How to test, maintain 7EA IGVs, auxiliaries to achieve top performance

Dave Lucier, founder/GM of PAL Turbine Services LLC, was the featured speaker from the opening bell to morning coffee on Day Two of the 7EA User Group’s 2014 Conference, October 21-23, in Murfreesboro, Tenn. A former GE field engineer, who later instructed and managed the mechanical training of future field engineers, Lucier shared his knowledge on the servicing and change-out of inlet guide vanes with the rotor in-situ, and on the maintenance of auxiliary systems.

He credited his former partner, Charlie Pond, recently deceased, for the significant cost-saving maintenance techniques described in the IGV segment of the presentation. Pond was well-known to 7B-EA, 6B, and Frame 5 owner/operators for his field work and user-group presentations.

Variable inlet guide vanes. With many first-timers among the 136 registered users, Lucier began “at the beginning,” telling attendees IGVs had the following purpose:

      • Prevent compressor stalls during startup.

      • Modulate air flow at partial load to maximize exhaust temperature on units in combined-cycle and cogeneration service.

IGV Fig 1He urged O&M personnel to inspect IGVs regularly and replace those components not operating properly, and to upgrade airfoils on ageing machines to improve performance. Example: The C-450 material used for vanes today is stronger than yesterday’s Type 316 and 403 stainless steels, allowing a slimmer airfoil and more air flow through the machine. Common problems with IGVs include frozen bushings, broken shafts, worn bushings, and excessive gear backlash.

Regarding frozen bushings, Lucier said it is not surprising to find one or more IGV shafts seized in their bushings on ageing outdoor machines. Legacy bushings, he continued, were steel and susceptible to corrosion and binding. More recent GE turbines have Teflon®-coated outer bushings and Chemloy bushings on the inner diameter (Fig 1). But keep in mind that while Chemloy bushings are much softer than steel, and never bind, they do wear out much faster.

IGV shafts can break at the pinion gear on machines suffering severe corrosion (Fig 2). If this happens at your plant, take immediate corrective action. Be aware that when a shaft breaks, the IGV generally reverts to the closed position, which is conducive to high-cycle fatigue of first-stage compressor rotor blades. Failure of an airfoil can cause catastrophic downstream damage.

Rack-and-pinion gears suffer wear and tear over the years and must be checked for excessive backlash. Lucier said 40 mils is the maximum backlash allowed; maintenance personnel should shim rack gear for 5-10 mils of clearance.

The speaker then reviewed the technique developed by Pond to service or replace vanes in the lower half of the engine without removing the rotor. This had been described at a user group meeting several years ago. In closing, he suggested O&M personnel become familiar with the following Technical Information Letters to assure proper inspection and maintenance: 421, 511, 515, 517, 522, 532, 1013, 1041, 1068, and 1132.

MIGV Fig 2aintaining auxiliary systems. A common thread linking Lucier’s presentations was the value of well-trained OEM field engineers to owner/operators. The speaker, mindful of the challenges facing O&M personnel new to the ageing 7EA fleet, walked the group through lessons learned and best practices, several documented on his company’s website.

Lucier began by saluting field engineers for their problem-solving abilities and linking their efforts to the fleet’s solid performance over the years. Judging from facial expressions, it came as a surprise to many in the room that critical parts of legacy units were made at different facilities and first met at the job site, where FEs were charged with assembling the units. He said the gas turbines were made in Greenville, SC; generators in Schenectady, NY; control cabs in Salem, Va; and the generator auxiliary cabs in Chamblee, Ga.

Also noteworthy: Factory testing was done on oil and full-power tests actually were conducted at about two-thirds of rated output because the turbine was driving its own compressor, albeit it not connected to a generator in the factory. That meant field engineers also were responsible for achieving contractual requirements and fulfilling the owner’s expectations. Theirs was an awesome responsibility, Lucier noted.

Next, he asked, and answered, the following questions to illustrate the importance of auxiliaries to unit availability (the ability to start and operate when expected) and reliability (the ability to achieve base rated load when called upon):

1. What is the most expensive part of a gas turbine? Most would say the rotor.

2. What would you say is the least expensive part of a GE gas turbine? Difficult to say.

3. What is the least expensive part, that if you needed one and didn’t have a spare, you learned it is no longer available to purchase and you can’t make one?

Lucier paused after the third question and asked attendees to think about it for a moment. Then, he said, “Chances are, a serious problem with a component in an auxiliary system will prevent the GT from starting, or initiate an emergency shutdown.” Startup challenges likely will occur during firing/flame/warmup and when ramping to base load, Lucier noted.

Electrical and controls equipment were at the top of the speaker’s list. Motor control centers, often overlooked in equipment assessments, require ongoing attention, he said, because they control the fluid systems critical to engine operation—including lube and hydraulic fluids; fuels (oil and/or natural gas), cooling and injection water; atomizing, cooling, and sealing air, fire protection, etc.

Lucier said GE motor starters are heavy duty and reliable, but they still need regular servicing. He stressed that plant startup hinges on DC motor-driven pumps. Gear drives may take over when the unit is up and running, the speaker continued, but achieving high starting reliability demands that motors run when required. He showed photos of equipment that was past-due for maintenance and how careless personnel had mistreated junction boxes.

Servicing of battery cells and charger for emergency DC power were the next topic. Maintenance practices published previously offer details; a more recent post addressed NERC’s proposed requirements for station batteries to assure grid reliability. Referring to transducers and sensors, Lucier said, “Don’t assume these things will last forever.” He urged users to (1) have a lab periodically test pressure transducers, (2) send servos to the manufacturer for regular maintenance, (3) stroke test LVDT position feedback sensors, and (4) verify the operability of fire sensors in both the accessory and turbine compartments, speed pickups, flame detectors, and vibration sensors.

The auxiliaries’ “action items” checklist presented next was a “keeper” for most attendees. It is of particular value in outage planning. Here are the highlights:

      • Accessory gearbox. Recall that during normal unit operation gears drive pumps supplying lube oil, hydraulic fluid, and water. Suggested action plan: (1) Remove accessory-gear cover; (2) Inspect gears, bearings, and seals; (3) Inspect lube-oil, hydraulic-fluid, and water pumps; (4) Check for proper gear contact and shaft alignment.

      • Jaw clutch. Jaws wear out over time, particularly on peaking engines. During your maintenance outage, inspect the jaw clutch and check its alignment to the starting device. When replacement is necessary, consider a SSS clutch or equivalent.

      • Lube-oil pumps (AC and DC). Inspect and test all motor-starter circuits and interlocks. Check coupling between the pumps and their motor drivers. Verify pump/motor shaft alignment and replace the intermediate coupling.

      • Hydraulic supply pump. Inspect and test the motor starter circuit; check the coupling between pump and motor; verify alignment and replace the intermediate coupling.

      • Lube-oil cooler. Remove the tube bundle periodically for flushing and maintenance. Specify a pressure test prior to reinstalling the tube bundle.

      • Package radiators. Remove radiator sections for flushing and pressure test. Reinstall radiators and conduct a leak-check.

      • Buffalo water pump. Remove to inspect impeller and replace seals.

      • Atomizing-air compressor. Remove the gear-driven atomizing-air compressor, disassemble the unit and inspect impeller and seals.

      • Cranking motor and torque converter. Megger test the motor, inspect the torque converter and replace seals, inspect the jaw clutch as described previously.

      • Combined gas stop/speed ratio and control valve. Replace servos with spare set and return to manufacturer for service, inspect gas-valve plugs and clean valve seats, stroke-test gas valves with hydraulics.

      • Hydraulic dump valve. Test solenoid valve 20HD and all electrical trips, verify that the stop/ratio valve will close on loss of trip-oil pressure.

      • Liquid-fuel stop valve. Remove cover behind spring and inspect the spring, actuate the valve with trip oil.

      • Generator. Remove the main circuit breaker (device 52G) from the generator auxiliary compartment for testing and service, maintain potential and current transformers as suggested by OEM advisories.

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