The overspeed trip device is a mechanical overspeed-protection apparatus for steam turbines, serving as a vital actuator terminal for DEH (Digital Electro-Hydraulic Control System) and ETS (Emergency Trip System). Generally mounted at the front end of the turbine rotor or inside the bearing box, it is directly interlocked with the oil-hydraulic control system of main steam valves and control valves.

• Overspeed protection: Automatically triggers unit trip when rotating speed exceeds 10%-12% above rated speed (e.g. 3300 r/min)
• Interlock protection: Receives signals of vibration, axial displacement, low oil pressure and other parameters to realize comprehensive emergency shutdown
• Manual shutdown: Operators can perform emergency trip via handle or push-button

Components: Trip weight, spring, adjusting nut, housing, striker pin, etc.

Working Principle:

• During normal operation, spring force holds the trip weight inside the rotor
• As rotating speed rises, centrifugal force increases
• Once centrifugal force exceeds spring preload, the trip weight flies out, strikes the trip lever, actuates the trip oil circuit, closes main steam valves and shuts down the unit

Advantages: Simple construction, reliable performance and power-independent. It acts as the final mechanical protection barrier for the turbine.

It follows similar operating principle as weight-type version, but adopts an annular mass block sliding axially for triggering action. Mostly applied in large-capacity units with higher sensitivity and smoother actuation.

• Observe status of overspeed-trip indicator lamp
• Perform reset operation before unit startup
• Forbid masking or short-circuiting trip signals

• Overspeed test: Mandatory for newly-commissioned units or after major overhaul, to verify actual tripping speed
• Oil injection test: Available on certain units under running condition, used for checking jamming risk of trip mechanism
• Manual trip test: Conduct at least once per month to confirm emergency-shutdown function