Normal & Emergency Shutdown Procedures
Two Very Different Ways to Stop
Modules 7.1 through 7.4 covered getting a turbine running. This module covers the two fundamentally different ways a turbine stops — one deliberate and controlled, the other immediate and protective — and the shared post-shutdown care both paths require.
Normal Shutdown
A normal shutdown — gradual unloading reverses the loading sequence from Module 7.4 in a controlled manner: load is reduced at a thermally-appropriate rate, the generator is desynchronized, and the unit decelerates through a deliberate, paced process. Because this is thermally paced just like the loading sequence it reverses, the same ramp rate and thermal stress principles from Modules 1.5, 3.5, and 7.4 apply symmetrically — unloading too quickly risks the same thermal stress consequences as loading too quickly.
Emergency Shutdown
An emergency shutdown — trip-driven results from a trip event (Track 5): stop valves close essentially instantly per the fail-safe mechanism (Modules 3.4/5.2), and the unit decelerates on an uncontrolled coastdown rather than a gradual, paced process. Unlike normal shutdown, there's no opportunity to control the rate of this deceleration — the entire point of a trip is immediate action, which necessarily means skipping the gradual approach normal shutdown uses. This is an acceptable trade-off given what a trip is protecting against.
An emergency trip's thermal impact isn't equivalent to routine cycling. A trip-driven shutdown skips the controlled unloading process, creating a more severe thermal transient than a normal shutdown — this is why post-trip inspection often includes additional scrutiny beyond what a routine planned shutdown would warrant.
Where Both Paths Converge — Post-Shutdown Care
Regardless of which path a shutdown took, turning gear engagement begins as soon as speed allows, continuing slow rotation to prevent the uneven cooling that would otherwise cause rotor bow (Module 2.6) — the same concept from Module 2.6 and the same pre-start permissive check from Module 7.2, now applied at the opposite end of the operating cycle.
Coastdown vibration monitoring continues throughout deceleration, just as during roll-up (Module 7.3) — the rotor passes back through critical speed on the way down, and this transit is watched for the same expected rise-and-fall pattern. This is essentially Module 7.3's critical speed transit monitoring in reverse, and it's especially important following an emergency trip's uncontrolled coastdown, since that deceleration rate wasn't planned or paced.
Roll-up (Module 7.3): accelerate through critical speed, monitor vibration. Shutdown (this module): decelerate through the same critical speed, monitor vibration the same way. The physics and the diagnostic approach are symmetric — only the direction of speed change differs.
After the rotor stops or transitions fully to turning gear, extended cooldown/soak time continues over hours or days, with metal temperature tracked (Module 7.1) to determine the eventual classification of the next start. This directly closes the loop back to Module 7.1 — the cooldown rate and duration following this shutdown is exactly what determines whether the next start will be classified as hot, warm, or cold.