ABB S26 Series Operating Instruction page 11

4.3.3 Closed Tank Installation – Pressure Service –
Single Seal
When a transmitter with one seal is used to measure level in a
pressurized tank, a compensating leg must be connected
between the vapor space at the top of the tank and the low
side of the transmitter.
The remote seal is on the high side of the transmitter, and the
seal element must be located near the bottom of the tank so
that the high seal datum line is at or below the minimum level.
If the process vapor is not readily condensable, or if the
compensating leg is at a higher temperature than the tank
interior, a dry leg can be used. A trap installed at the bottom of
the leg minimizes the possibility of condensate collecting in the
transmitter body.
When the process vapor is condensable, a wet leg is
recommended. The wet leg is filled with a suitable liquid to
maintain a constant pressure on the low side of the transmitter.
This approach avoids the problem of process vapor
condensing and collecting in the compensating leg resulting in
serious measurement error. The leg can be filled with process
liquid or any liquid with a low vapor pressure relative to the
process. A filling tee is required at the top of the leg. The wet
leg can be avoided if the transmitter can be located near the
top of the tank, so that condensate drains back into the tank.
Figure 6: Level measurement of a liquid in a closed tank - pressure service only
- condensate trap as option
Figure 7: Level measurement of a liquid in a closed tank - vacuum or pressure
service - dry leg
4.3.4 Closed Tank Installation – Vacuum Service
When the transmitter is used on a tank in which the operating
pressure range includes pressures below atmospheric, it is
essential that the transmitter be located below the high-
pressure seal datum line. The recommended minimum distance
between the primary datum line and the high seal datum line is
1ft (0.3m). Seal element location requirements for vacuum
service are the same as for pressure service.
Figure 8: Level measurement of a liquid in a closed tank - pressure service only
- wet leg installation
Figure 9: Level measurement of a liquid in a closed tank - vacuum or pressure
service - wet leg installation
Figure 10: Level measurement of a liquid in a closed tank - vacuum or pressure
service - dry leg installation
2600T Series Pressure transmitters | OI/S266-EN Rev. B 11