Not all problems can be attributed to a lack of information at the selection phase or to a lack of tight shut-off. Sometimes problems are introduced by the way the valves are operated
All control valves have a minimum operating point which, if observed, helps protect against the effects of what is called “low-flow erosion”. If the valve is opened only a minute amount off of the seating surface, the plug and seating surfaces can experience erosion. A rule of thumb that applies to most types of control valves: Do not operate your valves below 10% open. This ensures that the pressure drop occurs in the valve trim, not across the seating surface
Valves supplied with anti-cavitation trim can have other than a 10% minimum throttling flow requirement and may be an exception to the rule. So are valves that eliminate the formation of damaging cavitation by staging the pressure drop through the valve trim. In essence, this is similar to placing a number of orifice plates or elbows into a valve to reduce the pressure in a small amount of space
To gain the anti-cavitation effect, a certain number of flow passages must be exposed. If they are not, all of the pressure drops will occur across the last stage of the trim and the seating surfaces, creating high, local velocities that will erode the plug and seat ring. To protect against this effect, the valve manufacturer should provide the minimum travel point to the user
The photo shows a valve plug that has been operated too close to the seating surface. The “gear-tooth” damage at the bottom of the plug is what is commonly found in valves that have operated below or right at the minimum operating point for extended periods of time. This plug was removed from an h-p drum level valve and was in operation for six months. After reviewing the operating data, it was determined that this valve was operated below the minimum operating point for nearly an hour during each plant start-up
The damage did not occur only during plant startup. Because the seating surface had been damaged, any leakage through the valve only exacerbated the problem. This is most commonly experienced before the valve is operated during startup when the recirculation valve is bypassing feed pump flow as discussed in point 3
Similar damage can occur in the main drum-level valve when two valves are being used for level control. The cause of damage to the main drum level in a two-valve arrangement is either a lack of tight shut-off or an improper transition from the startup valve.
(the above is an extract from an Emerson Fisher technical paper)
Additional posts in this series:
- Part 1: Insufficient information at specification time
- Part 2: Oversizing control valves to meet maximum conditions
- Part 3: Failure to specify tight shut-off
- Part 5: Poor control arrangement
- Part 6: Entrained particles
Links for additional steps to take to improve your Critical Control Valve performance or for a copy of the control valve handbook. If you wish to discuss your leaking boiler feedwater valve issues, please contact us using the enquiry form or call Joe Walsh on +353 21 4510900 or Dave Ralph on +44 1633 877505