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MINOR OVERSIGHTS CAN BE COSTLY WITH A SMALL GAS TURBINE
By Sarma Krishnamoorthy
Commissioning of a gas turbine (GT) is often done against a rigid timeline. Even experienced engineers might miss certain details if they are engrossed in achieving project milestones. Despite this, steps should be put in place to maintain the integrity of the process.
Take the commissioning of Solar Turbine model Taurus 70 GTs installed in a high-pressure, high-temperature (HPHT) gas processing offshore platform situated offshore. Units A and B were duel-fuel with auto fuel changeover. Each had a rated capacity of 7 MW but provided 5.5 MW due to site conditions. Turbine exhaust was channelled to a waste heat recovery unit (WHRU) to heat a thermal fluid used for process heating.
A function check
An onsite technician began commissioning procedures with the powering up of panels, a function check of balance of plant (BOP) components, a logic command and sequence check, and a sensor and actuator health check. Various problems were detected and located, such as the replacement of a faulty battery charger card, and the addition of a generator protection fuse in the alarm control board.
The liquid inlet line, air line and igniters were opened, and the units were dry cranked for five minutes to drive out dust or metal particles. The liquid fuel pump Variable Frequency Drive (VFD) panel HMI was configured, and the fire-suppression-system control heads were connected and made ready for start-up.
An attempt to start Unit A resulted in failure. Repeated start attempts confirmed that a signal was not reaching the VFD control panel. Two control wires were found to not have positive contact. These wires were tightened. Another attempted start-up led to the discovery of diesel leakage near the spark plug housing. A gasket was replaced and tightened.
In a subsequent start attempt, the unit achieved full power. During this test run, the doors of the enclosure were kept open to physically check and ensure no liquid fuel was leaking. Two more leakage sources were detected near the spark plug, which were handled.
Another test run of the unit ensured readiness for operation in liquid-fuel mode. The unit operated for about 15 minutes at full power. All major operating parameters (vibration, temperatures, and so forth) were well within limits.
About five minutes into a normal shut down, a mild boom was heard. An emergency shut off was carried out. Enclosure alarms indicated a fire in the enclosure. The CO2 suppression system had been in operation.
This confirmed an abnormal rise in temperature inside the enclosure. Subsequent inspection confirmed that a UV sensor inside the enclosure had been actuated. Fumes were spotted emanating from a burst portion of insulation mattress around the turbine portion flange joint.
Technicians made a short crank check of the unit to confirm there was no indication of rotor jamming and that the unit was mechanically healthy. The insulation mattress around the flange joint was opened to visually check hardware for any tell-tale leakage indications.
Dark brown patches were seen in the inside surface of the insulation mattress at the location where fumes emanated after the incident. This indicated trapped liquid fuel had burned outside the turbine flange joint actuating the fire-suppression system.
A further effort to start and load Unit A tripped on “ignition fail.” Troubleshooting indicated the igniter needed to be replaced, but no spare was available. Work was transferred to the commissioning of Unit B.
It took months for the needed components to arrive. Gas production levels were constrained because a standby unit was not available.
Thermal expansion that happens during factory tests and tests at the platform build-up yard can cause slight looseness in the hardware. Therefore, control wires must be checked for tightness and all hardware of the hot side should be re-torqued.
In this case, the turbine flange joint covered with the insulation mattress, did not have its tightness checked by the commissioning engineer.