Page 432 - Boiler_Operators_Handbook,_Second_Edition

Controls 417

should be right at 50% at any firing rate immediately duced by the offset of the air flow zero.
after it’s set up. With this method the oxygen trim controller is
I can recall many an operator that was confused limited and it could easily wind up or down to the end
because the trim controller didn’t seem to be doing any- of its output range if there was a considerable change in
thing because only about 10% of its output range influ- the fuel or some other factor. If the operator notes that
enced the air to fuel ratio; talk about reset windup! Bias in condition the first action should be to check the fires to
the summer is set to 0.95 so the output is anything from see if their appearance indicates a condition indicated
0.95 to 1.0 and the air flow transmitter is set for a range by the analyzer output. If the fires appear normal then
16.87% higher than the actual differential (8.11% more the analyzer should be checked for calibration. It’s un-
flow) at full load at normal operating conditions so the common to need any more adjustment than that plus or
output of the multiplier is 100% under those conditions. minus 5%.
Recall that the oxygen content had to increase as With our experience getting LNG (liquefied natu-
firing rate decreased because of loss of turbulence? An ral gas) added to our normal there was no guarantee
oxygen trim control has to provide for that and it does that 5% would always be enough. Rather than allow the
so by introducing a variable set-point that’s fed to the trim controls more latitude, I added a manual station so
oxygen controller. Two methods are used, one adjusts a boiler operator could put another signal on the sum-
the oxygen set point as a function of firing rate (basically mer. Giving that input a gain of 0.1 and changing the
the boiler master signal) and the other adjusts the set summer’s bias from 95% to 90% allowed the trim control
point as a function of load using steam flow. I prefer the a ±5% and the boiler operator ±5%. If the fires indicate
latter. A boiler needs more excess air during cold starts the analyzer is right the operator can adjust the manual
because the fire doesn’t get the benefit of the heat reflect- air to fuel ratio adjustment (slowly) to restore the trim
ed back from a hot burner throat and furnace refractory. controller’s output to 50%.
Using steam flow to establish a variable oxygen set point Note that I used a gain on those inputs so the con-
produces a higher set point for the trim controls while troller or operator could adjust their respective outputs
the boiler is warming up. Once steam flow is established over the full control range, from zero to 100%. A large
tighter control can be accommodated and the result is number of trim controls and similar control loops use
the same as using the master. limits that will not allow that. You might be able to ad-
The variable set point is generated by a control just the output from 0 to 100% but the limits only allow
device called a function generator. A function genera- it to work from 40% to 60%, the rest of the time nothing
tor permits a control technician, or a savvy boiler op- happens. Operators complained to me when they were
erator, to produce preset outputs at multiple points of exposed to limits on the output so I modified the control
input. Some function generators provide for four points parameters so they got full range. Now they’re happy. A
while others allow eleven. Its setup is simple with new control technician shouldn’t set up a control system to
microprocessor controls because you can set an output impose limits that don’t make sense.
for each input by simply entering the input and output A 2-1/2% swing in air to fuel ratio is not something
values into the controller. In my early days we had to do that I would expect to see in the short term so regular
it with a cam, like on a valve positioner, to produce the checks and adjustment of the manual ratio adjustment
desired output. I believe we did a better job back then, (to restore the trim output to 50%) are not likely to hap-
even though it took a lot of time and tooling to deter- pen. Any significant changes should be discussed with
mine how then cut the cam. Why? Because the cam was your control technician because it may indicate prob-
smooth and a function generator that’s set with specific lems with the controls or (more likely) one of the flow
points has sharp changes at the points that can introduce transmitters.
instability in the control. A boiler with a high stack temperature (over 500°F)
In addition to the function generator the oxygen will benefit from oxygen trim control. Low pressure
set point generator should include a bias adjustment heating boilers and boilers with economizers or air
or independent summer that allows the operator to heaters have to burn a fair amount of fuel to justify oxy-
increase the oxygen set-point to accommodate upsets gen trim. Single burner boilers can be fired with stack
or unusual fuel conditions and decrease it when the oxygen content of 1/2 to 1% with a combination of full
boiler is firing at a fixed rate (manually set) to optimize metering and oxygen trim but don’t expect much in fuel
operation under that condition. The result is a set-point savings from oxygen trim if you have low stack tem-
that follows the same slope for excess oxygen that’s pro- peratures.