Design changes, retrofit solutions and performance evaluation techniques enable steam turbine owners to keep their machines in proper running order and sustain high plant efficiency.
By Brian K. Schimmoller, Chief Editor
Maintaining steam turbines in proper running condition is critical to achieving optimum power plant efficiency. As steam turbines age, leakage rates accelerate, mechanical rubbing increases and the likelihood of fatigue/corrosion/erosion increases. However, through design changes, retrofit solutions and performance evaluation techniques, turbine owners can recover much of the lost efficiency and output. For those purchasing steam turbines for new power plants, technological advances offer substantial improvements in efficiency, reliability and cost of electricity.
This article summarizes three papers delivered at POWER-GEN International 2005 that address steam turbine retrofit projects and design upgrades: (1) “Testing for Turbine Degradation and Improving Performance with Seal Optimization,” by Stephen Blachly with KeySpan Corp. and Mary Foley with TurboCare Inc.; (2) “A Large Steam Turbine Retrofit Design and Operation History,” by Jeffrey Cheski, Ramanlal Patel, Kent Rockaway and Henry Osaghae from Siemens Power Generation and Marc Christianson with Dairyland Power Cooperative; and (3) “Steam Turbine Technology Advancements for High Efficiency, High Reliability and Low Cost of Electricity,” by Michael Boss, Michael Gradoia and Douglas Hofer with GE Energy.
Long Island Rehab
At KeySpan Corp.’s Northport Station on Long Island, pre-outage performance testing and turbine condition observations provided the data for developing a successful overhaul of the Unit 3 steam turbine. The Northport Station has four General Electric G2 turbines, installed between 1967 and 1977, that burn either natural gas or No. 6 residual oil. The four generators have a combined output of approximately 1520 MW. KeySpan tests each of the four units regularly and schedules a major overhaul about once every seven years. Unit 3’s major overhaul occurred in 2004.
Keyspan Corporation’s Northport Power Station.
Field observations found that Unit 3’s output was limited by 15 MW with the main boiler feed pump at maximum speed throughout 2003 and early 2004. Thereafter, the unit output dropped steadily through the summer of 2004. Additionally, KeySpan noted an increased need to limit the main steam temperature due to ever increasing hot reheat temperatures.
KeySpan arrived at several pre-overhaul conclusions after analyzing historical and current modified turbine tests:
- A relatively continuous drop in first-stage pressure relative to the other three units indicated that Unit 3 might have a flow restriction prior to the first stage.
- Although the turbine efficiencies did not drop significantly, fluctuations in the IP section efficiency pointed to the possibility of some steam flow bypassing the HP turbine. A temperature leakage test on August 27, 2004 showed very high HP-IP leakage and very low IP efficiency, but the results did not reconcile with turbine heat rates and load.
- Boiler feed pump performance was about 8 percent below design, but this did not appear to be a major factor limiting load. KeySpan decided that fixing suspected boiler recirculation pump leaks and turbine restrictions was more prudent than replacing the feed pump.
When the Northport turbine was opened for the Fall 2004 outage, personnel found no flow restrictions upstage of the first stage; in fact, the nozzle area had opened up due to erosion. This sent KeySpan back to the drawing board, reviewing all the modified turbine tests for the previous two overhaul cycles and analyzing unit data and trend data to identify any possible patterns. Several key findings emerged:
- The Unit 3 feedwater flow calculation had been applied inconsistently in modified turbine tests over the years. By trending all the tests using the same calculation, KeySpan determined that the rise in corrected main steam flow during the past year was significantly outstripping the rise in corrected first stage pressure. This pointed to either HP-IP leakage or an increase in diaphragm partition areas after the first-stage nozzle.
- The reheater pressure drop decreased steadily from 8.8 percent in 1998 to 7.0 percent in 2003, before rising to 7.4 percent by the start of the overhaul. The initial decreased pressure drop pointed to additional flow bypassing the reheater (HP-IP leakage), in the range of 7-10 percent of the main steam flow.
- A sensitivity analysis performed by recalculating the results of all Northport modified turbine tests from before the 1997 overhaul to the present suggested HP-IP leakage of about 11 percent, with IP efficiency around 85 percent. While the HP-IP leakage figure seemed high, it was supported by a paper showing that HP-IP leakage at the Winyah Generating Station in South Carolina may have been as much as 22 percent.
- Analysis of on-line PI data revealed that, while there was a steady decline in boiler feed pump performance over the previous two years, the main culprit in overloading the feed pump was still likely the increased flow required to the turbine.
By reducing the estimated 11 percent HP-IP leakage down to 4 percent, and by improving the IP efficiency to near its design level of 91.3 percent, KeySpan predicted a heat rate reduction of 257 Btu/kWh and a 4 MW output boost. To accomplish this, the HP inner shell horizontal joint had to be machined to reestablish the contact needed to minimize HP-IP leakage. The inner shell was removed and sent off-site, where 0.031 inches was taken off the horizontal surface to achieve acceptable flatness.
To achieve additional leakage reductions, KeySpan teamed with TurboCare to develop and install an optimized seal package, consisting of a combination of conventional labyrinth packing rings, retractable packing rings, brush seals, conventional blade tip seals and brush tip seals. Brush seals were installed in the HP packing stages where the most performance impact would be gained, and brush tip seals replaced the conventional blade tip seals in the HP and IP stages.
Seal rubbing is common in most steam turbines, particularly in configurations such as the single casing HP/IP turbine Northport uses, where a large packing box seals the flow between the HP and IP sections. Because of a relatively long bearing span, these machines also can deflect, initiating rubs in the midspan seal area. This rubbing typically occurs in the first few turbine startups after an outage, leaving the conventional style packing and blade tip seals to operate with a 0.025 to 0.040 inch clearance above their normal design, until the next outage opportunity to refurbish the seals. Retractable packing addresses this limitation, using springs to force the packing ring segments away from the shaft during startup. Once the turbine reaches normal speed and pressure, when thermal distortion and vibration are no longer a concern, the segments then move to their design clearance (relative to the shaft), protecting them and preventing the bowed rotor.
Each TurboCare seal upgrade that includes brush seals is analyzed for a wide range of operating conditions. Seal clearance has long been recognized as one of the most important parameters when determining destabilizing forces from seals. Reliable turbine design requires the high-pressure rotor to withstand twice the expected destabilizing force from seals in a worst-case condition. The original Northport design did not satisfy this requirement. Therefore, to implement the seal upgrade package and bring the unit to current design standards, TurboCare added three anti-swirl features to the HP midspan seals (N2 rings) and to stages 2 and 3 in the HP section. This design change reduced the total destabilizing force by more than 40 percent. The final design with the anti-swirl ring can withstand 2.3 times the worst-case level of destabilizing force. The performance impact of the improved sealing package is estimated at 2.3 MW additional output over original unit design.
Table 1 shows the improvements achieved through the Unit 3 overhaul. Unit load capability increased by 14 MW, rather than the 4 MW expected, and heat rate dropped by 465 Btu/kWh, much more than the 257 Btu/Kwh expected. About two-thirds of the unit’s 465 Btu/kWh improvement was attributed to the turbine (about 300 Btu/kWh). KeySpan will continue tracking the post-overhaul performance to assess subsequent overhaul activities for the other three turbines.
Makeover at Madgett
The 365 MW Unit 1 steam turbine at Dairyland Power Cooperative’s J.P. Madgett station is a tandem-compound design, with a combined HP-IP section and one double-flow LP section. In service since 1979, the unit has been experiencing increased maintenance and reduced performance. Coupled with increased electricity demand in Dairyland’s service area, the utility decided to retrofit the unit with a Siemens Power Generation upgrade package that includes new rotors, inner casings, and high-efficiency stationary and rotating blades.
For the HP/IP turbine, Siemens’ BB44FA retrofit targets existing Westinghouse Building Block (BB) 44 steam turbines operating at inlet conditions up to 2,400 psi and 1,000 F. Major retrofit design elements include: full arc admission inlet section; 3D blading technology, fully integral inner casing; advanced sealing technology; elimination of separate nozzle chambers and nozzle blocks; elimination of the 180-degree steam turnaround to the HP blade path; and elimination of the impulse control stage.
A BB44FA fully assembled HP/IP steam turbine rotor. Photo courtesy of Siemens Power Generation Inc.
For the casing, all existing internal stationary components are removed and replaced with a single, fully integral casing. This reduces the number of parts and significantly reduces installation and outage time. Additionally, since the BB44FA design uses the same mating features as the original BB44 design, the new casing will match up to all axial, vertical and transverse anchor points in the outer casing. The BB44FA also re-uses the existing inner and outer glands. The seal segments are replaced with spring-back seal carriers to restore these seals back to the original design radial clearances. The inner gland ring was re-used, although the flow guide portion was re-machined with a new profile.
The BB44FA HP/IP rotor consists of a monoblock, fully integral no-bore forging that facilitates quicker start-up times and long fatigue life. Coupling flanges are integral to the rotor body and all rotating blades use either single or double tee-roots. Rotor weight is comparable to the original BB44 rotor, which allows the existing bearings to be re-used. The journal diameters and the coupling bolt holes are machined to match up with existing equipment, again reducing cost, field scope and installation time.
The first stage of the HP and IP blade paths utilize a low-reaction design with a diagonal arrangement to reduce rotor inlet temperatures while providing favorable flow conditions where the radial steam flow enters the reaction blading axially. The first stationary row is supported in the casing at the base and tip of the airfoil to minimize leakage.
For the LP turbine, Siemens retrofitted the existing five-stage BB73 design with a seven-stage BB73-8.7m2 design. The cross-over steam conditions from the HP/IP turbine remained the same as for the original design. Major retrofit design elements include: single inner casing with moisture removal features; increased resistance to stress corrosion cracking and high cycle fatigue; higher exhaust pressure limit at high loads (8.7 inches Hg); 10-year inspection interval; torsional compatibility with existing generator rotor; elimination of riveted shrouds on front-end blading; and elimination of riveted shrouds and lashing wires on large LP blading.
The LP inner cylinder design features single casing construction with a separate upper and lower half, and a bolted flange at the horizontal joint. In each flow direction, the casing supports and aligns three blade path stationary components: guide blade carrier for front-end drum stages, segmental assembly for L-1C row, and L-0C segmental and exhaust diffuser. Design using finite element methods resulted in a more rigid inner casing, which maintains proper alignment between stationary and rotating components to maximize inter-stage seal efficiency and eliminate rubbing.
Tip-to-tip seals provide inter-stage sealing for this design, which allows large relative axial displacements without rubs and improves the unit’s cycling ability. The LP rotor is a monoblock, fully integral, no thru-bore forging. Both coupling flanges are integral to the rotor body and all rotating blades use either a tee-root or side entry-root. The no thru-bore rotor eliminates the need for maintenance bore inspections and also reduces the operating stresses relative to an axial thru-bore. The LP blade path uses Siemens’ standard 37.7-inch free-standing L-0R, interlocked L-1R and integrally shrouded L-2R blades in the last three stages. The four front-end stages are integrally shrouded tee-root blades with customized 3D blade profiles. The entire blade path is made from 12 percent Cr and 13 percent Cr stainless steel, with the exception of the last-stage blade, which is made of 16 percent Cr.
Since the Fall 2004 upgrade project, performance has been excellent. HP and IP element efficiency have increased by 8 to 10 and 2 to 4 percent, respectively, resulting in a 15 to 20 MW output increase. In the LP stage, enhancements can be traced to the improved front-end blade design, the 25 percent increased annulus area in the last three stages, and reduced exhaust losses through the use of an improved exhaust flow guide. The Madgett Station did not upgrade its boiler to match the LP turbine retrofit capabilities, but if it had, an additional 7 to 8 MW of power output could be gained.
Gimme the HEAT
GE Energy has continued to improve its line of steam turbines, with the latest example being the High Efficiency Advanced Technology (HEAT) combined-cycle steam turbine product line. The HEAT turbines are reheat designs covering an output range of 90 to 300 MW, and are optimized for power plants using GE’s F-class gas turbines.
The A14 HEAT turbine has been operating since mid-2005. Its high-pressure section has demonstrated the highest section efficiency – 90 percent – of all previous HP sections for that average volume flow, validating the steam path advances developed for the A14.
To ease the rotor flexibility constraints associated with a disk and diaphragm construction for low volume flow combined-cycle machines, the HP section features a drum-type construction, which reduces the minimum difference between the steam path and the rotor solid diameter. The axial length of a drum-type stage is also about 50 percent shorter than a comparable disk-and-diaphragm stage. As a result, the HEAT steam turbines have a smaller steam path diameter and larger stage count with equivalent or improved rotor flexibility.
The HEAT turbines have fully automated start-up capability, as well as a long list of start-up permissives established to ensure safe and proper turbine operation. The benefits of this advanced control system include faster starts, consistent start-ups, sustained performance, reduced operator workload and reduced emissions.