Metal sealing in the application of low-temperature equipment
With the rapid development of industrial technology, we make more stringent requirements for the valve industry, especially butterfly valve in the use of low-temperature medium, in addition to meet the general valves performance under normal temperature state, more importantly, at low temperatures, the valve sealing reliability, movement flexibility and a number of other low-temperature valve special requirements.
Because the valve has compact structure, small size, light weight (compare with the same pressure, same diameter gate valve, it reduces 40% ~ 50%), small fluid resistance, open and close rapidly and a series of advantages, so it is widely used . But in some low-temperature devices, such as burning liquid equipment, air separation equipment, and pressure swing adsorption equipment, and other chemical industry, more than 80% use globe valve or gate valve, using a small number of butterfly valves. The main reason is the metal seal butterfly valve sealing performance is not well at low temperature, as well as medium inner leakage and outer leakage phenomenon caused by some irrational structures and other reasons, seriously affect the safety of these low-temperature equipment, and normal operation, can not meet the cryogenic equipments requests.
According to continuous developments of low-temperature devices in our country, the requirements of low-temperature valve grows more, so improve the metal sealing butterfly valve structure, develop three eccentric pure metals high sealing performance butterfly valve, this butterfly valve can meet their needs whether the medium is high or low temperatures. Now combined with its structural characteristics, we only make a brief introduction of the low-temperature performance.
一、requirements to low-temperature butterfly valve sealing performance:
low-temperature valve occurs leakage mainly with two reasons, one is internal leakage; second is outleakage.
1) The valve inner leakage mainly due to the valve seal pair occurs deformation at low temperatures. When the medium temperature dropped to make the material produce phase change and causes volume change, so the original high grinding precision sealing surface produces warpage and causes low-temperature seal not well. We have taken low-temperature test to DN250 valves, medium is liquid nitrogen (-196 ℃) butterfly plate material is 1Cr18Ni9Ti (not subject to low temperature treatment) found that the sealing surface warpage amounted to about 0.12mm, which is the main reason resulting in leakage .
The newly developed butterfly valve replaces flat sealing by cone sealing. the seat is an oblique cone elliptical sealing surface, and composes sealing pair with circular elastic sealing ring embedded on disc. Sealing ring can float in radial butterfly plate tank inner diameter. When the valve is closed, flexible sealing ring first connects with the ellipse sealing surface short axis, with the stem rotating, pushing the sealing ring gradually inward, forcing elastic ring to connect the long axis oblique cone surface again, eventually leading to elastic sealing ring contacts with oval sealing surface completely. Its sealing is achieved depends on elastic sealing ring deformation. So when the butterfly valve or plate occurs deformation at low temperatures, will be flexible to absorb the compensation by the seal ring, will not produce the phenomenon of leakage and stuck. When the valve opens, the elastic deformation disappears, in the hoist process there is basically no relative friction, so the service life is long.
2) Valve out-leakage: first is when valves and pipes use flange connections, because connections litter, connecting bolts, and connectors under low temperature between the materials are not synchronized contraction and relaxation so lead to a leakage. So we put the body and pipes connecting mode by welded structure, instead of the flange connection, avoiding the low-temperature leakage. The second is the stem and packing leakage. Generally the majority of the valve packing adopts F4, because of its good performance from the sliding friction coefficient (for steel friction coefficient f = 0.05 ~ 0.1), and also has a unique chemical stability, so widely used. But the F4 is also inadequate, first is cold flow tendency is large; second is large linear expansion coefficient, at low temperatures lead to leakage resulting shrinkage, resulting in a large number of frozen in stem and lead to the valve open malfunction. To this reason, developing low-temperature butterfly valve uses self-shrinkage seal structure,ie, using the large expansion coefficient of F4, through the reserved gap to reach purposes of room temperature, low temperature can be sealed.
二、Valve body, stem bushing design requirements
1) Low-temperature valve shell structure shape. Whether choose the correct material plays an improtant role in valves normal and reliable works. The structural characteristics of butterfly valves compare with globe valve, gate valve, not only avoid a result of irregular shape, uneven shell thickness, at low temperatures resulting from shrinkage, thermal stress caused the deformation, but also because small size, body shape is basically balanced, and thus heat capacity is small; the consumption of cooling capacity is also small; shape rules would also facilitate the valve cold measures. If the new model developed DD363H butterfly valve ensures reliable use at low temperatures, designed and manufactured fully according to cryogenic valves particularity, such as: shell material selects a cubic lattice of 1Cr18Ni9Ti austenitic stainless steel, etc.
2) Stem Bushing choice: according to the user reflection, and during the running, some low-temperature valve rotating parts occurr viscous, occlusal phenomena have always occurred, mainly due to: unreasonable matching material selection, reserved cold space is too small and processing precision and other reasons. In the development of low-temperature valve, and take a series of measures to prevent the aboved phenomenon. For example: we select SF-1-type composite bearings with small friction coefficient and self-lubricating properties for the upper and lower bushing of the stem, so that can be applied to some low-temperature special needs.
Metal sealed butterfly valve has the features that ordinary valves can not possess. In particular, flow resistance is small, sealing is reliable, open and close rapidly, and long service life, etc. The company develops three eccentric metal sealed butterfly valve sealing ability comes from the elastic deformation to achieve sealing, thus do not need medium force, it can be done with a two-way sealing. According to some characteristics of butterfly valves will be more paid attention to by people. In the future there will be more butterfly valves that are applied to the low-temperature devices.
