Emerson Rosemount 3051 Reference Manual page 46

Section 2: Installation
November 2012
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 2-25. 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 inH O
2
e = (Y)(SG)
= 100 x 0.9
= 90 inH O
2
Range = 90 to 540 inH O
2
Wet leg condition
Condensation of the gas above the liquid slowly causes the low side of the transmitter piping to
fill with liquid. The pipe is purposely filled with a convenient reference fluid to eliminate this
potential error. This is a wet leg condition.
The reference fluid will exert a head pressure on the low side of the transmitter. Zero elevation of
the range must then be made. See
Bubbler system in open vessel
A bubbler system that has a top-mounted pressure transmitter can be used in open vessels. This
system consists of an air supply, pressure regulator, constant flow meter, pressure transmitter,
and a tube that extends down into the vessel.
Bubble air through the tube at a constant flow rate. The pressure required to maintain flow
equals the liquid's specific gravity multiplied by the vertical height of the liquid above the tube
opening.
36
Figure 2-27 shows a bubbler liquid level measurement example.
Figure
20
SUPRESSION
ZERO
mA dc
4
0 90
Figure 2-26.
Reference Manual
00809-0100-4001, Rev JA
2-25.
X
Y
T
540
inH O
2
Installation