Reference Manual
00809-0100-4841, Rev. AA
May 2007
4-49
Rosemount 3490 Series
Figure 4-37. Stage 3 of Standby
split-off wet well example
When the measured level falls below 3.5 metres (P422, Off point), relay RL2
will de-energise to switch off Pump 2.
Relay RL1 is then energised to start Pump 1 again.
When the measured level falls below 2 metres (P412, Off point), relay RL1 will
de-energise to switch off Pump 1.
However, if Pump 1 kept the measured level below 8 metres, it would stay
switched on until the level is 2 metres. (Safeguards to prevent overuse of the
pump are in Section “Relay Safeguard Options“ on page 4-43.)
Filling Tank Application:
Consider an application with two relays, RL1 and RL2, connected to individual
valves controlling the delivery of liquid into a tank. In addition, the PV value
(D800) is a liquid level measurement in metres.
Initially, both valves are closed since the liquid level is at a satisfactory level,
which in this case is above 5 metres.
Figure 4-38. Stage 1 of Standby
split-off filling tank example
When the measured level falls below 5 metres (P411, On point), the relay RL1
will be energised to open Valve 1.
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