Liquid Level Measurement - Emerson Rosemount 2051 Reference Manual

Reference Manual
00809-0100-4101
Table 2-7: Three-valve configuration
Next, open the isolate valve on the low pressure
side of the transmitter.
2.9

Liquid level measurement

Differential pressure transmitters used for liquid level applications measure hydrostatic
pressure head. Liquid level and specific gravity of a liquid are factors in determining
pressure head. This pressure is equal to the liquid height above the tap multiplied by the
specific gravity of the liquid. Pressure head is independent of volume or vessel shape.
2.9.1 Open vessels
A pressure transmitter mounted near a tank bottom measures the pressure of the liquid
above.
Make a connection to the high pressure side of the transmitter, and vent the low pressure
side to the atmosphere. Pressure head equals the liquid's specific gravity multiplied by the
liquid height above the tap.
Zero range suppression is required if the transmitter lies below the zero point of the
desired level range.
2.9.2 Closed vessels
Pressure above a liquid affects the pressure measured at the bottom of a closed vessel.
The liquid specific gravity multiplied by the liquid height plus the vessel pressure equals
the pressure at the bottom of the vessel.
To measure true level, you must subtract the vessel pressure from the vessel bottom
pressure. To do this, make a pressure tap at the top of the vessel and connect this to the
low side of the transmitter. Vessel pressure is then equally applied to both the high and
low sides of the transmitter. The resulting differential pressure is proportional to liquid
height multiplied by the liquid specific gravity.
Dry leg condition
Low-side transmitter piping will remain empty if gas above the liquid does not condense.
This is a dry leg condition. Range determination calculations are the same as those
described for bottom-mounted transmitters in open vessels, as shown in
Reference Manual
(continued)
Figure 2-23 shows a liquid level measurement example.
Installation
March 2024
Figure 2-23.
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