MHPS offers alternative solutions to address fleet issues

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 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. Access this article also to learn more about the company’s two-ply belly-band upgrade and R4 blade root springs to reduce turbine-disc serration wear.

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. 

Posted in D5-D5A Users |

Comments are closed.