Entrained particulate matter has been linked largely to the startup and operation of combined-cycle plants
It causes a great deal of damage to all types of valves, and other equipment as well. Much of the particulate matter can be linked to FAC or inadequate pipe blowdown during plant startup and commissioning.
Inadequate pipe blows have caused the failure of many valves, not just feedwater valves. But feedwater valves are particularly susceptible to particulate damage because crud can become lodged in the flow passages and reduce capacity. In valves with flow passages large enough to remain open, particulate matter can become lodged between the plug and cage causing galling and scoring of those surfaces. This can cause jerky motion of the valves, a phenomenon that hampers control. The worst case is that it renders the valve completely useless if the plug becomes stuck
FAC occurs when the HRSG is operated at reduced loads for extended periods. What happens is that high-velocity flow in the preheater tubes removes material from the boiler tubes. The liberated material has to go somewhere and it often finds its way directly to the feedwater system
One plant in Texas removed nearly 50 pounds of fine particulates from the l-p drum during a scheduled outage. Fine particulate matter is not captured by the strainers typically installed in many new plants and it essentially “grit blasts” critical surfaces. Erosion of the valve plug can lead to the plug wedging into the seat ring and to leakage and additional valve damage
Solution:
There are several ways to deal with entrained particulate matter. The first is to ensure that proper piping blows are conducted. Prior to a pipe blow, it is necessary to remove the normal operating trim from the valve and install sacrificial trim. This will protect the finished surfaces from damage while also acting to either catch the particulates passing through the valve or to let them pass through without obstruction
Entrained fine particulates removed from the boiler tubes are not so easy to deal with. Over time fine particulates will be expelled through continuous and intermittent blowdown operations, but it can take several years to complete the process. Protected seats are available for control-valve trims that also provide anti-cavitation benefits. These solutions either remove the shut-off areas of the trim away from the flowing particulates or the seating surfaces are located away from any areas where a pressure drop may be taken
By ensuring that a pressure drop does not occur across the seating surface, the high-velocity sandblasting effect can not occur and the trim is protected against premature erosion. Research is ongoing to identify erosion-resistant materials that do not require the use of elaborate valve trims
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 4: Improper operation
- Part 5: Poor control arrangement
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