7EA vendor partners contribute to plant performance improvements, safe work practices

Presentations by suppliers expose users to new technologies and solutions for improving unit performance and personnel safety, providing valuable perspective on topics of importance. At the 2017 7EA Users Group meeting, 27 companies interfaced with owner/operators from the podium. There was GE Day, the Bus Duct Roundtable, the Rotor Life Roundtable, and more than a dozen individual presentations. Plus, the vendor fair brought users face-to-face with about 100 product/services providers on Wednesday evening.

Steering-committee members know how sensitive their colleagues are to sales pitches, so presentations are invited, then vetted to eliminate more than expected horn-tooting. Inviting experts to address specific topics minimizes duplication of subject matter and expands the learning experience. Subjects covered in last year’s vendor presentations included the following: gas-turbine inspection, DLN-1 troubleshooting, controls issues, fuel systems, training, generator circuit breakers, rewinding of generators, coupling best practices, exhaust-plenum improvements, control-system tuning, generator diagnostic, air filters, and actuator overhaul.

Quite a lineup for sure and what you can expect at the 2018 meeting. Register today! 

Short summaries of several 2017 presentations selected by the editors for coverage are below. To dig deeper, and to access other PowerPoints, go to the user group’s website. You’ll have to register if not already a member.

“Troubleshooting DLN-1 Primary Re-Ignitions,” Mitchell Cohen, Turbine Technology Services Corp. 

Mitch Cohen, one of the industry’s leading experts on the DLN-1 combustion system, always has been ready to share his considerable knowledge with 7EA owner/operators. CCJ has been covering Cohen’s presentations since he spoke on DLN-1 basics at the 2007 meeting of the 7EA Users Group in San Francisco. If you are new to the industry and the DLN-1 you might want to begin there; it’s a valuable backgrounder.

At the 2017 meeting in St. Augustine, Cohen spoke to the causes of primary re-ignition (PRI) and how to mitigate this operational anomaly. Operators should be concerned about primary re-ignition, he told the group, because it can move the engine out of emissions compliance; plus, extended lean-lean operation causes excessive heating of the venturi in the combustion liner and incurs a 10× maintenance factor (TIL-1443 R2).

Cohen listed the following as possible causes of primary re-ignition:

    • Spontaneous, unexpected ignition source in the primary zone—such as lube oil, hydrocarbon liquids in fuel gas, distillate in dual-fuel machines, and combustible matter in air (dry grass, for example). The speaker identified several common materials that might be drawn into the inlet duct through breaches in inlet filters or ductwork, or accidently left inside the machine during an outage, with auto-ignition temperatures below the compressor discharge temperature.

    • System instability—fuel, air, combustion dynamics; or an instability induced by faulty or mis-calibrated instrumentation. Cohen said PRIs after transfer from liquid fuel are relatively common. A possible remedy: Implement a temporary, higher-pressure purge in primary and lean-lean mode after fuel transfer.

    He also mentioned that wiring problems in the Mark V cabinet (so-called wire whiskers) are known to have spuriously energized the ignition transformer, thereby causing re-ignition. Don’t overlook the possibility of a faulty P-2 transmitter: It can cause a hiccup in speed-ratio-valve control.

    The most common cause of PRI during fuel transfer was said to be improper timing of purge valves. The specified opening time is 35 ± 5 seconds. Slow opening minimizes load and temperature spikes, while rapid opening (8.5 seconds) expels residual transfer fuel unevenly and in a slug-like manner causing a large load spike and high exhaust spread. The latter triggers the spark plug to fire, transferring to lean-lean extended mode.

    • Powering up of spark plugs, initiated by control-logic response to an alarm/fault condition such as high exhaust spread, thereby igniting a combustion source in the primary zone. Bear in mind that DLN-1 spark plugs cannot be retracted and that insertion depth is critical. Check the depth after a maintenance outage. Inserting a plug too far will burn up the tip and the spark plug will act as a glow plug to re-ignite primary fuel during the fuel transfer process.

    • Sub-optimal tuning.

“Considerations for the Rewind of Highly Cycled 7A6 Generator Rotors,” W Howard Moudy, National Electric Coil. 

Howard Moudy is a frequent presenter at user-group meetings and generally held in high esteem by owner/operators both for his knowledge of things generator and his willingness to share what he knows. Over the last several years, Moudy’s 7EA User Group presentations have focused on a variety of concerns with the 7A6 generators typically married to the 7EA, including the following:

    • Stator failures traced to housing leaks.

    • Stator-winding partial discharge.

    • Rotor main lead failure.

    • Stator endwinding looseness (dusting and greasing).

    • Rotor-turn insulation migration.

    • Rotor copper elongation and foreshortening.

The speaker moved quickly through his review to allow maximum time for coverage of the requested topic: rewinding of highly cycled 7A6 generator rotors. You can come up to speed on the subjects listed above with simple keyword searches in the online archives of CCJ ONsite and the International Generator Technical Community Forum. 

Moudy set the stage for his presentation with this brief introduction:

    • The rotor is the primary generator component most affected by cycling.

    • Many generators, the 7A6 in particular, were not designed for cyclic operation.

    • Rotational forces, or cyclic loading, can fatigue rotor components—especially those not designed for cycling.

    • Rotor endwindings not designed for cyclic operation can be modified and rewound to better withstand this demanding service environment and provide long-term reliable operation.

The last bullet point was Moudy’s segue to his 2017 presentation. Key points he made:

    • 7A6 rotors are experiencing issues in the rotor main lead, and cracking of the 90-deg right-angle-turn joint assembly of the coils, because of fatigue attributed to cycling. First step in avoiding a forced outage: Have a qualified technician borescope the main leads of your generators—regularly.

    • NEC reported having a coil-forming process to avoid the 90-deg right-angle turns on coils by forming a bend in the copper to eliminate the right-angle joint.

The speaker also mentioned migration of rotor-winding turn insulation because of insufficient bonding, noting the potential value of borescoping for identifying possible issues. Plus, he said poor design of the end-turn blocking contributed to copper elongation and foreshortening issues. Finally, Moudy suggested that users read IEEE C50.13 to learn more about the terminology used in lifecycle investigations and better understand what their machines are capable of in terms of start/stop cycles, in baseload and peaking service, etc.

“7EA Marriage Flange Load Coupling Hardware Solution,” Pat McCormack, Riverhawk Company.

Pat McCormack spoke to hardware issues associated with the 7EA marriage flange (turbine to compressor) and load coupling, and proven solutions. For example, marriage-flange bolting often is difficult to remove because of galling. Galled threads cause the hardware to seize, requiring that the bolts be cut off, which can add to outage time. Turnaround costs are increased as well because new hardware is required.

Recommended solution: Use hydraulically stretched bolting for the marriage coupling to eliminate galling. Tensioned studs were said to be reusable because no damage occurs to the threads even after multiple installation and removal cycles. Improved rotor balance is another reported benefit; plus stud design provides a convenient location for balance weights.

Tensioning tools also were covered in the presentation. For the 7EA, the load-coupling and marriage-flange tensioner are one in the same. The company developed a new, lighter-weight tool for easier handling.

Given the age of many machines in the fleet, McCormack said legacy tooling can be modified to permit marriage-coupling assembly with the new bolting.

The speaker also mentioned a solution for couplings requiring fitted bolts. The details: A sleeve is machined to the ID of the coupling hole (less a clearance). The bore of the sleeve is tapered and the bolt has an OD matching the sleeve taper. As the bolt is tightened, the sleeve expands to take up the clearance for a size and size fit between the coupling and bolt assembly.

“Overheat Detection in Air- and Hydrogen-Cooled Generators,” Christopher Breslin, E/One.

You don’t have to sit through many presentations at a meeting of the Generator Users Group to realize these machines often do not get the love they deserve. Forced outages occur more frequently than you’d like to believe, typically because there’s no one at the plant with a good understanding of how generators work, what inspections and maintenance they require to keep them operating reliably, and how to tell when problems are on the horizon.

This situation likely will get worse before it gets better because the relatively few generator experts in the power-generation community are retiring at an alarming rate, staff sizes are shrinking at virtually all plants, and the need for generator training can get a big “huh?” from owners without an O&M background.

Christopher Breslin discussed the importance of condition monitoring in protecting generators.

Operational risks increase as generators age and staff experience declines, he told the users. OEMs typically include the following condition monitoring tools with their equipment: temperature sensors to identify overheating indicative of insulation failure, overload (over-current) protection, vibration monitoring to warn of shaft/rotor imbalance, and hydrogen purity to alert about a possible safety issue.

But there’s still more monitoring equipment available to help mitigate risk, including: fiberoptic accelerometers to indicate end-turn vibration, partial discharge (PD) to warn of stator insulation breakdown, flux probe to identify shorted rotor turns, radio frequency (RF) to find arcing, dew-point sensor to track moisture level, and the generator condition monitors (GCM) offered by the speaker’s company for hydrogen- and air-cooled generators.

The GCM, Breslin said, can warn of back core burning, shorted rotor turns, foreign material, mechanical vibration, hot spots, and blocked rotor cooling. Several slides explained how the GCMs (one for hydrogen-cooled generators, one for air-cooled) work. What the GCM does is detect the sub-micron particles created during overheating and arcing; their numbers increase exponentially when those events occur.

An ion chamber is ideal for detecting sub-micron particles in hydrogen-cooled generators because their cooling systems are closed-loop. The solution is not so simple for air-cooled generators which can have open cooling systems. Their detectors must be highly sensitive, yet immune to false alarms created by the omnipresent dust and dirt in the atmosphere. Access Breslin’s presentation for details.

“7EA Exhaust Plenum Improvements,” Gary Martin, Cust-O-Fab Inc. 

Benjamin Franklin reportedly said, “. . .in this world nothing can be said to be certain, except death and taxes.” To that it’s safe to add “and gas-turbine exhaust system problems,” especially in cycling units more than about 20 years old and installed in areas with high humidity.

Knowing that one day you will have to make extensive repairs to your 7EA exhaust system, or replace same, it’s important to keep current on companies in the business of repair and upgrade and what they’re offering. Gary Martin’s presentation highlighted the exhaust-plenum improvements offered by COF Specialty Services, including the following:

    • Double-sealed wing door.

    • Forward and aft flex-seal insulating collar.

    • Drainable liner floor.

    • Full floating internal liner design.

    • Cold-flange design between components.

    • Material upgrade for flex seals.

    • Ability to install the exhaust plenum with turbine rotor and aft diffuser in place.

It’s difficult to understand these improvements without looking at the drawings Martin showed the group. The editors recommend accessing a copy of the presentation on the 7EA Users Group website to better understand COF’s design.

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