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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.
Core Functions
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
Main Types, Structure and Working Principle
Two widely-used types of overspeed trip devices are listed below:
Weight-type Overspeed Trip Device
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.
Ring-type Overspeed Trip Device
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.
Key Components and Common Spare Parts
Component
Function
Precautions
Trip weight / Trip ring
Senses centrifugal force to initiate tripping action
Smooth surface, free from jamming
Spring
Provides preload and sets tripping speed
Periodic spring-force test to prevent fatigue failure
Adjusting nut
Sets actuation speed
No arbitrary adjustment; calibration is mandatory
Striker pin & lever
Transmits triggering signal
Check flexible inter-linkage movement
Overspeed trip slide valve
Controls on-off of safety oil pressure
High sealing requirement; oil leakage prevention
Note: Disassembly inspection, cleaning and function test of overspeed trip device shall be completed in every major overhaul to guarantee sensitivity and reliability.
Operation and Maintenance Guidelines
On-site Monitoring
Observe status of overspeed-trip indicator lamp
Perform reset operation before unit startup
Forbid masking or short-circuiting trip signals
Periodic Tests
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
