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The application of metal sealing butterfly valve in cryogenic equipment
Source: | Author:zhongdeen | Publish time: 2015-08-23 | 1403 Views | Share:
With the rapid development of industrial technology, puts forward more strict requirements on the valve industry, especially in the butterfly valve is used in low temperature medium, in addition to general valve can satisfy the normal temperature condition of performance, more important is in the condition of low temperature valve sealing reliability, the flexibility of action and some other special requirements of cryogenic valves.

Because the butterfly valve has a compact structure, small volume, light weight (compared with the same pressure and gate valves of the same size can reduce 40% ~ 50%) fluid resistance small, open/close quickly and a series of advantages, so widely used. But in some cryogenic system in our country, such as natural liquefaction equipment, air separation equipment, and equipment such as pressure swing adsorption (psa) adopted by the chemical industry more than 80% or stop valve and gate valve, valve with small Numbers of butterfly valve. The main reason is the metal sealing butterfly valve sealing performance well under low temperature condition, and a few other due to reasons such as unreasonable structure within the medium leakage and leakage phenomenon, serious influence the safety and normal operation of cryogenic equipment, can not meet the requirements of cryogenic equipment.

According to China's development of low temperature device, the increasingly demand for cryogenic valves, so the metal sealing butterfly valve structure improvements, developed a triple eccentric high pure metal sealing butterfly valve, the butterfly valve in any medium is high or low temperatures can meet their needs. Combining with the characteristics of its structure, only make simple introduction on the low temperature performance.

A, the low temperature disc valve sealing performance requirements:

Cryogenic valves there are two main causes of leakage, one is the internal leakage; The second is leakage.

1) valve leakage within the main reason is that seal caused by deformation produced in low temperature condition. When the medium temperature drop to make the material produces phase change caused by the volume change, make originally high sealing surface grinding precision warping deformations caused by poor sealing at low temperature. We've DN250 valve and low temperature test, the medium of liquid nitrogen (196 ℃) butterfly plate material is 1 cr18ni9ti (without low temperature treatment) found that the sealing surface warping deformation around 0.12 mm, this is the main reason for the cause of leakage.

Newly developed variety of butterfly valve sealing instead of cone sealing by the plane. The seat is an oblique cone ellipse seal face, perfectly on the butterfly plate and a round of elastic sealing ring seal. Sealing ring can be radial floating in the butterfly plate groove. When the valve is closed, the elastic sealing ring and short axis of elliptic sealing surface contact first, as the rotation of the valve stem seal ring to push, forcing the elastic ring and long axis of the oblique cone surface contact again, eventually led to the elastic sealing ring and elliptic sealing surface contact. Its sealing is relying on the elastic deformation and reach. So when the valve or butterfly plate deformation in low temperature, will be absorbed by the elastic sealing ring to compensation, won't produce leakage and jammed. Immediately disappear when the valve is opened when the elastic deformation, basic no relative friction in the process of opening and closing, so the long service life.

2) of the valve leakage: one is the valve and pipe flange connection with the payment, due to the connection pad material, connecting bolts, and fitting materials under low temperature shrinkage of sync between relaxation and lead to leakage. So we put the body and the piping connections from flange connection to welding structure, to avoid the leakage in low temperature. The second is stem and packing leakage. Generally most of the filler of the valve adopts the F4, because it from sliding performance is good, small friction coefficient (on friction coefficient of steel f = 0.05 ~ 0.1), but also has unique chemical stability, so widely used. But the F4 also has shortcomings, it is cold flow tendency; Second, linear expansion coefficient is big, produce in the low temperature cold cause leakage, in a large number of ice, the stem to open valve failure. Therefore developed using the cryogenic butterfly valve sealing structure of the advantage of the characteristics of the F4 large expansion coefficient, through to stay on the sidelines of achieve the goal of normal temperature, low temperature can be sealed.

Second, the valve body, valve stem bushing design requirements:

1) cryogenic valves shell structure shape. Material selection of right for reliable valve normal work has very important significance. Compared the structure characteristics of the butterfly valve and stop valve, gate valve, not only avoid the because of irregular shape, shell wall thickness nonuniformity, produce in the low temperature cold, deformation caused by temperature stress, and because the butterfly valve is small in size, body shape about basic yes said, has a smaller thermal capacity and thus; In the cold quantity consumption is small; Shape rules and convenient for the valve of the anti-condensation measures. Such as a newly developed DD363H type disc valve to ensure the reliable use of valve under low temperature, completely according to the particularity of cryogenic valve design and manufacturing, such as: shell material selection has a cubic lattice 1 cr18ni9ti austenitic stainless steel, etc.

2) stem bushing choice: according to customer reflect, some cryogenic valves in operation, the rotation of the valve parts in viscous, occlusion phenomenon occurs frequently, the main reason is: matching material selection is not reasonable, to leave a gap is too small, cold and machining accuracy. In the development of cryogenic valves, adopted a series of measures to prevent the above phenomenon. For example: we under the valve stem, bush chose has a small friction coefficient and self-lubricating properties of SF - 1 type of composite bearing, it can be applied to some special needs of cryogenic valves.

Type metal sealing butterfly valve with what is the feature of some ordinary valve does not have. Especially the small flow resistance, reliable sealing, opening and closing fast, long service life, etc. The company of the triple eccentric metal sealing butterfly valve sealing force comes from the deformation of the elastic ring to seal, and therefore do not need to use medium force, so to do two-way sealing. According to some characteristics of the butterfly valve will be more and more people paid great attention. In the future will have more butterfly valve is applied to the low temperature equipment.
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