What Are The Requirements For Metal Sealing Butterfly Valves In Cryogenic Valves?

- May 18, 2018-

With the rapid development of economy and industrial technology, the ever-changing high-tech and people's demand for valves has made the valve industry face enormous challenges and opportunities. In particular, the use of a butterfly valve in a low-temperature medium not only satisfies the performance of a normal valve under normal temperature conditions, but more importantly, the reliability of the valve seal at low temperatures, the flexibility of the action, and some other aspects of the cryogenic valve. special requirements.

The butterfly valve has a series of advantages such as compact structure, small volume, light weight (compared with the same pressure, the same diameter of the gate valve can be reduced by 40% to 50%), small fluid resistance, rapid opening and closing, and the like, so it is widely used.

However, the number of valves used in some low-temperature devices, such as liquefied natural gas equipment, air separation equipment, and pressure swing adsorption equipment, etc., in the chemical industry is more than 80% of the valves or gate valves, the use of a small number of butterfly valves. The main reason is that the sealing performance of the metal sealing butterfly valve is poor under low temperature conditions, and other phenomena such as internal leakage and external leakage of the medium due to unreasonable structure, etc., seriously affect the safety and normal operation of these cryogenic equipments, and cannot meet the low temperature. Equipment requirements.

According to the continuous development of cryogenic equipment in our country, the requirements for cryogenic valves are increasing day by day. Therefore, the structural improvement of the metal sealing butterfly valve has been developed, and a butterfly valve with high eccentricity and high metal sealing performance has been developed. This type of butterfly valve regardless of whether the medium is high or low temperature To meet their needs.

Now with its structural characteristics, only a brief introduction to low temperature performance.

First, the requirements for the sealing performance of cryogenic disc valves:

There are two main reasons for the leakage of cryogenic valves: one is internal leakage; the other is external leakage.

1) The valve has an inner leakage

The main reason is that the sealing pair is deformed at a low temperature.

When the temperature of the medium drops to a phase change caused by the material, a volume change is caused, so that a sealing surface with a very high grinding accuracy warps and deforms, resulting in poor sealing at a low temperature. We have conducted low-temperature tests on the DN250 valve. The medium is liquid nitrogen (-196°C) and the disc material is 1Cr18Ni9Ti (without low temperature treatment). The warpage deformation of the sealing surface is found to be about 0.12mm, which is the main reason for internal leakage.

The newly developed butterfly valve is changed from a flat seal to a cone seal. The valve seat is an inclined conical ellipse sealing surface, which forms a sealing pair with a perfectly circular elastic sealing ring embedded in the butterfly plate. The seal ring can float radially within the disc groove. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptical sealing surface, and gradually pushes the sealing ring along with the rotation of the valve stem, forcing the elastic ring to contact with the long axis of the inclined conical surface, finally resulting in an elastic sealing ring All contact with the elliptical sealing surface. Its sealing is achieved by deformation of the elastic ring.

Therefore, when the valve body or the butterfly plate is deformed at a low temperature, it will be absorbed by the elastic seal ring, and no leakage or jamming will occur. When the valve net is opened, the elastic deformation immediately disappears, and there is basically no relative friction during the opening and closing process, so the service life is long.

2) The leakage of the valve.

One is that when the valve and the pipeline are flanged, the connection gasket, the connecting bolt, and the connection member are loosened at the low temperature, causing relaxation and leading to leakage. Therefore, we changed the connection between the valve body and the pipeline from a flanged connection to a welded one to avoid low-temperature leakage.

The second is the leakage at the stem and the packing. Most of the valve packing F4, because of its good self-slip performance, friction coefficient (friction coefficient of steel = 0.05 ~ 0.1), but also has a unique chemical stability, it has been widely used.

However, F4 also has its disadvantages. First, the cold flow tends to be large; second, the coefficient of linear expansion is large. Cold shrinkage at low temperatures leads to leakage, resulting in a large amount of ice at the valve stem and failure of the valve opening. The cryogenic butterfly valve developed for this purpose adopts a self-shrinking seal structure that utilizes the characteristics of a large F4 expansion coefficient, and can achieve sealing at room temperature and at low temperatures by maintaining a gap.

Second, valve stem bushing, valve body design requirements

1) Selection of stem bushings.

According to the user's feedback, some low-temperature valves are in operation, and viscous and occlusal phenomena occur in the rotating parts of the valve. The main reasons are: improper selection of mating materials, too small cooling gap, and processing accuracy.

In the development of cryogenic valves, a series of measures were taken to prevent the above phenomena. For example, we have selected the SF-1 type composite bearing with low friction coefficient and self-lubricating performance for the upper and lower bushings of the valve stem, which can be applied to some special requirements of cryogenic valves. Metal-sealed butterfly valves have characteristics that some ordinary valves do not have. In particular, small flow resistance, reliable sealing, rapid opening and closing, and long service life.

2) Low temperature valve housing structure shape.

Whether the choice of material is correct or not has extremely important significance for the normal and reliable operation of the valve. The structural characteristics of the butterfly valve compared with the cut-off valve and the gate valve not only avoid the deformation due to irregular shape, uneven shell wall thickness, cold shrinkage and temperature difference stress at low temperature, but also due to the small size of the butterfly valve body. The shape is basically right and left, so the heat capacity is small; the consumption of the precooling quantity is also small; the shape rule makes it easy to keep the valve cold. For example, the new DD363H type disc valve developed by Shanghai Vivendi Valve is designed and manufactured according to the particularity of the cryogenic valve to ensure the reliable use of the valve at low temperatures. For example, the shell material is 1Cr18Ni9Ti with a cubic lattice. Body stainless steel and so on.