Proper operation critical to avoiding steam-valve damage

Operational changes can lead to hardware issues deep inside system components. At Darling Downs, explained by Origin’s Pieter Wessels, valve damage seems to have occurred in the first 60 seconds of operation below the saturation line, with water going through the valve. The 630-MW power station in Queensland was commissioned in 2010 with three GE Frame 9E turbines, three HRSGs, a 270-MW GE steamer, and an air-cooled condenser.

First inspection of the main steam valve was in May 2014, five months before the first steam-turbine minor. The unit’s two main-steam stop valves, configured as combined stop and control valves with a common seat, were arranged in parallel. The inspection revealed damage to both stop-valve stems. Important to note is that Darling Downs had converted to cycling operation.

Stem damage (photo, left) was attributed to plastic deformation, erosion, and/or corrosion.

HP-bypass DRAG® valves were inspected in October 2014. Inconel 718 disk stacks had significant damage on all units (photo, right).

AHUG fig 3

Stop/control valves. Repairs and actions were both short- and long-term. For the stop valves, both stems were replaced and the plant increased its stock level from one stem to two. Operational actions included daily on-load valve-gear testing. Control-valve throttling was avoided by increasing minimum station load by approximately 10 MW and by continuous monitoring.

Long term, avoiding control-valve throttling to the degree possible is an ongoing operational goal. Additional inspections at 12,000 to 15,000 hours determined the need to install a modified seat and install a modified control-valve head assembly. Users may recall that steam-valve issues in the US focused on stellite delamination issues.

For the bypass valves, short-term repair included turning all disc stacks 90 degrees, purchase of replacement disc stacks, and a review of HRSG startup procedures. Longer term, stacks would be replaced at 12,000 to 15,000 operating hours and a design change would enhance HRSG superheater manual drainage. This would improve blowdown at shutdown (removing magnetite) and improve drainage at startup (removing water).

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