Over Speed Trip in Diesel Engines & Types of Over Speed Trips.
An over speed trip is a
safety feature provided on the diesel engine of the ship to restrict
uncontrolled acceleration of the engine, leading to mechanical failure or
untoward accidents. In order to prevent the speed of a diesel engine to go
beyond the pre-set speed range, an over speed trip is used in the diesel
engines.
How Over Speeding can be
Harmful?
A Diesel
engine is designed for the mechanical stress associated with the centripetal and
centrifugal forces of the moving parts inside it in a specified operational
range. Centripetal force is directly proportional to the square of the
rotational speed, stress increases rapidly with increase in speed. Mechanical
connection strength can be overcome by the exceeding stresses due to the
increase in operational speed. These can result in breaking of rotating parts
or damage to the machinery itself. Over speed is thus a serious safety hazard
and can lead to a fatal situation.
What does Over Speed Trip Does?
Due to
sudden changes in the load on the diesel engine, the speed of the engine may
vary. Though a governor is provided to control the speed of the diesel engine,
the speed might go out of control, damaging the engine. Thus, for this reason
over speed trips are used.
No
matter what type of the over speed trip the engine uses, the main aim of the
over speed trip is to cut the fuel supply to the engine cylinders in case the
engine speed rises above a specific level.
Preventing Over Speeding of Engine
Reducing
the likelihood of an uncontrolled and catastrophic over speed is essential and
can be done by two methods:
a)
Mechanical over speed trip
b)
Electronic over speed trip
In this
article we will have a look at the electronic over speed trip.
Electronic Over Speed Trip
To
understand the electronic over speed trip, a normal lay out of the system is
described below. The electronic over speed trip consists of
a) Fly wheel mounted speed sensor
Magnetic
speed sensor is preferred in generator engines. Due to the discontinuity of
actuator surface (gear tooth of flywheel) voltage is excited in the pick off
coil of sensor, producing an electric analog wave. This cyclic wave created by
the flywheel is read by the sensor.
b) Signal condition unit
This
unit act as a receiver to the speed sensor. Basic function of the signal
conditioner is to convert one type of electronic signal which may be difficult
to read into another type into a more easily read format. This can be achieved
by amplification, excitation and linearization of an electrical signal.
c) Detection and Comparison unit
There is
a set value which is normally 10 % above the rated speed and acts as base value
for this unit. Signal condition unit output is continuously detected and
compared with the set value.
d) Trip signal unit
If the
difference between the set value and detected value is above the limit, then
this unit gives a trip signal which in turn shuts down the generator.
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