Emerson Process Management 2051 Liquid Level Measurement, Open Vessels Closed Vessels, Rosemount

Reference Manual

00809-0200-4101, Rev AA July 2008

Rosemount 2051

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.

Open Vessels

Closed 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. Figure 2-19 shows a liquid level measurement example.

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, the vessel pressure must be subtracted 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 Figure 2-19.

Figure 2-19. Liquid Level

Measurement Example.

Let X equal the vertical distance between the minimum and maximum measurable levels (500 in.).

Let Y equal the vertical distance between the transmitter datum line and the minimum measurable level (100 in.).

Let SG equal the specific gravity of the fluid (0.9).

Let h equal the maximum head pressure to be measured in inches of water. Let e equal head pressure produced by Y expressed in inches of water. Let Range equal e to e + h.

Then h = (X)(SG)

=500 x 0.9

=450 inH2O

e = (Y)(SG)

=100 x 0.9

=90 inH2O

Range = 90 to 540 inH2O

X

Y

T

20

ZERO SUPRESSION

mA dc

4

0	90	540
		inH2O

2-31