Page 375 - Boiler_Operators_Handbook,_Second_Edition

360 Boiler Operator’s Handbook

crack the steam valve to any turbine, it takes very little a sign “Jacking Gear engaged” hanging on the throttle
steam to get it moving. The marine turbines I used to valves. Modern equipment has more serious interlocks
operate had a bunch of heavy gears, a fifty ton propeller, to prevent accidental admission of steam when the tur-
and long shaft holding it back so we gave it a bit of a blast bine shaft is connected to the jacking gear (remember
to get it started, opening the valve a quarter turn or so, where torque is greatest).
then quickly throttled back as it started moving. All electric utility steam turbines, including those
There’s still considerable force on the turbine in nuclear plants, are condensing turbines. That means
blades when a turbine is operating under load. If a pow- that at least some of the steam passes through the tur-
er generator trips off the line, instantly stopping any bine to a condenser. The water from the condenser is
generation of power, the turbine is bound to over-speed then pumped up to the deaerator, usually through a
because there’s nothing to stop it taking off. That’s why number of heat exchangers. To condense the steam all
they all have over-speed trips and some even have hy- the heat of vaporization (the latent heat) has to be re-
draulic brakes to limit the speed. Don’t skip that very moved. That heat is transferred in the condenser to river
important function of testing the over-speed trip when water or cooling tower water. On rare occasions it is
you start up a turbine. If it doesn’t work you could be dumped to air through air cooled condensers.
watching turbine blades flying out of the casing and all For maximum power generation the condenser
over the place. must operate under a vacuum so non-condensible gases
Power generating steam turbines are usually large and any air that might leak in must be removed from
pieces of equipment with very thick parts. The shaft of the condenser. That’s typically done with a steam jet
a turbine can be several inches thick so it’s important to ejector but may also be accomplished with motor driven
prevent quick temperature changes during the opera- vacuum pumps. The steam jet ejector (Figure 10-94) is
tion of those turbines. During startup the turbine should usually two or more stages to pull as much vacuum
be brought up to speed and loaded gradually. Always, as possible. The steam used to eject the air is then con-
and I do mean always, look for the instruction manual densed in a heat exchanger using condensate. The actual
to see what the manufacturer recommends for startup vacuum achievable is dependent on the temperature of
then confirm that the facility’s SOP’s comply with those the cooling water or air but 27 inches of mercury is a
guidelines (they normally allow for slower heating) and typical value to shoot for. At that pressure the steam will
stick with those rules. Luckily I never brought a turbine condense at 92°F.
up too fast but I do know what one sounds like when it Any condensing turbine requires special provi-
starts rubbing due to uneven temperature growth; that’s sions to seal the shaft of the vacuum pressure stages to
a sound you don’t want to hear, ever. prevent drawing air into the turbine. That’s usually ac-
The large shafts and heavy metal of steam tur- complished with a special regulator that supplies steam
bine casings also retain heat. If the turbine is stopped it from a high pressure bleed or a reducing station to keep
should be given a bump to force a few rotations at regu- pressure on the shaft seals. The regulator also dumps
lar intervals to prevent the shaft buckling because the excess steam leaking from high pressure seals into the
temperatures are higher in the top of the casing than in condenser during operation.
the bottom. When we were at a stop during maneuvers Maintaining a vacuum by providing adequate
to get into or out of a docking space we would give the cooling water or air and keeping non-condensible gases
main shaft turbine a little steam to rotate it once or twice and air out of the condenser is imperative for best power
at regular intervals. It should be noted that the propeller generation. A boiler plant that’s converted to generate
only moved a couple of degrees during that operation. power in addition to heat, as opposed to one that gen-
When a power steam turbine (or a gas turbine) is erates power as well as heat, may have a condensing
shut down it is normally connected to a “jacking gear” turbine although most of the steam will be used in the
A small electric motor with (typically) a screw drive is plant. Various schemes including bleeding steam and
engaged to rotate the turbine at a low speed (normally separating stages with piping and control valves are
less than one rpm) to provide for a balanced cool down. used to maintain pressure of the steam supply to the
The jacking gear is usually operated until the moment facility while the steam to the turbine is controlled for
before steam is admitted to the inlet of the turbine power generation. One such scheme is a goggle plate
throttle valve and some form of interlock is provided inside the turbine with slots that are opened and closed
to prevent opening those steam valves until the jacking to control flow to the lower stages right after the facility
gear is disengaged. Our typical shipboard interlock was steam is drawn off.