Best Practices – Lenzie

Steam-line drain valve inspection program

Chuck Lenzie Generating Station

NV Energy Inc 

1100-MW, gas-fired, two-unit, 2 x 1 combined cycle located near Las Vegas, Nev

Regional plant director: Steve Page

Lenzie plant photo

Challenge. Chuck Lenzie Generating Station had experienced repeated weld failures on the attachments to the hot-reheat drip leg. Historically the plant would identify a leak and perform NDE and a weld repair. But because of prior failures, Lenzie personnel conducted a pre-outage inspection of all drip legs and discovered yet another leak in the hot-reheat system. Insulation was removed after unit shut down and a circumferential crack was discovered in the SA335 Grade P91 base material (Fig 1).

The crack was located approximately 3 in. below the hot-reheat bypass valve drain line feeding into the drip leg (Fig 2). To determine the failure mechanism, NV Energy sent the affected section of the drip leg out for metallurgical analysis. The lab’s conclusion: The failure was caused by thermal fatigue initiated from the internal surface between the hot-reheat bypass valve drain line and the drip leg’s end-cap girth weld.

Best Practices - Lenzie - 1 Best Practices - Lenzie - 2

A root-cause analysis was performed and experts attributed the failure to a leaking manual drain valve, which emptied the hot-reheat drip leg. The drip leg normally maintains a condensate level at or above the drip-leg drain line to minimize thermal transients. Condensate level is controlled by a thermocouple, which opens the DCS-controlled drain valve when temperature reaches Tsat + 45 deg F. The drain line for the hot-reheat bypass valve drains to the drip leg which normally contains a minimum condensate level. Since the drip leg was empty because of a leaking bottom drain valve, it experienced thermal transients leading to thermal fatigue and finally base-metal failure.

Solution. Chuck Lenzie Generating Station implemented a comprehensive valve inspection program for all steam-line drain valves including the manual low-point drains on the drip legs. The maintenance and engineering departments evaluate valve inspection results and determine the urgency, nature, and priority of valve repairs.

Results. Plant safety and reliability were improved by performing steam-line drain-valve inspections in addition to the existing high-energy piping inspection program.

Project participants: 

David Hall, plant engineering and technical services manager

Dan Schiller, lead production technician

Jimmy Daghlian, staff engineer