Technical Requirements for Boilers Safety Valves

Technical Requirements for Boilers Safety Valves


Technical requirements for boilers safety valves are as follows:
1. At least one safety valve should be installed for boilers with a rated evaporation capacity lower than 0.5t/h; for boilers with a rated evaporation capacity higher than 0.5t/h, at least two safety valves should be installed besides the economizer. Safety valves must be installed at the outlet of the separable economizer, steam superheater, reheater and start-up separator of the once-through boiler. Safety valves shall be installed after the atmospheric thermal deaerator, blowdown expansion vessel and pressure reducing valve.
2. The boiler with rated pressure less than 0.10Mpa (1kgf/cm2) should adopt static weight or water sealing device. The inner diameter of the water seal pipe should not be less than 25mm; no valve should be installed and anti freezing measures should be taken.
3. The safety valve should be installed vertically at the highest position of the boiler drum or header; the location for installation should be convenient for daily maintenance and inspection and the discharge sound can be heard. The boiler safety valves being located at the highest position of the boiler drum are easy to maintain. No steam outlet pipes and valves for taking steam should be installed between the valve and boiler.
4. The safety valve must have the following devices: the lever type safety valve must have a device to prevent the heavy hammer from moving by itself and a guide frame to limit the amount of the lever. The spring type safety valve must have a device to prevent the screws and nuts from being turned randomly. The static weight safety valve must have a device to prevent the heavy piece from flying off. The globe valve on the inlet pipe of impulse type safety valves must be fully opened, locked or sealed. Reliable air and power sources should be provided for the safety valve controlled by compressed air.
5. When several safety valves are installed in the base of a short tube directly connected to a boiler drum, the cross-sectional area of the short tube should be 1.25 times greater than the exhaust area of all safety valves. For boilers with working pressure equal to or less than 3.9Mpa (39kgf/cm2), the inner diameter of the safety valve base should not be less than 25mm. For boilers with working pressure higher than 3.9Mpa (39kgf/cm2), the inner diameter of the safety valve base should not be less than 20mm.
6. The total exhaust capacity of the safety valve must be greater than the maximum continuous evaporation of the boiler. Ensure that after all the safety valves on boiler drums and superheaters are opened, increases of steam pressure in the boiler do not exceed 30% of high opening pressure of safety valves, and steam pressure of the boiler must not exceed 1.1 times higher than the design pressure. The reference formula for exhaust steam capacity is E=CA(P+1)K. E means the exhaust steam capacity of the safety valve; P refers to the steam pressure at the entrance of the safety valve (Mpa or Kgf/cm2); A is the exhaust area of the safety valve (mm2); C represents the exhaust steam constant of the safety valve, which is determined by the manufacturer; K is the correction coefficient of the specific volume of steam at the entrance of the safety valve. The exhaust steam volume of the safety valve at the outlet of the superheater and reheater should be controlled on the premise that the superheater and reheater are sufficiently cooled to prevent burnout. The cross-sectional area of safety valves of economizers is determined by the design unit.
7. The safety valve should generally be equipped with an exhaust pipe to prevent injury to people during exhaust. The cross-sectional area of the exhaust pipe should be at least double that of the safety valve. There should be a drain pipe connected to a safe place below it. It is not allowed to install any valve on the exhaust steam pipe and drain pipe. If the exhaust sound of safety valves cannot be heard by the worker at the work place, no signal device shall be installed. The safety valve of the economizer should be equipped with a steam exhaust pipe and led to a safe place. No valve is allowed on the drain pipe.
8. If working pressure of the boiler and design pressure of the sub cylinder connected behind are both lower than design pressure of the boiler, the boiler’s pressure may exceed the design pressure of the sub cylinder when the safety valve is exhausted. Therefore, strength of the sub cylinder must be checked and a safety valve should be installed on the sub cylinder.
9. In order to prevent the valve seat and valve core of the safety valve from sticking, the safety valve should be periodically tested for steam or water discharge manually or automatically.
10. The safety valve on the furnace barrel and superheater should be adjusted and verified according to the manufacturer's requirements. The opening pressure of the economizer safety valve is 1.1 times the working pressure of the installation site.
11. When the working medium of the special boiler is not suitable to be discharged into the cavity, the discharge pipe should be connected to a specially designed condensate recovery device.
12. When installing a lever type safety valve, you must strictly keep the center of the valve clack rod perpendicular to the horizontal plane. All bolts connecting flanges of inlet and exhaust pipes of safety valves must be evenly tightened to avoid additional pressure on the valve body destroying the concentricity of the valve body and affecting normal operation.
13. After the safety valve is checked, it should be locked or sealed, and the result should be filled in the technical file of boilers.
 
Common faults and causes of safety valves
1. Air leakage
(1) The contact surface of the valve core and valve seat is damaged or there is dirt in the middle.
(2) The valve stem and bushing are worn. The gap between the spring and valve stem is too big, and the valve stem is bent. The valve stem is tilted during installation, and the center line is not straight.
(3) The lever is skewed between the fulcrums, and the contact surface of the valve core and seat is damaged due to uneven pressure.
(4) The spring is permanently deformed and loses its elasticity.
(5) The contact surface of the spring and spring tray is not smooth.
(6) The plane of the spring is not parallel or the length of the struts on both sides is inconsistent, which makes the bearing force of the spring uneven and causes the valve core and bonnet to contact not well.
(7) The section of the spring is reduced after corrosion, and the elasticity is not enough.
 
2. Reaching the opening pressure without opening
(1) The valve core and valve seat are stuck.
(2) The gap between the valve stem and shell bushing is too small, and it will expand and become stuck after being heated.
(3) The tightness between the valve core and valve seat is severely damaged, resulting in long-term leakages. The pressure acting on the valve core is reduced, and the safety valve cannot be opened under predetermined pressure.
(4) The adjustment is improper; the spring is pressed too tightly, and the heavy hammer moves backward too much.
 
3. Other failures
(1) The adjusted opening pressure is not accurate, and the spring is not tight enough.
(2) The spring loses its original stress due to permanent deformation.
(3) The heavy hammer is not fixed.