Rough nozzle surfaces hurt turbine performance
Specifying maximum allowable roughness when inspecting and refurbishing gas or steam turbine components can help avoid costly efficiency losses
By J. Jeff Butler, Sithe Energies Inc.
Plant operations and maintenance personnel are constantly working to maintain design thermal efficiency or, in many cases, to reduce the rate of efficiency decrease. It is well known that keeping the airfoils of a turbine free-flowing helps to maintain pumping efficiency and subsequently helps to keep thermal efficiency and output capacity high. At major overhauls of both gas and steam turbines, it is common to grit blast airfoils. This is done as part of a nondestructive testing (NDT) inspection process and, in the case of steam turbines, to remove products of carryover. But grit blasting can create a rough surface finish, resulting in a loss of turbine efficiency. Plant operators can improve thermal efficiency at low cost by improving surface finish (roughness) during gas and steam turbine component inspection and refurbishment.
To date there has been little information or discussion about opportunities for maintaining free gas flow in the power turbine section. Unlike compressor cleaning, nothing can be done to maintain free gas flow in the power turbine section during normal operations. However, if proper care is not taken to assure smooth components after an inspection or refurbishment, an efficiency penalty may be-and often is-experienced.
Liners, transition pieces and turbine nozzles are the most frequently refurbished major components of a gas turbine. Turbine nozzles are the components that will have the most affect on efficiency where surface finish (roughness) is concerned. Although both nozzles and buckets have the highest ratio of surface area to the cross-sectional area of the gas path, buckets have a lower differential velocity between the gas and bucket surface, making them less affected by roughness than nozzles are. Also, the higher the pressure of the gas, the more important roughness becomes.
At a major overhaul, the compressor is thoroughly cleaned and several, if not all, of the turbine nozzles and buckets (blades) are replaced or refurbished. This restores much of the efficiency that had been lost over the previous several years` operation, but it usually does not restore efficiency to as-new condition. The reasons for this are many, but a major reason is that refurbished parts are unlikely to have the exact same measurements as those of new parts. Experience has shown that careful specification of turbine component refurbishment is very important in maintaining cycle efficiency.
Table 1 was published by a major gas turbine original equipment manufacturer (OEM)1 and shows actual test results of the effect of rough surfaces in a power section of a heavy-duty gas turbine. Note the significant impact rough finishes have on heat rate (efficiency). The table shows the performance loss that could be expected for each stage with a 260 micro-inch (µin) surface finish. If just one stage had a decrease of 1 percent in performance, the multistage turbine would have a performance loss less than 1 percent. If all the stages suffered a 1 percent performance loss, a compounding effect would cause a loss significantly more than 1 percent.
Gas turbine field readings
The following is a list of actual field readings the author accumulated and averaged to show comparisons of heavy-duty gas turbine nozzle roughness in various stages of component life cycle:
z nozzle (new, never used) roughness, 60 µin,
z nozzle (never refurbished be fore) roughness after 32,000 hours of service 110 µin,
z nozzle roughness, refurbished and grit blasted with 220 aluminum oxide, 160 µin,
z nozzle roughness, refurbished and ceramic coated with thermal barrier coating (TBC), 400 µin and
z nozzle roughness and ceramic coated (TBC) after 24,000 hours of service, 750 µin.
Nozzles are originally shipped from the OEM with a smooth finish. Once in service for a full operational interval, the nozzle becomes more rough, decreasing efficiency. At this point, if the nozzle is sent out for refurbishment and surface finish is not specified, the nozzle may come back from the shop in a rougher state than when it was sent. The field readings shown previously clearly indicate opportunities for increasing turbine output and efficiency. These readings indicate that, by following a strict specification, some refurbishment shops could improve performance by polishing nozzle segments after the last grit blasting and prior to reinstallation. Additionally, careful consideration should be given to specifying maximum roughness for TBC.
Figure 1, published by a major steam turbine OEM,2 shows that surface roughness can have significant impact on steam turbine performance. It is common practice during steam turbine overhauls to remove the rotor and to NDT dovetails, blade sections, diaphragms (nozzles) and other components. Many contractors use a 220 grit aluminum oxide (AO) blast to clean and prepare the surfaces for NDT inspection.
This can leave a surface finish of approximately 160 µin (typical 220 AO finish). The figure shows that this practice can be detrimental to steam turbine performance. An alternative blasting material, such as glass beads, could decrease the amount of roughness left on aerodynamic surfaces; or if a heavier blast material, such as 220 AO, is needed to remove products of carryover, blasting operations could be followed by polishing operations. This is particularly important on the high-pressure stages.
Although large generating plants have been measuring surface roughness and polishing high-pressure steam turbine airfoils for years, many smaller turbine owners and contractors have thought this unnecessary for lower pressure turbines (900 to 1,200 psi). In today`s competitive market, it may be time to reconsider this position.
Measuring surface roughness
Surface roughness, waviness, lay and flaws are elements of surface texture. Surface texture consists of repetitive or random deviations from the nominal surface which form the three-dimensional surface topography. Whereas surface roughness consists of the finer irregularities which are present. Ra is the universally and internationally recognized parameter of roughness. Roughness is defined as the arithmetic average (AA) deviation of the surface expressed in µin from a mean line or centerline. Surface roughness is still sometimes displayed with the symbols AA, CLA or c.l.a. and can be measured using several techniques. The most prevalent measuring technique for surface roughness employs a mechanical-electronic device with a readout indicating the roughness of the surface profile taken during the passage of a small-radius stylus over a short straight-line path on the surface. The most common diamond stylus has a 0.0004-inch (in.) (10 mm) radius and usually is used with a 0.030-in. (0.8-mm) cutoff width. The total stylus travel is usually 20 to 60 times the cutoff width with the electronic circuitry continuously averaging the readings over the set cutoff width. This type of instrument was used by the author to gather readings for this article. The electronic gauge is placed on the sample and, when activated, a diamond stylus is dragged across the sample. A digital readout shows the Ra value. This device is very accurate and costs approximately $1,300.
Grit blasting is commonly used in the refurbishment process to remove scale or debris and to clean the component in preparation for welding and NDT. The most common grade is aluminum oxide 220 grit, although some repair shops commonly use 180 grit (more coarse). Grit blasting removes metal and must be administered carefully to not change the airfoil shape. Some repair shops perform a cosmetic final grit blast after all repair operations.
Polishing turbine components
Polishing turbine nozzle segments will help to reduce the roughness resulting from grit blasting operations. Polishing is best directed at the low-pressure side in the divergent area of high-pressure nozzles. Rotating airfoils also benefit from polishing but to a lesser extent. Polishing would best be performed at the inspection/refurbishment facility. Polishing should be performed after the last repair or inspection operation and just prior to coating (if applicable) but could also be performed on site using local personnel.
Tests performed at the Sterling Energy Facility indicate that a millwright can polish by hand a 24-square-inch uncoated nozzle segment from 160 µin to 60 µin in less than 5 minutes using aluminum oxide cloth (emery paper). Power tools are much faster. Polishing uncoated turbine nozzle segments should typically be performed using a fiber wheel or drag finisher. A die grinder with pads of nonwoven abrasives (scotchbrite) attached could be used, also. It is important to remove only the high spots and not to remove any of the base metal. The Electric Power Research Institute (EPRI) has specified an 80 µin maximum roughness after refurbishment for first stage turbine nozzles for heavy-duty gas turbines,3 yet it is interesting to note that repair shop personnel re port that few customers specify a maximum surface roughness for their refurbished gas turbine parts. According to knowledgeable sources, polish ing three complete sets of nozzle segments (N1, N2, N3) of a medium-sized, heavy-duty gas turbine at the refurbishment shop should take approximately one man-day and not cost more than $1,000.
Although it would take extensive research to quantify efficiency gains, evidence presented here suggests that prudent maintenance management requires the ability to measure turbine component surface finish and issue a standard or specification when sending components out for refurbishment. Careful consideration to inspection and refurbishment procedures, including the type of grit blast material and post-blast polishing, could have a significant impact on future cycle efficiency.
J. Jeff Butler is a maintenance manager for Sithe Energies Inc. at the Sterling Energy Facility. He graduated from the Calhoon Marine Engineering School as a licensed marine engineer in 1983. He is president of the Northeast GE Steam Turbine Users Group and has been involved in cogeneration plant maintenance since 1985.
1 Distributed at GE Power Systems` 1996 Pacific Region Gas Turbine Users Conference, Feb. 1-8, 1996.
2 EPRI Specification 7BNOZ1, June 1992 Repair Specification for General Electric MS7001B First Stage Turbine Nozzles.
3 Schofield, P., "Efficiency maintenance of large steam turbines," General Electric Co., Large Steam Turbine-Generator Department, presented at the Pacific Coast Electrical Association 1982 Engineering and Operating Conference, San Francisco, Calif.
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Author measuring nozzle convex surface roughness