How to select the spring as the core part of safety valve?
Safety valve is an automatic valve, which uses the force of medium itself to discharge a rated amount of fluid without any external force, so as to prevent the pressure from exceeding the rated safety value. When the pressure returns to normal, close the valve again and prevent the medium from flowing out. Safety valve is mainly used in boiler, pressure vessel and pipeline equipment to ensure that the system will not be subject to accidents due to high pressure, which plays an important role in protecting the safety of equipment operation and operators. Note the safety valve can only be used after pressure test.
Safety valve plays a role of safety protection in the system. When the system pressure exceeds the specified value, the safety valve will be opened to discharge part of the gas / fluid in the system out of the atmosphere / pipeline, so that the system pressure does not exceed the allowable value, so as to ensure that the system will not have an accident due to the high pressure.
Spring, as the core component of safety valve, needs to meet a series of detailed requirements and strict testing process before being installed and used on the safety valve. When selecting the spring material for safety valve, the influence of working medium and temperature shall be fully considered, and the corrosion-resistant material or a layer of corrosion-resistant and antirust material shall be applied (plated) on the spring surface. The influence of temperature on the deformation of spring and the creep or cold brittleness of spring material should be fully considered for the spring used in high or low temperature situation.
The spring of safety valve shall meet the following requirements:
- Cylindrical spiral compression spring should be used;
- The structure of the safety valve can prevent the discharged medium from directly eroding the spring, especially the hot corrosive fluid;
- The spring design can ensure that when the safety valve is fully opened, the deformation of the spring is equal to 20% – 80% of the maximum deformation of the spring, and the maximum shear stress of the spring design is not greater than 80% of the allowable shear stress.
In order to ensure the stability of the spring during long-term operation, the spring shall be subject to strong pressure treatment, and a strong pressure treatment report shall be provided. The results shall meet the following requirements:
- After the spring is pressed from the free height to the parallel height at room temperature, the residual deformation of the free height shall not be greater than 0.5% of the free height for 10 minutes;
- For the spring of the same specification and the same heat treatment furnace, measure the deformation (or rigidity) under the specified load, and the deviation shall not be greater than 10%;
The spring manufacturer shall provide the performance report test of the spring one by one, the heat treatment report of the same specification and the same heat treatment furnace and the qualification. After receiving the spring, the safety valve manufacturer shall check the material report and download the relevant material specifications, test the size and material, and check the furnace batch number. The material report includes the material certificate, heat treatment record, strong pressure treatment record and strong pressure test record of the spring.
After receiving the spring, the safety valve manufacturer needs to recheck the size, stiffness coefficient and shape variable according to the national standard. The test report shall include the load, deflection, R value, free length, compressed length and other data of the spring. The spring can be installed on the safety valve only after it passes the factory acceptance.
There are three main types of safety valve structure: spring type, lever type and pilot type. Spring type refers to the sealing of valve disc and valve seat by the force of spring; lever type refers to the force of lever and heavy hammer; pilot type safety valve is designed to meet the needs of large capacity, which is composed of main safety valve and auxiliary valve. When the medium pressure in the pipeline exceeds the specified value, the auxiliary valve shall be opened first, and the medium shall enter the main safety valve along the conduit, and the main safety valve shall be opened to reduce the increased medium pressure.
According to different conditions such as structure, opening mode, discharge mode and action principle, safety valve has many different classification modes, among which the common safety valve types are:
- Common spring loaded safety valve: a safety valve that only relies on direct mechanical loading device (such as spring, etc.) to overcome the force generated by the medium pressure under the disc;
- Balanced spring loaded safety valve: a safety valve that takes measures to minimize the influence of back pressure on the action continuity;
- Spring type safety valve with pneumatic device: a safety valve opened when the pressure is lower than the normal setting pressure with the help of a power auxiliary device;
- Pilot operated safety valve: a safety valve driven or controlled by discharging medium from the pilot valve.
As the safety valve belongs to special equipment, it is required to strictly follow the international specifications and standards when selecting the safety valve. When selecting the safety valve, it is necessary to consider the working conditions of the operation site, the material and connection mode of the valve components, and the relevant media parameters.
Source: Best China Safety Valve Provider: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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