The generating station for a Midwest-based utility provider had five supercritical boilers that generated in excess of 3100 net MW. Each unit had a motor-driven boiler feed pump primarily used only in emergency situations, and two turbine-driven feed pumps which satisfied feed water demand during startup and normal operation.
However, running the motor-driven pump caused frequent pressure and flow surges in the feed water system that upset the unit. This extended downtime and caused excessive stress on piping and operating systems.
The unit’s control issues were primarily attributed to the boiler feed pump flow control valve. Because the valve’s flow characteristics were non-linear, it would go through abrupt changes in flow at mid-stroke. Control problems were also attributed to the electric actuator. It was not fast enough to respond to feed water flow requirements, and it would not reliably stroke to 100% capacity.
The utility company’s maintenance crew revised the electric actuator and tried some different trim assemblies, with marginal success. They turned to the valve manufacturer, Copes-Vulcan®, an SPX brand, for a solution.
Valve Trim Design Provides Optimal Solution
Copes-Vulcan recommended replacing the valve’s trim with its RAVEN™ Trim Technology. The RAVEN Control Valve design enhances the well-proven labyrinth disc stack type of trim with two main innovations: resistance to the effects of blockage and reduction in noise.
In addition to the trim upgrade from Copes-Vulcan, the utility company specified a particular hydraulic/electric actuator from another manufacturer. The Copes-Vulcan staff worked with the actuator manufacturer to make the project a success.
Team Effort
Retrofitting a valve is not as simple as selecting an off-the-shelf part and replacing the old one. Copes-Vulcan engineers helped the utility company evaluate their needs beyond what was included in the valve specification sheet.
They took into consideration the modifications that the company’s maintenance staff had made to try to overcome the surge problem on their own. They also worked with the actuator supplier to meet the customer’s requirements.
The retrofit allowed the utility company’s production team to operate the motor-driven control valve automatically with control of feed water flow for the first time in plant history.
The plant has since upgraded the remaining four units with retrofitted control valves.