Let us take a moment to think about control valve operation and maintenance. On a valve on gas service, provided the gas is clean, there should be no erosion damage to the control valve trim. So why does the damage, typically shown on the plug shank in the picture below, start to occur? A gas valve will be relatively dry, which means there is no natural lubrication between the plug and the cage. A liquid service valve, however, will naturally have a degree of lubrication through the fluid passing through the valve. The first consideration is that it could be "galling". In simple terms, galling occurs when two relatively soft materials are rubbed together. The rubbing effect causes the materials to "pick up". Galling is normally eliminated by applying a hard facing to at least one of the rubbing surfaces. A typical hard facing material is Stellite 6. So if the materials have been correctly selected, why would the damage shown below still occur? The answer is often line debris. Control valve trims are manufactured to close running tolerances, and if debris gets trapped between the plug and the cage, then this will cause score marks, mostly in the softer material. The scoring often starts to "ball up" the material, which in turn gathers between the plug and the cage. Over the period of operation, as the actuator continues to move the valve up and down, the process is repeated, and the damaged area grows in size.
Insightful
Technical Sales Engineer
1wGreat info, the solution? Buy a new trim, that is why control valve manufacturers almost ‘give their valves away’ initially