An overspeed trip is required to ensure that fuel supply will be cut off from engine if its speed exceeds the predetermined value.
Over speeding will result if there is
- · Sudden removal engine load such as propeller coming out of water in rough weather, failure of clutch, fracture in a drive shaft of engine.
- · Stuck fuel control shaft.
Any trip requires manual resetting. It should not be able to automatically reset after speed is reduced as reason for overspeeding may still exist and engine may race again. It needs to be tested periodically to ensure its correct functioning. Sometimes engines are fitted with two independent over speed trips, an electro-pneumatic device with trip setting at 15% above nominal speed and a mechanical device with trip setting at 18% above nominal speed.
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Types of Overspeed Trips
Following are the types:
Mechanical Type Overspeed Trip
A simple mechanical device may have a weighted spring loaded bolt set into the rotating shaft of engine. The centrifugal force exerted on the bolt
fitted in a rotating shaft exceeds the preset spring force when speed exceeds about 15% of the rated speed and the the bolt ejects out.The bolt in its outward position strikes a latch which releases the plunger which shuts off fuel supply to the engine.
Electro Pneumatic Overspeed Trip
The electro pneumatic device as fitted on Wartsila engine is as shown above. A spring loaded pneumatic cylinder is fitted at each fuel pump and when tripping occurs each fuel pump rack is moved to ‘no-fuel’ position. During normal operation the spring force keeps the operating end of the rod clear of the pin at the end of the fuel rack, which can then be moved to any desired position.
Should over speeding occur the solenoid valve is actuated which admits air into the cylinder to act on the piston, moving it against the spring force. This causes the operating end of the piston rod to pull the pin at the end of the fuel rack fully over, shutting off fuel completely.
Electronic Type Overspeed Trip
The electronic protection system monitors three individual magnetic pickups to provide overspeed protection. This system replaces mechanical over speed devices, especially in high speed applications, such as steam or gas turbines having speed more than 10000 rpm, where mechanical over speed devices are less reliable. System’s two out of three voting scheme ensures that prime mover will not shut down because of transient conditions or conditions affecting only one out of three isolated units.
Slowdown present in the engine
Ease of calibration and testing, accuracy and repeatability are benefits of system. Mechanical devices require considerable trial and error to set the set point of overspeed device. Because in this system digital speed sensing technology is used set point is configured simply by programming the value from touch panel (and hence much easier and time saving compared to mechanical trip setting) The accuracy of speed sensing is within 0.1% which no mechanical trip can match. To ensure its functionality each of three speed sensing units can be tested while prime mover is running, using its integral frequency generator (simulated testing).
A key lock prevents programming by unauthorized person. It needs to be tested periodically to ensure its correct functioning like other types of over speed trips.
In Sulzer engines
, a collapsible link is fitted between the governor and the fuel rack. If engine over speeds the collapsible link operates and fuel to engine is cut off. In B & W engines, a puncture valves are fitted on top of each fuel pump, which spill the high pressure oil back to suction side of the pump in case of over speeding.
So this was all about the over speed trip that are used in the engines. If you want to add more info in to this topic ‘overspeed trips’, please feel free to comment below.