Cryogenic butterfly valve is mainly used in liquefied natural gas and liquid nitrogen pipeline system, the control pipe opening and closing or regulating equipment. In view of the domestic market demand for cryogenic valve and its basically depends on the status of the imported products, developed a new type of cryogenic butterfly valve. Products adopted double eccentric structure and elastic metal seal structure, the double eccentric structure can greatly eliminate butterfly valve opening and closing butterfly plate and the sealing ring in the process of unnecessary excessive extrusion and the phenomenon such as scratches, elastic metal seal structure can effectively impact deformation compensation temperature change, the application of two kinds of structure can effectively improve the working condition of cryogenic butterfly valve sealing performance.
2 design
(1) butterfly plate
Butterfly plate for part of the sphere structure of double eccentric butterfly plate, its basic structure is shown in figure 1, point O is parallel to the axis of rotation by adjusting the cricket heart straight line perpendicular to the line of intersection point on the cutting plane, points O for double eccentric butterfly plate on the cutting plane intersection of rotational axis, A, B, C and D as the butterfly plate edge points on the cutting plane, e, C respectively with some point O O radial, axial eccentricity, butterfly plate is located on the circular radius R of the cutting plane, t, s respectively and points O AD, while the distance between the BC, alpha, beta, and lambda is corresponding Angle. Butterfly plate according to the vertical plane of rotation axis section are shown in figure 1 (b), figure 1 (c) two kinds of circumstances, figure 1 (b) as the butterfly plate in the middle section of the seal, figure 1 (c) as the butterfly plate rotating shaft seal near both ends up and down.
According to the structure characteristics of double eccentric butterfly valve and its effect on the double eccentric, figure 1 A, B, C and D in the rotation of the edge points radius R1, R2, R3 and R4 must satisfy the R1 > R2, R3 > R4 relative relationship was established at the same time. According to the geometric drawing double eccentric butterfly plate can be determined on the axis of rotation point O suitable area, as shown in figure 1 (a).
Axis of rotation (a) suitable for area (b) (c) butterfly plate in the middle period of the geometric relationship and limit position of butterfly plate geometry relationship
Figure 1 double eccentric butterfly plate structure
In determining the double eccentric butterfly plate axis of rotation is suitable for areas, on the basis of the rotation radius of butterfly plate edge point R1, R2, R3 and R4 for further calculations, when to figure 1 (b), there is
（1）
式中，
，对于c＞ 0或c＜0均适用。
（2）
式中
（3）
（4）
As shown in figure 1 (c) the seal is shown in figure 1 (b) the sealing situation of special circumstances, the edge points b, c, R2 = R3.
（5）
The calculation formula of R2 and R3 (5) is a type (2) and (3) the simplified form, that is the special case of the alpha 2 to 0. So for two kinds of double eccentric butterfly plate sealed case calculation formula of the rotation radius of the edge point is unified.
Through the analysis of double eccentric butterfly plate geometric relations, in determining the double eccentric butterfly plate axis of rotation is suitable for areas, on the basis of combining the seal compression in the sealing structure and comprehensive consideration of the double eccentric butterfly plate produced by additional factors such as hydrostatic torque, using type (1) to (5) to butterfly plate edge sealing surface limit position difference R1, R2, R3, R4 numerical calculation, such as a double eccentric butterfly plate structure can be set for the appropriate size.
(2) the sealing structure
Under cryogenic conditions, soft sealing material as well as the cold short happens, disable seal structure. Cryogenic butterfly valve adopts the elastic metal seal structure (figure 2), by adjusting the control pressure ring bolt position adjust the amount of compression elastic metal sealing ring in order to meet the sealing requirements. Elastic metal seal for key parts, it is composed of the outer, inner, and the internal spring (figure 3), the main material of the outer for copper, has a good hard seal performance, the inner main material for stainless steel for cryogenic application environment, guarantee the seals has certain rigidity. Internal spring make seal has a degree of elasticity that is a good way to reduce the temperature change of material heat bilges cold shrink of the impact on the sealing performance.
3. (1) butterfly plate (2) body retaining ring 4. Adjust the bolt
5. 6. Positioning plate pressure ring 7. The elastic metal sealing ring
Figure 2 elastic metal seal structure
Outer 2) inner layer (3) spring
FIG. 3 the elastic metal sealing ring structure
(3) selection of materials
Under the condition of low temperature, increased strength and hardness of the metal material, plasticity and toughness is reduced, showing a different degree of low temperature cold short phenomenon, the serious influence to the butterfly valve performance and security. In order to prevent the brittle fracture of materials at low temperature, the valve body and disc pressure parts such as austenitic stainless steel (such as 304 l, 304, 316 l, etc.), including the stability of the 316 l is best, no obvious low temperature cold short critical temperature, under the condition of low temperature, still can maintain a higher toughness, so the valve body and disc pressure parts such as selection of 316 l material
3 test
Reference BS6364, JB/T7749 cryogenic valves test standards, set up a cryogenic butterfly valve test unit (figure 4), through the pressure reducing valve and globe valve combined operation of butterfly valve sealing performance test. In the process of test, by thermocouple monitoring body, cover and temperature in areas such as the butterfly plate, to fulfill the requirements of test temperature, cryogenic helium test medium for butterfly valve sealing performance test.
1. Helium bottle 2. Cut-off valve 3. 4. The relief valve pressure gauge
Measured valve 5. 6. 7. Alcohol bottle flow meter
Figure 4 butterfly valve principle of low temperature test device
In double eccentric butterfly plate, seal structure and valve selection of main parts material, on the basis of research achievements, 12 - PN10 cryogenic double eccentric butterfly valve was developed, and in the test device using helium separately under the working condition of normal temperature and liquid nitrogen for the valve sealing performance test. The experimental results showed that 12 - PN10 cryogenic double eccentric butterfly valve leakage of room temperature is less than 16 ml/min. In liquid nitrogen temperature conditions, the valve leakage is less than 100 ml/min.
4 conclusion
Through the study on the relationship between the double eccentric butterfly plate geometry, this paper puts forward the numerical calculation formula, combined with the seal compression in the sealing structure and comprehensive consideration of the double eccentric butterfly plate factors, such as additional hydrostatic torque can better determine the structure of the double eccentric butterfly plate size. The elastic metal seal structure can reduce the temperature change of material well heat bilges cold shrink of the impact on the sealing performance. Selection of 316 l material as the valve body, disc, etc material applied in cryogenic condition is suitable. Developed at the 12 - PN10 cryogenic double eccentric butterfly valve sealing performance reached the ANSI/FCI70-2 level Ⅵ and JB/T7749 cryogenic butterfly valve for sealing performance requirements