Steam Turbine Retrofits Upgrade Reliability and Performance
By Brian K. Schimmoller, Associate Editor
Steam Turbine Retrofits Represent Big-ticket Items for Many Fossil and Nuclear Plants. Where
the invested capital value is high (such as for large LP nuclear steam turbines), the driving force for retrofit is long-term mechanical reliability. Where the capital value is more modest (such as for fossil HP, IP or HP-IP turbines and nuclear HP turbines), performance improvement can often justify retrofit. In all cases, turbine retrofits become more economically attractive for plants with existing or expected high capacity factors. Justification is also strengthened if the turbine retrofit is paired with boiler and/or balance-of-plant improvements to achieve the greatest efficiency gain. As mentioned in the previous article, such integrated whole-plant improvement is gaining popularity; Westinghouse, GE, Siemens, GEC ALSTHOM, Babcock & Wilcox, Foster Wheeler, DB Riley and Black & Veatch are some of the companies that are exploiting this technique.
Steam turbine retrofits typically result in secondary benefits. Efficiency gain, for example, translates into reduced environmental impact: less fuel for the same output, fewer emissions, less heat rejection to cooling water sources. Increased plant output reduces the need for new plant construction or enables companies to retire other, older units. Increased plant output also is typically achieved at much lower capital cost than greenfield generation and often at lower operating cost.
Problems and Solutions
A number of equipment or operational characteristics can lead to the need for steam turbine retrofits. Stress corrosion cracking (SCC) is the most common mechanical problem plaguing nuclear turbines. SCC is typically found in nuclear LP turbines featuring “shrunk-on” disk construction or certain types of blade root construction. The threat of SCC necessitates regular inspection and, when discovered, expensive repairs. Steam generator degradation, leading to reduced steam pressure and increased volumetric turbine flow, can also justify a nuclear turbine retrofit to match steam conditions to turbine design.
For fossil steam turbines, particularly in the United States, solid particle erosion of HP turbine nozzles or first-stage IP blades is the main reliability culprit. Exfoliated particles from boiler tubes (principally magnetite) are carried over into the steam path and cause erosion. In severe cases, erosion damage can cause HP section efficiencies to fall by 5 percent within three to four years. In fossil plants where poor boiler design necessitates reheater desuperheating sprays to prevent excessively high reheat temperatures, a turbine retrofit offering higher HP efficiency (leading to reduced steam temperatures at cold reheat) can permit the sprays to be turned down.
All types of steam turbines suffer from steam loss. Steam escapes around the valves, inlet and exhaust sections, crossover piping, and blade and tip seals. Obsolete seal designs, non-optimized flow paths and inadequate fastening are some of the fundamental reasons for excessive steam loss and diminished turbine life.
A variety of advanced technical approaches have been developed that offer improved performance and/or mechanical reliability after retrofit. GEC ALSTHOM, for example, currently employs the following advances for steam turbine retrofits:
Improved fixed and rotating blade profiles with reduced frictional losses and increased tolerance to variations in steam incidence angles;
Advanced 3-D stacking of blade profiles to reduce the influence of end wall losses;
Improved understanding of 3-D flow characteristics in long LP blading;
Stronger blade profiles and root fastenings that accommodate additional stages in the same axial space;
Stage addition at reduced root diameters, leading to lower steam velocities (lower losses) and blades with more favorable aspect ratios;
Integral blade tip shrouding that provides more effective tip sealing;
Diaphragm blades integral with root and tip platforms manufactured by five-axis machining to provide accurate blade pitching;
“Optiflow” turbine technology with single-flow section at LP turbine inlet; and
Improved hub seals with smaller radial clearances.
Other advanced technical approaches being applied to steam turbine retrofits include increased exhaust areas to reduce exhaust losses, optimized rotor geometries to minimize stress concentrations, spring-backed sealing and rotor steam pre-warming systems.
Half Empty, Half Full
Steam turbine retrofits are commonly referred to as partial or full. Partial retrofits include the replacement of rotating blades and/or fixed blade diaphragms. Older LP turbines, for example, can be retrofit with advanced diaphragms (tilted tangentially from the radial plane) that more effectively accommodate the high degree of radial swirl in the later LP stages. Such retrofits have provided heat rate improvements of more than 1 percent.
Full retrofits, more expensive but offering greater performance improvements, involve the replacement of the HP, IP, HP-IP or LP cylinder modules (comprising inner casing shells, diaphragm carrier rings, nozzle boxes, rotor and fixed blade diaphragms). Full retrofits provide greater design freedom within the constraints of the outer casing and enable consideration of blade-to-steam speed ratios, improved aspect ratios and lower steam speeds.
Because the investment cost per kW is normally lowest for the HP turbine, full retrofits are more readily justified, while partial retrofits are more common for IP and LP turbines. Full retrofits make the turbine “better than new” by injecting modern technology, thereby increasing performance and reliability and extending equipment life for another 250,000 operating hours or more. For certain operating problems such as solid particle erosion, turbine retrofits can provide significant extensions to the overhaul inspection interval (from every four to six years to every 12 years on HP turbines). Partial retrofits, though reducing expense through component reuse, do not normally confer additional lifetime to steam turbines.
The threat of SCC in the LP rotors and disk rims of two 1,170 MW pressurized water reactor units in California convinced plant management to perform a turbine retrofit. Some SCC had already been discovered on the stage 3 and 4 disk rims, and the rotor featured shrunk-on disk construction, increasing the likelihood of additional SCC. Various options were considered–including an ongoing repair program and the use of spare rotors–but retrofitting with new rotors was the only option that offered potential performance improvement in addition to long-term mechanical security.
GEC ALSTHOM has tested more than 200 specimens of varying tensile strengths in a steam environment to improve understanding of SCC. Whereas previous research suggested that SCC would not occur at stresses below 0.8 x yield stress, the GEC ALSTHOM research indicated that the stronger the material, the lower the applied stress necessary to initiate SCC. Therefore, the design objective is to minimize operating stresses in order to reduce the material`s yield stress. Replacing a shrunk-on disk rotor with a welded rotor eliminates assembly stresses in the disks. Welded, monobloc-forged rotors have much lower stresses in all areas previously susceptible to SCC, enabling the rotor material to be specified with a lower yield strength.
The GEC ALSTHOM Optiflow turbine design will provide an approximately 20 percent performance improvement compared with double flow designs. The number of inlet flows is reduced from six to three, allowing the blading on the first four stages to be about twice the length of corresponding blades in double flow designs. The new turbine also incorporates larger exhaust hood end sections to reduce exhaust losses. These sections were shipped and installed at the nuclear plant in early 1998 in advance of the December 1998 outage for the turbine retrofit. A 5 MW benefit is expected from the new hood sections.
A southern utility operates four 670 MW coal-fired units in Alabama to supply baseload power. Solid particle erosion of the HP nozzles had resulted in HP section efficiency losses of 3 to 5 percent between overhauls every five to six years. Such component degradation, plus the desire to improve the overall unit performance by increasing the HP section efficiency, led plant management to contract for a retrofit of the HP turbine inner cylinder modules.
The existing HP turbines are arranged for nozzle control with partial arc admission. Since the four units operate predominantly at full load, GEC ALSTHOM recommended conversion to full arc admission, which would increase the HP section efficiency. Partial load is achieved by throttling all four valves simultaneously, leading to much lower steam velocities through the HP nozzles than with partial arc admission, and reducing the extent of solid particle erosion.
The retrofit HP turbine will contain 11 stages, rather than seven, and utilize a single forward-flow first stage, rather than a double-row opposed-flow arrangement. The HP nozzles in the single forward-flow stage are much bigger and fewer in number than the existing nozzles, also reducing the likelihood of solid particle erosion. The new blade path is applied at a slightly reduced base diameter for optimum blade-to-steam speed ratios with longer blades and lower steam speeds. The rotating blades have integral blade tip shrouding, which provides better tip sealing than the existing riveted shrouding and facilitates coning of the blade passage ceilings to match the natural expansion of the steam through the turbine.
The retrofit contract guarantees a HP section efficiency improvement of 6 percent over baseline, which corresponds to an approximate 15 MW increase in output. Performance improvement will be verified using enthalpy-drop tests conducted immediately before and after the installation, which was completed in April 1998. p
New exhaust hood sections were air freighted to a 1,170 MW nuclear plant in California for installation prior to a 1998 outage for an LP steam turbine retrofit. Photo courtesy of GEC ALSTHOM.
Project personnel set a new HP/IP steam turbine into place at Omaha Public Power District`s Nebraska City Generating Station. Photo courtesy of Westinghouse.
Replacement inner cylinder assembly for an HP steam turbine retrofit at a 660 MW fossil unit in Alabama. Photo courtesy of GEC ALSTHOM.