Page 411 - Boiler_Operators_Handbook,_Second_Edition

396 Boiler Operator’s Handbook

ing the cam assembly. As the cams rotated a change in failure in the burner management system should also
the diameter of one would drop its spring to close the act to safely shut down the boiler or, if it’s failure does
motor circuit and another closed in the modulating mo- not present an immediate danger, prevent a subsequent
tor circuit to drive the controls to high fire. Another con- start-up of the burner.
tact then opened to stop the cam drive motor. Once the Of the many boiler explosions I’ve investigated
modulating motor got to high fire it closed its high fire only two were found to occur during operation, the rest
interlock contact which bypassed the open cam contact occurred on start-up and problems with the system ar-
and restarted the cam drive motor beginning the purge rangement (design) or an alteration of the design could
timing. be attributed to the explosion. Unlike airplane accidents
I won’t explain the whole operation, you’ll figure where the reason is regularly attributed to pilot error
the rest out if you happen to get in a plant that’s old I don’t find operator error to be a primary reason for
enough to still have one (I doubt it) and you’ll want to re- a boiler explosion. Many times the operator is present
place it anyway. Those cam contacts were always a prob- and doing something but that doesn’t mean the system
lem because the springs would stretch and get weak and operated flawlessly, usually the system prevented proper
the contacts on them would get dirty so they wouldn’t operation.
complete a circuit (a boiler room wasn’t that clean in the Those of us that design the burner management
good old days) and lower priced microprocessor units systems have a directive to make the system “damn
have replaced most of them. You can’t access the program fool proof and moron approved” so it’s supposed to be
or fix the microprocessor based units so you simply re- virtually impossible for operators to create an explosive
place them; it saves you fooling with those springs and condition unless they intentionally defeat limits and in-
cleaning those contacts about once a month. terlocks. Don’t get me wrong, I’ve seen many a bypassed
One last comment on the old cam operated con- interlock or limit switch. Why was it bypassed? Because
trollers; there was always a dial connected to the cam the damn burner wouldn’t work if it wasn’t!
shaft. The dial looked something like the bottom of an Speaking of those limits and interlocks reminds
aluminum can that was cut off and it had numbers on it me of the many ways they can fail to do what they’re
that corresponded to the timing of the motor so it could supposed to, mostly because of improper design or ap-
be checked. There was also a large black dot on the dial plication. Every other facility I visit for the first time is
that indicated where the timer was to stop for normal usually set up with minimum air flow limits and purge
firing operation. The beginning of the cycle, which was air flow switches that, quite frankly, don’t work. It’s be-
also where the timer stopped when the boiler shut down cause they don’t sense flow, they only sense pressure.
for any reason was always marked with a zero. The ex- I’m sure you’ve seen many burner assemblies
planation of the cam operated switches and timing does where the air flow switches are air pressure switches
help explain some of the instructions for the micropro- with one side connected to a burner windbox. Any
cessor based equipment because they were written to burner windbox, however, normally has burners with
help us old farts relate to what was going on in that new air registers that can shut off the flow. Even if they don’t
black box. a blockage in the boiler will ensure pressure to actuate
One of the most important elements of the burner those switches. I’ve seen many an installation where the
management control is the checking provisions. The operators closed the burner registers to produce enough
burner management controller had to include at least pressure in the windbox to trip the purge air flow
two relays, a power relay and a flame relay. The power switch. Needless to say, if the registers are closed there
relay could only be energized via a normally closed is no way to get purge air flow. I prefer air flow switch
contact on the flame relay (proving the flame relay was installations that measure the air flow, normally simply
de-energized) and it closed a normally open contact to sensing the pressure at the fan inlet as shown later for
bypass the flame relay contact to continue operation. adding air flow metering. See initial boiler start-up for
Once the power relay was up the flame relay closed more clues on proper operation of burner management
its contacts to power the main fuel valves and stop the controls.
timer motor. The key actions for a wise operator when it comes
Any indication of a flame when there isn’t sup- to burner management is 1) know what they’re sup-
posed to be one can mean the burner will continue to posed to do, 2) shut the boiler down when they don’t
operate when there is no flame present, generating an do it, 3) Report inconsistencies in operation and regular
explosive atmosphere in the boiler. Any component interruptions in operation, 4) don’t change switch or po-