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Analysis of Fixed Ball Valve Seal Specific Pressure

July 3, 2018

1 Overview

COSMOSWORKS is a design analysis system fully integrated in SOLIDWORKS. It provides pressure, frequency, constraints, heat, and optimization analysis etc. It provides designers with a relatively complete analysis method in the SOLIDWORKS environment. This article SINOV focuses on the valve seat of the fixed ball valve, because in the engineering design process of the valve seat, the calculation of the total force and the seal specific pressure on the valve seat sealing surface is used in the actual design is an experience or an amendment formula. With the means, these two parameters are not easily measured. The COSMOSWORKS finite element analysis software is used to calculate the seal specific pressure of the fixed ball valve under simulated conditions, and compared with the theoretical calculation formula, it can provide a relatively accurate reference for the design of the valve. 

2 Solid Modeling

The DN239mm, PN25M Pa fixed ball valve is designed as an example. In SOLIDWORKS, the solid parts of the fixed ball valve are first modeled and the parts are assembled (Fig.1). Interference checking is performed on the assembly to obtain an assembly without interference between the parts.

 

Fig. 1 Solid modeling of ball valve 

3 Finite element analysis 

3.1 Sealing mechanism

The fixed ball valve adopts the imported seal (Fig. 2) when the ball valve pressure difference (P – P1) > 0 (P, P1 is the fluid pressure in the front and middle chambers). When the pressure difference is large enough to cause a certain compression ratio pressure on the surface of the sealing sub, the specific pressure will cause the valve seat to be plastically deformed, and the microscopic roughness of the sealing surface will be filled to prevent fluid from passing between the sealing fittings. When the differential pressure is small or the seat is made of metal, reliance on the pressure difference cannot achieve a complete seal. At this point, an external sealing force must be added to increase the specific pressure. The empirical formula for calculating the necessary specific pressure qb for the seal based on the working pressure is qb=112PN=30MPa. In order to ensure the reliable sealing of the ball valve, there should be enough specific pressure on the connecting surface between the ball and the valve seat, but it must not exceed the allowable specific pressure of the sealing auxiliary material [q]. Theoretical seal pressure q1 is

 

Fig. 2 Ball valve sealing principle


In the formula Q—ball valve sealing force, N


P —Nominal pressure, M Pa

S —Seal surface area of the seat (S = 7634), mm2

d2 —Outer Diameter of Seat Ring ( d2 = 290) , mm

D2 —Outer Diameter of Seat (D2 = 270), mm

D1 —The inner diameter of the seat (D1 = 256), mm

Substituting each value, the value of q1 is 40117M Pa. 

3.2 Finite element calculation

Since the calculation of the sealing specific pressure is only related to parts such as ball body, seats, and valve seat rings, its finite element analysis model has been simplified (Fig.3), which not only saves computer resources, but also improves the accuracy of calculation results. By simulating actual conditions, the pressure value Qs of the surface of the seat ring connecting with the left and right body must be calculated.

In the formula,

Q1—Pretightening pressure on the connecting surface of the seat ring and left and right valve body, MPa

Q2 —The nominal pressure on the surface of the seat ring that connects the left and right bodies, MPa

S1 — The area of the seat ring that connects the left and right valve bodies (S1 = 21 189126, directly measured in SOLIDWORKS), mm2

D1 — The inner diameter of the seat ring ( d1 = 239), mm

Substituting each value, the value of Qs is 23101MPa.

 

According to actual conditions, it can establish a static analysis example in COSMOSWORKS, and determine the constraint load conditions(as followed)

 

fixed ball connecting surface with the upper stem and bottom cover.

limited seat and seat support ring can only be in the axial direction

Apply Qs to the surface of the seat ring connecting the left and right valve bodies, and mesh them.

Make sure that the material of the ball body and seat ring is No. 35 steel, and the seat material is Austenite stainless steel, and there is no sliding between sealing surfaces (the allowable specific pressure of austenitic stainless steel [q] is 150MPa). 

Click to run and the result is shown in Figure 3c.

 

(a) Analytical model  (b) Model gridding  (c) Model calculation results

Fig. 3 Finite element analysis

3.3 Extracted results

It can obtain 25 points equidistantly on the sealing surface of the valve seat (Fig. 4), then detect the pressure values of the 25 points in sequence along the X direction, and then import the detection data into Excel for analysis (Fig.5). The results of the sealing specific pressure are parabolically distributed over the seat sealing surface. The maximum value is at the inner diameter of the valve seat, which is 6211MPa. The minimum value is in the middle of the valve seat and its value is 40116 MPa. The design parameters of the valve seal must be combined with the pressure qb (30MPa) and the allowable pressure of the valve seat material [q] (150MPa). The value of the seal specific pressure is between qb and [q], which satisfies the design criteria.

 

Fig. 4  Taking a point on the valve seat

 

Fig. 5  The distribution of the seal specific pressure in the X direction along the seat sealing surface

4 Conclusion

Since the valve seat and the sealing surface of the ball are relatively fixed and cannot be moved during the sealing process, after the pre-tightening force and the fluid pressure are received, the inner diameter of the sealing ring is smaller in axial displacement with respect to the middle, and the pressing force is greater. The radial deformation of the seal ring relative to the center is large, and the pressing force received is also relatively small. In addition, although the ball body is in connecting with the valve seat, there is a fluid medium due to the capillary phenomenon. When the ball body moves relative to the flow direction, the ball body and the valve seat are in close connecting with each other. The fluid medium from the edge of the sealing surface to the middle is getting less, so the edge of the sealing surface is under the double action of the medium and the ball body, and the force is slightly larger than the middle of the sealing surface. Therefore, the specific pressure on the sealing surface is parabolically distributed. At the edge ring surface, due to the combined effect of the preload, the force exerted by the medium and the metal ball body, there is a fluctuating curve, especially at the outer diameter of the sealing surface. Due to the effect of the capillary phenomenon, it is reasonable to take the seal specific pressure at the middle of the seal face as the seal specific pressure of the entire fixed ball valve. According to Fig. 5, the pressure in the middle of the sealing surface is 40116M Pa, and the theoretical calculated seal specific pressure is 40117M Pa. The difference between the two is 0101M Pa and the error is < 3%. It can be seen that the value of the sealing specific pressure of the fixed ball valve obtained by the finite element calculation is credible. And it can fully reflect the distribution of the seal specific pressure on the entire valve seat, thus providing a reliable design basis for the design of the entire valve.


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