Page 434 - Boiler_Operators_Handbook,_Second_Edition

Controls 419

from other sources and there’s a lot needed to control the required range (less than two inches of water col-
that draft, other means of draft control, more expensive umn). The larger diaphragm transmitter costs a lot more
means, will lower the operating cost and pay for that than the standard differential pressure transmitter (like
expensive control. about three to four times as much) so many a plant is
In addition to accounting for a variation in flue fitted with one that saved the contractor a lot of money
gas flow, draft controls can maintain a parameter in the but doesn’t work worth a darn. The desired operating
boiler, such as furnace pressure, a requirement for bal- point for a furnace pressure is 0.05 to 0.2 inches of water
anced draft boilers. Many operators believe that’s the column below the boiler room pressure. Transmitters
only place you can control the pressure with draft con- with a wide range, like 50 inches or so, become too un-
trols but nothing could be further from the truth. If you stable for good control. Also, at the low pressures we’re
have two or more boilers capable of pressurized firing dealing with for draft control any pressure fluctuations
you can control the draft anywhere between the furnace due to a noisy fire create a very noisy pressure signal
and the outlet for individual boiler control. and considerable filtering is necessary to get a steady
If you’re controlling the common draft (at all boiler output. Frequently the location of a furnace pressure
outlets) it has to be controlled there or at a central point sensing connection has to be moved because the selected
in the breaching where there’s little difference in pres- spot just happens to be where heavy pressure waves
sure as flows change. I don’t recommend a common from combustion noise strike it. Of course there’s also
control because it can fail to prevent operation of all the the problem of incomplete combustion that can create a
boilers and it’s very difficult to get the large damper in noisy signal.
a common stack to handle all the turndown that’s re- Once any problems with the furnace pressure sig-
quired of it. nal are resolved there’s the problem of load changes.
Balanced draft boilers require a means of control- In the old days when controls were expensive we lived
ling the induced draft fan to keep a constant pressure in with that unless the boiler loads were constantly chang-
the furnace, something slightly less than atmospheric ing more than ten percent or so. When necessary we
pressure. A typical control loop looks no different than added cascade control where the output to the boiler
any other control loop; a transmitter senses furnace outlet damper became the output of the air flow con-
pressure and sends a signal to a controller which alters troller plus or minus the output of the furnace pressure
its output to an actuator for a boiler outlet damper or a controller. The summer which combined the air flow
damper at the inlet or outlet of the induced draft fan. It controller output and the furnace pressure controller
can also vary the speed of the induced draft fan. output also needed a bias spring to subtract fifty percent
The control isn’t that simple because there are a so the furnace pressure controller output would end up
number of factors that influence it. First the pressure at mid range just like two element and three element
in the furnace of the boiler should only be slightly less feedwater controls. Those draft control systems got
than the pressure in the boiler room outside the furnace. tuned with changes in gain applied to the air flow con-
That way any air that leaks into the furnace and boiler troller input and the bias to satisfy control requirements
passes is kept at a minimum. That air is heated to stack which varied between boiler start-up and operating
temperature and thrown away just like excess air so it’s conditions. Modern microprocessor controls can use the
a loss that should be minimized. The furnace pressure stack temperature as an input to help compensate for the
transmitter is really a differential pressure transmit- variation in conditions.
ter comparing furnace and boiler room pressure and it The use of balanced draft allows the boiler manu-
should have a maximum range of six inches water col- facturer to use open inspection doors and joints that
umn and, preferably, have a range of one inch. aren’t exactly gas tight in furnace construction. The
Don’t do like one plant I checked where they result is there are plenty of places for atmospheric air
mounted the transmitter in the control panel and sensed to enter the furnace. I’ve visited many a plant where
the pressure using draft gage piping. The control panel the furnace controller setpoint was at two tenths of an
was in a conditioned room in another building. The inch negative or more. The leakage at two tenths is three
boiler pressurized regularly, blowing smoke and soot times as much as the leakage at five hundredths where
out into the operating area. Of course it never blew any the setpoint should be. Operating at five hundredths
into the remote control room. will allow an occasional puff of furnace gases into the
The differential that transmitter measures is so low boiler room, especially during start-up, but will provide
it needs a large diaphragm to accurately measure it in far more efficient operation. With modern microproces-