Planning and executing a long-term layup program (a/k/a cold storage)

Stanwell’s 375-MW Swanbank E Power Station in Queensland had the largest gas turbine in Australia (Alstom GT26) when commissioned in 2002. It set a world record (unofficial) of 254.8 days of continuous operation.

But the station was removed from service in December 2014 and is not scheduled to return until 2017. In a public statement, Stanwell explained: “Analysis of the electricity and gas trading markets concluded that greater value could be achieved from Stanwell’s gas entitlements by selling the gas rather than using it to generate electricity.”

A comprehensive cold-storage and preservation program for all systems began; site labor was reduced to a caretaker team. Ongoing lessons learned, covered in detail by Stanwell’s John Blake, a member of the AHUG Steering Committee, could be helpful to other owners and operators.

To prepare for storage, a full baseline inspection documented component and system conditions. Major storage risks predicted are outlined below.

1. Pitting and general corrosion:

          • Steam/water side—HRSG, steam turbine, piping.

          • Gas side—gas turbine and HRSG hot gas path.

          • Under-lagging corrosion—steam, gas, and feedwater lines.

2. Corrosion fatigue:

          • Steam/water side—HRSG and steam turbine.

3. Acid dew-point corrosion:

          • HRSG gas side.

This storage process began with Australia’s AS3788, “Pressure Equipment: In-service Inspection” and Clause 4.6, preservation-plan requirements and return-to-service procedures. Blake outlined specific steps taken and lessons learned for both inspecting and preparing the HRSG and its associated steam/water and gas path, gas turbine, steam turbine, and generator. Balance-of-plant discussions included gas yard, plant air, control systems, and pumps.

Relative-humidity monitoring details also were given, showing equipment and locations (with ongoing lessons-learned updates). Cold-storage monitoring trends then were presented for all major equipment.

Blake stressed vigilance to every detail, such as valve tagging to identify those modified for air circulation. He listed areas easily overlooked, such as draining of the flash box on the side of the condenser.

Perhaps most beneficial were the cold-storage lessons learned:

1. New equipment is needed (dehumidifiers, for example). This should include critical spares (photo).

2. Plant staff must understand the impact of every change (for example, valve position) made during layup. Access in-depth article on the topic.

3. The entire process (including changes) must be clearly recorded and traceable.

4. “Don’t just set and forget.” Staff should always look for improvements. This means reviewing all ongoing strategies, not just the original plan.

5. Ongoing strategy reviews should include all site personnel.

AHUG fig 1

During comments and discussions, Barry Dooley, Structural Integrity Associates Inc and chairman of the AHUG Steering Committee, stressed the LP turbine as a critical risk location for any layup beyond three days. Dooley also cited continuing work by the International Association for the Properties of Water and Steam (IAPWS) on film-forming amines for protection during layup, a topic that would be discussed again later in the meeting.

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