Emerson Rosemount 3490 Series, Reference Manual

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Note
If Pump 1 had kept the level below 8 m, it would stay switched on until the level is 2 m.
Related information
Relay safeguard options
Duty assist with split off points
This function requires two or more Assist mode relays. Two or more of these relays can be
energized at the same time (assisting), and they all de-energize at a split off points.
An Assist relay uses its On Point and Off Point parameters in a similar way to a Set Point
mode relay. A typical application would be emptying a wet well/lift station.
The worked example here illustrates how the function works. To keep the example simple,
the auto-sequencing options are not considered.
Example wet well/lift station application (emptying operation due to rising level)
using assist mode relays and split (separate) off points
Consider an application with two relays, RL1 and RL2, connected to individual pumps in a
wet well. The control unit PV value (D800) is a liquid level measurement in meters.
In Figure 4-26 , both Pump 1 and Pump 2 are off because the liquid level is at a satisfactory
level, below 5 m.
Figure 4-26: Both Pump 1 and Pump 2 are Off (Level Okay)
A
B
C
E
D
F G
A. Level in m
B. 8.0 m (P421)
C. 5.0 m (P411)
D. 3.5 m (P422)
E. 2.0 m (P412)
F. Pump 1
G. Pump 2
When the level exceeds 5 m (On Point, P411), relay RL1 is energized to start Pump 1
(
Figure 4-27 ).
Getting started Reference Manual
January 2021 00809-0100-4841
102 Emerson.com/Rosemount