Rosemount 3491, Справочное руководство, Страница: 78 / 152

Reference Manual
00809-0100-4841, Rev. AA
May 2007
Rosemount 3490 Series
4-46
Figure 4-31. Wet well application
- Stage 3 of example
In an emptying application, the Common Off point is always the Off point of
the Standby, Common Off mode relay with the lowest On point, which in this
example is P412 of Relay RL1 at 2 metres
Pump 2 continues to pump until the measured level falls below 2 metres
(P412, Common Off), at which relay RL2 will de-energise to switch off
Pump 2.
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 over-use 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 4-32. Filling tank
application - Stage 1 of example
When the measured level falls below 5 metres (P411, On point), the relay RL1
will be energised to open Valve 1.