Cleaver-Brooks CBH 40 - Setting and Adjusting; PRESSURE CONTROLS-Steam Boiler

Chapter 5--Adjustment and Maintenance
controls clean. If any dust accumulates in the in-
terior of the control, remove with a low pressure
air hose taking care not to damage the mechanism.
Examine any mercury tube switches for damage or
cracks: this condition, indicated by a dark scum
over the normally bright surface of the mercury,
may lead to erratic switching action. Make certain
that controls of this nature are correctly leveled
using the leveling indicator if provided. Piping
leading to various controls actuated by pressure
should be cleaned if necessary. Covers should be
left on controls at all times.
Dust and dirt can cause excessive wear and over-
heating of motor starter and relay contacts and
maintenance of these is a requirement. Starter con-
tacts are plated with silver and are not harmed by
discoloration and slight pitting. Do not use files or
abrasive materials such as sandpaper on the con-
tact points since this only wastes the metallic silver
with which the points are covered. Use a burnish-
ing tool or a hard surface paper to clean and polish
contacts. Replacement of the contacts is necessary
only if the silver has worn thin.
Thermal relay units (overloads) are of the melting-
alloy type and when tripped, the alloy must be
given time to re-solidify before relay can be reset.
If over-loads trip out repeatedly when motor
current is normal replace them with new over-
loads. If this condition continues after replacement
it will be necessary to determine the cause of ex-
cessive current draw.
Power supply to the boiler must be protected with
dual element fuses (fusetrons) or circuit breakers.
Similar fuses should be used in branch circuits and
standard one-shot fuses are not recommended.
SETTING AND ADJUSTING
Burner controls properly set to match load
demands will provide operational advantages and
achieve the following desirable objectives:
(a) The burner will be operating in low fire posi-
tion prior to shut down.
(b) The burner will operate at low fire for a
brief period on each start during normal
operation.
(c) Eliminates frequent burner on-off cycling.
Normal operation of the burner should be with the
damper positioning switch in the "auto" position
rather than either "low" or "high". This allows
the burner to automatically shift from one firing
rate to the other in accordance with load require-
ments.
When firing a cold boiler, it is recommended that
the burner be kept under manual flame control un-
til normal operating pressure or temperature is ap-
proached. If the burner is not under manual con-
trol on a cold start, it will immediately move to
5-4
high fire as soon as the program control releases
the circuit that holds the burner in low fire during
ignition. The high-low fire control will be calling
for high fire and the burner will move to that
position as rapidly as the damper motor can com-
plete its travel. This rapid heat input can subject
the pressure vessel metal and refractory to un-
desirable conditions.
Any control setting must not cause the boiler to
operate at or in excess of the safety valve setting.
Settings that do not exceed 90% of the valve set-
ting are recommended, with lower settings greatly
desirable if load conditions permit. Avoid having
the operating pressure too near the valve set
pressure, because the closer the operating pressure
is to the valve pressure, the greater the possibility
of valve leakage. Continued leakage, however
slight, will cause erosion and necessitate early
valve replacement. The control settings on a hot
water boiler must be within the temperature and
pressure limits of the boiler.
Ideally, the burner operating controls should be set
under actual loaci conditions. Often, especially on
new construction, the boiler is initially started and
set to operate under less than full load
requirements. As soon as possible thereafter the
controls should be reset to provide maximum
utilization of the firing system.
In the setting of these controls, consideration must
be given to the time required for a burner restart.
Upon each start, there is a prepurge period of some
length, plus the fixed time required for the proving
of the pilot and main flame. This, plus approxi-
mately one-half minute required for damper motor
travel from low to high fire, may allow pressure or
temperature to drop below desirable limits.
The mechanics of setting the controls are:
PRESSURE CONTROLS-Steam Boiler
The pressure controls that serve as the operating
limit control and as the high-low fire control are
equipped with an adjustable differential setting.
See the following portion of this section for adjust-
ment procedures.
The switch in the control opens when the steam
pressure reaches a pressure equal to the main scale
setting and closes at a pressure equal to that
shown on the main scale, MINUS the amount of
differential.
In an installation that does not require very close
control of the steam pressure, the differential set-
ting should be adjusted to its maximum, since _this
will provide less frequent burner cycling.
The high-low fire control should be set to open at 5
to 10 psi. lower than the operating limit setting.
The suggested setting on a low pressure steam
boiler is 2 to 3 psi. The differential setting on this
control may be set low, to give quite close control