Introductory remarks

It seems fair to say most owner/operators attending the 2015 meeting of the Steam Generator Users Group in Orlando, August 24-27, knew more about GE and Siemens steam-turbine (ST) products and services than those offered by Mitsubishi Hitachi Power Systems Americas.

Mitsubishi has increased its presence in the Americas dramatically since the millennium. Organic growth was first. Market traction achieved, next steps included acquisitions (MD&A, Pratt & Whitney Power Systems); a merger with Hitachi involving the energy equipment businesses of both companies; and heavy investment in facilities, such as the sprawling Savannah Machinery Works, opened in 2011, which last week shipped the first US manufactured M501J gas turbine.

So it made good sense to start the MHPS track with an overview of the company’s activities, and that was done quickly and efficiently in four slides. The opening presentation also got into technology, with an update on the company’s 40-in. L-0 issue. Most ST users are aware that erosion is a common issue for all last-stage blades (LSBs), but some MHPS units with 40-in. LSBs observed higher erosion rates than others (not believed a fleet-wide issue). A few experienced shroud damage, which was attributed to the high erosion observed under the shroud on the leading-edge side of the airfoil.

The OEM’s engineers were said to be working actively with individual users to monitor turbine-blade erosion. The latest design of the 40-in. blade (so called Type 3), provided to users experiencing unexpectedly high erosion rates, has Stellite welded to the shrouds. Periodic inspections are scheduled to verify expectations.

A quick update on generators in combined-cycle plants closed out the opening presentation. The company’s 23 VPI air-cooled and TEWAC (totally enclosed water-to-air cooled) units generally were operating without issues, the speaker said. There have been no rotor rewinds, full or partial. However, two sites, each operating for about a dozen years, have required stator repairs because of partial-discharge damage.

Nearly three dozen hydrogen-cooled generators in service for more than three years were said to be operating without major issues. There have been no reports of core looseness, and no rotor rewinds (full or partial) have been necessary. Access presentation.

MD&A overview, Chris Elmore

If you’re unfamiliar with MD&A, access this short (12 slides) presentation and quickly come up to speed. Elmore spoke from a bullet-point list of services and shop capabilities, including: generator services and stator and field repair capabilities, turbine repair facility, steam-path services, high-speed balance facility (located in St. Louis), field machining, valve repair, parts restoration, etc. He also introduced the group to Turbo Parts LLC, a unit of MD&A and a respected supplier of steam- and gas-turbine stationary parts and components—such as packing rings and spill strips. Access presentation.

Hardware, upgrades, and capabilities, Mark Passino

Chris Elmore introduced Turbo Parts and Passino, the next speaker, provided details on the company’s expanded capabilities and improved lead times, thanks to a new facility in Danvers, Mass. The lead engineer for parts talked about upgrades for main-steam stop/control and other high-pressure valves, explained the company’s packing wear assessment process, and custom bolting. Access presentation.

Valve repairs and capabilities, Jason Wheeler

Inspection and repairs—both in the shop and onsite—were the focal point of this presentation. Wheeler said the company’s 2014 activities included servicing more than 220 valves from 40 units—mostly GE and Siemens (Westinghouse) bypass and main stop valves, but also ones from other manufacturers. Repair of large butterfly valves is another core competency.

Wheeler had separate slides with details on the inspection of stems, discs, and seats, as well as on the repair or replacement of stems and discs. He presented case histories on the re-machining of seating surfaces and determination of Flexitallic gasket crush, both in the shop and in the field, to correct leakage issues. Several slides on replacement parts/material analysis/weld repairs closed out the presentation. Access presentation.

Alignment methodologies and applications, Peter Oehman

The speaker began with a question, “Why align?” and then answered it:

• To correct bearing loading, thereby reducing vibration and the chance of bearing failure.

• To correct misalignment caused by turbine foundation settlement.

• To minimize rotor stresses caused by improper bearing position.

• To prevent new seals from rubbing.

Oehman went on to describe the methods of alignment—a good primer for in-plant training. The methods he discussed were alignment bar, optical, tight-wire, lead wire, electronic radial alignment gauge/concentricity alignment tool (ERAG/CAT), and laser.

Next, the speaker talked about the importance of understanding shell distortion and how it affects alignment. As you might have known, proper alignment of stationary components with respect to the rotor is essential for efficient sealing of steam. Improperly aligned steam-path components reduce efficiency and increase the likelihood of vibration caused by rubbing. Be sure to consider the effects of piping forces as well; they can contribute to shell distortion.

Oehman closed by describing traditional types of alignments—tops-off and tops-on/tops-off—as well as MD&A’s Topless Alignment®. The last was described this way:

• Centerline measurements are made tops-off.

• Shell horizontal joint distortion is measured and used to determine centerline component tops-on to tops-off movements.

• Calculated movements are used as offsets when aligning in the tops-off condition. Access presentation.

Turbine/generator vibrations, Dr Michael Hine

Hine began by listing the causes of unbalance: erosion, lost blade, bowed rotor, balance correction. He then presented a short primer that covered limits for pedestal and shaft vibrations, rotor position measurements, sub-synchronous vibrations, harmonics, and bowed-rotor correction.

This presentation probably would be of greatest value to someone familiar with turbine vibrations and looking for a refresher. Example: Oil whirl was one phenomenon mentioned. The speaker said it develops as a sub-synchronous vibration typically at 0.47 multiplied by the run speed; orbit diameter increases over time, tracking rotor speed (whirl). When it locks on to the system frequency (whip) the situation can become dangerous. One way to minimize sub-synchronous vibrations, Hine said, is to reduce fluid circulation in the bearing; changing the bearing design may be necessary. Access presentation.

D11R-HIP retrofit

Presentation begins with a list of D11 issues that could compromise unit reliability—including shell and head cracking, rotor bowing, diaphragm dishing. One suggestion for avoiding these problems: Retrofit the high-/intermediate-pressure turbine with one using advanced technologies for the airfoils, rotor, diaphragms, etc, available from MHPS. The speaker said benefits include a performance increase and reduced O&M costs. He closed by saying LP cylinder retrofits were available as well. Access presentation.

Generators, Andrew Adam

Adam identified the following issues with generators in combined-cycle plants that the GM of Generator Services said MD&A could repair easily—the first using a patented fix:

• 7FH2 spring migration. This problem affects the 7FH2 fleet with LCI start capability and is believed caused by heating and cooling cycles.

• 7FH2 and GE 324 dry ties.

• Alstom/ABB pole crossover cracking. Access presentation.

MD&A St. Louis shop overview, David Rasmussen, PE

Rasmussen covered much of the same territory as Chris Elmore (see above).

MD&A repairs, David Rasmussen, PE

Rotor bowing was the first topic introduced by the speaker. Bowing is created from surface stresses above the yield strength of the rotor material, Rasmussen said. Residual stresses must be relaxed, he continued, and if surface areas are rubbed and hardened, the excess hardness must be removed along with any surface cracks. Failure to remove excess hardness is conducive to rotor cracking and failure. The group was told MD&A’s St. Louis facility is equipped to straighten bowed rotors.

Next topic: Defects in stationary parts for A13, D10, and D11 steam turbines—specifically N-2 packing casing (1) male and female fits and (2) distortion (out of round), and diaphragm dishing-deflection downstream. Each of these generic weaknesses (speaker’s words) was described in detail with good photography. Then Rasmussen offered details on repairs the company uses to correct the issues; alternatively, it can supply new parts. Access presentation.

Last-stage-blade overview, David Rasmussen, PE

Challenges abound for designers of last-stage blades (LSBs), which are particularly important because they provide about 10% of the steamer’s output. Consider the following:

• LSBs are the longest and most massive blades in the turbine.

• Natural frequencies must be dampened.

• They operate in a wet-steam environment.

• High-strength materials are required.

• They are subject to high- and low-cycle fatigue.

As a result, LSB’s are expensive and delivery times are long.

Rasmussen identified the following design philosophies to provide frequency damping and discussed the generic weaknesses associated with each:

• Free standing vanes.

• Lashing lugs part way up the vanes.

• Grouped blade tips, up to continuous coupling.

He included in the presentation the generic weaknesses associated with finger-tip roots used on some long blades. CCJ ONsite previously covered this problem on 40-in. titanium blades and where to look for cracking during inspections. Note that such root cracking also has been identified in 400-series stainless steels.

To accommodate the wet steam environment, designers consider such materials as titanium alloy, Jethete (a martensitic stainless steel), and 400 series stainless steels; plus, leading-edge erosion protection. Access presentation.

Remaining life assessment, David Rasmussen, PE

The speaker provided several summary slides identifying the data required to perform the necessary engineering and metallurgical analyses associated with a remaining-life assessment (RLA), and several more slides outlining both the specific analyses and tests recommended and the computational methods used to make the necessary calculations. For anyone contemplating an RLA, reviewing the dozen slides Rasmussen prepared is a good place to start. Access presentation.