Page 100 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 85

switched to manual and firing rate is reduced to mini- combustion but it will assure a stable flame exists. Once
mum fire. The operator then begins the changeover by the operator observes the stable firing of the alternate fuel
turning a selector switch on the control panel to “Dual” and turns the selector switch to the alternate fuel the con-
or “Changeover” so the burner management system trols are released back to automatic. Switching to Manual
will energize both sets of fuel safety shut-off valves. The and manual adjustment of the firing rate controls is op-
operator then throttles the manual burner shut-off valve tional. I also inject ramping controls mentioned earlier. If
for the fuel being fired and slowly opens the manual the selector remains in the two fuel position for more than
burner shut-off valve for the alternate fuel. When obser- a minute after both fuel valves are energized the system
vation indicates the alternate fuel is firing the operator shuts down the alternate fuel and returns to automatic.
spins the alternate fuel’s manual burner shut-off valve There’s no reason to shut the boiler down.
open while simultaneously closing the valve for the Low pressure heating systems and similar applica-
fuel that was firing. The selector switch is then turned tions that do not have a critical steam pressure or water
to the alternate fuel position so the burner management temperature requirement can accept shutting down and
system will close the original fuel safety shut-off valves. restarting a boiler so the simple stop and restart method
The controls are adjusted to bring firing rate back to is satisfactory for them. The low fire changeover method
slightly above the rate before the changeover until pres- manages to eliminate the loss of heat input during the
sure or temperature in the boiler is near normal before purge period to reduce pressure or temperature loss
switching back to automatic firing rate control. but some drop is associated with holding operation at
The designers of burner management systems low fire. In my experience any facility that can’t afford a
incorporate additional logic in their systems to ensure drop in pressure or temperature has two other means of
a low fire changeover is performed properly. That logic switching fuels that will, unlike the previous methods,
requires the low fire interlock be maintained while the ensure a reasonably constant maintenance of pressure
selector switch is in the position to admit both fuels. and temperature. Smaller plants will have a spare boiler
They frequently add a timing sequence that limits the that can be brought up on the alternate fuel and placed
time when both fuels can be admitted. If the selector on line. Larger facilities normally don’t have spare boil-
switch remains in the two fuel position for more than a ers so a means of switching fuels on operating units
few minutes the boiler is shut down. I don’t like those while maintaining pressure or temperature is required.
standard provisions because logic is complex and the Larger facilities will have full metering combustion
time limit produces a sense of urgency in the operator control which allows dual fuel firing to maintain pressure
that may cause her or him to make a mistake. or temperature. Dual fuel firing is simply operating with
In low fire changeover systems I have designed both fuels at once. When equipped with a full metering
(keep in mind that I really don’t like this approach system the two fuel flows are measured, their values
to switching fuels) I allow the operator to initiate it added and the total fuel flow measurement is used by the
by turning the selector switch. The control logic then controls to maintain a proper air-fuel ratio. The alternate
knows controls have to be in manual and at low fire so fuel is started at low fire with its control in manual. The
the logic switches controls to manual and low fire. The ignitor is brought on, then the alternate fuel, and the
operator doesn’t have to do it. Once the low fire position boiler simply fires both fuels. Once the operator observes
is established the control energizes the ignitor and waits a stable alternate fuel the controls are adjusted to bring
ten seconds for it to be established. the alternate fuel up manually until the automatic control
Gas is normally admitted at the perimeter of the has reduced the original fuel firing rate to low fire. Once
burner while oil enters at the center; rather than accept the original fuel is at low fire the operator switches its
one will light the other I use the ignitor which is designed control to manual and transfers control of the alternate
to light both. After ten seconds, the normal PTFI (Pitot fuel to automatic. Finally, the original fuel valves are de-
trial for ignition) timing, the alternate fuel safety shut-off energized to complete the transfer.
valves are opened. Then, after the normal MFTI (main This method has been successfully applied on
flame trial for ignition) timing the ignitor and original multiple burner boilers with capacities of 250,000 pph.
fuel are shut down. Manual control of the fuel flows is When applied to multiple burners the second fuel is
not required but the operator may do it. The controls started one burner at a time to limit control upsets. An
should be set such that excess air at low fire is at least 150 interlock requires all burners be firing on both fuels be-
to 200%. During the period both fuels are firing the excess fore the alternate fuel firing rate can be increased above
air would be 25% to 50%; that doesn’t guarantee complete low fire. Safety shut-off valves for the original fuel are