Chemical Engineering October 2014 - 44

Cover Story
Maximum
level
100%
Maximum
level
DP
Transmitter
Vented
LP
HP
Minimum
level
0%
LP
HP
Datum
Line
FIGURE 2. A DP transmitter can be used to measure the liquid level in either an
open (left) or closed (right) tank or vessel
to a tap at the bottom of the tank
and the LP port to atmosphere, the
output signal can be related to the
tank liquid level.
Since the tank is open, any change
in the atmospheric pressure affects
pressures within the tank liquid. But
the LP port of the pressure transmitter
also connects to atmosphere, cancelling
out the effects of atmospheric
pressure on the open tank fluid level.
The liquid density must be constant
or be accounted for.
In the case of a closed tank or
vessel that is sealed from the atmosphere
(Figure 2, right), the lowpressure
side of the transmitter
can be piped to the top of the tank
rather than to atmosphere. Otherwise,
when a process fluid fills or
leaves a closed tank, the pressure
inside may go from positive to vacuum,
which would adversely affect
the level measurement. Connecting
the LP port of the transmitter to the
tank top easily compensates for any
changes in tank pressures.
A bubble tube of pipe offers another
way to measure liquid level
with a DP transmitter in open and
closed tanks (Figure 3). A tube inserted
into the tank maintains a
constant pressure of air or some
gas compatible with the tank contents.
As the liquid level changes,
the backpressure measured by the
transmitter directly corresponds to
the tank level. The advantage here
is that only the tube or pipe material
comes in contact with the process
liquid - not the pressure transmitter.
A second advantage here is
that it requires no connections to
the bottom of the vessel. However,
the process liquid cannot be sensitive
to the gas bubbling through it.
A disadvantage is that a source of
air pressure is required.
Level calculations. To calculate
the pressure at the bottom of the
tank in Figure 3, you must know
the value of h, the distance from the
liquid level to the end of the bubble
tube. For example, if h is 50 in., and
the material in the tank is water,
then we can express the pressure at
the bottom as 50 in. H2O or about
1.8 psig. But if the liquid in the tank
is not water, a conversion must be
made to specify in inches of H2O.
The formula for this conversion is:
h = h' × SG
(1)
Where:
h = liquid head, in. H2O
h' = actual liquid head, in.
SG = specific gravity (dimensionless)
of the fluid in the tank
Recall that specific gravity is the
relative weight of a unit volume of
liquid compared to the same volume
of water. Gasoline, for example, has
an SG of about 0.8. So a gallon of
gasoline weighs 80% of the weight
of a gallon of water. The point is
that you must consider the liquid's
density when measuring liquid level
with a DP transmitter.
Interface level measurement.
Figure 4 shows a technique for
measuring the level of the interface
between two liquids of different
42 ChemiCal engineering www.Chemengonline.Com oCtober 2014
DP
Transmitter
densities, such as oil and water. The
total level in the tank must remain
above the top tap of the DP transmitter
and the distance h must
be constant. As the interface level
changes, the lower tap sees a density
change along with a corresponding
change in the hydrostatic pressure.
Newer methods of performing this
measurement have evolved in the
relatively recent past.
Density. From the discussion
above, it follows that if the liquid
in the tank of Figure 5 is homogenous,
then any changes in the
process liquid density will be reflected
as a change in static pressure.
Again, as long as tank level
remains above the top pressuretransmitter
tap and as long as the
distance between LP and HP sensors,
h, is constant, the transmitter
will respond linearly to changes in
density. Typically, as in level measurement,
chemical plants use a
sensitive DP transmitter because
the spans are relatively small.
Remote seals
Generally, the sensing element
within a DP transmitter comes
in direct contact with the process
fluid. The sensing element, such as
a diaphragm within the transmitter,
responds to the force created by
the process fluid pressure. An associated
sensor generates a low-level
DP
Transmitter
LP
Regulated
air supply
Vented
HP
h
FIGURE 3. Liquid level can also be determined
using a bubble tube
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Chemical Engineering October 2014

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