This paper introduces the requirement and application of metal seal butterfly valve in low temperature valve

This paper introduces the requirement and application of metal seal butterfly valve in low temperature valve

With the rapid development of modern petrochemical industry, coal chemical industry and refrigeration industry, the use of low temperature valve is more and more, the dosage is more and more. Therefore, the correct and reasonable use of low temperature valve causes people’s attention. The use of low temperature valves in addition to the general use of conventional valves, there are some special requirements to pay attention to.
Cryogenic valves generally refer to valves operating at temperatures lower than -29 ° C. Some cryogenic valves can transport liquefied gases (such as nitrogen, oxygen and natural gas) at temperatures as low as -196 ° C and still ensure normal operation.
With the decrease of the temperature of the working medium, the material and structure of the low temperature valve used are different from the conventional universal valve, especially the temperature valve (t<-100℃), not only the selection of materials should be appropriate, but also have a special structure. In order to reduce the heat flow to the closing parts from the outside, the operating parts should be removed from the insulated body. So the neck of the bonnet of the cryogenic valve is very long. ** The temperature valve is operated under complex conditions from the outside temperature to the temperature. And it does so without interruption. The sealing of the closing part is usually guaranteed by the necessary contact pressure formed on the sealing ring, but if the force of the transmission device is fixed, the strength of the sealing ring may be different in the working temperature range, of course, the performance of the sealing ring material, especially the performance of the polymeric material is different. Therefore, the warm valve has strict requirements on the sealing ring, usually choose hard alloy (STL) material is better.
The temperature valve is quite limited in the opening speed of the shutoff. In spite of the adiabatic measures, there is still heat flow in a limited range, resulting in the formation of liquid phase in some parts of the system. Therefore, when the valve is opened quickly, the liquid may gain a great speed, once encountering obstacles or resistance like the valve will produce a water blow, which is to be paid attention to in use.
Generally used for refrigeration technology, the medium is nitrogen or freon refrigerant, the past commonly used cast iron valve, its applicable temperature is between -28 ~ 150℃, now use less, but the valve with ferritic ductile iron has a growing trend. This is because ferritic ductile iron has better impact toughness, wear resistance and corrosion resistance.
General carbon steel (WCB) material valve, its use temperature is -29 ~ 150℃; There are also low temperature carbon steel (LCB, LC1, LC2), its minimum service temperature is -46 ~ -73℃; Austenitic stainless steel (304, 316) is excellent performance of low temperature resistance steel, its minimum service temperature is below -196℃, some for -254℃.
General low temperature valve closing parts, the selection of metal seal valve seat on the valve body, the selection of fluorine plastic (F4) on the valve disc, there is no special requirements in the structure and the conventional universal valve is almost the same. Reasonable selection according to temperature and medium.
Cryogenic valve, only consider the working characteristics of the temperature valve. It can be installed in any position inside the incubator, but the stuffing box, operating mechanism (hand wheel, wrench) and indicator of valve disc lifting position should be placed outside the incubator. Easy to operate and replace packing. Most valves are installed with the stem horizontally. For warm electric valves, the valve stem must be placed horizontally.
Working conditions of packing device at low temperature are very complex, because packing is easy to lose elasticity, once due to evaporation medium leakage, the stem is frozen, it will lead to stuck phenomenon, the valve can not open and close normally. In order to improve the working conditions of the packing device, a thermal baffle can be installed to ensure the insulation of the packing. This is mainly in the design to increase stem length. The baffle is made of non-metallic materials with the lowest thermal conductivity (cloth, tape, etc.) so that the heat loss of the packing box is significantly reduced. Alternatively, the packing device is jacketed and a medium with a temperature sufficient to maintain the working capacity of the packing device is driven into the space formed. Its FUNCTION IS TO MAKE THE valve from the low temperature to the operating point has a temperature gradient, at a working point and beyond the operating point, the air pressure will be in a certain can make the liquefied gas is no longer liquid and return to the gas temperature.
Air PRESSURE IS ALWAYS OPEN AT ALL TIMES, GENERALLY COMMUNICATED WITH one end OF the VALVE OR WITH the other end, so that if the temperature in the air pressure rises, there will be no dangerously high pressure.
Because the valve parts are usually manufactured and tested at room temperature, and work under low temperature conditions, different parts will produce different expansion and contraction with the change of temperature, which will directly affect the normal work of the valve, which must be paid attention to and understood in use.
Low temperature valves use a variety of materials, ordinary materials: such as carbon steel (WCB) will become brittle as the temperature decreases. The most commonly used materials of choice are: austenitic stainless steel, bronze, and copper-nickel alloys, all of which are non-brittle and can be used properly. Experience shows that austenitic stainless steel, bronze and Cu-Ni alloy are the best materials for toughness and strength. So some important parts of the low temperature valve, such as: globe valve, safety valve, regulating valve, check valve and so on, choose this kind of material.
Production experience tells us that even for austenitic stainless steel, bronze and COPper-nickel alloy, if the finished products are not properly treated with low temperature, when the valve is operated at low temperature, the inner parts will be deformed due to the material phase transformation caused by low temperature, resulting in valve leakage.
In the assembly process of low temperature valve, due to the different materials of flange connection gaskets, connecting bolts and connecting parts, the contraction between various materials will not be synchronized, resulting in relaxation and leakage. Therefore, the low temperature valve, especially the low temperature valve used below -196℃, its connection method is best to use welding connection.
Low temperature valve packing gasket, the general majority of F4, because of its good self-lubrication, friction coefficient is small, and has a unique chemical stability. Therefore, it is used by the government, but F4 also has shortcomings, one is the cold flow tendency is large, the other is the linear expansion coefficient is large, cold shrinkage at low temperature leads to leakage, resulting in a large number of icing at the stem, so that the valve opening failure. Therefore, it is only used in general low temperature conditions. In case of temperature conditions, should choose flexible graphite woven filler, or stainless steel and flexible graphite winding pad. Some low temperature valve in operation, often found that the transmission part of the valve occurs sticky. The occlusal phenomenon occurs from time to time. The main reasons are: unreasonable selection of paired materials, too small reserved cold gap, and machining accuracy. If similar problems are found in the process of use, they should be put forward to the supplier in time to improve the structural design and replace the appropriate bearing (bushing) material.
Other requirements for low temperature valves are the same as conventional valves.
The requirement and application of metal seal butterfly valve in low temperature valve
With the rapid development of economy and industrial technology, the ever-changing high and new technology and people’s demand for valves, the valve industry is facing huge challenges and opportunities. Especially the use of butterfly valve in low temperature medium, in addition to meeting the performance of the general valve at room temperature, more important is the reliability of the valve seal at low temperature, the flexibility of the action and some other special requirements of the low temperature valve.
Butterfly valve has compact structure, small volume, light weight (compared with the same pressure, the same size gate valve can reduce 40% ~ 50%) fluid resistance, opening and closing quickly and a series of advantages, so get the use of.
But in some low temperature devices, such as gas liquefaction equipment, air separation equipment and pressure swing adsorption equipment used in chemical industry is more than 80% or globe valve or gate valve, butterfly valve is small. The main reason is that the metal seal butterfly valve has poor sealing performance at low temperature, and some other reasons such as unreasonable structure cause medium internal leakage and external leakage phenomenon, which seriously affects the safety and normal operation of these low temperature equipment and cannot meet the requirements of low temperature equipment.
According to the continuous development of the low temperature apparatus in our country, the requirement for the low temperature valve is increasing day by day. Therefore, the metal sealing valve has been structural improved, and the butterfly valve of three-eccentric pure metal with high sealing performance has been developed. This butterfly valve can satisfy its requirement no matter the medium is high temperature or low temperature.
Combining with its structural characteristics, the low temperature performance is simply introduced.
First, the low temperature disc valve sealing performance requirements:
There are two main reasons for low temperature valve leakage, one is internal leakage; The second is leakage.
1) The valve produces inner leakage
The main reason is that the sealing pair is deformed at low temperature.
When the medium temperature drops to the material phase change caused by volume change, so that the original grinding precision of the sealing surface warping deformation resulting in poor low temperature seal. We have carried out low temperature test on DN250 valve. 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 sealing surface is about 0.12mm, which is the main reason for internal leakage.
The newly developed butterfly valve is changed from plane seal to conical seal. The seat is a tapered oval sealing face, and a circular elastic sealing ring embedded in the butterfly plate to 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 with the short axis of the elliptic sealing surface, and with the rotation of the valve stem, the sealing ring is gradually pushed inward, forcing the elastic ring to contact with the long axis of the oblique conical surface, eventually leading to the full contact between the elastic sealing ring and the elliptic sealing surface. Its sealing is achieved by 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, and there is basically no relative friction in the opening and closing process, 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 relaxation of the connection pad, the connection bolt, and the connection parts which shrink out of sync between the materials at low temperature. Therefore, we changed the connection mode of valve body and pipeline from flange connection to welding structure to avoid low temperature leakage.
The second is the leakage of stem and packing. In general, 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 unique chemical stability, so it has been 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 temperature, resulting in a large number of icing at the stem, to make the valve open failure. The low-temperature butterfly valve developed for this purpose adopts 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.