Technical notes of interest to Marine Engineers
"Easy on the Sticks"
Why take your time to speed up or down...
Authored by: Martin Leduc, 2005
Brought to you by www.dieselduck.net, comments to [email protected]
The following suggestion were written up to help Bridge Officers
understand the effects of their actions, when briskly handling the propulsion
controls. These suggestions are for a large passenger ship with diesel electric
propulsion, but some of the ideas hold true for any ship.
Slowing down...
The engines onboard
the ship are massive on all account, and having that mass of steel and iron at
the speeds we do is quite amazing. 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.
Critical
Speed
“Critical speed” is a name given to a speed range where a phenomenon occurs in all rotating machines. At that speed range, a vibration sets up within the machine and can basically “feeds off itself”, to create higher than normal forces. This condition can cause damage to bearings and running gear, even broken shafts in certain cases. Although all machines are subject to this situation, local experience is best in determining the exact range of this critical area, as it is ship specific and depends on the machine and its installation.
For example, on this ship, the critical range of speed
for the propulsion motors has been determined to be 30-40 shaft RPM. When
manoeuvring through this range it is best to do it quickly, which will prevent
dangerous forces to build up within the running gear and assure it’s long
service life.