ABB 268D/V Operating Instruction page 42

. . . ADDENDUM FOR FLANGE-MOUNTED TRANSMITTERS
In open tank applications, mounting the transmitter on the tank
nozzle provides the HI side process connection, with the LO
side being vented to atmosphere. The hydraulic head pressure
acting against the process diaphragm is a direct measurement
of the liquid level. The effect of atmospheric pressure is
canceled because this pressure is applied to both sides of the
transmitter.
A recommended open tank installation is shown in Figure 3.
In the closed tank application, Figure 4, the effect of tank pressure is canceled by connecting the HI side and LO side of the primary
transducer to the tank. The HI side connection is made by mounting the transmitter on the tank nozzle. A compensating leg
connects to the LO side near the top of the tank. It is important to ensure that this leg is either completely free of liquid (dry leg)
or completely filled to a constant level (wet leg).
INSTALLATION WITH DRY LEG
Max. level
Min. level
Datum
line
For a better understanding, three applications of liquid level measure are shown, as follows:
°
Application n 1 : Liquid Level - Open Tank
Max level
Minimum
level
Minimum level must be at or above this datum
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Gate valve
Dry leg
Condensate trap
(optional)
Figure 4 - Liquid level transmitter in closed tank installation
Using a Flange-Mounted Transmitter
Datum
line
Figure 3 - Liquid level transmitter
in Open tank installation
INSTALLATION WITH WET LEG
Max. level
Min. level
Datum
line
Span = H1*G1, in inches w.g. if H1 is in inches
G1 = specific gravity of the process liquid
Lower range value = [H2*G1], in inches w.g. if H2 is in inches
Upper range value = Lower range value + span
Low side vented
Transmitter datum
Max. level
Min. level
Filling tee
Gate valve
Wet leg