Shanghai Brilliant takes you to understand the boiler safety valve
One page book
Boiler safety valve is one of the main safety accessories of boiler, which can automatically prevent the steam pressure of the boiler from exceeding the predetermined allowable range, so as to ensure the safe operation of the boiler, and is a safety device for overpressure release.
First, the structural principle
It is mainly operated by valve seats, discs and pressurization devices. The valve seat is connected with the pressurized medium, the valve disc is pressed on the valve seat by the pressurization device, and the valve disc is opened when the working pressure is greater than the pressurization, and the medium is relieved; When the pressure is greater than the pressure of the medium, the disc is pressed against the valve seat and the regulating valve is closed. It plays a role in safety and security.
Second, the type of boiler safety valve
According to the structural type, it is divided into static weight type, lever type, spring type and impulse type.
1. Static weight. Pressurization device is a set of annular iron parts, the pressure is increased, the pressure iron is heavy and bulky, the pressure adjustment is inconvenient, the sensitivity is low, because the starting point of the pressure can be made small, commonly used in the rated steam pressure less than 0.10Mpa (1Kgf/cm2) steam boiler.
2. Leverage. Composed of lever, hammer and fulcrum, it is lighter than static weight and has low return pressure. Its advantages are simple structure, easy to adjust, automatic stability, and widely used.
3. Spring type. The disc is spring-pressurized, compact structure, sensitive action, insensitive to vibration and shaking, easy to adjust, and can be used on mobile boilers. Its disadvantage type spring is affected by high temperature and corrosion, so that the pressure changes and starts the pressure drift. In addition, the opening height of the disc is proportional to the elastic pressure, and the overpressure is higher. The spring force should not exceed 20000N, and the hard spring cannot be prepared for work.
4. Impulse type (pilot type or pulse type). The main valve action is driven by the pulse action of the secondary valve. Advantages: sensitive action, good sealing, often used in large diameter and high pressure large capacity boilers. Its starting pressure cannot be calculated by impulse to the working pressure of the location, and should be calculated separately by the gas circuit and the operation test. Safety valves are divided into full opening type and micro opening type with different opening heights of their discs. The former's starting height h≥d/4, d is the throat diameter of the valve seat; The latter h=(1/20~1/40)d. The exhaust capacity is the same, and the full-opening D is much smaller than the micro-opening type. Fully open type is often used for steam, air compressible fluid, micro opening type is suitable for water, oil and other incompressible fluids, micro opening structure requirements are not strict, design, manufacturing, maintenance, test is convenient.
5. Water seal safety device: its pressurized water column of a certain height.
Third, the role of boiler safety valve
When the medium pressure in the boiler pressure attachment exceeds the allowable use, it can be automatically opened and discharged, and it can be automatically closed after the pressure drops, and a loud sound is made when discharged, which plays the role of automatic alarm and ensures the safe operation of the boiler.
Fourth, the technical requirements of boiler safety valves
1. The boiler with rated evaporation capacity <0.5t/h is equipped with at least one safety valve; For boilers with rated evaporation capacity > 0.5t/h, at least two are installed except for economizers. Safety valves must be installed at the outlet of the separable economizer, at the outlet of the steam superheater, at the outlet of the reheater and at the start-up separator of the DC boiler. Safety valves should be installed after atmospheric thermal deaerators, sewage expansion vessels, and pressure reducing valves.
2. The boiler with rated pressure less than 0.10Mpa (1kgf/cm2) should adopt a static weight or hydroseal device. (The inner diameter of the water sealing pipe should not be less than 25mm, and no valve should be installed, and antifreeze measures should be taken).
3. The safety valve should be installed in the highest position of the drum (or header), and the installation site should be convenient for daily maintenance and inspection, and the discharge sound can be heard. The boiler safety valve is located in the highest position of the drum (or box) and is easy to maintain; The outlet pipe and valve for taking steam shall not be installed between the valve and the boiler (joint box).
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 lever; The spring-loaded safety valve should have a device to prevent the adjustment screw and nut from being twisted at will; The static weight safety valve should have a device to prevent the heavy film from flying off; The shut-off valve on the impulse access duct of the impulse type safety valve should be kept fully open and locked or sealed; Safety valves controlled with compressed air must have a reliable air supply and power supply.
5. When several safety valves are jointly installed in the base of the short pipe directly connected to a drum, the cross-sectional area of the short pipe should be greater than 1.25 times the exhaust area of all safety valves. For boilers with working pressure ≤ 3.9Mpa, (39kgf/cm2), the inner diameter of the safety valve bottom should not be less than 25mm; For boilers with working pressure > 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 capacity of the boiler. And ensure that after all the safety valves on the drum and superheater are opened, the steam pressure in the boiler rises no more than 30% of the higher opening pressure of the safety valve, and the steam pressure of the boiler shall not exceed 1.1 times the design pressure. (Exhaust capacity refers to the formula: E=CA(P+1)K)
E-Exhaust capacity of safety valve (Kg/h)
P-Steam pressure at the inlet of the safety valve (Mpa) (Kgf/cm2) A-Exhaust area of the safety valve (mm2)
C-Exhaust constant of the safety valve (determined by the manufacturer) K-The correction factor of the specific volume of steam at the inlet of the safety valve
The exhaust steam volume of the safety valve at the outlet of the superheater and reheater shall ensure that the superheater and reheater are sufficiently cooled to not burn out under the exhaust steam volume; The cross-sectional area of the economizer safety valve is determined by the design unit.
7. Safety valves should generally be equipped with exhaust pipes to prevent injury during steam exhaust. The cross-sectional area of the exhaust steam pipe is at least twice the total cross-sectional area of the safety valve. There should be a drainage pipe under it to a safe place. No valves are allowed on both the exhaust and drain pipes. If the sound of the safety valve exhaust steam cannot be heard by the stoker at the workplace, the signal device is not installed. The safety valve of the economizer should be equipped with an exhaust pipe and lead to a safe place, and no valves are allowed on the drain pipe.
8. If the working pressure of the boiler and the design pressure of the sub-cylinder connected behind are lower than the design pressure of the boiler, the boiler pressure may exceed the design pressure of the sub-cylinder when the safety valve is exhausted, so the strength of the sub-cylinder must be checked and a safety valve is installed on the sub-cylinder.
9. In order to prevent the seat and spool of the safety valve from sticking, manual or automatic steam or water release tests should be carried out regularly on the safety valve.
10. The safety valve on the furnace barrel and superheater should be adjusted and verified according to the requirements of the manufacturer, and the opening pressure of the economizer safety valve is 1.10 times the working pressure of the device site.
11. When the working medium of the special boiler is not suitable for discharge to the cavity, the discharge pipe should be connected to the specially designed condensate collector.
12. When installing the lever type safety valve, the center of the valve disc stem must be strictly kept perpendicular to the horizontal plane. The bolts of the inlet and exhaust gas pipe connecting the flange of all safety valves must be evenly tightened to avoid additional pressure on the valve body, destroying the concentricity of the valve body and affecting the normal operation.
13. After the safety valve is calibrated, it should be locked or sealed, and the result should be filled into the boiler technical file.
5. Common faults and causes
1. Air leakage
1) The contact surface between the valve core and the valve seat is damaged or there is dirt in the middle.
2) The stem and bushing are worn, the spring is too clear from the stem, the stem is bent, the stem is tilted during installation, and the centerline is not correct. 3) The lever is deflected between the fulcrum, and the contact surface of the valve seat of the valve core is damaged due to uneven pressure.
4) The spring is permanently deformed and loses its spring.
5) The spring is uneven on the contact surface of the spring tray.
6) The plane of the spring is not parallel or the length of the struts on both sides is inconsistent, so that the spring is unevenly stressed. Cause improper contact of the spool bonnet.
7) After the spring corrosion, the section is reduced, and the elastic force is not as high.
2. Reach the opening pressure without opening
1) The valve core and valve seat are sticky
2) The gap between the valve stem and the shell bushing is too small, and the expansion is stuck after heating.
3) The tightness between the valve core and the valve seat is seriously damaged, long-term leakage, the pressure acting on the valve core is reduced, and the safety valve cannot be opened at the predetermined pressure.
4) Improper adjustment, the spring is pressed too tightly, and the heavy hammer moves too much back.
3. The following specified pressure is the exhaust steam
1) The adjusted opening pressure is inaccurate, and the spring is not pressed enough.
2) The spring is permanently deformed and loses the original pressure.
3) The heavy hammer is not fixed.