Page 75 - Boiler_Operators_Handbook,_Second_Edition

60 Boiler Operator’s Handbook

call the manufacturer. Assuming there are no leaks we including broken linkage, should establish a low fire
have now pressure tested the burner piping at operating condition.
pressure (or did you actually hydro test it?) and we can It pays to look at your equipment to see how it will
continue with the setup. Crack the recirculating control fail, if linkage comes loose and can fall to open the fuel
valve then slowly open it until you’ve established light- control valve add weights so it will close to minimum
off pressure at the burner piping (after the firing rate fire instead. The air flow controls should also rest on a
control valve) then continue slowly opening the supply mechanical stop at low fire so the dampers never shut,
valve while adjusting the recirculating control as neces- unless they leak so much at closed that low fire air flow
sary to maintain the pressure. is still achieved. I’ve run into a few new full metering
Nope, you’re not done. Establishing a pressure for systems where the designer or contractor felt a mechani-
light-off isn’t that simple. Remember the chapter on flow? cal stop was unnecessary, establishing minimum fire
You aren’t so concerned with pressure as you are with using control signals; of course most of those discoveries
flow and establishing the pressure doesn’t prove the flow. were on plants that had experienced a boiler explosion!
Use the oil flow indication on full metering systems or I was there to find out why and no low fire stops is usu-
take two oil meter readings at a set interval to determine ally one reason.
gpm to determine the flow; it should be the design flow
for fixed fire boilers and 20% to 50% (depending on turn- Fill Boiler and Test Low Water Cutoffs
down capability) for modulating boilers. Before starting a fire in the boiler, fill it with water
If there’s no meter I will count quarter-turns of the to a low level in the gauge glass, about an inch. Make
recirculating control valve on another identical boiler, sure the vent valve on the top of the boiler is open so
match that position and establish light-off pressure by air can get out to let the water in. When the water is
adjusting the control valve. Barring any other means of heated from cold to boiling it will have swelled so much
setting it I’ll listen to the recirculating control valve and that the level will rise to over the middle of the glass. In
set the low fire pressure while the squeal through the re- unusual boilers it’s sometimes necessary to drain some
circulating control valve sounds familiar. After you have water before the boiler reaches operating temperature
established a final position for the control valve you can because the boiler has a large volume of water compared
set the recirculating control valve to produce a pressure to the room for expansion in the steam drum. You’ll
that matches operating pressure at low fire for a good have to drain water to keep it in sight in the glass.
smooth light-off. From this point on you have to keep an eye on that
Since we don’t recirculate gas you can’t guarantee water level. When the water level is visible in the gauge
a light-off position by measuring the flow. We can es- glass it’s time to test the low water cutoff. Proving a low
tablish the pressure. For fixed fire units it’s a matter of water cutoff works on a new boiler is doubly important
setting the pressure regulator. The pressure regulator on because there are so many ways to defeat those devices.
a modulating burner should be set for the design supply The cutoffs should be tested without operating any by-
pressure. We’ll get light-off pressure refined when we pass buttons or similar provisions to ensure they oper-
perform the initial light-off. ate properly. Their failure is a primary reason for boiler
Light-off pressure is not necessarily low fire but failures.
it usually is. Some burners will operate at lower flows Be sure to test the low water cutoff properly; simu-
than that required for light-off and your plant may have late a loss of water due to evaporation by draining the
operating conditions where it is imperative to establish water column or cutoff chamber slowly so the water
a low fire position independent of light-off. If that’s the level drops gradually to the cutoff setting. If it doesn’t
case, your control provisions should includes means of shut the burner controls down, don’t continue the start-
proving the light-off conditions. up until it’s fixed.
Low fire is typically the light-off condition on most
boilers. It’s imperative that the low fire conditions are Prove Combustion Air Flow
fixed and reliable because many upsetting situations After the boiler is filled with water it’s time to start
could produce unstable fires and explosive conditions a burner cycle which always begins with establishing
otherwise. The fuel flow control valves should never and proving air flow through the burner and furnace.
shut, and I do mean never! Their minimum position In very small boilers, like your home hot water heater,
should be set mechanically so something has to break airflow is a function of combustion and is not proven.
before they shut. That way any upset in the controls, In most boilers, however, it amounts to starting a fan