1Open Vessels; 2Closed Vessels - Emerson Rosemount 3051 Reference Manual

Reference Manual
00809-0100-4001, Rev JA
5.
6.
2.12.1 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.
2.12.2 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, 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.
Installation
Close the equalize valve on
the high pressure (upstream)
side.
Finally, to return the
transmitter to service, open
the low side isolation valve.
Figure 2-25 shows a liquid level measurement example.
H
Test
(Plugged)
Equalize
(closed)
Isolate
(open)
Drain
Process
Vent
(closed)
H
Test
(Plugged)
Equalize
(closed)
Isolate
(open)
Drain
Process
Vent
(closed)
Section 2: Installation
November 2012
L
Test
(Plugged)
Equalize
(closed)
Isolate
(closed)
Process
L
Test (Plugged)
Equalize
(closed)
Isolate
(open)
Process
35