Chemical Engineering September 2020 - 56

Engineering Practice
Liquid Layers: Measuring Overall Level
and Interface Points
Accurate interface level measurement is crucial to optimizing separation processes.
Today's guided wave radar technologies make it possible to measure thin liquid layers
with greater accuracy than ever
Denny Nelson
Emerson
A
bottle of salad dressing can
illustrate a challenging level
measurement application.
Like the situation with oil
and vinegar separation, engineers
may be called to determine the
individual volumes of multiple immiscible
products in a single vessel.
Measuring the interface level in
tanks or vessels is a common and
vital objective across many process
industry applications where immiscible
products, like salad dressing,
form distinct layers. Examples include
oil over water, or low-dielectric
organic solvents over acid.
Some liquids do not mix due to
different molecular polarity or differences
in specific gravity. When
stored together, they separate into
defined layers due to gravity. Accurately
measuring the interface level
between such products can be
challenging, and if inaccurate, can
lead to cross contamination, which
can prove extremely costly. Newer
technologies provide solutions.
Measuring interfaces
Guided wave radar (GWR) is effective
and widely applied for interface
level measurement (Figure 1), but it
requires the top product layer to be
of a certain minimum thickness to
be measurable. Advanced technology
in the latest generation of GWR
devices can cut that minimum thickness
in half or even less, helping
users increase the efficiency of their
operations and reduce costs.
The need to measure interface
level arises when it is necessary to
know the volume of the upper product
layer, which can be determined
from its thickness and vessel diameter.
This could include situations
56
w h e n
a user
wants
to drain
off only
the top
fluid and
requires
an indication
of
when
to stop.
Interface
m e a -
sur e -
ment is
also esse
nti al
for efficient separation processes,
where it is critical to control the flow
of both fluids out of the vessel into
independent channels with minimal
cross contamination.
Oil Water
Figure 1. Guided-wave-radar
level instruments are an effective
solution for multi-product interface
measurement in tanks
In most cases, the two liquids
separate naturally because of their
differing densities, with the lowerdensity
liquid settling on top of the
higher. For example, when oil and
water occupy the same vessel, oil
floats on top of water. The interface
point to be measured in this example
would be the upper level of the water
and the lower level of the oil.
In an oil production process, a
separator upset combined with a
lack of visibility into the interface location
can result in oil being sent to
the water tank or water being sent to
the oil tank, both of which are undesirable.
Oil sent out with water gives
away product and could result in environmental
fines. Unrecovered oil in
the water tank from a multiple well
pad facility could siphon off more
than $1 million a year of revenue,
while excess water in the oil tank
could cause an unexpected capacity
loss, resulting in a spill or well shut-in
from a high-level alarm.
The accuracy of interface measurement
depends on the specific
process conditions, such as the difference
between dielectric constants
of the products, and the presence of
a distinct interface. Sometimes the
fluids do not separate entirely, and
instead form an emulsion or " rag
layer " (a mix of the two products) between
them. Typically, the thicker the
emulsion layer, the more challenging
it becomes to accurately measure
the interface level.
Technology selection
The most basic way to measure an
interface level is via a sight glass
on the side of a tank. However,
this method has some obvious disadvantages.
First, it must be read
manually, which is both time-consuming
and labor-intensive, as it
requires operator inspection. Second,
a sight glass will not always
give a clear indication unless there
are multiple connections to the
vessel to allow the interface in the
glass to match what is in the vessel.
Third, sight glasses need regular
cleaning, and in applications
where condensation can occur, this
can prevent the operator making an
accurate measurement.
Other level measuring alternatives
capable of identifying an interface
layer, at least to some extent, include
floats and displacers, capacitance
transmitters,
ultrasonic transmitters,
differential pressure meters and
magnetostrictive sensors. However,
these technologies have various limitations
in terms of their resolution,
accuracy, and reliability in challenging
process conditions, plus maintenance
requirements.
The many advantages provided by
GWR in comparison has seen it beChemiCal
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