Page 115 - Boiler_Operators_Handbook,_Second_Edition

100 Boiler Operator’s Handbook

When you do have many auxiliary turbines of dif- oil pump start it to start oil circulating through the bear-
ferent sizes using the Willians lines in their instruction ings. If it’s possible to get at the shaft, rotate the shaft a
manuals will help you determine ways to mix them for quarter turn every five minutes while it’s warming up to
maximum utilization. When you have an option of chang- help ensure uniform heating.
ing turbine nozzles (note the two lines in Figure 2-8) you Damage to auxiliary turbines is normally due to
determine when the extra nozzles are needed by when the alignment problems associated with thermal imbalance
turbine seems to be inadequate to power the pump. Note so take your time to ensure the casing and rotor are uni-
the feedwater flow or steam flow when that occurs so you formly heated. Large auxiliary turbines can have some
can determine when to adjust turbine nozzles. very thick metal parts, especially around the nozzle
Boiler feed pump turbines can help maintain the blocks and shaft seals so the larger the turbine, the more
heat balance because they’re equipped with controls. time you give it to warm up.
These vary from constant speed controllers which will When a bypass is provided on the exhaust valve
vary steam usage as the water flows change to special crack it to start warming up the casing. Admit only
control loops for maintaining a constant feedwater pres- enough to get steam at the vent then throttle down the
sure or constant differential between feedwater and vent so the air is pushed out the drains. If you don’t have
steam headers. As the boiler load increases the pump a bypass then crack the exhaust valve. Leave the vent
horsepower has to increase to pump more water. The open enough to dispel air that’s heated by the steam.
increased load will tend to slow down a speed regulated Don’t leave it wide open. With a wisp of steam coming
turbine so the controls open the steam valve more to out there should be enough pressure to push air out the
restore the speed. Similarly the steam supply to the tur- drains. The steam from a typical 100 to 150 psig supply
bine is increased to maintain feedwater header pressure (or higher) is about half the density of air when dropped
or water to steam differential as load increases. to atmospheric pressure. It’s so light that you need some
Very large auxilliary turbines may actually have push to force the air out the turbine casing drains.
control linkage that opens and closes turbine nozzles. Since most auxiliary turbines operate with exhaust
Those systems will open one nozzle control valve entire- pressures of 15 psig or less the steam will always be
ly before starting to open the next so only a small quan- less dense than the air. You want to be certain the entire
tity of steam is throttled. That increases the efficiency casing is flooded with steam so the rotor and casing are
of the turbine and improves the ratio of feedwater to heated uniformly. As the casing warms less steam will
turbine steam demand. be used to heat it up so the drains will begin blowing
The steps in starting up and shutting down auxil- more and more steam. Throttle the drain valves to limit
iary turbines are all pretty much the same. The first task steam waste but be sure to keep them open enough to
is deciding which one to start. You then set up it’s driven drain all the condensate.
equipment the same way you would in preparation for When there’s little to no condensate evident at the
starting one powered by a motor. The turbine casing drain valves open the exhaust valve; at this point the
vents and drains should be open but check that they steam has nowhere to go and isn’t condensing so the
are. Check oil levels in the turbine bearings or sump, casing pressure should be close to exhaust line pressure.
any reduction gear, and on the driven equipment. If the Open the drain valve above the steam supply shut-
turbine is fitted with an electric motor driven lubricating off valve to drain any accumulated condensate above
the isolating valve then throttle it until you’re primarily
draining condensate. If there’s a bypass on the steam sup-
ply valve crack it to bring steam up to the trip valve. Once
the supply line is dry, open the supply valve. If there is no
drain at the trip valve body don’t open the supply until
you’re ready to start rolling the turbine. While that sup-
ply piping is warming up open outlet then inlet valves
of any turbine bearing coolers, throttling the inlets if the
coolers are lacking temperature controls.
I’ve received many complaints that my timing is
off here because heating up the casing will heat up the
oil in the bearings. That’s true, and I want it to. If you
Figure 2-8. Willans line open the cooling water to the bearings first the oil may