Taking build and exchange to new heights4 June 2002
Teamwork and trust were key to the successful rehabilitation of Raccoon Mountain pumped storage plant in the US
THE Raccoon Mountain pumped storage plant units were originally manufactured by the Allis-Chalmers Corporation with the motor/generators constructed under a licence agreement by Siemens AG. The first machine, unit 2, was placed in commercial operation in December 1978. Units 1, 3 and 4 entered commercial operation in January, February and August of 1979.
In their 22-year operating life, the machines had never received a major overhaul and were approaching an age at which major repairs were becoming necessary. The advanced age of the equipment, and cyclic wear, was becoming critical - threatening availability. The extensive rehabilitation scope, required to simply maintain availability, provided an opportunity to upgrade with a modest increase in overall cost.
This team was charged with the objective of rehabilitating Raccoon Mountain and providing reliable world-class equipment with:
• A 12%+ increase in unit capacity (approximately 50 MW).
• A reduction in minimum load as well as improved reduced load operation.
• Automatic generation control (AGC) capability from minimum to maximum load.
• Condensing capability.
• No significant vibration or cavitation across the entire operating range.
• A post-outage availability greater than 90% with a forced outage rate of less than 1% for the five-year period following the outage.
• Annual outage durations of 120 days, or less, including a maximum 12 day plant unwatering outage for spherical valve replacement.
• Scope and budget discipline.
• No recordable injuries including full compliance with all applicable health and safety policies.
• No reportable environmental violations.
• Technical accountability on the part of each partner for a confidence factor greater than 99%.
• Strong communication and teamwork with responsible, professional and accountable execution of all activities.
• Model outage planning and execution structured to serve as an industry benchmark.
• Turnkey tasks, performed with precision, on critical components such as the spherical valve and stator.
• Aggressive completion of all work that can, and should, be accomplished pre-outage to minimise outage duration and facilitate unit return to service.
Scope of work
While three major entities share overall project responsibility, the partners and support organisations are actually comprised of TVA-Plant, Voith-Siemens Hydro, ABB-Alstom, TVA-Power Service Shop, TVA-Hydro Engineering, TVA-TOM, GE-Woodward, G-UB-MK, Acme Industrial Piping, and Weld-Mart. Additionally, dozens of vendors supply the support services necessary to keep a project of this type successful.
Build and exchange
The project was planned and executed using the relatively new 'build and exchange' concept of generator upgrading. This method incorporates the implementation of a site construction period prior to the upgrade outage, which means that a completely new component must first be constructed on site before commencement of the outage. Then, during a significantly shortened outage, the old and new components are 'exchanged'. This concept was first implemented, on a large scale, with the upgrade of the three world class 805MW, 19m bore, 740ton generator stators at Grand Coulee dam in Washington State.
Typically, extensive upgrades of large generators (windings and core) can take nearly a year to complete if performed in place in the generator pit. This results in a significant loss of generation revenue with higher construction costs due to the intense atmosphere of most long-term outages. By incorporating build and exchange, the outage can be shortened to 90 days or less, while the construction period can still last six to nine months. This allows reductions in premium site work pay, less duplicate tooling, and more time to get it right the first time.
This approach was chosen for Raccoon Mountain. Although only one new component is typically required for the first exchange, here four completely new stators would be provided. This was due primarily to an Alstom frame design, which prohibits re-using three of the old Siemens AG stator frames.
Voith-Siemens implemented significant design changes into the new pump/turbine design. These included:
• Increasing the number of runner buckets from six to nine.
• A modest increase in effective runner diameter.
• A longer runner band.
• New wicket gates with an optimised profile.
• Stay vane modifications.
• Head cover modifications to incorporate cartridge-type gate stem bushings and seals.
• A new hydrostatic mechanical shaft seal.
• Gate linkage modifications to allow slightly larger gate openings.
• New wicket gate servomotors.
• A new, deeper, stainless steel discharge ring incorporating state-of-the-art wearing rings.
Essentially, not a single component went without review from the bottom of the runner cone to the top of the speed signal generator (SSG). All three guide bearings received some level of modification as did the thrust bearing. Dozens of changes were implemented in an effort to enhance maintainability and reliability.
Construction at site
While most manufacturing tasks are performed at Voith-Siemens' facilities in York, Pennsylvania, site construction was particularly important. Replacement of the first (unit 1) spherical valve was the first order of business. This valve was entirely new to facilitate the build and exchange process. The old valve is subsequently modified and rebuilt for installation during the next scheduled outage.
Valve replacement necessitates complete draining of the upper reservoir, which also shuts down the entire plant. The upper reservoir is completely drained, the valve replaced, and the plant returned to operation, all within a period of 12 days.
The Alstom stator design incorporates oblique-element support legs that permit the radial frame movements caused by thermal cycles. The water-cooled stator utilises three separate cooling systems:
• The stator winding is directly water cooled through hollow strands located inside each bar.
• The stator core is cooled by water tubes partially imbedded in the outer surface of the core.
• The air housing is cooled with conventional water-to-air heat exchangers utilising remote fans to provide a high volume of air circulation. The cooled air is forced upward, over the stator, down through the air gap, under the stator, then back through the cooler.
The stator winding is part of a secondary cooling loop that requires de-mineralised water. The de-mineralised water is cooled by large plate coolers using raw river water for the primary loop.
The stator core back cooling pipes utilise raw river water directly without an exchanger. The generator air coolers also transfer heat directly to the raw river water.
The rotor is also a part of the secondary cooling loop that utilises de-mineralised water. New rotating water manifolds with rotating and stationary water distributors provide cooled de-mineralised water to the direct water-cooled rotor poles.
Slightly lengthening the existing speed signal generator (SSG) support with modifications to the attachment points allowed for installation of a longer stationary manifold. The stator frames are provided in three sections. The sections are assembled, aligned, and field-welded. The double-dovetail core attachment keys are then installed and the core laminations are stacked. Windings are installed and water system components complete the construction.
A custom, modular, lifting fixture was provided to transport the completed stator onto its final position, the existing stator sole plates. The lifting fixture transfers the load from the stator frame to the centre of the bore where the lifting fixture is pinned to TVA's existing spreader beam. This beam is, in turn, pinned to the two main sister hooks of both powerhouse overhead cranes.The level lifting and subsequent precision landing of these large, heavy, components require planning and proper monitoring of the load distribution. This is accomplished by:
• Providing a means by which to monitor load applied to each lifting fixture arm.
• Having the capability to adjust the centre point of the lift, as necessary, to bring the load up level and equal.
• Once the load is level, with the arms equally loaded, the elevations of the powerhouse soleplates are matched and corrected to the stator supports with shims.
Landing these components requires experience with the potential side shifting that can occur at contact if the load is not level enough for setting over multiple dowel pins.
The first installation outage - for unit 1 - was scheduled for 120 days. The machine was removed from service, with un-watering completed on 6 November 2000. With the unit 1 spherical valve replaced, units 2, 3 and 4 returned to service on 18 November 2000. Unit 1 was completely disassembled, rehabilitated, reassembled with testing and commissioning under way by 28 February 2000. Unit 1 returned to commercial operation, as planned, on 6 March 2000.
Lessons learned from this first outage resulted in additional planning, scheduling and manpower adjustments. These changes made it possible to plan a substantially shorter outage on unit 4 - one of only 88 days.
Unit 4 was removed from service, as planned, on 6 February 2002 with a scheduled return date of 6 May 2002. The unit 4 outage was actually completed slightly faster with unit 4 returning to commercial operation 29 April 2002.
The third outage will be slightly shorter but large improvements are difficult because the turbine component deliveries drive critical path. The next machine - unit 3 - is scheduled for an outage beginning 16 September 2002.
Extensive planning and scheduling
The joint project team (JPT) meets regularly during the construction phases of the project. These meetings form the nucleus of the outage-planning group that analyses every detailed task performed during the outage. High impact teams (HIT) look for new innovations, or new ways, to perform tasks faster with less manpower.
Planning an upgrade outage, especially a 'build and exchange' outage, requires consideration of many factors not of major concern in a normal 'build in place' outage. There is an additional lifting fixture to store and handle at site.
In some cases the old equipment must be modified during the outage to be lifted out. There is the real-estate issue of a potentially crowded erection area in the power house while old parts come out and new parts are going in. These issues add greatly to the need for early review of the space availability, crane usage, and manpower. But even with careful planning, communication is essential.
Scheduling documentation, whether it be paper scratch pad or Primavera is another essential tool of upgrade outages and even more so with build and exchange. However, the schedule is only as good as the data it represents. Experience and proficiency are the only things that count. Are the scheduled man-hours per task a true reflection - based on experience and insight - of the effort necessary to complete a safe, high quality and economical job? Throughout the project a strong team effort has prevailed. Contractors, and sub-suppliers alike, attend a daily progress meeting where constructive dialogue covers recent work completed and items planned for the near future.
The site safety programme has resulted in a minimum of accidents, all of which have been minor. Initially all personnel at site must attend a TVA orientation meeting where safety is a major issue covered. Weekly safety meetings, with the craftsmen, are conducted to review where problem areas lie with corrective actions initiated. Twice weekly safety walk-downs are conducted of the entire construction/outage area in the power house. These are performed with joint participation of all contractors and include craft representation. In addition a daily outage progress meeting always includes a discussion of any safety issues from prior walk-downs or other recent findings.
During the project, a strong quality assurance programme was implemented at site. Key elements included:
• Procurement control and vendor surveillance.
• Fully documented welding and NDE programmes because many of the components involve critical, high load lifts.
• Factory design review of component modifications and drawing control.
The whole purpose of the build and exchange concept is to save time and reduce lost revenue. Therefore, it is counter-productive trying to cut corners - only to incur a forced outage soon after startup and commissioning.
As a result, there were a number of design improvements made to some components during the outages with several additions to the scope of work. These changes resulted from the realisation that the component could be better, work better or save time and money on future maintenance.
Teamwork and trust are the real tools that made difficult tasks look easy. The joint project team at Raccoon Mountain pumped storage has agreed on a list of priorities to maintain focus. These priorities are: safety; quality; schedule; and budget. In this order, these priorities can help make a project work.
|Key responsibility assignments include: • Spherical valve and turbine - Voith-Siemens Hydro. • Generator, exciter, and demineralised water system - ABB-Alstom. • Governor - GE-Woodward. • Balance of plant - TVA and G-UB-MK. Key elements to this project’s success: • Build and exchange technology • State of the art design/construction and installation • Extensive planning and scheduling • Teamwork approach with support from all levels of management. • Safety-quality-cost|