Page 335 - Boiler_Operators_Handbook,_Second_Edition

320 Boiler Operator’s Handbook

Most manufacturers’ instructions state that you be able to determine how long that is for your particular
should replace the gaskets every time you change the boiler, burner and load combination.
burner. If you saw what they charged for those little I was in one plant that claimed they only changed
gaskets you would get the same impression that I have, their heavy oil burners once a month. One look into the
there’s more than preventing leaks on their mind! You furnace explained that. They had the atomizing steam
should keep a set of gaskets handy to replace them when running so high that the flame didn’t start until it was
what you’re using fails or you can tell they’re damaged about eight inches from the tip. The fire was just barely
but there’s no reason to replace them every time you stable. I didn’t analyze the situation to see how unstable
change out an oil gun. I’ve fired boilers with brass grips the fire could get on load changes nor how much it cost
on the oil guns that mated up with a steel yoke where for all that extra atomizing steam.
there’s no gasket and they don’t leak unless you get I should explain that it really isn’t all carbon, the
some dirt or grit in the joint. If they can last several hun- accumulation of unburned fuel that has been heated to
dred gun changes the copper wrapped gaskets should drive off much of the lighter fractions and leave mostly
too. I can recall changing guns every shift and seldom carbon is called “carbon” by boiler operators. Carbon is
changing gaskets. a common problem when firing oil. It is less of a prob-
A skilled operator can remove one oil burner and lem when firing light oils. There are many reasons for
install a fresh one in a matter of a few seconds; however, carbon buildup on burner tips, burner throats, and the
if the boiler has a single burner the speed of the operator floor and walls of a furnace when firing oil.
is not much of a consideration because the burner has to The most common reason for carbon buildup is
be shut down to remove the oil gun. To avoid the shut- poor atomization. That can be produced by dirty oil
down of the boiler along with the processes of purging, that plugs burners or ties up the oil like glue so it won’t
low fire positioning, and trials for ignition some burner atomize. Other reasons are using tips too large for the
manufacturers will provide single burners with the abil- load, worn tips, loose tips and tips and other burner
ity to accept two oil guns while others provide as many internals assembled improperly. One of our service en-
as four. gineers solved a poor atomization problem on a burner
The typical two gun arrangement is designed to by assembling it improperly. Nobody could get a decent
insert a temporary oil gun, transfer the fire to that gun fire out of the burner but he did almost immediately by
then transfer back to the main oil gun. The fire may be putting a couple of parts inside the mixer of the steam
lopsided or have voids when firing with the temporary atomizing burner in the wrong order.
gun. Other arrangements use guns with special tips Steam and air atomizing burners can also suffer
that produce a uniform flame pattern when all the oil from condensate in the air or steam, the wrong pressure,
guns are in position and operating. Changing out the oil and blockage of the atomizing medium piping. A prob-
burner so it can be cleaned is accomplished by switching lem we encountered regularly with differential controls
guns one at a time. During the period that one of those was a significant variation in the differential at the burn-
oil guns is removed there is a definite gap in the flame er tip due to pressure drop in the oil or steam piping.
pattern. While changing guns the operator should in- Usually the problem involved lighting the burner at low
crease the air to fuel ratio so the variations in fuel deliv- loads where the differential was so high the fuel air mix-
ery do not produce fuel rich conditions in some portion ture was always lean because the atomizing medium
of the furnace. broke it up too much. The solution to that problem is
Of course the reason we have oil guns is the tip adding an orifice nipple (steel bar simulating a piece of
gets dirty so they have to be removed for cleaning. Spare pipe with hole drilled through it) which allows adjust-
oil guns and tips are provided so you have a clean one ment of the differential at low fire to get stable firing. As
ready to put back in the burner to permit continued the load increases the nipple introduces a pressure drop
firing. Frequently I was asked “How often do we have in the oil that increases the differential at the burner tip
to change out the oil guns for cleaning?” The answer is as load increases.
always “as soon as they get dirty.” I know that’s a flip re- I do know that many operators create their own
sponse but there are no hard and fast rules for cleaning problems when it comes to cleaning oil burners; they
burners, it depends on the oil, contaminants in the oil, damage the tip so it gets dirty faster. Every hole drilled
the firing rates, and the condition of the burner itself. We in a burner tip comes from the factory fresh and sharp,
have to change guns and clean the tip of carbon before it with a pure 90° angle between the edges of each hole
builds up enough to start hampering atomization. You’ll and the face of the tip. That’s so there is a sharp separa-