Page 83 - Boiler_Operators_Handbook,_Second_Edition

68 Boiler Operator’s Handbook

When the outdoor temperature is swinging from warm with thermometers. Normally it is a psi per hour rate
to cold the amount of time the boiler is held at low fire that you can track. On large boilers it’s not at all uncom-
is almost proportional to the average heat load, it will mon to have to stop and start the burners to limit that
be less as the average temperature drops and the delay warm-up rate. Most boilers smaller than a quarter of
before release to modulation will decrease. Unless you a million pounds of steam per hour can be allowed to
are always on hand to control the warm-up of a boiler warm up at the low fire rate.
you should have low fire hold controls. One final note, Fixed fire boilers are absorbing the maximum heat
on some steam boilers where operating pressures are input every time the boiler is fired so they have to be
low you might want to use a temperature switch for low started and stopped to reduce the warm-up rate. If that’s
fire hold because pressures can swing more significantly required, other than on initial start-up, the manufacturer
generating control problems. should provide automatic provisions for it.
You really don’t want to suddenly switch from Multiple burner boilers can be warmed up slowly
light-off position to modulating because the controls by only operating one, or a portion of the burners. The
will simply run the burner right up to high fire when it burners should be switched regularly, according to the
isn’t necessary. If you’re controlling the boiler manually manufacturer’s instructions or every fifteen minutes to
you should allow it to come on line while at low fire. one half hour so the heating is more uniform. Always
Then, when it seems to have reached its limit, gradually start another burner before extinguishing the one it
increase the firing rate until the load is up to normal op- replaces so you don’t have to purge the unit. A purge
erating conditions, then switch to automatic. is blowing cold air over the metal you just heated to
When the boiler is unattended ramping controls produce a sudden swing in its exposure to temperature.
function the same way and are recommended for high That could produce stress cracks in the metal that you
pressure steam boilers that start and stop automatically. don’t want to have. A boiler should be limited to the
They control the rate of change of the firing rate so it number of starts and full stops it is exposed to. When
gradually increases at a constant rate (like going up a the manufacturer recommends limiting stops and starts
ramp) until it’s at high fire or, more normally, the set pres- it’s for high pressure boilers with very thick metal that is
sure is reached and the automatic controls take over. A more susceptible to damage from stress due to tempera-
ramping control should only function on the initial tran- ture variations across its thickness.
sition from light-off to automatic, or from low fire hold
to automatic. The transition rate should be adjustable Full Metering Switch to Automatic
and you should set it so the rate is as slow as possible to Simply switching from light-off position to firing
minimize thermal shock. Pneumatic and microprocessor rate control, whether it’s manual or automatic, can be
based systems are described in the section on controls. rough with a full metering control system. The fuel and
air controls are pre-positioned by the interface with the
Test Safeties Burner Management system and may be lower or higher
Never forget that the safety valves are the last line than the position that produces flow rates acceptable to
of a defense against a boiler explosion and test them as the control system. The result is what we call a “bump”
soon as possible. First, do a lift test on steam and high as the controls are suddenly allowed to react to the
temperature hot water boilers when the pressure has ex- difference and make some rather abrupt, and usually
ceeded 75% of the set pressure of the valves. Hot water excessive, changes in valve or damper positions in an
boiler safeties can usually be tested before firing by ap- effort to establish the required flows.
plying city water pressure. On almost any pneumatic or electronic (not mi-
As soon as possible in the start-up of a new boiler croprocessor based) controls you can also experience
run a pop test of steam and high temperature hot water problems with reset windup, where the controls detect
boilers. A pop test is described later. an error and try to correct it, but can’t, so the controller
output continues to increase or decrease until it reaches
Boiler Warm-up zero or maximum possible output. The outputs are out-
Your boiler manufacturer should have indicated side the control signal range (such as 3 to 15 psig where
a warm-up rate in the instruction manual. A problem the signal can drop to zero or climb to 18 psig—the stan-
with it is normally there’s no way for you to determine dard supply pressure. Similarly a 1 to 5 volts range can
if you’re actually doing it. If it were critical for tempera- be a negative voltage and go as high as 12). In either case
tures below 212°F then the boiler should be equipped there is no response to controller action until the control