Emerson Rosemount 3490 Series, Reference Manual

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Figure 4-32: Pump 1 is Off and Pump 2 is On (Rising Level Above 8 m)
A
B
C
D
E F
A. Level in m
B. 8.0 m (P421)
C. 5.0 m (P411)
D. 2.0 m (P412)
E. Pump 1
F. Pump 2
Pump 2 continues to pump until the level falls below 2 m (Off Point, P412), at which relay
RL2 will de-energize to switch off Pump 2. (Pump 1 is already switched off).
In this emptying application, the common off point is P412 (Off point, relay RL1) and P422
(Off point, relay RL2), both of which are at the 2 m level.
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
Standby, split off mode relay
This function requires two or more Standby, Split Off Point mode relays. Only one of these
relays is energized at any one time and the others are on standby to take over when
needed.
A Standby, Split Off Point relay uses its On Point and Off Point parameters, just like 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 standby, split off point relays
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 4-33 , both Pump 1 and Pump 2 are off because the liquid level is at a satisfactory
level, below 5 m.
Getting started Reference Manual
January 2021 00809-0100-4841
106 Emerson.com/Rosemount