Page 424 - Boiler_Operators_Handbook,_Second_Edition

Controls 409

of the fuel and air controls so a boiler operating on hand FIRING RATE CONTROL—ADD AIR METERING
can be trimmed (adjusted by the operators) to reduce
excess air. The full title of this control logic is parallel posi-
A principle disadvantage of parallel positioning tioning with air metering. The next evolution in control
controls is variations in response of the actuators, espe- systems after parallel positioning was to add air flow
cially for pneumatic actuators, which can produce tem- metering. Since air flow is influenced by so many factors
porary upsets in air to fuel ratio during load changes. it makes sense to measure the air flow and control it. The
Unlike the jackshaft system there is nothing to prevent measured air flow provides feedback to the control sys-
the fuel valve actuator from moving faster than the fan tem so the air flow controller can adjust the fan damper
damper actuator or vice versa. They also have all the actuator to produce a repeatable air flow. Instead of
disadvantages of the jackshaft system with one provi- simply positioning the fan damper the control system
sion to help offset the problems with maintaining air to adjusts the damper until the air flow signal matches the
fuel ratio, adjustment of the air to fuel ratio. plant master position signal.
By adjusting the resistance in the loop of a system The decision to measure air flow started the still
where the air flow actuator follows the fuel flow ac- standing arguments about where it should be mea-
tuator the relative position of the two actuators can be sured. The type of control system and boiler has some
varied to provide an excess air adjustment. The adjust- effect on the choice and you should be aware of all the
ment is a bias type adjustment in most systems but it variations. Air metering with measurement across the
does permit running the air to fuel ratio a little tighter burner windbox to furnace on multiple burner boil-
if the operating personnel choose to. It also allows the ers made it possible to compensate for the number of
operators to compensate for soot accumulation in the burners in operation. Because each burner throat is one
boiler, something that’s not easy to do with jackshaft orifice in the flow path; changing the number of burners
controls. To help overcome the problems with the in- doesn’t change the differential measured at the air flow
dependent actuators the controls can be enhanced to transmitter when the register on one closes, but it does
include a leading actuator provision so the fan damper change the air flow.
actuator follows the fuel on a decrease in load and the Measuring the air flow using a differential across
fuel actuator follows the fan damper actuator on an the boiler itself is measuring the flue gas flow, not just
increase in load. air flow, but that’s not a significant variation since the air
In addition to problems with response jackshaft is 93 to 94% of the flue gas. The problem with using the
and other modes of parallel control can suffer a degra- boiler is sooting can change the differential relative to
dation due to hysteresis associated with wear in link- air flow and other problems like refractory seals break-
age connecting the devices used to position dampers ing up can also alter that differential without the opera-
and valves. To overcome the problems with response tor being aware of it.
and hysteresis manufacturers have developed systems The best measurement is in a suitable metering
utilizing synchronous actuators which are, for the most run or venturi between the forced draft fan and burner
part, mounted directly on the damper or valve shaft. windbox but most boilers don’t have enough room
The synchronous motors are basically electric actuators there for any kind of precision flow measurement. I’ve
with an integral positioner that responds to a current always preferred using the inlet of the forced draft fan to
signal or digital instructions from a controller. I have measure air flow, provided it’s a single inlet fan. When
seen these devices operating smoothly and erratically a boiler is large enough to justify a double inlet fan
but don’t have enough experience with them at the then a good metering element between fan and burner
time of preparing this second edition to judge them one windbox is justified as well. Many operators don’t un-
way or the other. I think we all know that a mechanical derstand fan inlet metering and some even manage to
device that’s constantly changing position back and screw it up so I want to explain it well enough that you’ll
forth isn’t exactly working right so don’t accept one understand it.
that’s doing it. Within the boiler room there is air movement but
For all practical purposes a parallel positioning it’s very slow except for right at the inlet of the forced
control system is set the same way as a jackshaft control. draft fan or its silencer. In order for the air to accelerate
You need some way to set the fuel valve to match the air. from something very close to zero velocity in the boiler
Normally a parallel positioning system will have cams room to the speed it needs to get through the fan inlet
just like a jackshaft system. there has to be a difference in pressure between the two