Page 98 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 83

generate electric power so you better know the impor- temperature after you get the trip valve opened back up.
tant requirements of superheater operation. The first If your plant makes it a practice to lift check the
and foremost rule is the superheater has to have steam safety valves then do so with caution, waiting until the
flowing through it to absorb the heat getting to the tubes boiler has settled down after lifting each drum safety.
or the tubes will overheat and fail. Water in superheater Open the superheater vent first before starting to
tubes doesn’t help, it can block flow in some tubes to take a boiler off line, that’s first before anything else. If
permit them overheating or suddenly blow over at high other boilers are serving the load any reheater will have
velocities to create water hammer damage. to be set up to maintain steam flow as well. Whenever
The following guidelines should ensure proper possible keep serving the load after shutting off the fires
start-up of a superheated boiler. Note that some boilers, to keep the flow up, allow the turbine to drop off with
HRSGs in particular, can have special requirements so the boiler so you maintain maximum possible super-
be sure to read that instruction manual. When the boiler heater steam flow. Don’t open the other vents and drains
is equipped with a reheater you should have to adjust until the boiler is down to 25 psig when you’re ready to
valving to direct steam from the boiler through the re- open the drum vent. There are so many variables in su-
heater and open the reheater vents and drains. When perheater and reheater design today that I can’t begin to
starting up a boiler with a superheater make sure all ensure you these procedures are the best for your plant.
vents and drains on the superheater are open. Similarly, Be certain you follow manufacturer’s instructions.
check that all reheater vents and drains are open. Some superheaters are equipped with gas bypass
As soon as a reasonable flow of steam is evident dampers inside the boiler so you can control the su-
at the boiler vent, close it to develop maximum flow perheat temperature to a degree. Others will have an
through the superheater. When superheater drains intermediate desuperheater that injects feedwater into
appear to be blowing clear with no moisture present piping connecting two sections of a superheater to drop
(a slight gap between the pipe and the cloud of water the temperature coming out of the first stage and you
droplets) close down on the drain valves to increase may find desuperheaters on reheaters. Some of these
flow through the whole superheater. Similarly choke devices can produce a false sense of security by produc-
down on any intermediate vents. Constantly observe ing safe superheat readings at the boiler outlets but the
the superheater outlet temperature, paying close atten- temperatures upstream of the desuperheaters or in parts
tion after any change in firing rate, number of burners or of a superheater that aren’t affected by the dampers go
ignitors in service, and other activities that can change too high. In any kind of upset operating condition check
flue gas flow past the superheater. Close drains and as many temperatures as you can and don’t bet on the
vents except for the final superheater vent valve once lowest reading being the right one, always figure the
you have the turbine rolling over. Close the superheater highest reading is the right one.
vent valves after the turbine is carrying a load. Close the Desuperheaters are used to increase the supply of
bypass valve to the reheater as well, confirming reheater desuperheated steam (the added water evaporates and
flow from and to the turbine before closing the reheater becomes part of the steam). When the steam is used in
vent valve. heat exchangers and similar apparatus desuperheating
During operation note the superheater, and re- reduces the amount of heating surface required in the
heater if equipped, outlet temperature on a regular heat exchanger. They should always leave a little super-
basis. There are many things that can go wrong to pro- heat in the steam so you know there’s no water racing
duce a problem with overheating the superheater or re- down the piping looking for an elbow to run into. When
heater that aren’t necessarily going to be associated with you’re operating a superheated steam plant you have to
changes in sound. If the turbine trips, open the super- know what the saturation conditions are for every ser-
heater vent valve before trying to reset the turbine trip vice and what are the maximum temperature ratings of
valve. If the boiler has a reheater establish flow through the equipment and piping.
it as well.
Fooling around with a trip valve without super-
heater flow is dangerous. There’s no steam flow so the SWITCHING FUELS
superheater outlet temperature indication will fall even
though the metal a few feet inside the boiler is overheat- Any boiler plant of a reasonable size should be
ing. It’s very embarrassing and quite scary to see the capable of burning more that one fuel. It provides the
superheater outlet indication peak well above design owner or user with an alternative fuel in the event the