When gas turbines were first added to utility generation portfolios in large numbers, in the late 1960s and early 1970s, they were installed primarily for emergency service—a hedge against blackouts. You may recall that a Frame 5 owned by the Long Island Lighting Co (Lilco) is considered by many now-retired industry personnel as critical to the restoration of power following the Great Northeast Blackout (50 years ago this coming November).
Back then, 90% of the electricity produced in the US was generated by regulated electric utilities with a management corps dedicated to beefy fossil-fired steam/electric plants. They “tolerated” gas turbines but saw no future in them for bulk power production. A large GT in those days was a 20-MW machine. Today, gas turbines with 10 times that capability are commonplace and the domestic installed capacity of simple- and combined-cycle units powered by GTs exceeds that of coal-fired steam plants.
Over the last half century, the philosophy of asset managers has gone from “run gas turbines until they quit” with little to no investment after installation, to keeping GTs in service for the long haul. The latter requires a maintenance program that includes upgrades to assure competitive efficiency, meet ever-more stringent environmental regulations, assure a high starting reliability to keep favor with grid operators, maximize reliability/availability, etc.
Engine improvements demanded by owner/operators have kept OEMs and their third-party competitors busy over the last several years. Technologies incorporated into the designs of the latest gas turbines—H and J frames, for example—have been cascaded to mature fleets, helping to keep those units “in the money.”
A case in point is the work done to upgrade/uprate the Siemens W501D5 (89 units operating today, first installed in 1982) and W501D5A (61 units operating today, first installed in 1996). Advanced components offered both by the OEM and Mitsubishi-Hitachi Power Systems have been featured at the last two meetings of the 501D5-D5A Users, chaired by Gabe Fleck, manager of gas plant operations for Associated Electric Co-op Inc.
Siemens solutions
Judging from the number of entries in CCJ’s annual Best Practices Awards program, worker safety came of age as a top priority at powerplants eight years ago. Three years later, the number of entries for safety best practices topped all other categories—including O&M. More recently, many plants have earned Star status in OSHA’s Voluntary Protection Program (VPP), typically acknowledged as the platinum standard in personnel safety.
A plant’s safety net extends beyond its staff to all contractors and site visitors. Any injury is a black eye. This means contractors must have their safety programs approved by plant management before they can do any work. Violations will stop jobs for a sit down and review of the offenses. Serious violations will get a contractor kicked offsite. This doesn’t happen often but it does happen.
So, it was not surprising that the first presentation by Siemens at the 501D5-D5A Users’ 2014 annual conference and vendor fair in Key Biscayne, Fla, would review the field-service organization’s enviable service record and stress its continual improvement in critical areas such as job-hazard analysis (JHA), proactive safety stand-downs, and personnel protective equipment (PPE).
Closely related to the JHA program is Siemens’ pre-job briefing requirements, which includes its two-minute drill to focus worker awareness before starting a task. Here’s how it works: A card issued to all personnel that fits in a shirt pocket or can be attached to a badge shouts out, “Before performing work, ask yourself. . . .” On the back of the card are 10 questions such as these two: Where could there be stored energy? What’s in or under my load path? You get the picture.
The speaker next discussed safety policies established by the Siemens service organization in the last year—such as special shrouded hoods, eyewear, and air-purifying respirators for welding and grinding activities. Most important, perhaps, was the work done by the OEM to further ensure safe rotor lifts. This effort was encouraged by several recent rotor “drops” experienced by the industry—most offshore.
Siemens engineers developed an improved rotor lifting process with designed-in checks and holds during the lift. The procedure stresses the requirement for individual accountability of all actions. Modifications to the lifting beam and rotor skid were part of the effort. More specifically, a bubble level was permanently installed on the lifting beam to guide those conducting the lift and a piece of neoprene sheet is positioned between the sling and the clean rotor to improve friction.
Plus, a special plate is installed on the turbine end of the rotor prior to the lift to eliminate the unlikely possibility of the sling sliding off the shaft during a lift. A coupling prevents that from happening at the other end of the spindle. The so-called “disaster plate” is removed after the lift is complete. For more information on this development, and on other topics discussed below, visit the company’s Customer Extranet Portal (CEP).
A fleet statistics update is part of every Siemens user-group presentation and it seems to be of interest to attendees judging from facial expressions. The OEM tracks fleet reliability, availability, and starting reliability. These percentages were mid to high 90s for both the D5 and D5A fleets, but the statistical base could be better. Example: only 26% of the D5 engines reported operational data requested by the manufacturer, 49% of the D5As. By contrast, data are received from all units in the 501G fleet and from nearly three-quarters of the 501F engines.
This information is critical to the OEM to facilitate identification and prioritization of potential improvements. The user-group leadership understands the importance of the data acquisition program and it is working with Siemens to improve response. Making data entry easier is one initiative. Expectations are that later this year reporting of unit information will be done online via the CEP.
Turbine component sealing was the subject of a presentation encompassing more than a dozen slides. Minimizing leakage is important because of its negative impact on power output and fuel consumption. Seals should be checked seasonally during borescope inspections and replaced if damaged. At a minimum, replace seals during hot-gas-path inspections at 24,000 equivalent operating hours or 900 equivalent starts, the group was told. Interestingly, some D5s and D5As may never require an HGP inspection because they don’t start often and operate most years at a very small fraction of the period hours (2% or less).
The speaker said the seals on the inside and outside diameters of the R1 vane segments impact performance most. About half of the total performance loss caused by ineffective sealing occurs at these locations. Another 11% of the total loss occurs at the R1 ring segments and yet another 10% from leakage by static and ring seals. This means about three-quarters of the performance loss attributed to ineffective sealing typically occurs in Row 1. Here’s a summary of the material presented:
- R1 vane seals reduce the loss of cooling air between vane segments thereby preventing distortion of the airfoils.
- R1 static seals prevent ingestion of hot gas into the disc cavity, thereby minimizing the potential for rotor-disc damage.
- R1 isolation ring seals reduce the leakage of cooling air between isolation right segments, helping to prevent blade-ring distortion.
- R1 ring-segment seals reduce the flow of cooling air from between ring segments, thereby minimizing the potential for blade-ring distortion.
- R1 seal pins prevent hot-gas ingestion into the blade shank cavity, minimizing the possibility of blade cracking.
- R1 blade seal plate prevents hot-gas ingestion by the blade shank cavity to help prevent blade cracking.
The same seals in Row 2 have a relatively minor impact in performance loss when they leak—roughly one-fifth of the average R1 loss.
Strut and splitter-plate cracks in the compressor inlet manifold (both top and side entry) have been reported by some users. An inspection service bulletin available on the CEP was suggested reading. Corrective action likely will be based on strut features used on the company’s 501F.
Diaphragm retention screws prevent the diaphragms from rotating in their cases. A small number of screw-fracture incidents reported on W501FC units resulted in diaphragm rotation and related wear and disassembly issues. A durability retrofit mod was developed for the FC based on the W501FD. While no similar incidents have been reported on W501D5-D5A units, users requested a similar durability enhancement for these frames. An alternative suggested preventive measure: Replace screws at every major inspection.
Adjustable root springs for R4 turbine blades, which force airfoils radially outward when the unit is on turning gear (less than 125 rpm), are designed to reduce wear and tear on blade and disc fir trees as well as the potential for an unbalance issue caused by a shift in blade position. The latter can increase vibration on engine restart. Work on the development of springs for air-cooled blades in Rows 1 through 3 continues.
Spring installation in R4 is relatively easy because of access to the uncooled fourth-stage blades from the exhaust end of the unit. The springs are designed for long life with replacement expected only during HGP inspections. They can be tightened when necessary to compensate for creep. Heat treatment of springs after manufacture relaxes the metal and minimizes the potential for creep.
Rotor and casing inspection and evaluation, RCIE in Siemens lingo, reviewed the findings from inspection of 23 rotors—including coupling-hole galling, marriage-coupling spigot wear, unacceptable curvic-clutch findings, belly-band slot wear, and compressor journal scoring and pitting. The goal of the presentation was to show users that certain things could happen with their engines that likely would not be identified unless the units were disassembled.
The speaker’s suggestion was to thoroughly inspect the unit during the HGP prior to the second major to identify action items for the upcoming major and assure the necessary components would be available without adverse schedule impact.
Modernization and upgrade products for extending scheduled inspection intervals, improving operational flexibility, and increasing the thermal performance of W501D5 and D5A gas turbines also were discussed. The presenter summarized experience with the following recent product introductions—all inspection-interval extension capable:
- Si3D blading was successfully implemented for the first time on the D5 and met design expectations: GT power output increased by about 7.5 MW, heat rate improved by approximately 500 Btu/kWh.
- A new bolted-design compressor rotor with improved vibratory response is capable of a fast start to full load.
- Redesigned DF42 components have addressed durability concerns with D5A water-injected combustion systems. The new-style transitions are in service and meet design parameters. New-style baskets and WI nozzles are ready for their planned first installations.
Mitsubishi-Hitachi
Wendel Zolyomi of Mitsubishi Hitachi Power Systems (MHPS) had a great deal to tell W501D5-D5A users at their organization’s winter meeting in early February. For newcomers, it seemed like a lot to assimilate in one sitting. O&M personnel with fleet experience were familiar with at least some of what the speaker had to say; however, the review of the company’s parts, reliability enhancements, and operational solutions was timely with spring overhauls not far off.
The improvements Zolyomi discussed were products of the large investment made by the company in gas-turbine development. Hector Valenzuela, manager of service sales and marketing, told the editors during a break that “as MHPS progressed through the engine designs and product evolution from the M501D to M501F to M501G to the current M501J, it has continued to pioneer the use of materials, coatings, and cooling schemes to advance GT technology.
“Since all of these engine platforms share the same design basis and product evolution,” he continued, “many of MHPS’s advancements can be retroactively applied to prior-generation machines.” This enables D5 and D5A owner/operators to benefit from the leading-edge technology derived from the manufacture and support of the company’s fleet of advanced GTs.
O&M personnel at plants equipped with 501D5s and 501D5As who weren’t able to get all the necessary details, or were not able to attend the conference, can access the presentation at the user group’s website. If you’re qualified to receive this information but not registered to access the secure parts of the site, do that as soon as possible. Refer questions to Chairman Gabe Fleck, manager of gas plant operations for Associated Electric Co-op Inc.
Zolyomi began with a rundown of components offered by MHPS for the D5 and D5A engines. For the latter, these included 16k hot-gas-path (HGP) parts of improved design that have been validated in 501F and 501G engines. The speaker focused on blades and vanes for Rows 1 and 2 and R1 ring segments; plus, rotor components and outage kits. For the D5, HGP parts and major stationary and rotor components were covered.
D compressor section. Issues related to welded R1 diaphragms can be solved by switching to the MHPS design, Zolyomi said; it relies on mechanical fasteners to dampen vibration. Mitsubishi engines with this type of diaphragm have accumulated more than 2-million hours of service since its introduction in 1999. The first assembled diaphragm for R1 of a W501D5A dates back to 2007. A year later the same technology was installed in Rows 1-3 of a W501FD2; in 2010, Rows 4-6.
To deal with seizing of IGV (inlet guide vane) roller bushings encountered by some users, the speaker offered his company’s enhancement: It features a bearing material to reduce friction and an enlarged cross section for added strength. This solution has been installed on 10 F- and G-class units, the group was told. Perhaps the highlight for maintenance managers is that it’s possible to mix existing and enhanced roller/bushing/bolt combinations to produce a specific assembly.
Turbine parts. Zolyomi next compared materials used by the OEM for its D5A turbine parts with those provided by MHPS to achieve extended life. Vanes for Rows 1 and 2 are made from Mitsubishi’s proprietary MGA2400, which also serves in F-class engines. The OEM was said to have supplied parts made from ECY768, a proprietary alloy which predates the Siemens acquisition of Westinghouse. For turbine blades, the OEM relies on IN-738, Mitsubishi offers MGA1400, also with advanced-frame experience. Materials for blades and vanes in Rows 3 and 4 are the same for both companies.
The speaker continued: Materials for W R1 ring segments are Hast X/X-45, M uses MGA2400; R2 ring segments from W are of Hast X and those from M, X-45. Again, both suppliers supply parts for Rows 3 and 4 made from the same materials.
Zolyomi then offered some best practices/lessons learned regarding exhaust systems. He said some maintenance managers have found exhaust casing drains so corroded that hot exhaust gases were leaking into their GT enclosures. Suggestion was to inspect for leaks around the exhaust drain piping when possible. Look for color changes, he said, on surrounding insulation and/or oxidation on the floor below.
Also, inspect drain piping using a borescope to inspect for internal corrosion, oxidation, or blockage. If you find damage, the speaker continued, replace the drain piping with a material having improved oxidation and corrosion resistance.
A few troubleshooting slides offered possible solutions to nagging issues. The speaker asked: Are you experiencing varying vibration signatures on startup and/or shutdown? A bolted air separator may be the solution.
Use of root springs is not the only way to reduce wear and tear on both blade and disc fir trees. The speaker explained the value of intermittent turning-gear operation, which contributes to fir-tree health in Rows 1-3 as well as R4. At this time, the spring solution is viable only for R4 because of challenges associated with both cooling-air flow and access to root areas of blades in the first three rows.
As a general recommendation for the 501D5-D5A fleet, Zolyomi said, MHPS engineers suggest running on turning gear for 96 hours after a GT stop, to cool the rotor and prevent rotor bending. Put the unit on turning gear for two hours every week it doesn’t operate to prevent rotor sag, he continued. Before restart, put the unit on turning gear for 10 minutes to put a lube-oil film on the bearings. Then take the unit off turning gear and restart. Controls logic changes likely will be required to operate the unit in this manner.
The foregoing procedure has been used successfully by a Mitsubishi Hitachi D5 at the Makiminato plant in Japan since 2003. That generating facility confirmed that shaft deformation around the torque tube, and shaft vibration were not experienced; plus, there have been no abnormal issues reported since implementation of the procedure. Groove wear in the fourth-stage disc was identified because of long-time turning gear operation and the fix applied was installation of a root spring.
The topic of compressor-rotor disc material was discussed by Zolyomi. He provided details on latter-stage material degradation and cracking that has been observed on high-hours units during service work. When implementing a rotor life-extension project and the compressor section will be replaced, he said MHPS recommends upgrading to discs made of MGA10D, which has improved material properties.
The company also reported success with its new nozzle purge-air system, which was commissioned in January 2014. Nozzles were removed and inspected at the end of April. They were clean and there was “almost no sign of burnt/coked oil on them.” Previously, nozzle coking caused blade-path and emissions excursions after about 10 start/stop cycles.
There was a brief case history on the successful refurbishment of a D5 rotor with more than 165k hours of service. New Mitsubishi turbine discs for rows 1 to 4 were installed. Inspection and refurbishment of service-run components included attention to the air separator, disc adaptor, torque tube, and assembly hardware. Updated belly bands were installed and the rotor was low-speed balanced.
Zolyomi wrapped up his presentation with about 10 slides dedicated to several significant engine and component inspections, repair services, shop capabilities, etc. CCJ
