Page 410 - Boiler_Operators_Handbook,_Second_Edition

Controls 395

label can be inappropriate. I’ll call them self-checking burner management system. The system then waits for
when they contain a device that blocks the light from the the prescribed period of time for a purge.
sensor at intervals and the detector circuit has to sense a Some are set with fixed timing but modern units
no flame signal during that interval. Some scanners sim- have provisions for setting the purge time to comply
ply block the light at regular intervals so the detector cir- with the code requirements. The controller supervises
cuit can determine a flame is present because the signal the purge by requiring the damper open or purge air
from the scanner is constantly swinging to produce an flow switch contacts remain closed during the purge
alternating current. A constant signal from the scanner period. Some will simply restart the purge timing if the
indicates no flame or scanner failure including failure input is interrupted while others will stop the start up.
of the self checker. Self checking of scanners shouldn’t Once the purge timing is complete the contacts for high
be confused with self checking of the flame detector cir- fire are opened and another set close to instruct the fir-
cuitry which is different. ing rate controls to go to a low fire position for ignition.
Someone will also come up with a device and call When the firing rate controls are at low fire they close
it self checking by virtue of some software scheme. I a low fire position (or ignition permissive where low
spent several hours fooling with some smoke indicators fire is lower) switch contacts to provide an input to the
years ago and found they tested well but didn’t indicate burner management system.
the smoke I could see by looking at the top of the stack. During all of that portion of the start-up sequence
Their scheme for calibrating a black stack consisted of the scanner should be looking for a flame. If it sees one
shorting out a terminal on the sensor, not turning off the the system should lock out. The reasons can be anything
light at the other end of the stack. When I finally decided from defective scanners to oil dripping out of a gun and
the system wasn’t turning the light off and put a blank lighting to glowing hot refractory from the previous
in front of it the indicator happily showed a clear stack! firing. On more than one occasion an operator figured
If the scanner doesn’t block the sensor’s view of the fire out that he could prevent the lockout by pulling out the
it isn’t a self-checking scanner. scanner and covering it with his glove during the purge
A self-checking circuit simply confirms a flame and low fire positioning. Needless to say, he didn’t have
isn’t detected when it shouldn’t be. Of course I’ve dis- any accurate flame sensing and eventually an explosion
covered several systems that were installed and con- occurred. If that scanner thinks it sees a flame where
nected in such a manner that the self checking functions there isn’t one it’s not safe to operate that boiler.
of the circuit were not allowed to work. If, on a burner With low fire position proven the controller closes a
that’s turned off, your scanner doesn’t detect a flame on contact to energize an electric spark in the ignitor and an-
a candle or cigarette lighter that you hold in front of it other contact to energize the ignitor gas shut-off valve(s)
(you have to remove it from its mount) and it doesn’t (if the burner is equipped with an ignitor). The controller
alarm and lock out as a result, yours is one of those. then waits ten seconds to see if the valves open to admit
Since many of them were only found after a boiler explo- gas that is lit by the spark to create an ignitor flame. If the
sion I urge you to perform that test. If the system doesn’t flame isn’t detected in that time it stops operation and
lock out it isn’t safe. energizes an alarm horn. If the flame is detected it closes
The typical BMS (burner management system) another contact to energize the main fuel valves. Some
provides for automatic operation of the burner, per- also de-energize the electric spark. At a prescribed time
forming all the steps described in the section on boiler after the main fuel valves are open it de-energizes the ig-
start-up. When a pressure control switch closes the BMS nitor gas valves. If a flame remains detected the controller
should first determine there is no flame in the burner. opens the low fire contact and closes an automatic contact
Provided operating limits like low water cutoff, high to permit automatic operation of the modulating controls
steam pressure and low fuel pressure are all satisfied to control the firing rate.
(contacts closed in a series circuit) it closes an output How it did it in the early days was clumsily and
contact to start the burner fan or fans. When air flow is with a lot of errors. The timing was all controlled by a
proven by closing contacts on an air flow switch the fir- timer motor powering a shaft with several epoxy im-
ing rate control system is instructed to increase damper pregnated fiber discs on it that served as cams. Each cam
position to high fire. Some systems may include provi- had a flat metal spring riding on it and that spring made
sions to start an oil pump and prove it operating as contact with another one where the cam was notched.
well. The open damper or a purge air flow switch senses The program was initiated when the boiler’s pressure
purge air flow to close a contact for another input to the control switch closed to energize the motor to start rotat-