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BEST PRACTICES CAN PREVENT CATASTROPHIC FAILURE OF AERODERIVATIVE GAS TURBINES
By Sarma Krishnamoorthy
Aeroderivataive gas turbines (GTs) are being deployed in power plants and petrochemical facilities in increasing numbers. However, onsite personnel skilled in their maintenance are often in short supply. The preservation and distribution of best practices, therefore, has taken on greater importance.
Consider a Rolls-Royce Avon GT used as a prime mover for process gas compressors in an offshore oil gas platform off the coast of Mumbai, India. There are over 90 Avons operating in the Mumbai High offshore oil gas field of Oil and Natural Gas Corporation (ONGC) of India.
All three of the GTs compressor trains were continuously operating and dispatching 6.9 Million Metric Standard Cubic Meter Per Day (mmscmd) of natural gas to shore facilities via a trunk line.
Consumption was monitored daily on all GTs. The oil consumption volume in one of the compressors in this GT was higher than the other two and steadily rising.
Leakage, spillage or both
Normally, an unusual increase in lubricating oil consumption is due to leakage, spillage or both. Detailed inspection of all components in the engine lubrication system showed no visible increase in leakage compared to the other two operating units.
The facility contacted the authorized Rolls-Royce repair and service agency in India. Their instructions were to continue observation and inform them if an internal inspection was warranted.
Due to incessant demand, all process gas compressor trains had to be operated continuously. However, doubt remained that there must be some kind of internal oil leakage, as yet undetected. Management approved a one-day borescope inspection of the engine internals.
A service engineer was called onboard for the inspection. Meanwhile, the one spare engine onboard was readied in case a rapid change out was needed. Borescope inspection revealed a thick dark brown or black coating on the internals of the turbine portion of the engine. Service personnel concluded that this oil sludge was from burnt oil.
The service engineer referred to cross-sectional drawings of the engine (Figure). He noted oil transfer piping running inside the turbine which distributed lube oil between three bearings: front, rear and middle.
This pipe had a joint, which was sealed with an elastomer O-ring. The service engineer considered it most likely that the O-ring had broken, and that this was the cause of leakage.
The borescope inspection proven to be timely. It helped the facility avoid a catastrophic failure traced to the following causes: Oil cakes deposited on hot path components over a period of time led to differential expansion of components; and the engine bearing downstream of the leaking seal lacked adequate oil and was close to premature failure. Both could have resulted in a lengthy outage.
The decision to take out the unit reduced the outage to four days. Borescope inspections were done after the engine cooled. It took one day for cool off, one day for inspection and two days for engine change out. The replacement engine was ready at the platform as soon as the inspection had been completed.
The lesson learned is the importance of isolating something as minor as over-consumption of oil. In this case, the willingness of maintenance personnel to dig for the reason behind a potential problem saved at least ten days of production time. The cost of the outage was far less than the massive repair expenses that would have followed a catastrophic engine failure.
This is a good example case on how gas turbine O&M engineers and maintenance personnel must continue to be vigilant. Every variation in operating parameters should be investigated. Doing so will lower overall costs and lead to higher productivity due to the avoidance of catastrophic failures.