Slowing
down...
So when slowing down one must
consider the simple fact that such a large amount of weight needs a bit of time
to react to speed adjustments. Many efforts in design and maintenance are made
to make sure the response time of the engine matches the load as quickly as
possible - considering the laws of physics.
Others things to
consider when slowing down; the Propulsion Electric Motor (PEM) are connected
to main electrical system of the ship, which means that along with lights and
propulsion, are connected computers and other sensitive electronic systems.
These systems do not respond well to changes in frequency / voltage and that is
why numerous safety devices exist.
When abruptly slowing
down on the propulsion motor telegraphs, as if downshifting in a "sport
car", the load on the PEM is rapidly reduced. This in turns reduces the
load on the generator, which is sensed by the engine's governor, reducing the fuel
needed to turn the engine at the steady RPM needed. Slight performance
imperfection in any of the many components of the system may slow that
response, and the result would be a governor delivering too much fuel for a
load that is no longer there, therefore speeding up the engine.
This slight surge may
be enough to activate the various safeties designed into the system such as
over voltage, frequency fluctuations and reverse power. The reverse power
safety system detects when other engine(s) are driven by the faster engine,
which will trip the circuit breaker, this generally causes an overload on the
remaining generators, tripping them as well, resulting in, of course, a total
black out.
So the moral of this
little story is "take your time slowing down", although the bridge is
far removed from the sounds and feels of the propulsion system, please consider
the mass of steel and iron that needs to adjust. Avoid the temptation of
slowing rapidly from 140 shaft rpm to 0 in one fast swoop. Even in an emergency,
take a "minute" and prevent a bigger problem, like a black out, and a
possibility of frying anything electronic.
If a speed change can
be planned, it is always best for operational needs to give the Engine Room a
ten minutes advance warning. That time allows the engineer and motormen to
adjust water production, or steam consumption to match the engine load,
assuring a constant service to all steam users.
Speeding
Up...
Speeding up needs the
same considerations as slowing down, although the results may not be as
dramatic as a black out. A lack of care when speeding up could be very damaging
to the company in government imposed fines and sanctions, due to excessive
visible pollution. Not to mention the real worries, thermal load of the
numerous engine components.
Much more time is
required when speeding up to full speed on the PEM; in particular, the upper
range of load. Generally, the engines will easily adjust to loads up to 50%
therefore usually not an issue during a "Stand By" condition. The
problem of overloading the engines usually arises after "Stand By",
where people just want to "get on with it!" when in fact its the most
critical time for loading the engines.
There is no real rule
of thumb of how many minutes it takes to reach full load in a safe and
considerate manner, but the telegraph operator should expect 1/2 hour after
"Stand By", before safely reaching full load on the engines.
The best guide for the
average person is smoke from the stack. Anytime smoke is visible, it is an
indication of overload, because fuel is not being burnt properly for whatever
reason; "cold steel", reaction time of temperature devices, speed of
turbo-charger, etc. If speeding up and a thick black smoke is noticed from the
stack, the step taken was too much, reduce a little and wait until the smoke is
not visible in order to take the next step.
Remember that the
anxiousness to get up to full speed is usually the highness, when you need to
take the most time speeding up, towards the 80-90% load on the engines. In this
power range, a 5 shaft rpm increase may take 10 minutes to achieve without overloading
the engines.
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