Applications of Metal Seal Butterfly Valves
Here are the applications of the metal seal butterfly valve in low-temperature equipment.
With the rapid development of industrial technology, we put forward more strict requirements on the valve industry, especially for the low-temperature medium used in butterfly valves. Generally, it is more important in the condition of the sealing reliability of valves in low temperatures, the flexibility of action, and some other special requirements of cryogenic valves.
As we all know, the butterfly valve has a compact structure and small volume. The weight is light under the same pressure. The gate valve of the same size can reduce 40% to 50% fluid resistance, and the opening and closing are quick and other a series of advantages. However, more than 80% of the valves used in the chemical industry of some low-temperature devices such as gas liquefaction equipment, globe valves, or gate valves are used in the air separation equipment and pressure swing adsorption equipment. The number of butterfly valves is small. The main reason is that the metal sealing butterfly valve has poor sealing performance at low temperatures. Some other reasons such as unreasonable structure cause medium internal leakage and external leakage, which seriously affects the safety and normal operation of this low-temperature equipment and cannot meet the requirements of low-temperature equipment.
According to China's development of a low-temperature device, the demand for cryogenic valves is increasing. In order to adapt to the needs of the development of the market economy, the structure of the metal sealing butterfly valve is improved and we developed a high sealing performance of the triple eccentric pure metal butterfly valve ( It has been declared a national patent). Regardless of high or low temperature, their needs can be met. Combined with its structural characteristics, low-temperature performance is simply introduced.
1. The sealing performance requirements of low-temperature butterfly valves
There are two main reasons for the leakage of the low-temperature valve. One is internal leakage and the other is outer leakage.
1) The main reason for the leakage of the valve is the deformation of the sealing pair at low temperatures. When the medium temperature drops to the material phase change caused by volume change, the original grinding precision of the sealing surface warping deformation results in poor sealing at low temperatures. We have carried out a low-temperature test on DN250 valves. The medium is liquid nitrogen (-196℃) and the butterfly plate material is 1Cr18Ni9Ti (without low-temperature treatment). It is found that the warp-deformation of the sealing surface is about 0.12mm, which is the main reason for internal leakage.
The newly developed butterfly valve is changed from a plane seal to a conical seal. The seat is a tapered oval sealing face. A circular elastic sealing ring embedded in the butterfly plate can form a sealing pair. The seal ring can float radially in the disc groove. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptic sealing surface. With the rotation of the valve stem, the sealing ring is gradually pushed inward, forcing the elastic ring to contact the long axis of the oblique conical surface. It eventually leads to full contact between the elastic sealing ring and the elliptic sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the body or butterfly plate is deformed at low temperature, it will be absorbed and compensated by the elastic sealing ring, and will not produce leakage and stuck phenomenon. This elastic deformation disappears immediately when the valve is opened. There is basically no relative friction in the opening and closing process, so the service life is long.
2) The outer leakage of the valve: one is the valve and the pipeline using a flange connection. As the connection pad and connecting bolts and parts in low temperature between the material contraction are not synchronized, the outer leakage is produced. Therefore, we changed the connection mode of the valve body and pipeline from flange connection to welding structure to avoid low-temperature leakage. The other is the leakage of stem and packing. Generally, most of the valve packing is F4, because of its good self-sliding performance and small friction coefficient (friction coefficient of steel F =0.05 ~ 0.1). It has unique chemical stability, so it has been widely used. However, F4 also has shortcomings. First, the cold flow tendency is large. Second, the coefficient of linear expansion is large, resulting in cold shrinkage leakage at low temperatures. With a large number of icing at the stem, we make the valve open failure. The low-temperature butterfly valve developed for this purpose adopts a self-shrinking seal structure, that is, it can seal at both normal temperature and low temperature through the gap left by taking advantage of the large expansion coefficient of F4.
2. Design requirements for the valve body and stem bushing
1) The shell structure shape of low-temperature valves
The correct choice of material is very important to the normal and reliable operation of the valve. Compared with globe valves and gate valves, butterfly valves not only can avoid deformation due to the irregular shape, uniform shell wall thickness, and cold contraction at low temperatures. As the butterfly valve size is small, the shape of the body is basically symmetrical, the heat capacity is small. The cooling consumption is also small. The regular shape facilitates the cooling of the valve. For example, the newly developed DD363H disc valve is designed and manufactured in accordance with the particularity of the low-temperature valve in order to ensure the reliable use of the valve at low temperatures. For example, the shell material is 1Cr18Ni9Ti austenitic stainless steel with a cubic lattice.
2) The selection of bushing of valve stems
According to the user's reflection, during some low-temperature valves in operation, the rotating part of the valve occurs sticky, which leads to the occlusal phenomenon occurring from time to time. The main reasons are as follows. The selection of paired materials is unreasonable. The remaining cold gap is too small, and the processing accuracy is not accurate.
In the development of low-temperature valves, a series of measures have been taken to prevent the above phenomenon. For example, we choose SF-1 composite bearings with the low friction coefficient and self-lubricating properties for upper and lower stem bushing, which can be suitable for some special needs of low-temperature valves.
The metal seal butterfly valve has the characteristics some ordinary valves do not have, like small flow resistance, reliable sealing, rapid opening and closing, long service life, etc. The sealing force of the three-eccentric metal seal butterfly valve developed by our company comes from the deformation of the elastic ring to achieve the seal, so it does not need to use the force of the medium. It can be used as a two-way seal. In the future, there will be more butterfly valves applied to low-temperature equipment.
With the rapid development of industrial technology, we put forward more strict requirements on the valve industry, especially for the low-temperature medium used in butterfly valves. Generally, it is more important in the condition of the sealing reliability of valves in low temperatures, the flexibility of action, and some other special requirements of cryogenic valves.
As we all know, the butterfly valve has a compact structure and small volume. The weight is light under the same pressure. The gate valve of the same size can reduce 40% to 50% fluid resistance, and the opening and closing are quick and other a series of advantages. However, more than 80% of the valves used in the chemical industry of some low-temperature devices such as gas liquefaction equipment, globe valves, or gate valves are used in the air separation equipment and pressure swing adsorption equipment. The number of butterfly valves is small. The main reason is that the metal sealing butterfly valve has poor sealing performance at low temperatures. Some other reasons such as unreasonable structure cause medium internal leakage and external leakage, which seriously affects the safety and normal operation of this low-temperature equipment and cannot meet the requirements of low-temperature equipment.
According to China's development of a low-temperature device, the demand for cryogenic valves is increasing. In order to adapt to the needs of the development of the market economy, the structure of the metal sealing butterfly valve is improved and we developed a high sealing performance of the triple eccentric pure metal butterfly valve ( It has been declared a national patent). Regardless of high or low temperature, their needs can be met. Combined with its structural characteristics, low-temperature performance is simply introduced.
1. The sealing performance requirements of low-temperature butterfly valves
There are two main reasons for the leakage of the low-temperature valve. One is internal leakage and the other is outer leakage.
1) The main reason for the leakage of the valve is the deformation of the sealing pair at low temperatures. When the medium temperature drops to the material phase change caused by volume change, the original grinding precision of the sealing surface warping deformation results in poor sealing at low temperatures. We have carried out a low-temperature test on DN250 valves. The medium is liquid nitrogen (-196℃) and the butterfly plate material is 1Cr18Ni9Ti (without low-temperature treatment). It is found that the warp-deformation of the sealing surface is about 0.12mm, which is the main reason for internal leakage.
The newly developed butterfly valve is changed from a plane seal to a conical seal. The seat is a tapered oval sealing face. A circular elastic sealing ring embedded in the butterfly plate can form a sealing pair. The seal ring can float radially in the disc groove. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptic sealing surface. With the rotation of the valve stem, the sealing ring is gradually pushed inward, forcing the elastic ring to contact the long axis of the oblique conical surface. It eventually leads to full contact between the elastic sealing ring and the elliptic sealing surface. Its sealing is achieved by the deformation of the elastic ring. Therefore, when the body or butterfly plate is deformed at low temperature, it will be absorbed and compensated by the elastic sealing ring, and will not produce leakage and stuck phenomenon. This elastic deformation disappears immediately when the valve is opened. There is basically no relative friction in the opening and closing process, so the service life is long.
2) The outer leakage of the valve: one is the valve and the pipeline using a flange connection. As the connection pad and connecting bolts and parts in low temperature between the material contraction are not synchronized, the outer leakage is produced. Therefore, we changed the connection mode of the valve body and pipeline from flange connection to welding structure to avoid low-temperature leakage. The other is the leakage of stem and packing. Generally, most of the valve packing is F4, because of its good self-sliding performance and small friction coefficient (friction coefficient of steel F =0.05 ~ 0.1). It has unique chemical stability, so it has been widely used. However, F4 also has shortcomings. First, the cold flow tendency is large. Second, the coefficient of linear expansion is large, resulting in cold shrinkage leakage at low temperatures. With a large number of icing at the stem, we make the valve open failure. The low-temperature butterfly valve developed for this purpose adopts a self-shrinking seal structure, that is, it can seal at both normal temperature and low temperature through the gap left by taking advantage of the large expansion coefficient of F4.
2. Design requirements for the valve body and stem bushing
1) The shell structure shape of low-temperature valves
The correct choice of material is very important to the normal and reliable operation of the valve. Compared with globe valves and gate valves, butterfly valves not only can avoid deformation due to the irregular shape, uniform shell wall thickness, and cold contraction at low temperatures. As the butterfly valve size is small, the shape of the body is basically symmetrical, the heat capacity is small. The cooling consumption is also small. The regular shape facilitates the cooling of the valve. For example, the newly developed DD363H disc valve is designed and manufactured in accordance with the particularity of the low-temperature valve in order to ensure the reliable use of the valve at low temperatures. For example, the shell material is 1Cr18Ni9Ti austenitic stainless steel with a cubic lattice.
2) The selection of bushing of valve stems
According to the user's reflection, during some low-temperature valves in operation, the rotating part of the valve occurs sticky, which leads to the occlusal phenomenon occurring from time to time. The main reasons are as follows. The selection of paired materials is unreasonable. The remaining cold gap is too small, and the processing accuracy is not accurate.
In the development of low-temperature valves, a series of measures have been taken to prevent the above phenomenon. For example, we choose SF-1 composite bearings with the low friction coefficient and self-lubricating properties for upper and lower stem bushing, which can be suitable for some special needs of low-temperature valves.
The metal seal butterfly valve has the characteristics some ordinary valves do not have, like small flow resistance, reliable sealing, rapid opening and closing, long service life, etc. The sealing force of the three-eccentric metal seal butterfly valve developed by our company comes from the deformation of the elastic ring to achieve the seal, so it does not need to use the force of the medium. It can be used as a two-way seal. In the future, there will be more butterfly valves applied to low-temperature equipment.
