Improvement of Ball Valves Sealing Performance

Present Situation and Analysis of Ball Valve Sealing

Cast stone ball valve is an ideal disc for ash pipeline and its back seal will directly affect the quality and service life of itself. In the past, it adopted an O-shaped rubber seal structure in which an O-shaped sealing ring was placed in an annular groove machined on the shaft and then fitted it into a hole formed in a sleeve-like part to form the seal of the shaft and the hole. The sealing performance of this structure depends on the amount of interference at the time of design, that is, the compression of the O-shaped seal ring and the machining accuracy of the hole and shaft parts. If interference is too small, the seal performance will be poor; too large, the assembly will be easy to squeeze the O-shaped seal ring, causing sealing failure.

Although the structure of cast stone ball valve is very simple and easy to assemble, if the machining accuracy of the shaft and the hole is not high enough or difficult to be improved, the basic sealing performance will be hard to be ensured or further improved. Cast stone ball valves are suitable for use in cylinders and liquid cylinders, but their performance is difficult to be guaranteed if they are used in the ash water. The reason is that the ball valve switch is not the completely rotary type and it needs 90-degree switch. There is a certain gap between the stem and the valve body since the coordination of them is dynamic. When the valve is opening and closing, the huge thrust acted on the ball by the medium will inevitably produce a certain tilt of the stem, bringing abrasion for the stem. In addition, a large number of suspended solids contained in ash water will also scour the seal ring. All these factors will shorten the service life of O-shaped rubber seal, causing leakage and some effects on the environment. Thus, it was decided to improve the seal structure and materials. The main point after improvement is to make the shaft and the hole adjustable by its compression when they are assembled, instead of improving the accuracy of the shaft and the hole to ensure the performance of sealing.

Construction Improvement and Abrasive Tools Design

1. X-shaped Sealing Ring
With lubrication groove on the inner ring and outer ring, X-shaped sealing ring has sealing surface around it, whose structure is better than O-shaped sealing ring. Due to its low installation accuracy, good operation reliability, maintenance-free performance, low price, adjustable compression ;by sealing ring, it is widely applied to help seal cast stone valve in different stress grades, preventing them from leakage, making valve service life much longer, and improving valve operational reliability.

2. Material of X-shaped Sealing Ring
Material of X-shaped sealing ring is CPU, which can effectively solve the problem of traditional O-shaped sealing ring. The casting CPU is a kind of sealing material widely applicable in the world these years. It has properties of large elastic modular, great physical and mechanical performance and heavy impact load. Moreover, the hydrolysis resistance problem of CPU material is solved basically while its temperature resistance performance keeps enhancing, expanding applicable scope to a large scale. Producing X-shaped sealing rings by casting CPU materials is a simple and low-cost process and we can get the rings’ of up to 600Mpa or higher class rating, excellent valve pressure-holding performance and leakage-free property.

3.Sealing Principle of Ball Valve
When you tighten valve bolt, packing gland moves downwards then pushes the sealing ring. Both internal and external grooves cling to sealing surface of valve axle and seal bore through the impact of V-shaped packing seat, ensuring a good sealing condition. The sealing performance depends on pressing force of tightening bolts during the assembly process, almost not relative to dimensional accuracy of axle and seal bore.

4. Design of Abrasive Tool
To ensure the operating requirement and design requirement of abrasive tools.
To be easy to demould, clean, and operate.
To have low manufacturer cost
To be made from DIN 45(GB 45) steel;
To design according to the shrinkage rate: 1.5 to 1.7 percent;
To ensure the accuracy and high yield of products.
To make molds comprised of upper die, lower die, and outer mold.

5. Dimension of Packing Chamber
Packing quantity of packing chamber depends on both pressure and diameter of ball valve and the width of packing chamber is decided by axle size. It is necessary to place one X-shaped sealing ring with pressure from 0.6 to 2.5Mpa, diameter smaller than DN250mm. Two sealing rings are needed under the pressure from 2.5 to 10Mpa, size more than DN300. Packing seat, pressure ring and X-shaped sealing ring decide the depth of packing chamber.

6. Pressure Ring
With high pressure and large diameter, a pressure ring is placed between two sealing rings. Its main characteristic is to reach its sealing performance by transferring pressure from above, pressing X-shaped sealing ring and cause radical deformation by change stress condition of X-shaped sealing ring.

7. Finite Element Analysis(FEA) of X-shaped Sealing Ring
An X-shaped sealing ring refers to a lip-type seal, which operates in combining with pressure ring(packing gland) and supporting ring (packing seat), taking effective measures to prevent medium leakage.

8. Stress Analysis of X-shaped Sealing Ring
An X-shaped sealing ring is forced by the pressure ring(packing seat). Due to three points supporting ring(packing seat) at the bottom of the sealing ring, sealing surface between axle and valve bolt is stressed uniformly, causing an excellent sealing performance.

Conclusion of Improvement of Ball Valve Sealing Condition
After improvement, ball valves can satisfy more customers with more reasonable structure, better sealing performance, less cost, prolonged operating life, accelerated speed of restructuring, and less investment.
Apr 28,2017
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