High-delta-p investigation identifies HRH piping cracks
Tenaska Central Alabama Generating Station
Tenaska Alabama II Partners LP
885-MW, gas-fired, 3 × 1 combined cycle located in Billingsley, Ala
Plant manager: Robert Threlkeld
Key project participants: Brian Pillittere, plant engineer; Alan Foether, LCRO; Cecil Boatwright, operations manager
Plant personnel found that the steam turbine experiencing higher than normal exhaust backpressure. The operators noticed that the backpressure dropped whenever the hot reheat (HRH) bypass valves were opened.
The LCRO came up with the idea to perform a test to determine the relationship between the HRH bypass-valve operation and the turbine-exhaust backpressure. During a plant startup, the steam pressure was increased in the HRH piping and steam was observed coming through the insulation downstream of the Unit 3 HRH bypass valve.
The pipe insulation was removed and it was determined that the steam was coming from a crack in the weld that connects the HRH bypass valve to the downstream P91 piping. The valve was closed and the sound of air being sucked into the crack was heard.
A plant outage was taken to grind out the cracks and make weld repairs. The repaired areas will be inspected during future scheduled outages. In addition, manual block valves will be installed downstream of the HRH bypass valves in 2011 to allow repairs to the valves and piping to be made without requiring a full plant outage.
Results. Steam-turbine backpressure has since returned to normal. Backpressure is monitored regularly during plant operation using an Excel® model that compares the actual backpressure to the condenser manufacturer’s design backpressure curves based on the circulating-water temperature and condenser heat load.