Page 423 - Boiler_Operators_Handbook,_Second_Edition

408 Boiler Operator’s Handbook

ning on up to high fire. Now, supposedly, the boiler present most of the time. That wasn’t my choice, how-
is warm enough that it won’t experience any thermal ever, I only designed the systems.
shock or excessive thermal stresses in the process, but If a sudden increase in plant load required the
I’m never certain of that. I’ve never had occasion to boiler firing rate to increase once it was on automatic
even think about a better way to do that before writ- the low signal selector would not allow the firing rate to
ing this book. It’s probably because I’ve never been increase any faster than the ramp rate. Once the pressure
required to design any for the smaller boilers. Now that in the volume chamber bled up to supply pressure the
I’ve thought about it, I would do something a little dif- boiler operated automatically as if it wasn’t there, the
ferent. automatic signals were always the low signals.
All of the large boiler systems where I designed Another nice feature of the system was it drained
the controls, and they included an automatic start-up off pressure from the volume chamber as slowly as it
provision, we used a ramping provision. Once the pres- filled it up. If the boiler tripped for some reason, then
sure exceeded the setting of the low fire hold switch started back up, the ramping simply swung back up but
vent valves were automatically closed and the ramping allowed the already hot boiler to fire at higher rates.
system put in service. It simply allowed a very slow With modern digital controls the same feature can
increase in the firing rate and prevented a more rapid be added. It can be augmented to provide a ramp rate
increase until it had completely ramped out. from a cold start and a different ramp rate from a restart.
The first ones were applied on pneumatic control All it takes is some additions to the software. It’s one of
systems and consisted of a low signal selector, three-way the times when computers are wonderful.
solenoid valve, and a volume chamber with a metering
valve. The solenoid valve dumped the contents of the
FIRING RATE CONTROL—
volume chamber to atmosphere while the boiler was
PARALLEL POSITIONING
off and applied supply air, usually at around 18 psig, to
the chamber via the metering valve when the boiler had Parallel positioning controls perform about the
started and the low fire hold switch released. The pres- same as a jackshaft control system because they simply
sure in the chamber was piped to the low signal selector establish the position of the fan damper and fuel valve.
along with the boiler master output or the plant master Large boilers and boilers with air heaters or fans located
output. The low signal selector then fed either the fuel away from the windbox can’t easily use a jackshaft type
and air controls or the boiler master. Which one de- of control because the weight of the linkage becomes a
pended on the plant master operation. If all the boilers problem. A parallel positioning system allows the fan
were operated off the plant master then the output of the and fuel valve to be located convenient for the construc-
ramping control was fed to the boiler master. Otherwise tion and for other reasons.
it went to the fuel and air controls so the operators could The most commonly used parallel positioning sys-
set a firing rate manually at the boiler master. tem is an electric positioning system which uses potenti-
Once the low fire hold was released the air bleed- ometers like the modulating motor controls to compare
ing into the volume chamber slowly increased the firing the position of the fan damper and fuel valve actuators
rate. Where we could we set that ramp rate as slow as and adjust them to match the position of a boiler master.
possible, sometimes to get it out to two hours to reach Plants with parallel positioning control usually have a
high fire (and that’s still faster than some boiler manu- plant master for steam pressure control which actuates a
facturers specified) we would have to use a larger vol- bunch of potentiometers that are matched in position by
ume chamber. Once the firing rate exceeded the plant the boiler masters or fuel valve controllers on each boiler
master or boiler master output it was no longer the low plus or minus any bias introduced by adding resistance
signal and the boiler was operating on automatic. in the potentiometer loop.
The ramping control actually allowed a boiler to Some advantages of parallel positioning controls
automatically come on line, generating steam and pick- include the ability to run on a plant master and bias
ing up load without upsetting the operation of the other boilers, it doesn’t constrain the location of fuel valves
boilers. I should mention that this control feature wasn’t and fan, it permits isolation of gas and oil control valves
all that was required. Steam traps to drain the boiler on two fuel boilers so both aren’t operating to influence
steam headers and many other features were required operation of another boiler firing a different fuel, and
for fully automatic control. Oh yes, some of those boiler they permit maintenance on the valve and actuator for
plants were actually unattended, no licensed operator the alternate fuel. It also permits independent operation