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Foreign Objects Can Severely Damage Gas Turbines

Within 24 hours of resuming operation after rotor out maintenance on a 180-MW gas turbine (GT), a drop in performance and compressor efficiency and a step change in vibration were observed. Inspections revealed damage to the inlet guide vanes and all 17 downstream stages of the compressor section of the GT. Debris consisting of rusted metal parts, including scaffold nails, a nut and various metal pieces and flakes, were found on the air inlet filter plenum floor. Rotor removal and repairs were needed, estimated to take 12 weeks at the original equipment manufacturer (OEM) repair shop.

At this plant, compressor section offline water washes are conducted twice a year as part of the spring and fall outages, and at the beginning of an outage to clean the machine for work. The floor of the gas turbine inlet plenum has two 2-in. drain lines. The drains connect to a common discharge pipe fitted with a manual ball valve. This discharge pipe connects to a common header to which other drain lines are connected, including extraction drains from compressor stage 9 and 13. Each drain line also has a manual ball valve. The common drain header connects to an open ended 4in. pipe, fitted with another manual ball valve. A suction truck connects to this pipe to suck remaining water from the header and drain pipes from the systems during outages.

So what happened? The various drain line ball valves are opened during the compressor wash process to remove water. Prior to startup, according standard operating procedures (SOPs), the various valves need to be closed for normal GT operation. However, after this event, valves in the 2-in. drain lines, the purge air manifold and the inlet bleed heat manifold were found open. The ninth stage compressor drain line valve indicated closed, but was partially open. The pressure from this stage of the compressor is 160 psi.

Debris may have accumulated in the drain lines/header over time and/or entered the header through the open 4-in. pipe outlet. During operation, the debris was allowed to enter the inlet plenum by the pressure from the ninth stage extraction line and the other open valves. Debris was subsequently sucked into the gas turbine inlet causing foreign object damage (FOD).

Ways to Mitigate FOD Potential:

  • Understand system weaknesses and vulnerabilities to human error, and seek opportunities to make systems inherently safer to protect critical equipment. In the above described configuration, a separation of the drain lines was recommended.
  • Ensure SOP include “sign-off” procedures and locking of critical valves.
  • Provide adequate contractor management and oversight.
  • Properly maintain piping systems, including valves.
  • Implement a complete Foreign Material Exclusion program to prevent foreign objects from entering critical systems.

Although they are fairly infrequent, FOD events in gas turbines are costly, averaging almost US$5 million.

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