Develop a robust preservation program to protect plant equipment during lay-ups

Inactive powerplant equipment can suffer rapid and oftentimes irreversible damage when left unprotected and exposed to the natural environment. Such exposure can occur before the plant has been completed (Fig 1), as well as after commissioning.

Prior to commercial operation, preservation procedures should be implemented and sustained from the time equipment is manufactured until the plant for which it is intended is declared operational. During this period, warranties, insurance, and other considerations typically dictate that the OEM’s and EPC contractor’s procedures be followed.

1. Construction was stopped on several plants in the early 2000s because of changing economics. In some cases, equipment was not protected well from the elements

1. Construction was stopped on several plants in the early 2000s because of changing economics. In some cases, equipment was not protected well from the elements

 
Preservation Fig 2

2. Proper storage both indoors (left) and outdoors (right) requires rigorous procedures to protect equipment

Recall that construction on several combined cycles was halted in the early 2000s when the gas-turbine ordering boom went bust and equipment was put in storage, both indoors and outdoors, (Fig 2) as well as in partially completed plants. Following proper lay-up, equipment monitoring programs were put in place under the watchful eyes of preservation professionals to assure degradation mechanisms were being suppressed as intended. A great deal that was learned about equipment preservation during the bust—at plants such as Lenzie, Hillabee, and Dell—is of value for post-commissioning lay-ups.

After COD, the owner generally controls decision-making regarding preservation procedures—except possibly in cases where the maintenance of major equipment is governed by long-term service agreements. A big difference between preservation activities before and after commercial operation is that for the former, procedures are specific to individual pieces of equipment; for the latter, a more holistic approach is likely. Example: One plan for the steam side of the heat-recovery steam generator (HRSG) also may cover HP and reheat steam piping and valves, and the steam turbine.

There are many reasons a plant might be laid up for anywhere from a month to several months—possibly longer. A typical retrofit project—even a major inspection—might require an outage of from four to six weeks; a good snow-pack might keep fossil-fired plants out of service for an entire spring to maximize hydro generation; legal issues could keep a unit from operating for years after completion.

An effective preservation program requires careful planning and diligent management. The fundamentals of these activities recently were compiled into a guidelines document by the engineers and scientists at European Technology Development Ltd, “Preservation Guidelines for CCGT and Conventional Power Plants during Short- and Long-Term Shutdowns,” prepared by ETD’s W Moore, J Ford, T Callagy, and Dr D G Robertson. The guidelines for both wet and dry preservation are based on the practical experiences of industry experts at power producers in both North America and Europe. Write Eur Ing David Slater for more information on contents and price of the 69-page ETD Report 1274-gsp-188.

In a telephone interview with the editors, ETD’s Dr Ahmed Shibli said the major issue with long periods of inactivity is prevention of corrosion and its associated damage, which would adversely impact the plant’s reliability during re-commissioning and subsequent service. He said ETD found information available on powerplant lay-up procedures fragmented, dividing the issues between chemistry and engineering considerations. Some information also was subjective, often linked to the use of a company’s products. The objective of the EDT guidelines, Shibli continued, is to provide a broad view of the options together with the pros and cons.

The guidelines cover storage procedures to accommodate the type of plant, its history, outage time, locally available facilities, materials of construction, etc. Most plant shutdowns are planned, the metallurgist said, giving advanced notice that lay-up procedures will be required. However, there are generating facilities—combined cycles in particular—that need procedures ready for immediate application to accommodate the vagaries of the markets they serve.

Shibli stressed that owner/operators consider the entire plant when developing their lay-up procedures, not just the major equipment. Here’s a checklist of components/systems, among others, he recommended they include: HRSG water and gas sides, lube oil, steam turbine, condenser, generators, condensate/feedwater, water treatment plant, cooling water, transformers, switchgear, etc.

A few of the takeaways from report follow:

• Lay-up procedures depend to a large degree on the materials specified for plant equipment and systems. Example: If austenitic steels are present, then there will be a preference for all-volatile storage solutions (ammonia, for example) and chlorides should be limited to 0.3 ppm to minimize the risk of stress corrosion attack. Take care, Shibli said, to exclude solid alkalis, such as tri-sodium phosphate. The preferred storage method for plants containing copper alloys (condenser tubes, for example) is dry storage. If wet storage is unavoidable, bear in mind that high-pH ammonia-based procedures are not suitable—particularly under aerated conditions.

• The length of the lay-up period and the local environment significantly influence lay-up procedures. In general, wet lay-up is preferred for short shutdowns, dry lay-up for longer periods. However, today’s world is not so straight forward. Grid demands may make it impossible to determine just how long a lay-up will be at the planning stage; sometimes generating units are recalled to service with little notice. Alternatively, if a wet lay-up is prolonged, it is necessary to have a circulation system installed to keep water chemistry balanced throughout the HRSG and to assist in the mixing of any additional chemicals that may be required for the duration of the outage.

• Whether storage is wet or dry, routine checks are required to assure that the preservation plan is working correctly and that the optimal lay-up environment is being maintained (Figs 3, 4) In general, equipment should be checked daily until the storage team is satisfied that the preservation program is stable and reliable. At that point, the frequency of monitoring typically can be relaxed to twice weekly.

Preservation Fig 3

3. Pitting and rusting in a boiler steam drum after a hydraulic test and no preservation treatment

Preservation Fig 4

4. Breakdown of the passive oxide layer in an HP steam line that stood for six months without preservation treatment

• Some plant modifications likely will be required to implement either wet or dry lay-up. The guidelines summarize available storage procedures and provide an overview of required modifications for each. If the planned storage is expected to be of limited duration, temporary solutions may be satisfactory. But if you expect to put a unit in storage regularly (such as each spring to maximize hydro production), well-designed permanent facilities should be considered. A series of logic diagrams is provided in the guidelines to assist in the development of suitable storage procedures.

 

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