Page 80 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 65

what you have to do. fire purge it twice to get all that fuel out of there before
I say you can’t be timid or quick in this operation trying again.
because you don’t want to create a flammable mixture Use the purge period to think about why you didn’t
that doesn’t light right away. If you open the manual get a fire. It might be because the gas piping was full of
valve too slowly you will allow so little fuel in that the air and you forgot to purge it. It might be that you simply
mixture at the burner will always be too lean to burn. forgot to start the oil pump (in which case, why did the
However, the fuel can settle or rise and accumulate to low oil pressure switch not prevent an attempt at igni-
create a mixture that’s just right, waiting for you to finally tion?) or you forgot to open a fuel or atomizing medium
get ignition. If you open the valve rapidly you can shoot valve. Maybe you saw a little light burning indicating
right past the point where the mixture is right and into you didn’t have enough fuel or a lot of smoke indicating
a fuel rich condition that won’t burn; that only happens you had too much so you can adjust the controls accord-
if the controls admit too much fuel, and they have half a ingly. One problem with steam and air atomized burners
chance on an initial start-up. That fuel in its rich condi- is not enough fuel but it’s not apparent because the steam
tion can mix with some air in the furnace to produce a or air is breaking it up. Make some corrections then, after
flammable mixture and accumulate in preparation for an a purge, try again until you get it going.
explosion, suddenly lighting when you don’t expect it. Now for an operation that many service techni-
If you’re going to be the one operating that valve cians fail to do, mainly because it can take some time and
do a few practice runs before doing it for real and time several light-offs, do a pilot turndown test. It’s a process
yourself. You should operate a plug valve or butterfly where you prove to yourself that, if the ignitor fire has
valve from closed to open in approximately five sec- decayed to the point where it can’t light the main burner,
onds. That gives you enough time to stroke through the scanner will prevent an attempt at ignition on a faulty
all the potential mixture conditions within the trial for pilot. Throttle the gas supply to the ignitor until you note
ignition period without going so fast that you miss the a drop in the flame. Make sure the ignitor can light the
proper point of ignition. When lighting off on oil you’re main flame. Continue dropping the pressure and check-
usually using a multiple turn valve that’s really open ing to be sure the main flame ignites. If, during the pro-
enough for low fire in two turns so practice getting it cess, the scanner fails to detect the ignitor flame and the
two turns open in five seconds. Also train yourself to Burner Management locks out the test is complete.
close the valve at the same speed. That seldom happens, what usually happens is
Keep in mind when you’re operating that valve the ignitor fails to light the main fire. Now you have
that there is a delay involved; the fuel has to displace to repoint or orifice the scanner so it will not detect the
the air in the burner piping and burner parts before it ignitor flame when it isn’t adequate to light the main
can enter the furnace and start mixing with the air. An burner. Matching that scanner position or orificing so it
assistant watching the fuel gauge can read off pressures also allows reliable detection of the main flame can fre-
to you so you can get an idea of where you are. You want quently be a problem. You have to do it, however, or the
to stop opening the valve the instant you see that main system can be forced to repeatedly attempt light-off of
fire light and be prepared to close it a little or open it a a main flame with an inadequate ignitor and the results
little more depending on your perception of the fire. If have been very devastating in some installations.
the fire is bright and snappy, an indication that it’s air Now that you have managed to light a main burn-
rich, you should open the valve more. If the fire is lazy, er you want to establish proper firing conditions so you
rolling, and smoking to indicate it’s fuel rich you should can repeat them for every light-off. If you managed to
close down on the valve. open the manual valve completely without changing the
If you didn’t get a fire then allow a full purge of condition of the fire you’re past the need to balance the
the boiler. It’s not uncommon to have several attempts manual valve and controls. If not, then note the burner
at starting that first fire. My service technicians were pressure and close down on the main fuel control valve
artists and knew what they were doing but they always a little (or adjust the minimum pressure regulator) then
upset me by shortening the purge time so they could open the manual valve a little to restore the pressure and
get back to trying the main burner faster. Please don’t repeat the process until the manual valve is wide open.
do that! Every missed fire leaves an accumulation of Once you know what the proper conditions for start-up
fuel in the boiler that can produce a healthy explosion are the only reason for operating the manual valve is
when it’s lit by the next operation of the ignitor. Always, when you question the ability of the system to repeat
please, allow for a full purge; and …if you saw a smoky those conditions. You should get a smooth light-off ev-