Page 428 - Boiler_Operators_Handbook,_Second_Edition

Controls 413

so the fuel flow controller doesn’t see any change in its shut down the boiler if the fuel and air flow signals don’t
remote setpoint. match within limits; I don’t favor shutting the boiler
The master signal is higher than the fuel flow down. The choice we made was to compare the fuel
signal so it passes through the high signal selector to flow signal with a prescribed minimum and drive the
become the remote setpoint of the air flow controller. boiler to low fire if the signal was less than that value. It
The air flow controller then responds, changing its doesn’t result in a boiler shutdown and gives an opera-
output to increase air flow. As the air flow increases the tor a chance to correct the situation or fire the boiler in
transmitted flow signal increases to raise the setpoint of hand rather than running around trying to get another
the fuel controller. If the master signal stops changing boiler on line. The limit also prevents a shift above low
the air flow signal will eventually come up to match the fire in the event of loss of the control signal after start-
master and the fuel flow signal will follow it. Look at up. We don’t worry about loss of an air flow signal be-
the diagram and see how the air will follow the fuel on cause we haven’t had it happen… yet.
a decrease in master signal. That’s how the system with
cross limiting works.
On a decrease in load the fuel flow goes down and FIRING RATE CONTROL—DUAL FUEL FIRING
the air flow controller follows the fuel flow signal (as its
remote setpoint) down. Therefore, cross limiting pre- First let me explain that dual fuel firing means firing
vents a fuel rich condition. Some engineers try to think two fuels at the same time and under control. Boilers that
of these as lead-lag systems because the air leads the can fire gas or oil are two fuel boilers, they can fire gas or
fuel going up and lags it going down. They’re incorrect they can fire oil but they can’t fire both at the same time.
because we’ve had lead-lag systems for years and it has Low fire changeover systems are discussed in the section
nothing to do with fuel and air. on operating wisely and aren’t dual fuel firing either.
Since all the control signals have to match we have To fire two fuels at once you have to have a full me-
a problem when the air to fuel ratio has to change. Any tering system. In addition you need a fuel flow summer
change in the master signal between air and fuel control- that combines the two fuel flow signals so the total fuel
lers will upset the cross limiting. To resolve that problem flow is the feedback signal to the fuel controllers and to
we modify the air flow signal to indicate an air flow that the high selector of the air flow controller. One of the
is less or more than what it actually is. A typical method two fuels has to be considered the primary fuel and the
is to insert a ratio control between the air flow transmit- other fuel flow signal has to be adjusted with a gain so it
ter and the fuel and air controls with their signal selec- produces an output that equates to the air flow demand
tors as shown in Figure 11-36. of the primary fuel. Some engineers call the summer a
When we had systems that used our ratio totalizer Btu summer because it takes about the same amount of
we used a special one with a threaded shaft through air to produce a Btu whether you’re firing oil or gas. The
the pivot point extended to a knob on the panel. By rest of the controls don’t know that they’re looking at
turning the knob we changed the totalizer pivot posi- two fuels so they operate normally.
tion, sort of adjusting the gain, so the flow signal to the When dual fuel firing you’re usually switching
controllers was equal to the air flow transmitter times fuels. There are other operating conditions that favor
the totalizer gain. dual fuel firing but the common one is switching fuels.
Despite the fact that a full metering control elimi- A dual fuel firing system is the ultimate in control for a
nates many of the variables of pressure effects (people boiler and you should have it unless you only fire one
opening and closing windows and doors and other fuel or almost never switch. I believe it’s the best way
situations), there is one serious problem with full meter- to transfer fuels because you’re always operating with
ing controls that you must be aware of. If the fuel flow an inert furnace environment. It’s safer than stopping
signal is lost the controller will drive the fuel valve wide then restarting the boiler and a lot safer than the low fire
open almost instantly! If the air flow signal is lost the changeover systems.
air controller will drive the damper wide open and can The standby fuel is brought on the burners at low
blow the fire out or produce a lot of unburned fuel by fire then manually adjusted upward until the fuel flow
quenching the fire. controller output equals the manual output for the
Either situation is hazardous but the loss of fuel standby fuel; the controller will automatically reduce
flow signal is the most dangerous. Many system design- the firing rate of the leading fuel to compensate for the
ers incorporate differential sensing devices that will added standby fuel. When the two fuel flow signals are