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Introduction of Bellows In Control Valves

Bellows in the petroleum, chemical has a wide range of applications, play a pressure sensor and isolation role, production control and safety have an important role, with the petroleum, chemical and other industries continue to develop and innovate, the production equipment put forward higher requirements, so the importance of the choice of bellows is also much attention, this article mainly discusses the application of bellows in the control valve.

1、The role and classification of bellows

A bellows is a cylindrical, thin-walled, folded shell with multiple transverse corrugations. The bellows on the control valve plays only one role - isolation, mainly divided into three kinds: one is to prevent the valve medium diffusion to the outside world, that is, for the valve flow medium for the chemical nature of the active or poisonous and harmful gases, volatile liquids and valuable media; another is to prevent external impurities and air and other into the valve, affecting the valve The environment in which the valve and the purity of the medium requirements; the last is to protect the internal parts of the valve, to prevent the medium on the valve stem caused by scouring and corrosion.

There are two kinds of installation position of bellows, one is installed inside the valve body, the bellows is made of PTFE, the upper end is integrated with the fluorine lining of the upper bonnet, and the lower end is connected with the valve spool, which plays the role of fixing the spool, and at the same time, protects the valve stem from erosion and scouring; one is installed outside the valve body, most of the bellows are made of metal, and one end is fixed in the upper part of the valve stem, and one end is the free end, and there is a layer of protective layer on the periphery of bellows. Protective layer, the protective layer not only play a role in protecting the bellows, but also play a role in ensuring that the bellows damage within a certain period of time the valve medium will not pollute the environment.

According to the material of the bellows can be divided into two categories, which are plastic bellows and metal bellows.

Plastic bellows, also called PTFE bellows, are made by heating and pressurizing tubular PTFE and block PTFE in molten state and then shaped. Plastic bellows is simple to make, in some cases can replace the metal bellows cost savings, cheap, expansion and contraction performance and other outstanding features. However, the application of plastic bellows is limited by three factors, one is the temperature of the environment in which the valve is located and the temperature of the medium, the temperature is too high and easy to cause aging and deformation of the bellows, so its long-term use of the temperature is generally not more than 180 ° C, the low-temperature case there is no specific requirements; the second is subject to the limitations of the pressure in the pipeline (general selection of the plastic bellows working pressure is not higher than 12 kg), the pressure is too high it will be the Valve lining material from the valve wall suction, resulting in valve blockage; another factor is the chemical nature of the medium, plastic bellows can be applied to formic acid, hydrochloric acid, chloride and other strong corrosive media, but when the medium is similar to its molecular formula of the medium, it will be similarly dissolved, at this time, cannot be applied. The life of plastic bellows is generally 5 years, plastic bellows has a wide range of applications in various industries, much larger than the application of metal bellows.

2、Classification of metal bellows and related differences

Because the use of plastic bellows has certain limitations, one is affected by the temperature, pressure, and the second is inside the valve body will affect the valve's flow capacity, on the contrary, metal bellows is different, it is not subject to temperature and pressure limitations, a wider range of materials to choose from, and not subject to the media limitations, so most of the bellows in the harsh working conditions are metal bellows.

Metal bellows can be divided into two forms according to the different processing forms: forming bellows and welding bellows. Forming bellows, also called mechanical forming bellows or disposable bellows, is made by mechanical stretching and pressing; welding bellows is the two pieces of bellows parts welded in turn. The selection of the two is the same, the difference in the selection of materials, precision, price and so on.

Introduction of bellows in control valves

Welded bellows than forming bellows and plastic bellows application of long life, its life can reach more than 10 years, forming bellows than welding bellows performance is almost, but the price has the advantage of welded bellows price is higher, especially special materials, if the company has strong capital can choose welded bellows, if the use of a shorter period of time can choose to form the bellows, special working conditions are also the choice of welded bellows is appropriate. In highly toxic conditions and some special circumstances must be used to ensure the safety of metal bellows, which cannot be replaced by plastic bellows.

The choice of bellows is crucial, some bellows damage is not just replacing the bellows, may lead to the scrapping of the entire valve, and generally need to be replaced together with the valve stem, so it is essential to strengthen the maintenance and inspection of the bellows in daily work.