Safety valve material

The basic requirements of safety specification for safety valve material

1. Bearing parts and spring materials

According to TSG zf001-2006 safety valve technical supervision regulations and relevant technical standards, the pressure parts and spring materials of the safety valve shall meet the requirements of relevant safety technical specifications and table 2.6-1.

When adopting the materials not listed in table 26-1, there shall be examples of use, which shall be submitted to aqsiq for record after being approved by type test institutions

The pressure-bearing material of the relief valve shall be adapted to the working medium, the highest operating pressure and the highest and lowest operating temperature to ensure the reliable operation of the relief valve.

For security reading of spring material, its selection should be fully aware of the working medium and temperature, the influence of spring should be of the corrosion of materials or the spring surface coating (plating) a layer of corrosion resistant anticorrosive material used in high temperature or low temperature occasion of the spring, should fully consider the influence of temperature on the spring deformation and creep or cold spring material brittleness

Material for disc and seat

The relief valve disc and seat material must be impact and corrosion resistant. Iron material is not allowed. For safety valve with high technical parameters, the sealing surface is recommended to use hard corrosion resistant material

3. Guide the surface material

The guide surface material of the relief valve (including the surface between the guide sleeve and the recoil disc, and between the guide sleeve and the bushing) shall be corrosion-resistant, and the surface shall be wear-resistant and of a hardness that prevents obstruction.

4, welding parts and materials

The carbon content of the welded parts used in the relief valve should not be less than 025%

Second, the choice of safety valve body material

Relief valve body material is mainly divided into casting and forging two major categories, in the design of the selection of materials, should be based on the use of the safety valve temperature, working pressure, corrosion of the medium inside the installation system to choose. Commonly used for steam, water, air, and the operating temperature is less than 425℃, the body is generally selected WCB and other carbon steel materials, when the operating temperature is more than 425℃, should choose the high-temperature low-alloy pot or austenitic stainless steel, and for the ultra-low temperature occasions below -100℃, should choose austenitic stainless steel. This is a general principle in the selection of safety valve materials, and sometimes there are specific options depending on the system. Number of commonly used body materials

Table 2.6-2 shows.

Iii. Selection of sealing materials

1. Basic requirements of materials

The material used for the sealing surface of the relief valve closure shall meet the following requirements

(1) corrosion resistance and corrosion resistance

When selecting material for seat and disc sealing surface, corrosion resistance of the material should be considered first. The best corrosion resistant materials are 18-8 series stainless steels and the following cemented carbides: stellite surfacing electrodes, wires, and iron - and nickel-based surfacing materials. Therefore, the sealing surface of the valve seat is mostly made of stainless steel or surfacing material, and the valve disc is made of stainless steel or embedded with a softer corrosion material. Superhard sterlite alloys are surfacing welded to disc and seat under high and ultra-high pressure conditions. This alloy has excellent thermal strength, corrosion resistance and corrosion resistance, and can withstand high specific pressures. However, the application of this alloy as a relief valve sealing material is limited due to its high cost and shortage, as well as the high labor cost in the surfacing process.

(2) when there is no medium pressure in the system, it can withstand the huge specific pressure generated by the spring.

The closing member is required to have sufficient strength and stiffness, and will not be plastic deformation under large specific pressure. Superhard surfacing alloys are often used as sealing elements when the pressure exceeds 300MPa.

(3) good machining performance as well as grinding and grinding performance

(4) elastic deformation capacity

2. Metal village materials

Table 263 lists the pressure, temperature and corrosive degree of medium are different, corresponding to the sealing surface of the valve closure material. For steam condition with temperature below 200-250℃. The sealing surface of the safety reading closure can be made of brass or bronze. For the refining process below 350℃, 2Cr13 and 3Cr13 stainless steel can be used: up to 600℃, with 18-8 stainless steel surfacing, such as D801, D802, etc. Table 2.6-4 shows the classification and chemical composition of surfacing electrodes for sealing surface of common valves

3. Non-metallic materials

Although the sealing surface made of elastic material can only be applied to lower temperature, it has better properties than the metal sealing surface. For example, the sealing surface can be kept sealed under the pressure of smaller sealing surface ratio. The processing and grinding of the sealing surface are simple.

Compared with steel, elastic material can guarantee the sealing of the closing part with less sealing force.

According to the results of many tests on the safety valve seal, the most suitable elastic materials for the sealing surface of the closing parts used with stainless steel are determined: tetrachloroethylene and F46, hard rubber, B83 babbitt alloy and various brands of rubber. One of the hard rubber is mainly recommended for high pressure media and temperature in the 100℃ below the valve. In a neutral medium with a temperature of 50℃ and a pressure of 300MPa, polycarbonate was successfully tested as a sealing material.

In use, it must be considered that over time, rubber will break down into sticky substances. These substances can make rubber adhesive on the metal, which can lead to shut down a stick, depends on the nature of the rubber adhesion, and with the temperature and closing parts increase with the increase of the pressure on the sealing surface defect is polymeric material in residual deformation happens when high pressure, and more sensitive to temperature changes, this will lead to the relief valve opening pressure change over time. Therefore, the "metal to polymer" seal is limited to temperature

In order to overcome the relaxation effectively under the condition of little movement, the sealing surface can be compressed in advance and the closing parts with special structure can be used. The surface roughness of the sealing surface of the closing piece is not more than Ra0.1

Fluoroplastics are not active for corrosive media or even oxidants. They do not lose elasticity at very low temperature, have no tendency to absorb other substances, and will not form various sediments on their surface. So chlorine plastic is also commonly used in relief valve closure, can be used in highly corrosive conditions