Page 76 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 61

which will produce a measurable air flow that can be windbox and similar parts the inside is mostly air so
proven. Proof typically consists of a fan motor starter you can determine its volume by simply measuring the
interlock contact and an air flow switch. outside and multiplying length, width and height to get
Note that I said air “flow” switch, on many sys- the volume. Do the same thing for the boiler. The manu-
tems a simple pressure switch is used and pressure facturer’s instruction manual will list the weight of the
doesn’t prove there’s flow. Too often I see boilers with a boiler empty and flooded so you can calculate the vol-
simple windbox pressure switch used to prove combus- ume of water, steam, steel, and refractory then subtract-
tion air flow. It’s contacts will close when the fan runs ing that to get the volume of the gas space in the boiler.
and open when the fan is shut down because a pressure Divide the dry weight by 500, the approximate weight
switch simply compares pressure at the point of connec- of a cubic foot of steel to determine the steel volume and
tion and atmospheric pressure. If one of those switches divide the difference between flooded and dry weight
is giving you difficulty (they seldom do) you can usually by 62.4 to determine the volume of water; subtract the
get it to function by closing the burner register. I’m not results from the outside volume of the boiler. The total
saying you should do that, you shouldn’t; there’s no air gives you the volume of the setting.
flow through the burner when the register is shut… but For single burner oil and gas fired boilers you can
the switch is made! use the required combustion air flow rate for full load air
I’ve seen many installations where the operators flow, see the section on fuels. If the boiler fans cannot be
have pulled similar tricks to get the boiler operating or operated at full load air flow on a purge determine the
keep it operating. Air flow should be proven by a means actual purge air flow rate (as a percent of full load) using
that’s independent of such conditions and my favorite the processes described for estimating the minimum air
method is using the differential pressure across a fixed flow. For multiple burner and coal fired boilers use 25%
(not adjustable) resistance somewhere in the air flow of the full load air flow as a purge rate. Now you have
stream. I’ll mention some methods later in the book. a volume in cubic feet and a rate of flow in cubic feet
per minute. Divide the volume by the flow rate and you
Purge the Boiler know how many minutes it takes to displace the volume
Once air flow is proven we “purge” the boiler. A of the setting which is one air change. Multiply that re-
purge is a constant flow of air through the boiler that sult by the required number of air changes (4, 5 or 8) to
must occur long enough to ensure any combustible ma- determine the purge timing.
terial is swept out the stack so it can’t be ignited by the Maybe you aren’t starting a new boiler but you
starting burner. On an initial start-up some math has to would like to know what the required timing for your
be done to determine the purge timing and the flow rate existing unit is. The means is described and it’s always a
may have to be established. Your state law and frequent- good thing to know. For many of you the result is going
ly insurance company requirements dictate the flow to be a surprise. The required purge timing is usually a lot
rate and timing of a purge. These are the more common longer than what the boiler is originally set up for. I’m the
requirements: Single burner boilers can be purged at the only engineer I know that actually performs those calcu-
maximum combustion air flow rate unless they are coal lations to determine the required purge timing.
fired. Multiple burner and coal fired boilers purge air It wasn’t a big deal in the days of pneumatic timers
flow requirements vary but the basic rule is 25% of full where an operator could reach in the panel and read-
load air flow. ily turn the timer back… most of them did it. When we
Single burner fire tube boilers must purge for suf- started installing microprocessor based (programmable
ficient time to displace the volume of the setting four controller) systems near the end of the twentieth century
times. Single burner water tube boilers must purge for some of our customers got very excited; the operators
sufficient time to displace the volume of the setting eight couldn’t reach into the control system to change the pro-
times. Multiple burner and coal fired boilers must purge gram memory and shorten the purge. As a result, they
for sufficient time to displace the volume of the setting had some very long purge times to go through after a
five times and for at least five minutes. power interruption or any boiler trip. There’s more on
So how to calculate the purge air timing? First this subject in the section on why boilers blow up. Try to
calculate the volume of the setting. The setting is ev- live with the legally binding number if you can.
erything from the point where combustion air enters Once you’ve done the math, calculated the correct
enclosed spaces leading to the furnace to the exit of the purge time and set the controls for it, use the purge to
stack. For all the fans, ductwork, air heaters, burner clear the boiler every time before attempting ignition.