By Morgan Hendry, SSS Clutch Company Inc
Gas-turbine (GT) manufacturers work continuously to improve their product lines. Likewise, users take steps at the plant level to reduce their O&M costs, extend maintenance intervals, and enhance the operation of their machines— often growing revenue in the process. Many equipment upgrades/ retrofits can be made at relatively low cost and can be accomplished during a short, planned outage.
Over the last 40 years or so, more than 7000 servo-actuated jaw clutches have been supplied for GE Energy’s (Atlanta) GT starting and turning-gear systems (Frames 3, 5, 6, 7, and 9) and for its steam-turbine turning-gear systems. Although servo- actuated jaw clutches are simple devices and effective when new, they are known for developing problems over time and compromising engine reliability.
A jaw clutch that fails to engage on a turbine starting or tuning-gear system can have costly consequences to the operator—such as an inability to meet a specific start requirement or a bowed rotor because the turning gear does not engage.
One way to avoid the risks associated with the jaw clutch on an existing GT is to replace it with a totally automatic, gear-tooth, overrunning clutch. The synchronous, self-shifting (SSS) clutch fits into the exact same space as all the existing GE jaw clutches so no equipment must be moved to enable the retrofit. For a new unit, the SSS clutch can be ordered as an option.
The existing jaw clutch consists of two hubs, each with three or four teeth. These hubs are mounted on opposing shafts. One jaw is fixed and the other is able to slide. The sliding piece is pushed with servos axially along straight splines to mesh with the stationary jaw, thereby enabling the transmission of power through the clutch. Once the output side of the clutch accelerates above the speed of the input side, the geometry of the teeth push the clutch out of engagement; disengagement is assisted by the servos (Fig 1).
Jaw clutches used on starting systems for Frames 3, 5, 6, and 7 normally are installed between a torque converter and the No. 1 shaft of the accessory gear (see Frame 6 report in this issue, p 23, Figs 30-32). The starting-system prime mover can be a diesel engine for black-start capability, an electric motor, or a steam turbine. The power of these various starting devices is approximately 250-500 hp. The clutch input is connected to the torque-converter output shaft and the clutch output to the No. 1 shaft of the accessory gear.
Plants that continue to operate GE machines with jaw clutches typically are familiar with their failure modes, including these:
- Worn/leaking actuating cylinders used to engage the clutch.
- Displaced metal on the jaw-clutch teeth, caused by engaging when slightly misaligned.
- Dirty or corroded splines on the torque-converter shaft, preventing the jaw on the input side from sliding properly.
Once a jaw clutch has damaged teeth, it is less likely to stay engaged in service. Some machines require a pneumatic system with the sole purpose of operating the servo-actuated cylinders that engage and disengage the clutch; this system can be a maintenance headache in itself.
Frame 5 starter-clutch retrofit
In the early 1990s, Consolidated Edison Co of NY Inc’s contemplated replacement of servo-actuated starting-system jaw clutches on its Frame 5 GTs at the Gowanus and Narrows peaking facilities with an automatic clutch that would be more reliable. There was no clutch alternative available from the OEM or in the aftermarket, that (1) would physically fit into the existing startingsystem space, (2) would provide separation between shafts (like the jaw clutch) when disengaged, and (3) was completely automatic. Con Edison also wanted to retrofit the new clutch in five days, to minimize downtime and revenue loss.
The 48 MS5001 Model N GTs installed on barges at these two East River sites had to be started and accelerated to full speed in seven minutes and brought up to full power in an additional four minutes. To receive NY ISO (Independent System Operator) compensation for “guaranteed power generation within 30 minutes,” Con Edison had to prove that each unit could restart in the event of a high-speed flame out, or some other minor problem. Therefore, it was advantageous to install an automatic clutch with the capability to start and/or restart the turbine within the 30-min window.
Because the Frame 5 took about 17 minutes to coast to rest from full speed on a unit trip, the automatic clutch had to be able to restart the turbine “on the fly”—something the jaw clutch could not do. The re-engagement had to be below the purge speed of the turbine; Con Edison determined that a 500-rpm reengagement speed was logical. The GT would coast to that speed in about five minutes (Fig 2).
The challenge was to fit an automatic clutch into the space vacated by the original jaw clutch and not be too heavy for the existing system’s shafts. Note that the jaw clutch for the Model N is nestled between the starting system and the turbine accessory gearbox with 1.312 in. axial distance between shaft ends.
If the new clutch did not fit this space, the starting system would have to be moved back to increase the distance between the torqueconverter output shaft and the No.1 shaft of the accessory gear. This would make the retrofit prohibitively expensive and potentially exceed the allotted time for the outage. Con Edison selected a 64T clutch from SSS Clutch Company Inc, New Castle, Del, which met all requirements (Fig 3).
Although the space between the torque-converter output shaft and the respective input shaft of the accessory gearbox is minimal, the clutch retrofit was relatively easy to accomplish for two reasons:
- The respective shafts were relatively long and there was no existing turbine auxiliary equipment in close radial proximity to interfere with the new clutch.
- The original offset torque-converter gearbox was still used, thus the starting system did not have to be moved to accomplish the retrofit.
Regarding the second point, the gearbox was rotated 90 deg on its mounting spigot to achieve axial access to each respective clutch mounting shaft. Fig 4 shows the offset gearbox with SSS clutch installed. Prior to installation of the new clutch, the diesel/torque converter and accessory gear were realigned and the torque converter and its gearbox were dowelled together to help maintain drive-train alignment.
Installation. The new clutch’s output sleeve was mounted on the existing accessory gearbox input shaft, the clutch sliding component with main torque transmitting teeth was mounted on the clutch output, and both are contained by an end plate bolted to the shaft (Fig 5). The clutch input hub was rigidly mounted to the existing torque-converter gearbox shaft and retained with an end plate bolted to the shaft. For details on how the new clutch works, visit www. sssclutch.com.
The replacement clutch is oil-lubricated; the jaw clutch needs no lubrication. The jaw-clutch guard and servos were removed and replaced with an oil-tight, horizontally split clutch cover. A one-piece adaptor plate was bolted to the accessory gear in place of a labyrinth seal, and the cover, with floating seals on the input end, was cantilevered from this plate.
An oil feed connection was installed on the adaptor plate to deliver 2 gpm of turbine oil from the existing accessory-gear supply. A drain in the bottom of the cover was plumbed into the existing accessory-gear drain pipe. Finally, a proximity switch was provided to indicate when the clutch was disengaged, signaling when the diesel could be shut down.
Operating results. All engines installed at the Gowanas and Narrows facilities ultimately were retrofitted with SSS clutches. The new owner of those plants, New York City-based US Power Generating Co LLC, reports that the 64T is meeting expectations—no failures since the first installation was completed in 1992. Each retrofit was achieved in the projected outage time of one week; about two days to install the clutch and three for starting-system work and realignment.
US Power Gen receives a credit from the New York Power Pool for guaranteeing full power within 30 minutes of an engine start. The ability to restart the turbine in only five minutes after an aborted start (the restart permissive is set at 500 rpm) enables multiple starts in the 30-min timeframe. The old jaw clutch could only engage at 0 rpm, permitting only one start attempt in the 30-min window.
The success of this project was the foundation for GE’s decision to adopt the SSS clutch as a standard upgrade for its Frame 5 fleet (refer to GER 4196, “Performance and Reliability Improvements for the MS5001 Gas Turbine”). Interestingly, the large number of interface configurations on Frame 5s as supplied or modified since the machine’s introduction requires 30 different clutch designs to serve the fleet.
The latest version of the Frame 5, the Model P, manufactured in Italy by GE Oil & Gas subsidiary Nuovo Pignone SpA, is supplied with a SSS clutch as standard equipment. The 64T starting clutch is used on engines driving compressors as well as generators.
Frame 6B starting system
In 1993, GE adopted the same 64T SSS clutch for all new Frame 6B machines being built in Greenville, SC. Today, Nuovo Pignone uses it as well for the starting systems on all Frame 6B units that it makes. The GE manufacturing facility in Belfort, France, has supplied a few 6Bs with 64Ts, but has not adopted it as standard; hence, it has not discontinued use of the jaw clutch.
Starting systems with jaw clutches on several pre-1993 Frame 6B GTs have been retrofitted with 64T clutches. The procedure for retrofit is exactly the same as the Frame 5 starting system with the Twin-Disc torque converter and horizontally offset gearbox. The standard distance between shaft ends between the torque converter and the respective input of the accessory gear on the 6B is 0.533 in. The clutches are slightly different from those used on the Frame 5s but operate exactly the same.
To date, more than 700 size 64T SSS clutches have been supplied for Frames 5 and 6 GT starting drives.
Frame 7B, 7EA starting and turning-gear retrofit
The Frame 7B and early 7EA GTs are equipped with two jaw clutches— one for the starting system, one for the turning-gear system.
The starting-system jaw clutch is used similarly to that on the Frames 5 and 6—that is, between the torqueconverter output shaft and the No.1 shaft of the accessory gear. This frame’s larger starting motor (900 hp) requires the slightly larger size 74T clutch.
The first SSS starting clutch was installed in a Frame 7B in 2002 at Midwest Electric Power Inc, Joppa, Ill. The starting system of this machine can be moved axially away from the accessory gear or the No. 1 shaft of the accessory gearbox can be removed to install the clutch. The 74T also is designed to fit into the existing space, thereby eliminating the need to relocate the starting-system machinery. More than 10 SSS clutches have been installed on Frame 7B and 7EA starting drives.
Turning-gear clutch. On these machines there also is a jaw clutch installed in the so-called tombstone turning-gear system located at the non-drive-end of the generator. The turning gear consists of an electric motor (10 hp at 1800 rpm) which drives through an epicyclic gear to a chain-drive/sprocket assembly. It is housed in what looks like a grave tombstone to reduce the turning speed to approximately 6 rpm. The tombstone unit is cantilevered off, and bolted to, the generator housing (Figs 6, 7).
The clutch designed for this retrofit is a 55T. To ensure reliability, it was modeled after the same 55T used successfully on more than 500 Frame 7F turning gears. The clutch for the tombstone turning gear can be installed in several ways. The two methods used to retrofit more than two dozen 7Bs to date are these:
- Slide back the entire tombstone to gain access to the generator and turning-gear shafts.
- Remove the tombstone lid, then the chain sprocket wheel, and finally the jaw clutch.
Steam-turbine turning-gear retrofits
GE also used jaw clutches on the turning gears of some steam turbines with ratings up to about 100 MW; SSS clutches have been retrofitted on several of these units. One project required the replacement of the three-jaw-clutch engagementtype turning gears in the front standards of two 85-MW steam turbine/ generators at Puerto Rico Electric Power Authority’s Palo Seco Power Plant and four 100-MW units at the utility’s San Juan facility (Figs 8-10).
The complete turning-gear gearbox and jaw clutch were replaced on each machine with a double-reduction worm-gear-type reducer turning gear equipped with an 80T SSS clutch. The turning-gear systems with jaw clutches had been problematic for years, and because they were inaccessible, maintenance and downtime were costly. Since this retrofit project was completed in the 2001-2002 period, no turning-gear problems have been identified. ccj