The bellows control valve is suitable for the occasions where external leakage is strictly controlled. It is suitable for toxic and harmful media, and it is easy to react with air to produce strong corrosive media, and the application conditions are harsh. This paper from UTMOST control valve factory introduces the characteristics of the valve stem seal of the bellows control valve and the related design of the bellows, and describes the failure form and cause analysis of the bellows.
1. Introduction
The stem part of the bellows control valve is characterized by the double seal of the bellows and the packing, which is used in the occasions where external leakage is strictly controlled. Metal bellows contain metal-specific mechanical properties, have good sealing, pressure resistance, corrosion resistance, high/low temperature resistance, good elasticity, and can withstand a certain torque. The metal bellows can produce corresponding displacement under the action of pressure, lateral force or bending moment, which can improve the sealing performance of the valve stem and protect the valve stem from corrosion by the medium. It is mainly used in the polysilicon industry, PVC, benzene hydrogenation, Hydrogen sulfide, chlorine and other industries.
2. How it works
Bellows control valve is mainly composed of valve body, support, upper bonnet, valve seat, valve core, bellows parts, etc. The bellows parts are welded by support ring, valve stem, anti-rotation pin, connecting ring and bellows The sealing performance of the valve stem is improved by the bellows, and the dynamic seal is converted into a static seal. The compression form of the bellows is mostly external pressure. It is recommended to compress 70% of the total displacement when the bellows is opened and 30% of the total displacement when it is closed. The well-known brands of bellows on the market are: Weitzman, Jiangsu Junlong, Shenyang Huibo, Nanjing Eddy; UTMOST adopts Weitzmann bellows, and the fatigue life can reach 100,000 times.
The bellows control valve is a special type of stem seal based on the ordinary control valve. In view of the working conditions where the packing part is easy to leak, the seal of the stem and the packing is isolated from the medium, which makes the function of the seal and isolation. The functions of the valve are distinguished. The packing part focuses on the sealing between the packing and the valve stem, and the isolation part focuses on the isolation between the medium and the valve stem. The two do not interfere with each other but are indispensable. Since the failure characteristics of the packing part are different from those of the bellows part, and the packing part can be regarded as the secondary protection of the bellows, in the actual design of the bellows control valve, both should exist at the same time.
3. Design ideas
The bellows control valve is a variant based on the ordinary control valve. Therefore, a mature ordinary control valve is required first, which can meet the needs of various working conditions, and can meet the requirements of toxic and harmful media and strong corrosive media without considering the sealing condition. Adjustment and sealing tasks. On this basis, the design of the bellows control valve is carried out. As shown in Figure 1, it is the basic structure that the bellows control valve needs to have.
As the core of the bellows control valve, the size of the bellows is the key point. However, the design tasks of most bellows control valves do not include the design of the bellows. At present, there are many well-known bellows suppliers in the market, with high quality and low price. , has rich experience in the service life and strength of bellows, so it is recommended to purchase the bellows part.
The parameters that determine the size of the bellows are: service life (the number of times of tension and compression), operating pressure, valve stem diameter, operating stroke (tension and compression stroke), medium parameters, etc. The above parameters determine the wave number, number of layers, inner and outer diameters of the bellows, Length, thickness, stroke. With these parameters, the design of the support can be carried out. The inner and outer diameter of the bellows determines the diameter of the bellows support, and the length and stroke of the bellows determine the length of the bellows support. In order to ensure that there will be no leakage between the bellows and the valve stem, a more reliable welding form is usually used to make the bellows and the valve stem a whole. Therefore, in the design process of the bellows control valve, the bellows seal and the bellows should be taken into account. Pipe stroke, valve adjustment stroke and other working parameters.
Since the bellows support is equivalent to the extension of the upper bonnet and the valve body in the ordinary control valve, the flange part of the support can refer to the connection size of the valve body and the upper bonnet, and the structure of the support can be approximated as two flanges A combination of a piece of steel pipe. The bellows support can be in the form of welding, and the mature structure can be in the form of casting. Since the bellows support is used as the connecting part between the upper bonnet and the valve body, in terms of strength, in addition to meeting the pressure bearing requirements, it is also necessary to consider the weight of the upper bonnet and the actuators and components installed on it, and when the actuator switches The generated pulling force and pushing force need to take into account the strength and wear, and find a balance between cost, strength and life.
