Page 118 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 103

tors and the gearing to drive the other rotor and the piping connecting to shaft seals exposed to lower pres-
main shaft. That jacking gear constantly rotates every- sures or vacuum. This permits recovery of the steam
thing taking about 10 minutes for rotating the main shaft that would leak through seals exposed to high pressures
and propeller over one revolution. Many power tur- while preventing air entering seals exposed to a vacu-
bines are equipped with the jacking gear and these can um. Normally the turbine will have a regulator installed
be manually engaged or automatically engaged. Even if to maintain a constant pressure in the seal piping.
there is no vacuum or steam on the turbine the jacking The first steps in putting a back-pressure power
gear should be operated continually when the turbine turbine in operation is to ensure the lube oil pumps and
is out of service. On board ship we always had a sign jacking gear are running, admit cooling water to the seal
hanging on the two throttle control valves (ahead and steam condenser, and open the bypass of the turbine ex-
astern) that indicated that jacking gear was engaged. haust valve to admit steam to the turbine casing to start
Power turbines can be used to drive large pieces warming it up.
of equipment, everything from sewage treatment plant The first steps in putting a condensing power tur-
pumps to chillers. Every electric utility plant whether bine in operation is to ensure the jacking gear is running,
burning hydrocarbon fuels or generating steam with start the fans on an air-cooled condenser or the cooling
nuclear energy use power turbines to generate electric- water pumps for a water-cooled condenser, and admit
ity exclusively. In order to maximize power output and makeup steam into the turbine seal system.
efficiency those plants use condensers. Some plants Once turbine seal steam pressure is established the
with back-pressure turbines will also use condensers. next step to take is to start the vacuum pumps or air ejec-
Condensers can be air-cooled or water-cooled but most tors. The typical vacuum pump uses water to seal it and
are water-cooled. Water cooling is preferred because as the pump rotor turns if forces changes in the water
the temperature of the water is normally much lower level so the water acts like pistons to pump the air and
than air temperature. The water used for cooling the other non-condensible gases out of the condenser. Steam
condensers can be drawn from a well, a city water sup- from the condenser is condensed into the water in the
ply, a river, a lake, or the sea. When using well water or vacuum pumps which typically makes up for any water
city water a closed loop system utilizing cooling towers that’s lost in them. There is normally some automatic
dramatically reduces that water consumption. means of providing makeup water to the pumps to ac-
Condensers for power turbines operate at a vacu- commodate leaks and evaporation and the Operator
um. The vacuum in the condenser is related directly to should make sure the makeup isolating valves are open.
the temperature of the cooling air or cooling water. The The Operator should also open isolating valves for any
lower the temperature the higher the vacuum and there- lube oil coolers on the vacuum pumps.
fore, the higher the pressure drop through the turbine On shipboard we utilized steam powered air ejec-
and correspondingly higher power output. Operating a tors to produce the vacuum in the condenser. Shore-
turbine with a condenser introduces a number of pieces side plants can also use steam ejectors. These typically
of other equipment to maintain the vacuum. consist of two steam jets that pump the air by simply
Unlike auxiliary turbines power turbines with con- accelerating it with a flow of steam. Two jets are used
densers have low casing exhaust pressures and, there- in series to increase the vacuum in steps. The jet steam
fore, lower casing temperatures. The ships I operated and any steam drawn from the condenser is recovered
normally maintained vacuum in the main condenser by condensing it. Condensate from the condenser is nor-
at all times exclusive of requirements for maintenance mally used to condense the steam in the air ejector con-
and long-term shutdowns of the turbine. This had very denser. The air ejector condenser contains two separate
little effect on steam consumption and stabilized casing condensing chambers to serve the two steps in vacuum.
temperatures. Temperatures throughout the rotors (we The recovered condensate from the higher pressure con-
had a high pressure turbine rotor and another rotor that denser drains to the lower pressure condenser through
handled low pressures for propulsion and reversing, a loop trap and the condensate from the lower pressure
cannot be expected to be stable because the pressures condenser flows to the main condenser through another
along those rotors and the corresponding temperatures loop trap. A loop trap is simply piping that loops down
vary from boiler outlet conditions to the saturated con- and back up so the difference in pressures between the
ditions in the condenser. two chambers the loop connects is offset by static head
Shaft seals on a power turbine will consist of those in the loop.
exposed to pressure and they normally incorporate Before starting a steam jet air ejector its cooling wa-