Common faults of safety valves
Thread Content
The main causes of safety valve failures are improper design, manufacturing, selection, or use. If these faults are not eliminated in a timely manner, they will affect the efficiency and lifespan of the valve, and it may even fail to provide any safety protection. Common faults and their solutions are as follows: (1) Leakage. At the normal operating pressure of the equipment, leakage occurs between the valve disc and the valve seat sealing surface beyond the allowable level. The reasons include: there is dirt between the valve disc and the valve seat sealing surface. Use a lifting wrench to open the valve several times in order to flush out the dirt ； Damage to the sealing surface. Repair should be carried out by grinding or by grinding after turning, depending on the degree of damage ； The valve stem is bent, tilted, or the lever is offset from its pivot point, causing the valve core and valve disc to be out of alignment. It should be reassembled or replaced ； The spring loses its elasticity or its resilience decreases. Measures such as replacing the spring and readjusting the opening pressure should be taken. (2) Does not open when the specified pressure is reached. The reason for this situation is inaccurate pressure setting. The compression amount of the spring or the position of the weight should be adjusted again ； The valve disc is stuck to the valve seat. The safety valve should be regularly tested for manual venting or water discharge ； The lever of the lever-type safety valve is stuck or the weight has been moved. The position of the weight should be readjusted to allow the lever to move freely. (3) Does not open at the specified pressure. The main issue is inaccurate constant pressure ； The spring loses its elasticity as it ages. The adjustment screw should be tightened appropriately or the spring replaced. (4) The pressure continues to rise after exhaust. This is mainly because the safety valve selected has a small discharge capacity, and the device’s safe discharge volume requires that a suitable safety valve be chosen again ； If the valve stem is not aligned properly or the spring is rusted, preventing the valve disc from reaching the proper height, the valve stem should be reassembled or the spring replaced ； The exhaust pipe is not long enough; an exhaust pipe with a size suitable for safe emissions should be used. (5) Frequent jumping or vibration of the valve disc. It is mainly due to the excessive spring stiffness. A spring with appropriate stiffness should be used instead ； The adjustment ring is not set properly, resulting in excessive reseating pressure. The position of the adjustment ring should be readjusted ； The resistance in the exhaust pipeline is too high, resulting in excessive exhaust backpressure. The resistance of the emission pipeline should be reduced. (6) The valve disc does not return to its seat after discharge. This is mainly caused by the spring bending the valve stem, or the valve disc being installed in the wrong position or getting stuck. It should be reassembled.
The nominal pressure of a safety valve indicates the maximum allowable pressure of that valve at normal temperature; for safety valves used in high-temperature equipment, the reduction in the allowable stress of the materials at high temperatures should not be taken into account. Safety valves are designed and manufactured in accordance with nominal pressure standards. Opening pressure: also known as set pressure, it is the pressure of the medium at which the valve disc of a safety valve begins to rise under operating conditions. Discharge pressure: The pressure on the inlet side when the valve disc reaches the specified opening height. Seating pressure: The inlet pressure at which the valve disc presses back against the seat after the safety valve has released, causing the flow of fluid to stop. The return seat pressure is an important parameter that indicates the quality of a safety valve’s performance; it is generally required to be at least 80% of the operating pressure, with an upper limit set such that frequent jumping of the valve disc does not occur. Opening and closing pressure difference: the difference between the opening pressure and the returning pressure. The setting pressures for the safety valves of the boiler drum and superheater are specified as follows: (1) When the rated steam pressure is less than 1.27 MPa, the setting pressures are respectively the operating pressure + 0.02 MPa and the operating pressure + 0.04 MPa ； (2) When the rated steam pressure is greater than 1.27 MPa but less than 3.82 MPa, the opening pressures are 1.04 times and 1.06 times the working pressure, respectively ； (3) When the rated steam pressure is greater than 3.82 MPa, the opening pressures are 1.05 times and 1.08 times the operating pressure, respectively. The opening pressure of the economizer safety valve is 1.5 times the operating pressure of the economizer. The opening pressure of the safety valve on a pressure vessel must not exceed the design pressure of that vessel.
The safe discharge volume of a pressure vessel refers to the amount of gas that must be discharged per unit time when the vessel is under overpressure, in order to prevent its pressure from rising any further. The discharge capacity of a safety valve refers to the amount of fluid discharged per unit time under the discharge pressure when the valve is in its fully open position. For boilers, it is required that the total discharge capacity of the safety valves be greater than the boiler’s maximum continuous evaporation rate, and that the steam pressure inside the drum shall not exceed 1.1 times the design pressure once all safety valves on the drum and superheater have opened. For pressure vessels, the discharge capacity of the safety valve must be greater than or equal to the vessel’s safe discharge capacity. When selecting a safety valve, the following aspects should be taken into consideration: (1) The type of structure – the choice of safety valve type is primarily determined by the process conditions of the equipment and the properties of the working medium. Under normal circumstances, spring-loaded safety valves are commonly used for boilers and pressure vessels. If the working medium in the container is toxic, flammable, or explosive, a closed-type safety valve should be used. For boilers and high-pressure vessels, as well as medium and low-pressure vessels with a large safety discharge volume and relatively thin wall thicknesses, full-opening safety valves are the best choice. (2) Pressure range: Safety valves are designed and manufactured according to nominal pressure standard series, and each type of safety valve has a specific operating pressure range. When selecting, an appropriate safety valve should be chosen based on the maximum allowable operating pressure of the boiler and pressure vessel. (3) The discharge capacity of the safety valve selected for emissions must be greater than the equipment’s safe discharge capacity; this ensures that when the boiler or pressure vessel is under overpressure, the safety valve can open to discharge a portion of the fluid in a timely manner, preventing the pressure inside from rising further.
Safety valves can be classified in the following main ways: (1) Based on their overall structure and the manner in which they are loaded, they can be divided into three types: weight-lever type, spring type, and control type； (2) Based on the ratio of the disc opening height to the valve flow diameter, safety valves can be divided into two types: slightly open type and fully open type ； (3) Based on the gas emission method, they can be divided into three types: fully enclosed, semi-enclosed, and open type. Safety valves must come with a product quality certificate upon leaving the factory, and a sturdy metal nameplate must be installed on the product. The metal nameplate shall indicate the following: (1) the name of the manufacturing unit, the manufacturing license number ； (2) Model, type, specifications ； (3) Product number ； (4) Nominal pressure, MPa ； (5) Valve seat throat diameter, mm ； (6) Emission factor ； (7) Qualification mark ； (8) Date of manufacture. For safety valves used in pressure vessels, the metal nameplate shall also indicate the applicable medium and temperature. The discharge coefficient of the safety valve shall be determined through testing by the manufacturer of the safety valve.
Safety valves should be tested at high speed before use. (1) Before installation, the safety valve shall undergo a pressure test and a leak test to verify its strength and sealing performance. Correction and adjustment can only be carried out after passing the inspection. (2) On the gas test bench, the opening pressure of the safety valve is adjusted by modifying the load applied to the valve disc. Lever-type safety valves adjust the position of the weight, while spring-type safety valves adjust the degree of spring compression. The opening pressure of the safety valve shall comply with the relevant provisions of the Safety Technical Inspection Regulations for Boilers and Pressure Vessels. (3) On the container, the discharge pressure and seating pressure of the safety valve are adjusted by modifying the gap between the safety valve adjusting ring and the valve disc. If the safety valve only leaks at its opening pressure without activating, or activates but experiences severe vibration and a \"buzzing\" sound after the pressure drops, it means the gap is too large; it should be adjusted to be smaller. If the return seat pressure is too low, it is due to an excessively small gap in the adjusting ring, which should be increased appropriately. The discharge pressure of the safety valve for steam should be less than or equal to 1.03 times the opening pressure, and the pressure difference between the closing and opening pressures should not exceed 10% of the opening pressure. The discharge pressure of a safety valve for air or other gases should be less than or equal to 1.10 times the opening pressure, with the pressure difference between the opening and closing states not exceeding 15% of the opening pressure. (4) When calibrating and adjusting the pressure valve, technical personnel in charge of boiler and pressure vessel safety at the user’s facility must be present. The accuracy of the pressure gauges used for adjustment and calibration should be no less than grade 1, and safety protection measures must be in place during on-line calibration. (5) During the verification process, the verifiers should keep proper records in a timely manner. A safety valve that has passed the inspection should be sealed with lead after the relevant accessories are installed, in order to prevent any unauthorized changes to its adjusted settings.