What are the requirements and applications of metal sealed butterfly valves under low temperature conditions? With the rapid development of economy and industrial technology, the ever-changing high-tech and people's demand for valves, the valve industry is facing great challenges and opportunities. In particular, the use of butterfly valves in low-temperature media, in addition to meeting the performance of general valves at normal temperature, more importantly, the reliability of valve seals at low temperatures, the flexibility of action and some other special requirements for low-temperature valves.

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

However, more than 80% of the valves used in some low-temperature devices in China, such as Tianyan liquefaction equipment, air separation equipment and pressure swing adsorption equipment, are still globe valves or gate valves, and the number of butterfly valves is very small. The main reason is the poor sealing performance of the metal sealed butterfly valve at low temperature, and some other reasons caused by unreasonable structure and other media leakage and leakage phenomenon, seriously affecting the safety and normal operation of these low-temperature equipment, can not meet the requirements of low-temperature equipment.

According to the continuous development of low-temperature devices in China, the requirements for low-temperature valves are increasing, so the metal sealed butterfly valve is improved on the structure, and a three-eccentric pure metal butterfly valve with high sealing performance is developed, which can meet its needs whether the medium is high temperature or low temperature.

Combined with its structural characteristics, only the low temperature performance is briefly introduced.

First, requirements for sealing performance of low-temperature disc valves

There are two main reasons for low-temperature valve leakage, one is internal leakage; The second is external leakage.

1) The valve produces internal leakage

The main reason is the deformation of the sealing pair at low temperature.

When the temperature of the medium drops to cause the material to undergo phase change, the volume changes, and the sealing surface with high grinding accuracy is warped and deformed, resulting in poor low temperature sealing. The low-temperature test of DN250 valve was carried out, and the medium was liquid nitrogen (-196℃) and the butterfly plate material was 1Cr18Ni9Ti(without low temperature treatment). It was found that the buckling deformation of the sealing surface was about 0.12mm, which was the main cause of internal leakage.

The butterfly valve is changed from plane seal to cone seal. The valve seat is an oblique conical elliptic sealing surface, and the circular elastic sealing ring embedded on the butterfly plate constitutes a sealing pair. The sealing ring can float radially in the slot of the butterfly plate. When the valve is closed, the elastic sealing ring first contacts the short axis of the elliptic sealing surface, and gradually pushes the sealing ring inward with the rotation of the valve stem, forcing the elastic ring to contact the long axis of the inclined conical surface, and eventually the elastic sealing ring contacts the entire elliptic sealing surface. Its sealing is achieved by the deformation of the elastic ring.

Therefore, when the valve body or the disc is deformed at low temperatures, it will be absorbed and compensated by the elastic sealing ring, and there will be no leakage and stuck phenomenon. When the valve net is opened, this elastic deformation immediately disappears, and there is basically no relative friction in the process of opening and closing, so the service life is long.

2) Leakage of the valve

The first is that when the valve and the pipeline are connected by flanges, the leakage is caused by the non-synchronous contraction between the connecting gaskets, connecting bolts, and connecting parts at low temperature. Therefore, we changed the connection mode between the valve body and the pipeline from flange connection to welding structure to avoid low temperature leakage.

The second is leakage at the stem and packing. Generally, most valve packing uses F4, because of its good self-sliding performance, small friction coefficient (friction coefficient of steel f=0.05 ~ 0.1), and has a unique chemical stability, so it is widely used.
But F4 also has some shortcomings. First, the tendency of cold flow is large; Second, the linear expansion coefficient is large, resulting in cold shrinkage at low temperatures leading to leakage, resulting in a large amount of ice at the valve stem, so that the valve fails to open. The low-temperature butterfly valve developed for this purpose adopts a self-shrinking sealing structure, which takes advantage of the large F4 expansion coefficient and achieves the purpose of sealing at both room temperature and low temperature through the reserved gap.

2. Design requirements of valve body and stem bushing

1) Structure shape of low-temperature valve shell

The correct selection of materials is of great significance to the normal and reliable operation of the valve. Compared with the globe valve and gate valve, the structure of butterfly valve not only avoids the deformation caused by the cold contraction and temperature difference stress caused by the irregular shape, uneven shell wall thickness at low temperature, but also because the butterfly valve is small in size and the valve body shape is basically equal, so the heat capacity is small; Cooling capacity consumption is also small; The shape is regular and easy to keep the valve cold. For example, the DD363H type disc valve newly developed by Shanghai Vivendi Valve is designed and manufactured in full accordance with the particularity of the low-temperature valve in order to ensure the reliable use of the valve at low temperatures, such as: the shell material selected is 1Cr18Ni9Ti austenitic stainless steel with a cubic lattice.

2) Selection of stem bushing

In the operation of some low-temperature valves, the rotation part of the valve is sticky, and the occlusion phenomenon occurs from time to time, mainly because of unreasonable selection of matching materials, too small reserved cold gap, and processing accuracy.

In the development of low temperature valves, a series of measures have been taken to prevent the above phenomenon. For example, the SF-1 composite bearing with small friction coefficient and self-lubricating performance is selected for the upper and lower bushings of the valve stem, which can be suitable for some special needs of low-temperature valves. The metal sealed butterfly valve has characteristics that some ordinary valves do not have. In particular, the flow resistance is small, the sealing is reliable, the opening and closing are fast, and the service life is long.