starting the process from scratch. Once the coating is removed, the underlying part is more clearly visible. At this stage, the roundness of the ball is verified, and a final evaluation is conducted to determine if any additional work is needed on the part before the new coating is applied.
A finish size is determined by a print, and the part is then prepared for thermal spraying. Spray parameters are set, the part is coated, and then it is finished to tolerance through grinding or lapping. Ceramics require finish lapping to achieve tolerance, while carbides are finished through grinding and lapping. The final thickness will vary depending on the number of previous repairs. Customer specifications typically define the maximum allowable thickness.
Grinding is a machine driven process which involves using an abrasive to wear away the surface of a workpiece to reshape it. Spherical grinding is specifically used to finish ball valve balls. Grinding serves as a finishing process and helps achieve the correct final size for the ball. A smooth surface finish on the ball is crucial for achieving a proper metal-to-metal seal. After grinding, a lapping process typically follows to further refine the finish.
Lapping is a precision abrasion process used to achieve a specific surface finish or dimensional tolerance by removing a very small amount of material. This process is essential for creating a proper sealing surface. There are two types of lapping: rough lapping, which is machine-driven and designed to remove significant material quickly (similar to grinding), and finish lapping, which removes minimal material to create a smooth, precise sealing surface.
Once the repair process is complete, the assembly and testing phase begins. This phase includes tasks such as stack fitting, bolting, labeling, pressure testing, and, in some cases, O2 cleaning.
The Repair Cycle
Valve repair is a cyclical process, meaning the
same valve may need to be serviced multiple
times. Therefore, repair decisions should be
made with a long-term perspective. If a valve
is failing prematurely, recurring tell-tale signs
will surface, prompting a failure analysis and
possibly even a redesign. Throughout the repair
process, there is always an opportunity to modify
and improve the valve for the end user.