Ringhals retrofit boosts output by more than 40 MW
Westinghouse and Vattenfall AB team up for steam path modernization
By R. Subbiah, B. P. Maxwell, L. G. Fowls, J. C. Groenendaal and Ken Johnson, Westinghouse Electric Corp., and Benno Persson, Vattenfall AB
Around the world, many power generation utilities faced with aging equipment, increasing maintenance costs and decreasing reliability, are seeking cost-effective ways to modernize their units and improve thermal performance. Fortunately, advances in metallurgy, analysis, design tools and testing technology have presented an opportunity to address these objectives, thus delaying or avoiding the costly option of erecting a new facility.
Confronted with increased operating costs and reliability issues associated with its low-pressure turbines, the management at the Ringhals Nuclear Power Plant in Sweden decided to approach all turbine equipment manufactures, including the Original Equipment Manufacturers, for recommendations to modernize and improve thermal performance of Units 11 & 12 at the facility.
The Ringhals Power Plant is one of several generating facilities owned and operated by Vattenfall AB. Ringhals has four nuclear reactors, each of which supplies steam to two 3,000-rpm tandem turbine generators. Ringhals 1 is a boiling water reactor (BWR), whereas Ringhals 2, 3 and 4 are pressurized water reactors (PWR). The combined generating capacity of the plant was in excess of 3,500 MW.
Bids were submitted by a number of suppliers, including Westinghouse`s Power Generation Business Unit in Orlando, Fla. The contract`s requirements were demanding because of its aggressive retrofit schedule. Furthermore, several special requirements were necessary due to the BWR environment that impacted both the design and manufacture of the proposed retrofit components:
– The BWR operating environment required the elimination of cobalt in the erosion shield material;
– All steam flow path surfaces had to meet special contaminant criteria; and
– There had to be minimal impact on any non-replaced equipment and the balance of plant (BOP) systems.
The original equipment for Ringhals 1 was constructed in the early 1970s. The Ringhals Unit 1 consists of two 3,000-rpm turbine trains, each having a single-flow high pressure (HP) turbine, three low-pressure (LP) turbines, a generator and an exciter. The steam is passed through two single-stage moisture separator reheaters (MSR) at the HP exhaust before entering each LP turbine. Both turbine trains receive their steam directly from a single BWR resulting in a radioactive environment throughout the steam side`s components.
In August of 1991, a team of Westinghouse personnel undertook a trip to Sweden to take preliminary measurements of Ringhals Units 11 and 12 for bid purposes. After the initial inspection, the team recommended that the inner cylinders be retained because they were similar to Westinghouse design. Moreover, the cylinders were in good condition and could be machined in place to accommodate the new stationary components. This approach reduced the cost of the modernization significantly. On the basis of this assessment and the resources available, it was decided that Westinghouse could supply the equipment required to replace the blade path and rotors within the time allotted.
Based on the results of the assessment trip, Westinghouse developed a bid for the supply and installation of the required equipment along with performance guarantees that resulted in the contract award on March 30, 1992.
Westinghouse`s proposal was designed to keep the cost of modernization within the customer`s budget constraints while at the same time providing the improvements required to enhance reliability and performance. The proposal included retaining the inner and outer LP cylinders and machining them to accommodate the replacement components. It also included a guaranteed thermal performance improvement of more than 40 MWs or 5 percent, interchangeability of rotors from unit to unit and longer maintenance inspection intervals.
The customer required that the six rotors and associated components be installed during a very tightly scheduled maintenance outage. Westinghouse agreed to this schedule after careful planning of not only resources but having available the critical portable boring bars, milling machines and other tools for the complex retrofit.
For detail design work to begin, accurate measurement of the parts to be replaced and the parts to be retained is fundamental and very crucial for the success of the retrofit. Since the existing units were not Westinghouse designs, a complete set of measurements was necessary to accurately obtain all relevant geometrical dimensions and the mechanical fits of certain crucial parts. Since the information required by various groups such as design, manufacturing, material engineering, field service, etc., dictate different requirements, a well-coordinated effort was necessary to understand the needs of each function. Accordingly, a list of all the data required by various groups was collected and logically laid out. Missing information was identified and the list was updated and finalized prior to a second measurement trip.
The purpose of the second outage trip was clearly defined to everyone involved in the project. The effort and time spent up front was crucial in obtaining all of the detailed information required in a timely fashion. This not only assured that all data necessary would be obtained, but also ultimately paved the way to minimization of time required for installation.
A second team of Westinghouse personnel was sent to the site in July 1992 to obtain detail measurements during a time when all six LP turbines were disassembled for maintenance. The measurement team completed the work within the schedule`s limits and did not affect the outage. Some parts and assemblies were videotaped for clarity. It was found that the inner cylinder machining differed among each of the three LPs on each unit as well as between each end of each cylinder. This increased the engineering and manufacturing work, since it reduced the number of identical parts.
Each cylinder and rotor was measured independently. Measurements were taken with respect to discrete reference points. Reference points were identified during the pre-planning stage prior to the measurement trip. Measurements were obtained between each reference point and a complete layout was made using all the measured data in conjunction with the reference points. Since each cylinder reference was related to an axial rotor position, this simplified the incorporation of the rotor geometry into the cylinder layouts. During the design stage, the layout created from the data obtained was used to verify fits and clearances for each new component relative to the retained existing equipment.
Measurements were taken such that the original rotor shaft ends would be duplicated as much as possible in order to reduce the need for replacement equipment such as coupling guards, glands, supervisory instrumentation, cylinder-lifting equipment and rotor jacks. This approach reduced the total cost of the retrofit.
Engineers focused on four design areas to improve reliability and increase thermal efficiency:
1. Blade path and blading design. An eight-stage flow path replaced the existing six-stage path. Once the flow path was optimized thermodynamically, blades were designed to fit the flow envelop and achieve improved aerodynamic properties. New blade designs were developed using Westinghouse design techniques. Other improvements included optimized root and blade attachment designs, which contributed to lower component peak stresses and reduced susceptibility to stress corrosion cracking. The blade path was also optimized to provide the thermal performance gains and allow close matching of the original temperatures and pressures to minimize impact on the balance of the plant equipment.
2. Cylinder and stationary component design. Four of the original six diaphragm grooves in the existing inner cylinders were used to support the new stationary blade rows. The retrofit blade rings and welded blade assemblies all have bolted horizontal joints compared to the original diaphragms that were supported in halves with no bolted joints. The bolted-joint design assures that these rings maintain their curricular shape during operation without using massive inner rings. The bolted-ring design also reduces leakage in comparison to the original split design diaphragms. Both inlet and exhaust flow guides were replaced in order to conform to the new blade-path diameters as well as optimize flow field characteristics.
3. Rotor design. At the heart of the modernization, the team used Westinghouse`s integrated rotor design technology. The rotors are made of single piece (mono-block) construction and finish-machined to accept the customized blades and balanced to ISO acceptance levels or better. At Ringhals, the rotors and modernized blade path were designed to optimize the required flow path pressures and temperatures. The integral rotor disc shapes were optimized using finite element method (FEM) analysis to minimize peak stresses. Shaft overhangs, bearings and couplings were detailed and verified to meet design requirements.
4. Bearings. The new rotors are 28 percent heavier than the original rotors, which required bearing load evaluations to verify the adequacy of the existing bearings. Analysis determined the original LP bearings would support the heavier rotors. However, before the retrofit, engineers found that some of these bearings had been operating at temperatures as high as 114 C–above acceptable industry and Westinghouse operating limits.
It was decided to run a test at Westinghouse`s Charlotte facility to determine the behavior of an original-type bearing as compared to a Westinghouse viscosity pump-style bearing. This design provides an increased oil film thickness that contributed to the lower temperatures by as much as 7 C cooler than the original type bearing in the tests. Therefore, the bearing design of all 12 bearings was changed to the Westinghouse design. Due to the compressed installation schedule, six spare Ringhals bearings were modified to the Westinghouse design, and six new bearings were supplied.
In April of 1994, the six LP bladed rotors and more than 150 crates were shipped directly from the port of Charleston, S.C., to a site adjacent to the Ringhals facility in Sweden. The majority of the components were shipped in accordance with strict cleanliness requirements, free of any preservatives. Each rotor was enclosed in a metal container and environmental bag with continuous circulation of dry air provided by a dehumidifier. As a backup in case of a power failure, desiccant bags were packed with the rotors. Stationary-bladed components were packed in air-tight containers, also with desiccant bags.
Upon arrival at the site, the parts were inventoried and carefully stored awaiting the scheduled installation.
The installation began July 9, 1994. Westinghouse hired local craft workers from GOTFAB, a Swedish company. Because of the aggressive schedule, 50-man crews worked on the installation in two 12-hour shifts. The project began with the disassembly, removal and disposal of the components to be replaced. Then began the simultaneous boring and milling operations of the six existing outer and inner cylinders. The portable boring bars and milling machines were required to machine the cylinders to close tolerances in order to maintain the drawing clearances required between the rotors and stationary components. This was followed by the tedious fitting of the stationary components and final alignment of the rotors.
The workscope included the following:
– removal of inner and outer cylinder covers, inlet and exhaust flow guides, rotors and bearings,
– removal of 12 rows of stationary blading from each LP turbine,
– machining of the inner cylinders of each unit to accommodate the replacement blade rings and welded-blade assemblies. The schedule required that this operation be carried out in parallel for all six LP turbines using six boring bars,
– installation of the modified bearings, blade rings, rotors, and inlet and exhaust flow guides,
– honing of 56 coupling bolt holes per rotor and installation of new hydraulic radial fit bolts,
– sizing of coupling spacers between the LP rotors to maintain proper axial alignment of the turbine-generator train,
– setting of rotor and stationary clearances and alignment of the rotor train, and
– planning the entire installation effort in detail, well in advance of the outage. This included detail breakdown of all tasks, amount of work required, duration of each task, and the type of labor required for each task. This estimate was closely monitored and continuously updated as the outage progressed. Additional work-safety and cleanliness practices were required because the reactor was a BWR.
The following efforts contributed and supported the success of the scheduled outage:
– During the design process itself, careful consideration was given to minimize the amount of field machining. For example, existing component contours, bolt holes, etc. were used to the extent possible.
– Six original spare journal bearings were altered for oil supply passages and new babbitt design in advance, and six new bearings were supplied. These bearings were installed in the existing spherical bore supports, reducing the retrofit effort.
– Final radial seal clearances were set during manufacturing, thus eliminating seal machining during installation.
– Spare axial sealing rings with extra machining stock were provided, allowing axial shifting of the blade rings during field work if required.
– Spare seals, support keys, bolting, shims, etc., were provided.
– Rotor train alignments along with the permissible tolerance ranges were sent to the field well in advance, providing flexibility for the field crew.
– Design team members in Orlando were available around the clock to support field engineers in the event that assistance was required.
The installation went very well without major problems. Replacement parts assembled accurately, well within required design fits and tolerances. The installation workscope was completed within the refueling outage schedule and culminated with a trouble-free start-up of the two units. Units 11 and 12 were rolled with steam on September 3 and September 5 respectively. Both units were overspeed-tested at 110-percent speed and synchronized to the grid the same day. The units were brought to full load without any problems. Maximum lower half bearing metal temperature recorded was 99 C, average temperature was 83 C. Rotor balancing was not required, with the highest recorded bearing cap vertical vibration at 4.9 mm/s (1.7 mills p-p).
Westinghouse Project Manger Ken Johnson said, “The completion and success of this strategic project was made possible by a significant number of dedicated people in Westinghouse and the customer`s organization. The cooperative effort of the purchaser and supplier during the project cycle proved to be a key to the project`s success.”
In 1995, the Ringhals Power Plant set a new production record when the four units generated collective power of 3551 MW, or 17 percent of Sweden`s electricity production. The additional increased power outage of Units 11 and 12 were key to this record output. Johnson added, “The additional 40 MW of power that the retrofit produced is enough power to light a city of 30,000.”
Working within a very tight schedule, Westinghouse was able to do a complete steam-path uprate on Ringhals` OEM turbines.
One of six low-pressure turbine sections fitted in the modernization program.