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Utilitiesman Basic Volume 02 - Manual for electric, plumbing, water and other utilities | Next
Table 1-1.—Operation of a Boiler Circuit |
Figure 1-7.—Typical water columns.
Figure 1-8.—Combination float and mercury switch type of
feedwater control.
reacts to changes made within a maintained water level
by breaking or making a complete control circuit to the
feedwater pump. It is a simple two-position type control,
having no modulation or differential adjustment or
setting. As all water level controllers should be, it is
wired independently from the programmer. The control
is mounted at steaming water level and consists of a
pressurized float, a pivoted rocker arm, and a
cradle-attached mercury switch. The combination float
and mercury switch type of water level control functions
as follows: As the water level within the boiler tends to
drop, the float lowers. As the float lowers, the position of
the mercury switch changes. Once the float drops to a
predetermined point, the mercury within the tube runs to
its opposite end. This end contains two wire leads, and
when the mercury covers both contacts, a circuit is
completed to energize the feedwater pump. The pump,
being energized, admits water to the boiler. As the water
level within the boiler rises, the float rises. As the float
rises, the position of the mercury switch changes. Once
the float rises to a predetermined point, the mercury runs
to the opposite end of its tube, breaking the circuit
between the wire leads and securing the feedwater pump.
The feedwater pump remains off until the water level
again drops low enough to trip the mercury switch.
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