Fault analysis and treatment of pneumatic high pressure ball valve

Fault analysis and treatment of pneumatic high pressure ball valve

1. Introduction to valve and system functions

According to the technical specifications, in case of a loca event of the main system, the high-pressure safety injection ball valve must be fully opened within 10 s to ensure the availability of the safety injection system. Therefore, the function of the valve needs to be verified through regular logic switch test. If the valve fails to open or the opening time exceeds 10 s and cannot be recovered within 8 h, the unit must reduce power and enter the cold pressure relief mode within 24 h. The fault valve is an 8-inch 600 pound nuclear grade 3 three-stage butt welded soft seal pneumatic ball valve. According to the design requirements, under the working condition of design pressure difference of 4.2 MPa (inlet side), the opening torque of the valve shall not exceed 12 N • mo. the pneumatic actuator is a piston type single acting cylinder, the loss of air spring opens, and the valve is opened by the return force of the cylinder spring. The pneumatic actuator outputs an initial opening torque of 2070 n • m and an end of stroke opening torque of 1071n • M. The maintenance manual specifies that when installing the valve, the recommended grease Krytox GP 05 must be used between the valve seat and the ball. The pre tightening torque of flange bolts at both sides of the valve body is required to be 171 n • M. under this pre tightening torque, the installation clearance between flanges at both ends of the valve body and the valve body can be guaranteed to be 0.

2. Failure analysis and treatment of ball valve

Before the valve is disassembled and removed from the system, the opening torque of the valve manually tested by the torque wrench is greater than 20 N • m, far exceeding the design value. At the same time, the output opening torque of the cylinder spring can reach more than 2000 n • m, which meets the design requirements, thus eliminating the possibility of insufficient output torque caused by the degradation of the pneumatic actuator. Subsequently, the system was decompressed and the valve was disassembled. The inside of the stainless steel pipes connecting the front and rear of the valve was clean. The surfaces of the valve seat and the valve ball were coated with sufficient grease. The ball and the valve seat were intact, with only slight contact scratches, no obvious corrosion, and other parts were intact. In view of the fact that only the valve seat was replaced in this overhaul, it was preliminarily judged that the difference between the old and new valve seats was the main reason for the increase of the valve starting torque and the inability to open. The old valve seat was reused and reinstalled (without defects). The starting torque of the tested loaded valve was less than 500 N • m, which could be opened quickly. The final cause of the valve failure was to lock the size difference between the new valve seat and the old valve seat.

3. Comparison and difference between new and old soft seal seats

The new and old valve seats were tested and compared in many aspects. It was found that the materials, friction coefficient, elastic modulus and most dimensions of the new and old valve seats were not much different. The main differences were as follows

(1) There is a big difference in the radius r of the inner surface of the valve seat. The old valve seat R is 125 mm, and the new valve seat R is 121 mm, with a difference of 4mm

(2) Under the same positive pressure of the valve ball, the axial deformation of the new valve seat is significantly less than that of the old valve seat, that is, the axial stiffness of the new valve seat is significantly greater than that of the old valve seat. This is closely related to the different radius of the inner surface, because the reduction of the radius of the inner surface will increase the thickness of the cantilever part of the inner ring of the valve seat extending out of the inner diameter of the flange, and the bending section coefficient of the valve seat will increase accordingly.

(3) Under the condition of lubricant, the starting torque of the valve is obviously less than that under the condition of no lubricant, but under the condition of grease, the value of starting torque has obvious time effect. The starting torque at the beginning of the test is very small, and the torque gradually increases with the increase of static time. Finally, a relatively stable starting torque value will appear, which is consistent with the actual fault situation.

4. Conclusion

The axial deformation stiffness of the new valve seat is significantly greater than that of the old valve seat, resulting in a significant increase in the pre tightening starting torque of the valve under the same pre tightening clearance and the starting torque after medium loading. After the valve is just installed, the starting torque is close to the critical value, resulting in the failure that the valve cannot be opened.

The radius of the inner curved surface of the valve seat is the key factor affecting the axial stiffness of the valve seat. The smaller the radius of the inner curved surface, the greater the stiffness, and the greater the preload under the same preload clearance.

Under the condition of lubricant, the starting torque of the valve has obvious time effect. The starting torque at the beginning of the test is very small. With the increase of the static time, the torque gradually increases, and finally a relatively stable starting torque value will appear.