Performance matters in a power plant. Every process, piece of equipment, and valve must function at an optimal level to ensure continued success. Maintenance is a standard part of operations but specifying the right product designs can reduce the need for costly repairs and shutdowns.
Turbine bypass valves like the Fisher CVX and TBX are essential to controlling the steam bypassing the turbine during start up and shutdown. When combined cycle plants transition from no load to full load throughout a day, the resulting rapid temperature changes lead to the continuous expansion and contraction of the body and trim. Over time this may cause the valve to stick. These cycles can also lead to the release of magnetite from the boiler tubes, eventually eroding valve internals.
Per ANSI/FCI-2, all control valves are engineered with a specific leakage classification. In the past, some of the older turbine bypass valves were designed to provide CLIV shutoff. While Class IV shutoff may be acceptable, it can affect the overall efficiency of the plant.
Leakage is expensive and frequent valve repair and replacement is also costly, adding a layer of critical maintenance the plant manager must oversee. The bore seal solution from Emerson raises the standard for steam conditioning valves, in that it achieves CLV shutoff and decreases the potential of sticking. This was done by redesigning the plug and cage clearances/materials to limit the effects of thermal expansion and modifying the seal to prevent excess leakage.
Consider the potential cost impact of each leakage class:
The bore seal upgrade moves the level of leakage and corresponding cost to a highly desirable range. Better control of leakage and heat lost has a ripple effect for the plant including improved efficiency overall, reduced maintenance cost, fewer valve technicians on site and more streamlined operations.