An Introduction to Safety Valves for Boilers

An Introduction to Safety Valves for Boilers


safety valve is one of the main safety accessories of the boiler. It can automatically prevent the steam pressure of the boiler from exceeding the predetermined allowable range, thereby ensuring the safe operation of the boiler. A safety valve is a safety device for overpressure relief.
 
Structure principles
A safety valve relies on the valve seat, valve clack and pressurizing device to act. The valve seat is in contact with the medium under pressure, and the valve clack is pressed on the valve seat by the pressurizing device. When the working pressure is greater than pressurization, the valve clack is opened and the medium is released; when pressurization is greater than  pressure of the medium, the valve clack is pressed on the valve seat. The regulating valve will be closed, which plays a security role.
 
The types of safety valves for boilers
According to the structure type, a safety valve is divided into the static weight type, lever type, spring type and impulse type.
 
1. Static weight types 
The static weight safety valve directly acts on the valve clack with the load force of the heavy hammer. When the medium force and heavy hammer force are balanced, the static weight safety valve will be opened. After the set pressure value of the static weight safety valve is given, the weight of the heavy hammer and opening pressure will not change regardless of the use occasion, and the performance is relatively stable. The static weight type is commonly used for the steam boiler whose rated steam pressure is less than 0.10Mpa (1Kgf/cm2).
 
2. Lever types
The lever safety valve is composed of a lever, a heavy hammer and a fulcrum, it is lighter than the static weight type and has lower return pressure. It has advantages of being simple in structure, easy to adjust, automatic, stable and widely used.
 
3. Spring types
The valve clack of the spring type is pressurized by springs, which has compact structure, quick operation, is insensitive to vibration and shaking, easy to adjust, and can be used on mobile boilers. The shortcomings of the spring type is that pressurization will be changed due to the impact of high temperature and corrosion, which causes the initial pressure to drift. In addition, the opening height of the valve clack is proportional to the elastic pressure, and the overpressure is high. The spring force should not exceed 20000N, and the spring cannot be prepared for work if it is too great or too hard.
 
4. Impulse or pilot types
The main valve of the impulse type is driven by the pulse action of the auxiliary valve. It has advantages of sensitive operation and good sealing, which is often used in large diameter, high-pressure and large capacity boilers. The impulse type can be classified into the full opening type and slight opening type according to the opening height of its disc. The seat height of the former h is equal to or greater than d/4, and the d is the diameter of the valve seat throat; the latter h is equal to (1/20 to 1/40)d. The exhaust capacity is the same; the d of the full opening type is much smaller than that of the slight opening type. The full opening type is often used for compressible fluids such as steam and air, while the slight opening type is suitable for incompressible fluids such as water and oil. The slight opening type has less strict structural requirements and is convenient for design, manufacturing, maintenance and testing.
 
5. Water seal safety devices
They pressurize a certain height of water columns.
 
The role of safety valves for boilers
When medium pressure in the pressure attachment of the boiler exceeds the allowable range, the safety valve can be automatically opened and the pressure will be discharged. The valve can be automatically closed when the pressure drops. When the pressure is discharged, there will be a loud sound, which acts as an automatic alarm to ensure the safe operation of the boiler.