Page 426 - Boiler_Operators_Handbook,_Second_Edition

Controls 411

taken into account and most control systems had provi- windbox to furnace differential to restore the air to fuel
sions (adjustable cams) to compensate for it so inferen- ratio again. The only problem came when someone put
tial control provided many of the features of metered a burner in service and forgot to open the register.
control without the expense (and difficulty) of square
root extracting.
They also solved some problems that were prin- FIRING RATE CONTROL—
cipally associated with multiple burner boilers. There STEAM FLOW/AIR FLOW
were a lot more multiple burner boilers in the middle
th
of the 20 century because they were either converted Inferring fuel flow by pressure worked fairly well
from firing coal or designed to be convertible to coal. for oil and gas but it didn’t help with coal firing. Steam
Coal fired designs use a reasonably square furnace, not flow/air flow systems were developed for coal firing
the long skinny ones we’re used to on most boilers to- and are basically inferential metering systems because
day. The shorter furnace required use of multiple burn- the steam flow could be equated to fuel flow. If the
ers. boiler efficiency and steaming conditions were constant
Inferential metering is accomplished by consider- then a fixed relationship between steam flow and fuel
ing the fuel delivery systems as an orifice with a pres- flow would exist because the fuel would generate a pro-
sure drop that can be measured and comparing that portional amount of steam. The systems eliminated the
with the air side pressure drop. These systems were only problems with, or impossibility of, measuring the coal
applied to oil and gas fired boilers and they used the flowing to the fire. Coal fired boilers larger than about
burner header pressure as a variable that equated to fuel 90,000 pph can justify the expense of metering the coal
flow. After all, the oil burner tip is an orifice or group of but smaller units still use steam flow/air flow control.
them and a gas ring or spud has orifices in it, and the One problem with steam flow/air flow is the lag
pressure in the furnace (downstream of the orifice) was in response associated with load changes. If the plant
relatively close to zero so it is reasonable to treat the master output increases there is a delay associated
burner header pressure as a value of differential. with the inertia of the boiler. It takes a little time for the
Some gas fired systems used gas at such low pres- higher coal flow rate to heat up the boiler a little more
sures it was essential to include a furnace pressure in- and increase steam flow rate. If the system was set up
put to the measuring device so the changes in furnace so air flow followed fuel flow the boiler would probably
pressure didn’t upset the flow signal although they did smoke on a load increase.
experience some difficulty with pressure fluctuations The systems normally use a parallel positioning
(see draft control). control methodology where plant master changes pro-
Modern instruments have erased the cost ad- duce a proportional change in fuel feed, primary air flow
vantages of inferential metering systems so you will (on pulverized coal fired boilers) and combustion air
see fewer of them. When inferential metering is used flow and maintain the ratio of fuel and combustion air
today the differential is treated as a flow signal and the flow signals with the steam/air flow ratio on a slow reset.
square root is extracted by the transmitter or controller Some engineers refer to steam flow/air flow systems as
to produce a linear flow signal. One of the more serious parallel positioning with steam flow trim because the
problems with inferential metering systems was their steam flow is used to trim the ratio between fuel and air.
lack of linearity. The control response was normally It’s the timing problem that dictates how tight air
tuned for the high end of the boiler operation and to fuel ratio can be maintained with a steam flow/air
swings accepted at low loads. flow system. Gas and oil fired systems could actually
In dealingx with those multiple burner boilers they run a little tighter than a system with air flow metering
had a distinct advantage, even over today’s full meter- added because changes in fuel input produced a rapid
ing systems. The fuel flow based on the burner header change in steam rate. Pulverized coal fired boilers have a
didn’t account for the number of burners in service and delay in load changes associated with changes in coal in-
the differential from windbox to furnace didn’t account ventory in the pulverizer so they typically operate with
for the number of registers open. If a burner tripped the excess air rates around 30%. Stoker fired boilers have a
control backed down to restore the header pressure, ef- larger inventory change effect and have to operate closer
fectively decreasing the flow so the air to fuel ratio at to 50% excess air to eliminate fuel rich firing conditions
the other burners was restored. Later the operator could (and smoking) during load changes.
close the register and the air control would restore the Of course you don’t run all boilers at that rate; the