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Shenyang Taike Fluid Control Co.,Ltd

Application Of Lip Seal In Fixed Ball Valve

1 Overview

Lipseal (Lipseal) also known as spring storage ring. Because of its superior performance, as "plug" as widely used in various fields, and to achieve sealing function, it is also known as "pan plug ring." The pan collar is made of PTFE, RPTFE or other high-performance polymer. The internal spring can be made of different materials. Due to the resistance to media erosion and aging resistance, the pan plug ring is generally used in special conditions that can not be used on rubber O-rings in the valve industry, such as low temperature and strong corrosive media.


2 performance

Applicable temperature -270 ~ 340 ℃

Applicable pressure 0 ~ 350MPa

Applicable size 0.635 ~ 3000mm

Applicable media Almost all media

This article only discusses the use of a pan seal as a seal on a fixed ball valve seat.


3 sealing principle

When the pan seal is in working condition, the sealing lip and the metal spring are compressed. The spring inside the jacket provides an initial pre-tightening force, resulting in a good sealing effect at low pressure conditions. In addition to the spring load, the media pressure exerts an additional load on the sealing lip. In theory, the higher the media pressure, the better the seal (Picture 1).

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4 structural features

4.1 spring

There are three types of springs in the pan plugs: circular, V-shaped and oval (Picture 2). They are made of alloy steel such as AISI301, Elgiloy, Inconel718 and HastelloyC276. Three kinds of spring load and compression rate difference between the larger (Picture 3).

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Circular springs, also known as coil springs, are wound with flat steel strips to provide greater load than other springs but with a reduced range of deformation. Therefore, circular springs are suitable for use with gas and low temperature media.

      V-shaped spring works similar to the "cantilever", so that the spring load to focus on the leading edge of the lip seal, the spring has a medium load and deformation range, more widely used. In addition, when there is reverse media pressure, the spring is compressed by the media pressure, will have a "pressure relief" function.

Elliptical springs are made from a round wire wound into a tilted coil and provide a relatively constant load over a wide range of deformations so that the seal can not be overly deformed and damaged. Therefore, the oval spring can be applied to the closed groove design.

4.2 support ring

      When there is reverse medium pressure, in order to protect the jacket lip is not damaged, need to design the support ring in the rear of the spring. Support rings are rectangular, L-shaped and T-shaped structure (Picture 4).

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Rectangular support ring section similar to the rectangular, with only the V-shaped spring with the use of more economical approach is to use a metal strip welded together. The disadvantage is that the assembly is not positioned, prone to eccentricity, in order to prevent the assembly off and eccentric, rectangular support ring and V-shaped spring contact parts need to be spot welding, but this is easy to spring load impact. In addition, reciprocating larger occasions should not be used, such as ball-type fixed ball valve seat parts.

L-shaped and T-shaped support ring made of metal or plastic processing, to make up for the shortcomings of the rectangular support ring can be reciprocating larger travel occasions, but the higher manufacturing costs.

4.3 Retaining ring

     High pressure and / or high temperature media conditions, metal parts of the gap between the occlusal is very important. The snap fit clearance is a one-sided clearance between the mating parts. In high pressure and / or high temperature media conditions, excessive clearance may cause the pan collar to be squeezed into the snap-fit clearance, resulting in seal failure.

     Increasing the thickness of the root of the pan collar can improve the occlusion, and a more effective way is to use the collar to compensate for the occlusion gap. The retaining ring should be harder than the jacket material and usually use a highly filled RPTFE or PEEK (Picture 5).

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5 assembly

The groove structure of the pan plug should be installed and disassembled to avoid the damage of the sealing lip of the jacket. When the pan plugs are initially installed in the valve body, the parts of the valve body in contact with the pan plugs should be designed with a smooth chamfer and fillet. At this time, the pan plugs are free and have a certain deformation range. In the compact In the case of design, this part of the valve body can also be designed only for mounting fillets. In the area where the seat is in contact with the pan collar, a large enough chamfer and fillet must be designed (Picture 6).

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6 applications

6.1 since the relief valve seat

      API6D provides that the fixed ball valve cavity pressure should not exceed 33% of the pressure at both ends of the pipeline (Table 1), the use of self-relief valve seat can achieve this function. The lower the pressure level of the valve, the greater the difficulty of self-relief.

Table 1 valve cavity maximum allowable overpressure MPa

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Note: The valve body material is A105 at 38 ℃.

      An ordinary pan seal self-relieving valve seat seal groove is an open design. Sealing the right side of the valve seat than the left side of the pressure area, so by the media pressure piston effect, the valve seat to the ball seal (Picture 7 (a)).

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When the valve chamber is overpressured, the pan seal is pushed to the valve body as the pressure on the left side of the pan seal is greater than the pressure on the right. In this case, the pressure area on the right side of the valve seat is still larger than the pressure area on the left side, and the valve seat will still be tightly sealed against the ball (Picture 7 (b)). Self-relief must be achieved by pressure relief in the pan collar (Picture 7 (c)). Self-relief structure of the valve seat of its pan plugs must choose the V-shaped spring, because the circular and elliptic spring pressure valve and the valve seat will only support the more tight, until the valve chamber pressure crushed so far.

     V-shaped spring plug ring both to ensure sealing, but also to achieve pressure relief, so the spring force of the design and control is particularly important. After many tests, the V-shaped spring plug to achieve a reliable seal, the reverse pressure relief value is usually not less than 1.5MPa. Therefore, Ball Valves of Class 150 and Class 300 in Table 1 should not be used with V-Spring pan plugs. For self-pressure relief, three solutions are given.

(1) increase the area difference. The self-relief of the seat is achieved by increasing the size of the mating face of the pan collar with the seat (Picture 8 (a)). The inner diameter of the pan plug needs to be larger than the size of the contact area between the valve seat and the ball. So that when the valve overpressure, even if the pan can not be pressure relief ring, the left seat pressure greater than the right area, but also to promote the seat away from the ball. The disadvantage of this solution is the significant increase in valve opening torque. The practical application is limited to low pressure valves.

(2) increase the stopper. Add a stopper to the end of the valve seat, and screw the stopper and valve seat together (Picture 8 (b)) so that the pan seal and the valve seat can move integrally. When the valve cavity is overpressured, although the pan collar is pushed away by the pressure from the valve cavity, the resultant force still acts on the seat, allowing for a force analysis of the pan collar and seat as a whole. The disadvantage of this solution is the complicated structure. Some small-caliber valves do not have design space, and the valve body must be designed with a large enough pan plug to install chamfer to prevent damage to the sealing lip of the pan plug.

(3) closed groove. The use of closed trench, simple structure, compact design, suitable for some small-caliber valves. In addition, the pan plug ring selection of oval, you can take full advantage of the characteristics of this spring, the spring out of the end of the pan plug in order to protect the sealing lip under reverse pressure. The disadvantage is the disassembly inconvenience, disassembly easy to damage the pan plug.

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Picture 8 Pan plug circle self-relief valve seat solution

6.2 double piston seat

Double piston seat requires a single seat to cut off the medium from both ends, so do not have self-relief function (Picture 9), in order to facilitate disassembly, the seat and the valve body should be designed to large enough installation of reverse angle. When applied to double piston seats, the pan collar is usually mounted back-to-back with both sets (Picture 9). When the structure of Fig. 9 (a) is replaced with a round spring pan plug, pay special attention to the system pressure can not exceed the circular spring load, otherwise it is easy to crush the circular spring.

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7 Conclusion

In recent years, the rapid development of fixed ball valve, the structure and form of its seat with the use of a lot of changes have emerged, the correct, reasonable and flexible use of pan plug ring, can improve ball valve performance, expand the ball valve range of applications and reduce production losses.

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