Which sequence best describes a typical safe gas turbine shutdown?

Which sequence best describes a typical safe gas turbine shutdown?

• A. Reduce load, shut off fuel, idle to cooldown, purge systems, secure flame, monitor alarms.
• B. Purge systems, shut off fuel, idle to cooldown, reduce load, secure flame, monitor alarms.
• C. Reduce load, monitor alarms, purge, shut off fuel, idle to cooldown.
• D. Idle to cooldown, purge systems, reduce load, shut off fuel, secure flame.

Answer: (A)

The sequence tests safe management of energy and fuel to prevent ignition hazards and thermal stress during shutdown. Start by lowering the turbine’s load so the machine slows gradually and can be brought to a safe idle without abrupt changes. Then shut off the fuel supply to stop any new combustion as the unit decelerates. Allowing the turbine to idle and cooldown gives hot components time to drop to safer temperatures before other actions, reducing the risk of thermal shock. Purging the fuel system and combustor with clean air at this point clears residual fuel vapors from lines and chambers, lowering the chance of an unintentional ignition during shutdown. Securing the flame afterward removes the ignition source, ensuring that no flame can start from any remaining vapors. Finally, monitoring alarms throughout confirms there are no faults and that the shutdown is proceeding safely. Other orders that purge too early or before fuel is shut off, or skip the cooldown before purging, raise the risk of fuel vapors lingering or ignition occurring, or place undue stress on components, making them less safe than the sequence described.

The sequence tests safe management of energy and fuel to prevent ignition hazards and thermal stress during shutdown. Start by lowering the turbine’s load so the machine slows gradually and can be brought to a safe idle without abrupt changes. Then shut off the fuel supply to stop any new combustion as the unit decelerates. Allowing the turbine to idle and cooldown gives hot components time to drop to safer temperatures before other actions, reducing the risk of thermal shock. Purging the fuel system and combustor with clean air at this point clears residual fuel vapors from lines and chambers, lowering the chance of an unintentional ignition during shutdown. Securing the flame afterward removes the ignition source, ensuring that no flame can start from any remaining vapors. Finally, monitoring alarms throughout confirms there are no faults and that the shutdown is proceeding safely.

Other orders that purge too early or before fuel is shut off, or skip the cooldown before purging, raise the risk of fuel vapors lingering or ignition occurring, or place undue stress on components, making them less safe than the sequence described.