Legacy Turbine Forum, No. 7 in a series
By Luke Williams, PE, Consultant
www.geLegacyGasTurbineSupport.com
IN THE BEGINNING
Engineers who joined the GE gas turbine department in the early days often came from steam turbine design, where cooldown procedures were essential to prevent rotor thermal bow, a condition that can lead to vibration on startup.
Several approaches were available to cool the unit adequately and avoid rotor bow. Early criteria focused on rotating the rotor for an extended period using turning gear. The turning gear engaged the rotor shaft and rotated it at low rpm using an electric motor.
Turning gear was evaluated but had drawbacks. Because the MS5001 was conceived as a black-start unit, the turning-gear motor would need to be DC. A smaller DC motor could be considered, but it might not overcome breakaway torque. Estimates suggested that a 5.7-hp motor would be required to meet turning-gear requirements.
If the unit tripped during a black start, the emergency lube-oil pump would be required to maintain lube-oil pressure during coastdown. If the outage continued, the battery and charger would have to power both the 5-hp emergency pump and the 7.5-hp turning gear. Combined draw was estimated at about 60 DCA, while the battery charger was nominally rated at 35 DCA. The result was rapid battery discharge.
The solution was development of the accessory-gear hydraulic ratchet. The ratchet used a push-pull hydraulic cylinder to index a gear on the No. 1 shaft of the accessory gear. A pneumatic piston pushed the ratchet gear into mesh with the accessory gear. At the end of the forward stroke, the ratchet gear disengaged and retracted. The system indexed the shaft 45 degrees every three minutes.
Design considerations for the original hydraulic ratchet system included the following. The ratchet cycled every three minutes. The permissives for ratchet operation were lube-oil pressure above 63QL, 6 psig, and zero shaft speed. The ratchet disabled as soon as shaft rotation was detected to prevent damage to the accessory gear and the ratchet. Recommended cooldown time was 48 hours. A cooldown timer was not provided. If the AC cooldown pump failed to start, the emergency pump was enabled for both turbine coastdown and ratchet operation. To conserve battery power, the DC pump operated only during the three-minute ratchet cycle.
FUEL REGULATOR UNITS MS5001D, E, G, H, J, K, L, AND LA
Problem. What if the AC cooldown pump failed to start on shutdown?
Solution. The DC cooldown pump was sequenced by the ratchet timer to provide lube-oil pressure during ratchet operation.
Problem. Troubleshooting the ratchet.
Solution. The ratchet design was straightforward. Components such as the solenoid valve, limit switch, and actuators were easily accessible by removing the ratchet cover. As a result, the ratchet was reliable, easy to troubleshoot, and rarely caused problems.
Problem. Cold lube oil could result in low lube-oil pressure during a black start
Solution. To ensure that 63QL and 63QT could be picked up, a boost circuit was added to the DC pump MCC. The boost increased bearing-header pressure to pick up 63QL, lube-oil pressure low, at 6 psig, and 63QT, low lube-oil pressure trip, at 9 psig, avoiding a trip on low lube-oil pressure.
Problem. How black start worked with cooldown enabled.
Solution. With cooldown enabled, the ratchet stroked every three minutes, with the DC pump enabled by ratchet timer 2HR and ratchet position 33HRX. A start signal enabled continuous DC pump operation through relay 1Y-2. Boost output increase was enabled when the pump started and disabled when 63QT picked up at 9 psig on rising pressure with a four-second time delay. The ratchet was also enabled for continuous operation.
FUEL REGULATOR UNITS MS3002F
The MS3002F five-bearing design was not expected to be subject to rotor bow because rotor span was relatively short. Therefore, a hydraulic ratchet was not provided. However, the mid No. 2 and No. 3 bearings between the compressor and HP turbine were subject to high temperatures on shutdown. A cooldown timer ran auxiliaries for four hours to cool the bearing housings.
Problem. What if the AC pump failed?
Solution. If the AC cooldown pump failed, the DC pump started on low lube-oil pressure, 62QL. Expected draw of the 5-hp DC pump was about 29 DCA. The charger was rated for 35 DCA, which could support the cooldown period.
SPEEDTRONIC MARK I AND II UNITS MS5001M, N, AND P
Mark I was introduced in 1968 with the MS5001M. Accessory equipment, including the piggyback AC/DC lube-oil pump and accessory-gear-mounted ratchet, remained the same on the 5001M. The 5001P was upgraded to Mark II in 1980 and adopted the torque-converter-mounted ratchet. Ratchet hardware and cooldown logic remained broadly similar to fuel regulator units, with incremental improvements.
When Mark I was upgraded to Mark II in 1980, the cooldown sequence used in Mark I was not carried over. The elementary index listed 62CD cooldown on sheet 3F, but the logic was not present. The sequences for the hydraulic ratchet and lube-oil pumps also did not include cooldown logic.
Problem. The cooldown logic was energized to run.
Solution. Mark I introduced 1Z-1 to replace 1X-1. The key difference was that 1Z-1 was normally closed to enable cooldown. This made cooldown logic fail-safe in the event of relay failure.
Problem. Could the cooldown sequence be turned off?
Solution. With 1Z-1 enabled, both the AC cooldown pump and ratchet sequence were disabled. Cooldown could be terminated by selecting Stop after the unit reached zero speed, 14HR. Cooldown could be re-enabled by Master Select OFF and START.
Problem. On older units, the clutch and ratchet could enable before the shaft stopped rotating.
Solution. Addition of the zero-speed relay, 14HR, addressed multiple issues related to clutch and ratchet engagement before the unit reached zero speed.
Problem. When the ratchet cycle indicated retracted, rotor rotation continued and disengaged the clutch, resetting the sequence and causing a double cycle.
Solution. Delete the 33CSE normally closed contact in the 33HRX rung. If the clutch failed to close, ratchet permissive 4RP disabled and ratchet trouble alarmed 30 seconds later.
Problem. New ratchet components included 20CS for clutch engagement and ratchet pump, 72HR, sequence solenoid valve 20HR, and ratchet position limit switch 33HRF. How could the ratchet be troubleshot when the mechanism was internal to the assembly?
Solution. A screeching noise from the ratchet pump indicated relief valve VR5-1 had lifted at 1325 psi. Check clutch engagement, install a 1500-psi gauge on the F forward-stroke port, and verify hydraulic pressure. If pressure was satisfactory, suspect an issue with the hydraulic cylinders or possible rotor-to-stator contact. Disable 20CS and manually close 33CSX-1, then enable the ratchet with the clutch disengaged. If the ratchet cycled, rotor-to-stator contact was a likely cause. If the unit remained hot, allow it to cool while engaging the ratchet every 15 minutes in an effort to break the unit away. Solenoids could be checked by forcing them on and off and confirming movement.
Problem. Fuel regulator control specifications recommended 48 hours of cooldown. Mark I and II specifications did not provide a recommendation.
Solution. Select from the cooldown procedures described above and disable cooldown when the hottest wheelspace reached 200F or after five hours, whichever occurred first. Cooldown logic could be disabled by a Stop selection, disabling the ratchet and AC and DC pumps. Cooldown could be enabled by Master OFF and START.
SPEEDTRONIC MARK I UNIT MS3002J
The MS3002J four-bearing design was not expected to be subject to rotor bow because rotor span was relatively short. Therefore, a hydraulic ratchet was not provided. However, the mid No. 2 bearing between the compressor and HP turbine was subject to high temperatures on shutdown. A cooldown timer ran auxiliaries for four hours to cool the bearing housings.
Problem. Did the DC pump back up the AC pump in case of failure?
Solution. If the AC cooldown pump failed, the DC pump started on low lube-oil pressure, 62QL. Expected draw of the 5-hp DC pump was about 29 DCA. The charger was rated for 35 DCA, which could support the cooldown period.
Problem. The MS3002J experienced HP rotor vibrations early in the program.
Solution. Tests and data analysis concluded the issue was not rotor bow but was related to the balance procedure in use.
SPEEDTRONIC MARK II AND IV UNIT MS5002B
The MS5002B HP turbine was similar to the MS5001P. The hydraulic ratchet was mounted on the torque converter, and the sequence included a 10-hour cooldown timer. Timer 62CD picked up cooldown logic 1Z-1, which disabled the ratchet sequence and the AC cooldown pump.
Problem. What if the AC pump failed during the cooldown period?
Solution. If the AC cooldown pump failed during the 10-hour cooldown period, the DC pump started on the ratchet cycle, 20CS, and low lube-oil pressure, 62QL.
SPEEDTRONIC MARK II UNIT MS6001A
The MS6001A was introduced in 1979 with Mark II. Sequencing was essentially the MS5001P approach, with ratchet hardware upgraded to the torque-converter-mounted design. Cooldown logic was listed in the elementary index on sheet 03F, but 62CD did not appear on that sheet. No cooldown time recommendation was provided in the control specification. It appears that a cooldown timer similar to the MS7001 was considered but not implemented.
SPEEDTRONIC MARK II UNIT MS6001B
The MS6001B was introduced in 1981. Changes included a full-size AC lube-oil pump. The unit experienced startup vibrations, often above the alarm level of 0.5 ips. As the unit warmed, vibration tended to decrease, usually to below 0.5 ips. The situation was similar to the MS3002J. The issue was investigated, including design and manufacturing processes, but root cause was not immediately evident. Early Mark IV MS6001B elementaries listed cooldown logic in the elementary index on sheet 03F, but 62CD did not appear on that sheet. No cooldown time recommendation was provided in the control specification.
SPEEDTRONIC MARK IV UNIT MS6001B
The MS6001B Mark IV was introduced in 1984. Changes included a self-sequencing ratchet, which eliminated the 20HR solenoid. Cooldown logic was listed in the elementary index on sheet 03F, but 62CD did not appear on that sheet. No time recommendation was provided in the control specification.
Problem. How to troubleshoot the self-sequencing ratchet when major components were built into the assembly.
Solution. To support troubleshooting, the ratchet was supplied with plugged taps on all four hydraulic-cylinder ports. Pressure gauges could be installed on the forward and return ports to check operation of the internal 33HR transfer valve. As outlined in the MS5001P procedure, checking clutch engagement, confirming lifting of relief valve VR5-1, and operating the ratchet with the clutch disengaged typically identified the cause.
Problem. Cooldown logic appeared in the elementary index, but 62CD and a time recommendation were absent.
Solution. Select from the cooldown procedure options below and disable cooldown when the hottest wheelspace reached 200F or after five hours, whichever occurred first. Cooldown could be disabled by selecting COOLDOWN OFF, disabling the ratchet and AC and DC pumps. Cooldown could be enabled by selecting COOLDOWN ON.
The presence of cooldown logic, 62CD, in the Mark II, IV, and V elementary indexes indicates that a cooldown timer was considered for the MS5001 and MS6001. A cooldown timer was implemented in the MS7001. However, logic termination similar to the 10-hour timer on the MS5002B was not added. One suggested reason for vibration on the MS6001 was that customers were taking units off cooldown prematurely, resulting in thermal bow. The decision was made not to add cooldown-timer logic to the MS6001. If a customer experienced startup rotor vibration, insufficient cooldown time was a likely contributor.
SPEEDTRONIC MARK V UNIT MS6001B
The MS6001B Mark V was introduced in 1992. There was no reference to cooldown logic in the cross-reference, CSP, or control specification. Startup vibration remained an issue, but both frequency and amplitude became less pronounced after 1995. The root cause remained unresolved.
Some speculation centered on the compressor-to-turbine rotor distance piece. Historically, rotor designs used a cylindrical distance piece. The MS6001 distance piece was tapered, possibly to accommodate canted combustion liners. The theory was that machining of the tapered distance piece was not always consistent, introducing balance variability. It is believed that machining processes were investigated and improved around 1995.
Problem. Could the cooldown sequence be turned off?
Solution. Select from the cooldown procedure options below and disable cooldown when the hottest wheelspace reached 200F or after five hours, whichever occurred first. Cooldown could be disabled by selecting COOLDOWN OFF, disabling the ratchet and AC and DC pumps. Cooldown could be enabled by selecting COOLDOWN ON.
COOLDOWN PROCEDURE OPTIONS
Standard cooldown. Monitor wheelspace temperatures and disable cooldown when the hottest wheelspace reached 200F. The Mark V MS6001B control specification, ratchet section, recommended a minimum five-hour cooldown to prevent damage to bearing babbitt.
Forced cooling. Forced cooling was developed on the factory test stand when the test stand was the critical production facility. Cooldown could be reduced from several hours to a few hours. Engineering determined that cooldown could be terminated when the hottest wheelspace was less than 200F.
Forced cooling was also used when a unit had a problem and required disassembly for corrective action. Because starting-diesel operating time was not limited, the unit could be put on crank and run until cooldown criteria were met. With an electric start, the starting motor was limited by a 60C temperature rise. In that case, a thermometer was placed in the motor exhaust, and cooldown was paused to allow motor cooling. In most cases, wheelspace criteria were met before the motor reached 60C.
Intermittent cranking. Intermittent cranking could be used if the diesel or starting motor was limited. One example was high diesel cooling-water temperature on hot days exceeding cooling-system capability. The same approach could be used for the starting motor. Pause cranking and allow the motor to cool by 10C, then crank until 60C was reached, and repeat as needed.
Warm-start procedure to work out rotor bow. If the rotor could not be moved because of a major failure, such as a lube-oil-system failure, correct the failure and place the unit on the ratchet for 24 hours. Then attempt a start. If vibration reached the alarm level of 0.5 in/sec, shut down. Attempt another start and repeat. After two warm starts, vibration should begin to decrease. Continue starts until the unit reached FSNL with vibration below 0.5 in/sec.
This warm-start approach was often successful in working bow out of the rotor. Two examples requiring warm starts included an accessory-base fire caused by a failed torque-converter hose that tripped the unit at base load, and an accidental energizing of both AC and DC piggyback motors that disabled both. CCJ
