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Example 2 Level Monitoring and Control (up or down)
A vessel, containing a liquid that has a variation in level that is to be
monitored, and when the level reaches a specific point, the vessel is pumped
down, with the fluid being transferred to another process. The pump will be
assigned to Relay 1 a High Alarm to Relay 2 and Low Alarm to Relay 5.
empty distance (P105), 3.5m
85%, high alarm on (P223), 2.38m
80%, high alarm off (P224), 2.24m
80%, control (down) on (P213), 2.24m
15% , low alarm off (P254), 0.42m
10%, low alarm on (P253), 0.28m
20%, control (down) off (P214), 0.7m
In this example, there is a pump (relay 1), which will come on if the level
rises to 2.24 m, and go off when the level drops to 0.7 m. (control down). If
the level rises to 2.4 m, then the high-level alarm (relay 2) will come on
until the level drops to 2.24 m. If the level falls to 0.28m, then the low-level
alarm (relay 5) will come on until the level rises to 0.42 m.
Alternatively, if it is a control up application, then the on and off points for
the control relay are reversed, so the control relay comes on when the level
is at 0.7 m and goes off when it rises to 2.24 m.
The display will show the level in the tank and the mA output will be
representative of level where 4mA = empty level (0%) and 20mA = 2.8m
(100%).
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