The ELECTRODE PROBE TYPE of feedwater
control (fig. 1-9) and low-water cutoff consists of an
electrode assembly and a water level relay. The electrode
assembly contains three electrodes of different lengths
correspondin g to high. low, and burner cutout in the
To understand the operation of a boiler circuit, refer
to figures 1-9 and 1-10 as you read the information in
table 1-1. Although this information is not complete. it is
presented here to acquaint you with the operation of the
electrode type of boiler water level control.
The location of the try cocks is shown as item 6 in
figure 1-6. The purpose of the try cocks is to prove the
water level in the boiler. You may see water in the gauge
glass. but that does not mean that the water level is at that
position in the boiler. lf the gauge glass is clogged up. the
water could stay in the glass giving a false reading. The
try cocks, on the other hand, will blow water. steam. or a
mixture of steam and water out of them when they are
Figure 1-9.Electrode type of water level control.
manually opened. When steam is discharged from the
lowest try cock, you have a low-water condition.
Table 1-1.Operation of a Boiler Circuit
A c t i o n
When the feed pump switch The feed pump motor is
The feed pump will operate under control of the
is in the auto position.
water level relay (item #6 in fig. 1-10).
When the water level in the The circuit through the All of the CONTACTS labeled #6 change position.
boiler reaches the level of electrode is grounded and this The three feed pump contacts that are normally
completes the circuit.
closed, i.e., 6-1, 6-2, and 6-3 open, and contact 6-4
closes which maintains the grounded circuit through
When the water level falls The circuit through relay #6 This de-energizes relay #6, so all of the CONTACTS
below electrode #2.
will no longer be grounded
labeled #6 return to their normal positions. Contacts
because the water is not in 6-1 through 6-3 close and 6-4 opens. The feedwater
contact with the electrode.
pump is energized and water is pumped into the
When the water level rises Relay #6 will energize again. The cycle continues and the water level in the boiler
again to electrode #3.
When the water level falls Relay #5 will be de-energized. CONTACT 5-1 will open. This action de-energizes
below electrode #1.
the entire control circuit. The boiler is now shut
down and the low-water alarm is sounded.