Introduction
1-8
750-382
CBT
1.4.3 — Water level and limit controls, all boilers
The pressure transmitter provides an analog steam
pressure signal to the controller for modulated firing
(full modulation units) or on/off operation.
The high limit control (HLC) provides safety shutdown
when steam pressure exceeds the maximum operating
range of the boiler. It includes a manual reset switch
and adjustable set point.
The operating control (OLC) is set below the maximum
pressure setting of the HLC. This control is auto reset
and includes adjustable set point
Standard water level controls comprise an automatic
reset primary low water cutoff (LWCO) and a manual
reset auxiliary low water cutoff (ALWCO). A manual
reset LWCO is available where required by code.
Four individual conductance probes are mounted
within the LWCO water column to indicate levels for
low water, pump on, pump off, and high water alarm
(optional). The solid-state level control boards reside in
the boiler control enclosure.
A panel mounted ALWCO reset/test switch is provided.
The water column blowdown should be piped to a
boiler blowdown system. Please refer to service and
maintenance section for blowdown procedures.
FIGURE 1-8. Low water and limit controls
Steam pressure gauge
Manual valve
ALWCO probe
HLC
OLC
LWCO/pump control
Gauge glass
Gauge glass
Pressure transmitter
blowdown valve
WC blowdown valve
(modulating burner
only)
holder
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