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Improvement of Ball Valve Sealing

Improvement of Ball Valve Sealing

I. Present situation and analysis of ball valve sealing

The cast stone ball valve is an ideal opening and closing device in the ash pipeline, and its sealing directly affects the quality and service life of the valve. In the past, the O-shaped rubber seal structure was adopted, which means the O-ring was put into the ring groove machined on the shaft, and then it was assembled into the hole machined on the sleeve parts to form the seal between the shaft and the hole. The sealing performance of this structure depends on the interference which is the compression of O-ring and the machining accuracy of hole and shaft parts. If the interference is too small, the sealing performance is poor. If the interference is too large, it is easy to crush the O-ring during assembly, which contributes to the seal failure.

The sealing structure is simple and easy to assemble. However, if the machining accuracy of shaft and hole is not high or difficult to improve, the basic sealing performance is difficult to guarantee or the accuracy cannot be further improved. The main disadvantage is the sealing material. This kind of sealing structure is very suitable for cylinder and hydraulic cylinder. It is difficult to ensure the sealing performance of cast stone ball valve used in ash water. The reason is that the switch of ball valve belongs to partial rotary type, and it often needs to turn 90 °to switch. Due to the dynamic fit between the valve rod and the valve body, there is a certain gap. When the valve is opened and closed, the huge thrust of the medium acting on the ball inevitably causes the valve rod to incline to a certain extent. The side that force of the valve rod deviates will be severely worn. Moreover, a large amount of suspended solids in the ash water will erode the sealing ring. As a result, the life cycle of O-ring at the valve stem is short, and leakage often occurs, which also affects the site environment to a certain extent. Therefore, it is decided to improve the sealing structure and materials. The main starting point of the improvement is that it can be adjusted through the amount of compression in the assembly of shaft and hole. The effective sealing can be ensured without greatly improving the machining accuracy of shaft and hole.

II. Structure improvement of ball valve and design of grinding tool

1. X-ring structure
There are sealing faces around the X-ring and lubrication grooves inside and outside the sealing ring. This structure is superior to the O-ring, with low installation accuracy, good use reliability. There is no need for maintenance and it is of low price. The amount of compression can also be adjusted by the gasket. It has been widely used in the sealing of cast stone valves of various pressure grades, which can effectively prevent leakage, greatly extend the service life of the valve, and improve the reliability of the valve.

2. X-ring material
The use of polyurethane (CPU) as the X-shaped sealing material can effectively solve the problems of the traditional O-shaped sealing ring. Castable polyurethane is a kind of sealing material which has been adopted internationally. It is of large modulus of elasticity, good physical and mechanical properties, and can bear large impact load. At present, polyurethane has basically solved the problem of hydrolysis resistance, and its temperature resistance grade is also constantly improved, which greatly expands the application field of polyurethane. The production of X-shaped sealing ring with pouring polyurethane is simple in technology and low in cost. Its pressure resistance level can reach 60MPa or higher, and its pressure maintaining effect is quite good, basically reaching zero leakage.

3. Sealing principle of ball valve
Its working principle: when tightening the bolt, the gland moves downward to press the adjusting gasket. By pressing the V-shaped packing gasket, the inner and outer lips of the X-shaped sealing ring are pressed against the sealing surface of the shaft and the hole at the same time to ensure the reliable sealing. Its main sealing performance depends on the tightening force of the bolt during assembly, and it is irrelevant to the dimensional accuracy of the shaft and the hole.

4. Abrasive design
Ensure that the product use requirements meet the design requirements;
Easy to demould and clean, easy to operate;
Low manufacturing cost;
The abrasive material is 45 steel;
The shrinkage is designed as 1.5% ~ 1.7%;
There is no flash in 10 ° slope positioning, the accuracy of products is guaranteed, and the yield is high;
The grinding tool is composed of upper mold, inner mold and outer mold.

5. Packing chamber size
According to the pressure and caliber of the ball valve, the amount of seasoning in the packing chamber is determined. The width is determined by the size of the axis. One X-ring shall be prevented under the pressure of 0.6 ~ 2.5MPa and dn250mm; two x-rings shall be placed under the pressure of 2.5 ~ 10MPa and DN300. The dimension and depth of the packing chamber are determined by the height of the seasoning pad, the pressure transfer ring and the X-ring.

6. Pressure transfer ring
The pressure transfer ring is set at high pressure and large diameter. It is installed between two sets of X-ring seals. Its main function is to transfer the upper pressure and change the stress state of X-ring seals. It makes the X-ring bear the force and deform radially to achieve the sealing performance.

7. Finite element analysis of X-ring
X-shaped seal ring is a lip type seal ring, which is used in combination with pressure ring (packing gland) and back-up ring (packing pad). It can effectively prevent medium leakage.

8. Force analysis of X-ring
The force of the X-ring is transmitted to the X-ring through the pressure of the pressure ring (packing gland). The bottom of the sealing ring is provided with a three-point force application method of the support ring (packing pad). The force on the sealing surface of the shaft and the valve hole is even, which can obtain a better sealing effect.

III. Summary of ball valve sealing

Through the improvement, the valve structure is more reasonable and the sealing is more reliable. It can save raw materials, reduce costs, increase service life, and meet the needs of users.

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