SPX MARLEY LLC, User Manual

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6
operation
Operation
The LLC water level control system consists of special purpose liquid sensing
relays on one or more individual circuit cards connected to a probe assembly
located in the cold-water basin. Each circuit card contains one relay and external
signaling is provided by each of these special purpose cards. The individual
relay provides a “Form C” normally open and normally closed dry contact. The
circuit card activates the relay using “through the water” continuity by way of
the senor probes located in the cold-water basin of the cooling tower.
Utilizing water’s ability to conduct electricity, a circuit path can be established
between one probe tip and the other. Current conducts through the water
across probes of dissimilar length. One common or reference probe is present
in all systems and is shared by all functions of the system. This probe can be
identified by its length. It is the longest probe in the system and extends the
deepest into the basin. The current path is routed between all other probe tips
and this one “common”. When the water level reaches the shorter probe, the
circuit is completed and the relay responds, opening or closing relay contacts
corresponding to a fixed level. For low-level control, the ground reference
probe and a slightly shorter probe provide the circuit. When the water level
drops below this tip, the continuity between this probe and the reference
probe is interrupted and the relay contacts transfer. The distance from the tip
of the low probe to the floor of the basin determines the minimum water level
that is allowed before an alarm is produced or pump operation is interrupted.
The number of additional probes is determined by the individual application.
As an example, in a “water makeup” system there are three probes. One
reference and two standard or short-tipped probes. The tip of the reference
probe is normally positioned slightly above the basin floor with the additional
probe tips positioned at different heights dictated by their specific function.
The Makeup system would have one probe at a height to begin or start filling
the basin and another positioned higher to complete or stop filling. A probe
for a High Alarm or High Cutoff would be positioned at a level to activate
when the basin water exceeds its normal operating level and logically a Low
Alarm or Low Cutoff would be positioned to detect a low water level nearer
the bottom of the basin. Again, signaling is achieved in two ways. High Level
and Makeup cards react when the water provides a completed circuit or
continuity between its sensor and the reference probe. The second type of
signal is for Low Level detection. The Low Level cards react when the water
is not present and opens the circuit or disrupts the current flow between its
probe and the reference.