The material of copper valve suitable for the production of bellows must have good plasticity, high elastic limit, tensile strength and fatigue strength, good welding performance, and stable elastic performance. Because of these performance characteristics, austenitic stainless steel is widely used to make bellows.
In the metal bellows copper valve, the metal bellows assembly provides an axially movable metal shell between the valve stem and the process fluid in the valve body to form a dynamic seal. The bellows bear the pressure, temperature and corrosion of the medium while bearing the compression and extension of the valve stem, ensuring that the copper valve achieves zero leakage during the life cycle. With the widespread application of bellows in the copper valve industry, the technical requirements for bellows are constantly increasing. The metal bellows for brass pipe nipple are constantly developing towards high pressure resistance, high temperature resistance, corrosion resistance and long life.
High pressure resistant
Pressure resistance is the basic requirement for copper gate valves and bellows, but as the pressure level of copper gate valves increases, the wall thickness and rigidity of the bellows increase, which increases the difficulty of opening the copper gate valve and manufacturing the bellows. Bellows manufacturers generally do not simply increase the wall thickness, but increase the pressure resistance of the product by increasing the number of layers, while reducing the axial stiffness of the bellows and reducing the opening force of the copper gate valve. The wall thickness of austenitic stainless steel bellows with a nominal pressure below 15MPa (900 lb) is generally 0.1～0.3mm, and the number of layers is 1～6. The bellows under this pressure level has a certain versatility and has formed a series of products. When the pressure reaches 25MPa (1500lb) or more, the appropriate material should be selected according to the diameter of the copper valve and the working temperature, and designed and produced by the professional manufacturer of bellows.
Non-metallic materials of aluminum weld pipe fittings have excellent corrosion resistance, fluorine-plastic anticorrosive copper valves are widely used, and PTFE bellows copper valves also have certain applications. However, due to the low strength and heat resistance of non-metallic materials, the general use temperature does not exceed 150 ℃, and the scope of application is limited. Austenitic stainless steel represented by 304 and 316L can cope with weaker corrosive working conditions, but to resist wet chlorine corrosion in the chlor-alkali industry and other harsh corrosive working conditions, corrosion-resistant alloys with excellent corrosion resistance must be used material. The pitting resistance and crevice corrosion resistance of corrosion-resistant alloys are often characterized by the pitting resistance equivalent (PRE) value, which is calculated according to formula (1).
PRE = %Cr + 3.3 ×%Mo + 16 ×%N (1)
The higher the PRE value, the higher the critical crevice corrosion temperature (CCT) and critical pitting temperature (CPT) of the alloy. The PRE values of several corrosion-resistant alloys are much higher than those of 304 and 316L.