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The boiler safety valve is one of the main safety accessories in a boiler; it automatically prevents the steam pressure in the boiler from exceeding the predetermined allowable limits, thereby ensuring the safe operation of the boiler. It is a safety device designed for overpressure relief. I. Structural principle: It operates primarily through a valve seat, a valve disc, and a pressure-acting mechanism. The valve seat is in communication with the pressurized medium, and the valve disc is pressed against the valve seat by a pressing mechanism. When the operating pressure exceeds the pressing force, the valve disc opens, allowing the medium to be released ； When the applied pressure is greater than the medium pressure, the valve disc is pressed against the valve seat, and the control valve closes. It serves as a safety guarantee. II. Classified by structural type, they are gravity-type, lever-type, spring-type, and impulse-type. 1. Dead-weight type: It consists of a set of ring-shaped iron parts that are used to apply pressure. As the pressure increases, these iron parts become heavier and bulkier, making it difficult to adjust the pressure. Their sensitivity is also low. Since the starting point for applying pressure can be set at a low level, this type is commonly used in steam boilers with a rated steam pressure of less than 0.10 Mpa (1 Kgf/cm2). 2. Lever type: It consists of a lever, a weight, and a fulcrum; it is lighter than the dead-weight type and has lower reseating pressure. Its advantages include a simple structure, easy adjustment, stable automatic operation, and wide application. 3. Spring-type: The valve disc is pressed by a spring, offering a compact structure, rapid response, insensitivity to vibrations and shocks, and easy adjustment; it can be used in mobile boilers. Its drawback is that the spring is affected by high temperatures and corrosion, which causes changes in the pressing force and drift in the starting pressure. Furthermore, the opening height of the valve disc is proportional to the spring force, resulting in a high overpressure. The spring force should not exceed 20,000 N; overly large and stiff springs cannot be used for preparation work. 4. Impulse type (pilot-type or pulse-type): The main valve is actuated by the pulsating action of a secondary valve. Advantages: Fast response, good sealing performance; often used in large-diameter and high-pressure, high-capacity boilers. Its starting pressure cannot be calculated based on the working pressure at the point where the impulse is applied; rather, it must be determined separately through mechanical tests, hydraulic circuit tests, and operational test. Safety valves are classified into full-opening type and slightly-opening type based on the opening height of their valve discs. For the former, the seating height h ≥ d/4, where d is the throat diameter of the valve seat ； For the latter, h=(1/20～1/40)d. With the same exhaust capacity, the fully open type d is much smaller than the slightly open type. Fully open types are commonly used for compressible fluids such as steam and air, while slightly open types are suitable for incompressible fluids like water and oil. The design requirements for slightly open types are less stringent, making them convenient for design, manufacturing, maintenance, and testing. 5. Water seal safety device: It creates a water column of a certain height under pressure. III. Function of the boiler safety valve: When the pressure of the medium inside the pressure-containing components of the boiler exceeds the allowable level, the valve opens automatically to allow the excess pressure to be released. It closes automatically once the pressure drops. A loud noise is produced during release, serving as an automatic alarm to ensure the safe operation of the boiler. IV. Technical requirements for boiler safety valves: 1. For boilers with a rated evaporation capacity of 0.5 t/h, at least two safety valves shall be installed, in addition to the economizer. Safety valves must be installed at the outlet of the fractional economizer, at the outlet of the steam superheater, at the outlet of the reheat apparatus, and at the start-up separator of the once-through boiler. Safety valves should be installed after atmospheric thermal deaerators, blowdown expansion tanks, and pressure relief valves. 2. Boilers with a rated pressure of less than 0.10 Mpa (1 kgf/cm2) should be equipped with a deadweight device or a water seal. (The inner diameter of the water seal tube shall be not less than 25 mm, no valves shall be installed, and anti-freezing measures shall be in place.) 3. The safety valve should be installed vertically at the highest point of the boiler drum (or header), in a location that facilitates regular maintenance and inspection, and from where the sound of discharge can be heard. The boiler safety valve is located at the highest point of the boiler drum (or header), ensuring easy maintenance ； No steam extraction pipes or valves shall be installed between the valve and the boiler (water tank). 4. The safety valve must be equipped with the following devices: lever-type safety valves must have mechanisms to prevent the weight from moving on its own, as well as guides to limit the movement of the lever ； Spring-type safety valves should have mechanisms to prevent the adjustment screws and nuts from being turned arbitrarily ； Gravity-type safety valves must have a device to prevent the weight plate from coming loose ； The shut-off valve on the impulse inlet conduit of the impulse-type safety valve must remain fully open and locked or sealed with lead ； A safety valve controlled by compressed air must have a reliable air supply and power source. 5. When several safety valves are installed together on a short pipe base that is directly connected to a boiler drum, the cross-sectional area of the passage in the short pipe should be 1.25 times greater than the total exhaust area of all the safety valves. For boilers with a working pressure of ≤3.9 Mpa (39 kgf/cm2), the inner diameter of the safety valve seat should be no less than 25 mm ； For boilers with a working pressure of >3.9 Mpa (39 kgf/cm2), the inner diameter of the safety valve seat should be no less than 20 mm. 6. The total exhaust capacity of the safety valve must be greater than the boiler’s maximum continuous evaporation rate. It is also ensured that after all the safety valves on the boiler drum and superheater open, the increase in steam pressure inside the boiler does not exceed 30% of the highest opening pressure of those safety valves, and the steam pressure must not exceed 1.1 times the design pressure. (The exhaust capacity is calculated using the formula: E=CA(P+1)K). E represents the exhaust capacity of the safety valve in Kg/h; P is the steam pressure at the inlet of the safety valve in Mpa (or Kgf/cm2); A is the exhaust area of the safety valve in mm2; C is the exhaust constant of the safety valve, determined as provided by the manufacturer; K is a correction factor for the specific volume of steam at the inlet of the safety valve. The exhaust volume of the safety valves at the outlets of the superheater and reheater must be such that the superheater and reheater receive sufficient cooling to prevent damage ； The cross-sectional area of the economizer safety valve is determined by the design agency. 6. Safety valves should generally be equipped with exhaust pipes to prevent injury during steam discharge. The cross-sectional area of the exhaust pipe must be at least twice the total cross-sectional area of the safety valve. Below it should be a drain pipe leading to a safe location. No valves are allowed to be installed on either the exhaust pipe or the drain pipe. If the sound of steam discharge from the safety valve cannot be heard by the boiler operator at his work location, no signaling device shall be installed. The safety valve of the economizer should be equipped with a steam discharge pipe leading to a safe location; no valves are allowed to be installed on the drainage pipe. 7. If both the operating pressure of the boiler and the design pressure of the steam distribution cylinder connected to it are lower than the boiler’s design pressure, it is possible for the boiler pressure to exceed the design pressure of the steam distribution cylinder when the safety valve releases steam. Therefore, it is necessary to verify the strength of the steam distribution cylinder and install a safety valve on it. 9. To prevent the valve seat and valve core of the safety valve from sticking together, it is necessary to regularly perform manual or automatic venting or water discharge tests on the safety valve. 10. The safety valves on the furnace drum and superheater shall be adjusted and calibrated in accordance with the manufacturer’s requirements; the opening pressure of the economizer safety valve shall be 1.10 times the operating pressure at the installation site. 11. When it is not appropriate to discharge the working medium of special boilers into the atmosphere, the discharge pipe should be connected to a specially designed condensation recovery unit. 12. When installing a lever-type safety valve, it is essential to ensure that the center of the valve disc shaft remains perpendicular to the horizontal plane. The bolts of the inlet and outlet pipe connection flanges on all safety valves must be tightened evenly to prevent additional pressure from being applied to the valve body, which could disrupt its concentricity and affect its proper operation. 13. After being calibrated, the safety valve should be locked or sealed with lead, and the results should be recorded in the boiler’s technical documentation. V. Common faults and their causes 1. Air leakage 1) Damage to the contact surface between the valve core and the valve seat, or dirt trapped in between. 2) Wear of the valve stem and bushing, excessive gap between the spring and the valve stem, bending of the valve stem, and inclination of the valve stem during installation, resulting in an incorrect center line. 3) The lever is skewed between the pivot points, causing the contact surface between the valve core and the valve seat to be damaged due to uneven pressure. 4) Permanent deformation of the spring, resulting in loss of elasticity. 5) The contact surface between the spring and the spring tray is uneven. 6) The planes of the springs are not parallel, or the lengths of the support rods on both sides are different, resulting in uneven stress on the springs. It causes improper contact between the valve core and the valve seat. 7) After corrosion, the cross-section of the spring decreases, resulting in reduced elasticity. 2. The pressure required to open the valve is reached, but the valve does not open: 1) The valve core and seat are stuck together; 2) The gap between the valve stem and the housing bushing is too small, causing it to get stuck due to expansion when heated. 3) The tightness between the valve core and the valve seat is severely damaged, resulting in continuous leakage; the pressure acting on the valve core decreases, and the safety valve cannot open at the predetermined pressure. 4) Improper adjustment: the spring is too tight, causing the weight to move backward too much. 3. The following conditions indicate exhaust steam: 1) The set opening pressure is inaccurate, and the spring compression is insufficient. 2) The spring undergoes permanent deformation and loses its original pressure. 3) The weight has not been properly secured.