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Analysis and improvement of flange leakage in high pressure ball valve

Aiming at the middle flange leakage problem of high pressure three piece ball valve in the process of manufacture and use, this paper analyzes and discusses the structure characteristics, test method and field use of the valve, and puts forward the solutions.

In the hydrotreating process of refining and chemical units, the valves used are large diameter and high pressure metal sealed ball valves. Due to the particularity of working conditions, the research and development and manufacturing process are very difficult. Considering the safety of the device operation, the high-pressure ball valves are forgings, and castings are rarely used. At the same time, considering the cost, most of them adopt the three piece structure, and the top cylinder press with high efficiency is used for pressure test. However, flange leakage often occurs in the process of manufacturing and operation, which has a great impact on the reliability of production and the safe and stable operation of the device.

Problem analysis 

Because of the high test pressure, the claw of claw press can not bear the high test pressure, and the valve space is limited, the claw can not be inserted into the end flange, so it is less used in the test of high pressure ball valve. In the way of pressure test, two typical test methods are often used: top cylinder press and blind plate pressure test (Fig. 1).

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Figure 1 pressure test mode of typical ball valve
With the characteristics of high efficiency and flexible implementation, the top cylinder press is widely used at home and abroad. Its working principle is to apply axial thrust on the 2 ends of the valve to hold the valve tightly. However, the application of thrust on the top cylinder often depends on the experience of the operators, and there is no unified standard and regulation, which will have a certain impact on the seal of the middle flange or even damage it.
In the process of machining, the inner hole of the forged two-piece valve body is a small hole which is smaller than the diameter of the end flow channel and has machining allowance. It is not a step type with large middle body and small flow channel. Therefore, the processing capacity of the inner cavity of the valve body is large and the processing cost is high. Therefore, considering the processing cost, three piece valve body structure is usually used in production. Because the valve structure has a certain length, if the middle flanges of the left and right valve bodies meet the strength requirements, the length left for the middle body is short, which often leads to insufficient space left for the valve stem, and interference problems occur in the threaded holes of the left and right valve bodies. In product manufacturing, the method of weakening the flange thickness of the left and right valve bodies is often used to increase the space of the middle body stem. Therefore, the thickness of middle flange is reduced.

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Fig. 2 typical flange sealing form
In the test, it is found that when the middle flange is of spiral wound gasket sealing structure (Fig. 2a), the top cylinder press will play an auxiliary sealing role in the middle flange, which will not leak in the first test. When the axial force of the top cylinder is unloaded, the middle flange screw will relax, and it will leak when it is tested again.
When the middle flange is of octagonal ring sealing structure (Fig. 2b), there is a gap between the valve body and the valve cover. Under the action of the axial force of the top cylinder, due to the limited displacement of the octagonal ring, the middle flange will warp when the thickness of the middle flange is thin, resulting in the leakage of the middle flange when using the top cylinder test press for pressure test. In the whole test process, the middle flange bolt does not bear the axial tensile force produced by the internal pressure of the shell, so it is impossible to check whether the strength of the middle flange bolt meets the pressure requirements.
The blind plate pressure test method can test the stud strength and sealing surface leakage of the middle flange, and will not have any external force damage effect on the middle flange, but it has the disadvantage of low efficiency in the test process. At the same time, the valve in the pipeline not only bears axial pressure, or in the pipeline extrusion, bending and other complex stress state, so the blind plate pressure test method can not really reflect the actual use situation, so the valve assembly to the pipeline, use for a period of time, the phenomenon of leakage.


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Figure 3 distribution of valve body and bonnet of ball valve
In terms of valve structure, it is generally considered that the cost of three piece ball valve is lower than that of two piece ball valve. In its cost accounting, it is found that when the stainless steel valve body is used, the price of high-performance studs for medium flange accounts for a large proportion, and the number of bolts for three piece ball valve is twice that for two piece ball valve. Moreover, in the design specifications and standards, it is pointed out that the wall thickness of the three piece ball valve body should be based on the inner diameter of the middle body, and the space occupied by the stud should be deducted, which causes the middle body to be too large. Therefore, in order to save the processing cost of the middle body, the overall cost is increased. At the same time, if the flange in the designed valve structure is too thin, it is not coordinated as a whole and lacks aesthetic feeling (Fig. 3b, d).
When the two-piece structure is adopted, the thickness of the middle body is based on the diameter of the end flange flow channel, the outer circle thickness of the valve body will be greatly reduced, the thickness space left for the middle flange will be increased, the strength of the middle flange will be increased, the cost will be reduced, and the shape of the valve body will become beautiful (Fig. 3a, c). The structure of the two-piece ball valve body is continuous, so the thickness of the middle body meets the strength requirements. Therefore, the use of three chip is not in line with the optimal economic and application performance requirements.

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Fig. 4 test of socket type cylinder jacking press (improved)
In terms of pressure test, blind plate test can be used to test the strength of flange bolts, but the efficiency is low, so it is only used when there is a lack of large diameter pressure test equipment. Combined with the two test methods, the pressure test blind plate is improved. Instead of sealing with the end face of flange sealing surface, the pressure test blind plate is sealed with boss corresponding to different flow channel specifications and wrapped with O-ring (Fig. 4). The top cylinder test press will no longer completely force the two end flange faces, and will not damage the middle flange strength due to improper test method. Meanwhile, the stud strength and middle flange strength of the middle flange can also be tested in the shell strength test. In the aspect of valve structure, the valve body adopts two pieces instead of three pieces. Under the premise of limited structure length, it can ensure that the middle flange has enough strength. When octagonal ring seal structure is used, it will not be deformed due to pressure, or triangular ring without seam treatment is used to avoid flange warping or deformation.

Concluding remarks

No matter from the point of view of reliability or safety, the ball valve should adopt two-piece body structure. The pressure test method can neither assist the sealing of middle flange nor destroy the strength of middle flange. By adjusting the valve body structure and improving the pressure test device, the work efficiency is provided, the product quality is improved, and the reliable and stable operation of the refining and chemical device is ensured.
Authors: Dou Wenjiao, sun Fengwei, Jiang Xianxian, Chen Zhenxing

Source: Best china ball valve supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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