Planning for a major inspection in spring 2017 of the Frame 5 at a paper products plant in the Southeast was well underway Oct 10, 2016 when the unit tripped on high vibration, just as the day shift was arriving at the facility. An original Row 2 bucket failed and caused considerable downstream damage (Figs 1, 2). Management decided to begin necessary repairs immediately and to conduct the machine’s third major inspection at the same time.

The Frame 5 cogen unit had accumulated nearly 115,000 total fired hours and 1500 starts since commissioning in 1997. The 24.5-MW (on gas) MS5001PA engine was equipped with a DLN-1 combustion system and capable of dual-fuel firing. It was one of the most advanced Frame 5s in the fleet when installed.

Plant personnel told the editors that the bucket failure was the first major issue suffered by the gas turbine in its lifetime. Few significant modifications had been made to the basic engine in its two decades of service. The 12 combustion, six hot-gas-path (HGP), and three major inspections since commissioning typically revealed little beyond normal wear and tear.

Chris Mancini of Mechanical Dynamics & Analysis Ltd was informed of the unit trip and likely damage shortly after it occurred. He and a superintendent were onsite within two days to assist in damage assessment.

Field service personnel and their tools/parts/equipment containers arrived the day after Mancini. The following day, the day shift completed its site/safety orientation, organized tools and work areas, and ran power and compressed-air lines as needed. The night shift received its site orientation, set up lighting, and began unit disassembly. The project would proceed at an aggressive pace from that point forward given the black-start cogen unit’s importance to mill production.

MD&A was awarded a turnkey contract for repairs, the major inspections of the turbine and generator, and for some additional projects. Removal of the liquid-fuel and trip-oil systems was one of the latter. The mill never had success operating on liquid fuel. Burning of fuel nozzles and liners were two outcomes of oil combustion. The most practical solution: Don’t burn liquid fuel. That decision, made years ago by plant management, was easy given the ready availability of quality gas.

Oil infrastructure remained until it ran afoul of the company’s goal of continuous improvement. It took three shifts to remove liquid-fuel components to conduct a combustion inspection and three to reinstall it before engine restart. The facility has been performing CIs at 8000-hr intervals, so the cost (labor and outage schedule impact) of this activity adds up quickly. The plant engineer is guardedly optimistic about doubling that interval as promised by the more robust coating applied by ACT Independent Turbo Services Inc on hot-section parts in its LaPorte (Tex) shop during the outage. But this requires approval by the facility’s insurer, which is currently considering the coating’s merit.

MD&A was credited with developing the plan to eliminate oil capability (including fuel-nozzle mods) at less than half the cost estimated by an alternative supplier. This portion of the overall project would not have gone forward otherwise. Bonus: Eliminating the parasitic power associated with the liquid-fuel system increases unit output by 280 kW.

Issues with fuel valves equipped with hydraulic actuators motivated the mill to replace that equipment with electrically actuated valves when the change to gas-only firing was made. A bonus for this upgrade: Less gas is burned to produce a given amount of power than with hydraulic valves in the circuit.

Replacing the mechanical overspeed bolt and trip-oil system with an electronic overspeed trip enables operators to now verify trip functionality at 500 rpm without stressing the unit.

Converting from dual fuel to gas only. The liquid fuel system (LFS) for this Frame 5 included the following subsystems: primary and secondary liquid fuel and purge, atomizing air, and water injection and purge. LFS decommissioning, a first step in the conversion of the unit to gas-only operation included deactivation or removal of all hardware associated with oil supply as well as of equipment in the subsystems noted.

During the forced outage, key components of the LFS were removed, but because of schedule constraints and the physical location of some hardware, it was not feasible to remove everything at that time. Others who have performed similar conversions told the editors it’s important to disconnect/remove components that would consume power when inactive—such as the fuel pump and atomizing air compressor—and simply abandon in place piping that would have no adverse impact on gas-only operation.

The end covers and piping inside the turbine compartment were modified during the outage to reflect elimination of the LFS; Mark V controls software was reconfigured to accommodate the changes made.

Checklist of LFS hardware removed:

• • • Primary liquid-fuel lines from the flow divider to the fuel nozzles.

    • The accessory-gear-driven fuel pump—together with the electric clutch, coupling, bypass valve, and gear and its bearings.

    • The accessory-gear-driven atomizing air compressor—together with its drive gear and associated bearings.

    • Extraction piping from the compressor to the atomizing- and purge-air subsystems.

    • Atomizing-air pre-cooler and its cooling-water supply piping. Source-air piping from the atomizing-air pre-cooler inside the turbine compartment also was removed.

    • Atomizing-air booster compressor driven by the starting diesel, along with related piping and valves.

    • Accessory-gearbox oil-vapor eductor; a desiccant breather cap was installed in its place.

    • Gas-fuel purge system hardware.

    • Water-injection piping to the fuel nozzles.

Fuel valve upgrades. The mill’s Frame 5 was equipped with a combined, hydraulically actuated gas stop speed/ratio (SRV) and control valve (GCV) and gas fuel splitter valve. Recall that the SRV and GCV are independent valves. Gas flows through the SRV to the GCV, which regulates the amount of fuel flowing to the ring manifold serving the 10 combustion chambers. The splitter valve serving on DLN machines divides gas flow between the primary and secondary fuel systems.

Turbine Technology Services Corp was retained to remove the liquid fuel system, as described above, and to replace the existing hydraulically actuated, 3-in. SRV/GCV and splitter valves with new electronic valves from Young & Franklin Inc. Existing gas supply strainers and valves were retained inside the compartment. A 3-in. stop valve was required in addition to electronic primary- and secondary-fuel control valves (Fig 3).

The company’s Dave Simmons told the editors TTS has deep experience in this work, having removed liquid-fuel capability on about 50 GE Frame 5s through EAs over the years and retrofitted electronic valves from different suppliers on perhaps 20 machines.

Simmons said elimination of liquid-fuel capability on a non-DLN gas turbine is relatively easy, but experience counts when a DLN engine is involved. This project was unique: It was the first time to his knowledge that a Mark V-equipped DLN-1 machine was converted to electronic valves for fuel control—and it took only four weeks from initial request to startup.

TTS proved it could satisfy project goals by running tests on its reconfigured Mark V simulator. No empirical testing was involved. There were no surprises, Simmons said. The Y&F valves performed the way the company said they would.

He added that an increasing number of plants are investigating conversion to electronic valves and most projects can be justified based on opportunity costs. One of the first things to do, Simmons continued, is to determine the availability of physical space to accommodate the new equipment. This shouldn’t be challenging for non-DLN machines, he said.

Some demolition and install of the new valves and electrical conduit/wiring are key elements of the physical project. The editors were told that most wiring generally can be reused; an exception might be that for an old non-DLN unit. Note: Shielded cable is strongly recommended for use with electronic valves.

Finally, if considering electronic fuel valves for your plant, don’t forget to audit the control system logic file to see if it can accommodate the switch from hydraulics to electric. There was no such issue on this project because of all the liquid-fuel infrastructure removed.

TTS modified the gas control software in the Mark V panel and HMI operator screens and then performed functional and operational tests of the new gas control system. Other activities required to complete the project included the following:

• • • Disable piping to the gas control valve for the existing hydraulic- and trip-oil systems. Note that the mechanical overspeed trip was disabled when trip-oil supply to the gas control valve was terminated.

    • Install two magnetic speed pickups and independently connect to the Mark V overspeed “hardware” trip.

    • Install an emergency-stop pushbutton inside the accessory compartment.

To convert the dual-fuel end covers to gas only, the liquid-fuel and water-injection distributors were removed (Figs 4-6). The tubing runs connecting the distributors to the corresponding five primary-fuel nozzles on each end cover also were removed and caps installed in their place at the openings created. Secondary-fuel nozzles attach to the center of each end cover; their liquid-fuel and water-injection connections also were removed and capped.