Emerson Rosemount 3490 Series Reference Manual page 113

Reference Manual
00809-0100-4841
Scum line prevention (Rosemount 3491 and 3492 only)
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This special control function is normally used with a pump control application to provide a
small variance in the start and stop level for the pump. It will help to prevent a build-up of
scum on the side of a wet well or sump.
Table 4-19: Scum Line Prevention Parameters
Parameter
P277
P278
Related information
Auxiliary functions for relay modes
Pump-down (Rosemount 3491 and 3492 only)
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In a pump-down application, the lowest relay Off Point ('pump off' level) is normally a fixed
level above the Transmitter Bottom Reference point. However, it is sometimes required to
make the pumps continue to run for a period past the Off Point or run down the level to
the Transmitter Bottom Reference point. This can clear a sump of sludge that may have
collected at the bottom.
Pump-down can be initiated automatically at pre-set intervals. A digital input can also
initiate pump-down at any time and this will re-set the interval before the next pump-
down.
Pump-down automatically stops when the control unit PV (D800) is zero or after 20
minutes if a duration (P274) has not been programmed, whichever is the shorter period.
Note
For details on configuring a digital input to initiate a pump-down operation, see
inputs IN1 and
The relay mode must be Set point, Assist or, Standby, and parameters P272 to P274 edited
to set-up the pump down operation.
Emerson.com/Rosemount
SETUP → [CONTROLLER] → OUTPUT → RELAY → OVERRIDES
Description
Scum line var This parameter defines a maximum variance in the
On Points and Off Points of selected relays (P278).
The variance is entered in the units of the On/Off
Point, spaced in ten equal increments within the On
Point and Off Point limits. Each time the selected
relays (P278) de-energize, the variance moves on an
increment.
Scum line RL Select relays for the operation associated with
parameter P277.
Each digit represents a relay. Relay RL1 is selected by
editing the first digit to be a "1". Similarly, relay RL5
is selected with the fifth digit. To de-select a relay,
change the appropriate digit back to a "0".
SETUP → [CONTROLLER] → OUTPUT → RELAY → OVERRIDES
IN2.
Getting started
January 2021
Factory default
0.0
00000
Digital
113