainly: (1) The set pressure is inaccurate. Countermeasure: Readjust the compression amount of the spring ； (2) The elasticity of the spring decreases due to aging. Due to long-term use or medium corrosion, the spring ages, resulting in a decrease in its elasticity. Countermeasure: Tighten the adjustment screw appropriately or replace the spring. (3) An increase in operating temperature leads to a decrease in the opening pressure. Countermeasure: When adjusted at normal temperature for use at high temperatures, the set pressure value at normal temperature should be slightly higher than the required opening pressure value. (4) Due to leaks or other reasons, the operating environment of the safety valve is above the allowable operating temperature of the spring, resulting in a decrease in the spring’s elasticity. Countermeasures: Grind the sealing surface promptly to eliminate leaks and replace it with a new spring. (5) The safety valve is tilted, due to installation or being pressed or touched, causing it to tilt. Countermeasures: Safety valves, especially weight-type and lever-type safety valves, must be installed vertically, with a sturdy protective ring in place. Safety valve leakage: At the normal operating pressure of the equipment, leakage occurs between the valve disc and the valve seat sealing surface in an amount that exceeds the allowable limit. (1) There are impurities between the valve disc and the valve seat sealing surface. Countermeasure: Use a lift wrench to open the valve several times in order to flush out impurities or debris ； (2) Damage to the sealing surface. Due to factors such as the material and hardness of the sealing surface as well as the way it is assembled, the deformation or misalignment of the sealing surface that occurs during the opening and closing cycle of the safety valve, or the expansion and deformation of the sealing surface caused by changes in the temperature of the sealing medium, can all lead to leakage of the medium. Countermeasures: Select a safety valve made of an appropriate material based on the properties of the sealing medium and possible temperature changes. Defects or damage to the sealing surface of the safety valve should be repaired by grinding or by grinding after turning, depending on the situation; attention should also be paid to the grinding method to prevent the formation of grooves. (3) Damaged gasket. The seal gasket may be damaged due to impurities or hard objects that get stuck in it, squeeze it, or press on it; replace it with a qualified seal gasket. Also, make sure to clean the installation area and the objects onto which it will be installed before placing the seal gasket ； (4) The valve stem is bent or tilted, causing the valve disc to be out of alignment with the valve seat. Countermeasure: Reassemble or replace ； (5) The spring loses its elasticity or becomes less elastic, as the spring is corroded by the medium or softens due to an increase in the medium’s temperature, resulting in a decrease in preload and leakage. Countermeasures: Strengthen rust prevention and heat insulation measures for the springs, and replace the springs in a timely manner or readjust the opening pressure. (6) Spring breakage. Remedial action: Replace the spring again. (7) Improper assembly or inappropriate dimensions of related parts; during assembly, the valve core and seat are not fully aligned, or there is light leakage at the mating surfaces, as the sealing surfaces of the valve core and seat being too wide hinders proper sealing. Countermeasures: Check the size and uniformity of the clearance around the valve stem to ensure that the apex hole of the valve stem is perfectly aligned with the sealing surface ； Check that the clearances in all areas do not allow the valve core to lift ； The drawings require appropriately reducing the width of the sealing surface to achieve effective sealing. (8) Leakage at the valve body joint surface; insufficient or uneven tightening of the bolts at the joint surface results in poor sealing ； The sealing gasket at the valve body joint does not meet the standards. Countermeasures: Adjust the bolt tension; when tightening the bolts, do so in a diagonal pattern. It is best to measure the gaps at various points while tightening, and keep tightening until the bolts can no longer be moved further, ensuring that the gaps across the joint surfaces are uniform. Additionally, standard-compliant gaskets should be used. (9) Rust at the contact point between the valve stem and the valve disc causes the valve disc to lose its ability to return to its proper position; when the exposed valve stem is disturbed, the valve disc shifts, resulting in a failure of the seal and leakage. Countermeasures: Regularly inspect the contact groove between the valve stem and the valve disc, and take appropriate anti-rust measures. (10) The set pressure is too close to the equipment’s operating pressure. Since the fluctuation range of the equipment’s operating pressure is close to or exceeds the sealing pressure of the safety valve, leaks may occur. Countermeasures: Select an appropriate safety valve and set its setting pressure reasonably. The safety valve disc is unstable, causing frequent jumping or fluttering of the disc. Frequency jumping refers to the phenomenon where, after the safety valve returns to its seated position, it opens again as soon as the pressure rises slightly; this happens several times in a row, and such behavior is known as \"frequency jumping\" of the safety valve. Frequent pulsation is extremely detrimental to the sealing of safety valves, as it can easily lead to leakage at the sealing surface. The oscillation that occurs in a safety valve during the discharge process is known as flutter of the safety valve. The occurrence of such flutter can easily lead to metal fatigue, thereby degrading the mechanical properties of the safety valve and posing serious risks to equipment. The instability is mainly due to: (1) the spring stiffness being too high, which prevents the valve disc from reaching the opening height; as a result, the discharge volume is low, and the pressure continues to rise after the safety valve opens, leading to its reopening. Replace the countermeasure with a spring of appropriate stiffness ； (2) If the adjusting ring is not set properly and the channel area is too large, once the safety valve opens, the pressure drops sharply below the operating pressure. The valve then closes as the valve disc strikes the valve seat violently. Since the cause of the pressure increase has not been eliminated, the valve disc opens again, resulting in frequent opening and closing. Remedial action: Readjust the position of the adjustment ring. (3) The resistance in the discharge pipeline is too high, causing the momentum of the flowing medium to decrease sharply and resulting in excessive discharge backpressure. Countermeasures: Reduce the resistance in the exhaust pipeline and remove obstacles from it. (4) Improper use of the valve; a valve with an excessively high discharge capacity was selected. Countermeasure: Ensure that the rated discharge capacity of the selected valve is as close as possible to the discharge volume required by the equipment. The safety valve fails to return to its normal position after activating, and the pressure continues to rise even after venting. This is mainly due to: (1) the discharge capacity of the selected safety valve being less than the equipment’s maximum allowable pressure release volume; as a result, when the safety valve opens, there isn’t enough time to discharge the excess pressure, and the pressure inside the pressure vessel continues to rise. Countermeasure: Select a suitable safety valve again. (2) An incorrect centerline of the valve stem or rusted springs prevent the valve disc from rising to the proper height, resulting in a reduced discharge capacity of the safety valve. Countermeasures: Reassemble the valve stem or replace the spring, and pay attention to anti-rust measures for the spring ； (3) The cross-sectional area of the exhaust pipe is insufficient, resulting in the discharge capacity of the safety valve being less than the equipment’s safe discharge capacity. Countermeasure: Use an exhaust pipe that meets the safety discharge area requirements. (4) The spring grade does not meet the requirements; due to its low stiffness, excessive compression of the spring occurs during adjustment, resulting in an insufficient opening height. Countermeasure: Replace the spring of the appropriate grade, ensuring that its stiffness is suitable. The valve disc does not return to its original position after discharge. This is mainly due to: (1) the spring bending the valve stem, or the valve disc being installed in the wrong position or stuck. Countermeasure: It should be reassembled, and the moving parts should be carefully inspected for defects such as dents, scratches, rust, or burrs, which must be removed promptly. (2) The safety valve spring breaks during use; after it opens, the broken part shifts out of place, preventing the valve disc from returning to its proper position. Countermeasure: Replace with a new spring promptly.