If the process liquid
contains suspended solids or is chemically corrosive or radioactive, it is
desirable to prevent it from coming into direct contact with the level
transmitter. In these cases, a bubbler level measurement system, which utilize
a purge gas , can be used.
Purge-type
liquid-level measurement:
Usage: for corrosive liquids, liquids that
contain suspended objects, high viscosity liquids, and underground tanks.
Principle of
operation:A bubble tube is
inserted into a tank, and a fixed flow of air is forced through the tube such
that bubbles emerge from the end of the tube. If the air flow rate is not
extremely large, the air pressure in the bubble tube can be considered to be
the same as the pressure of the liquid at the end of the tube. The liquid level
can be measured by measuring the air pressure.
Open tank application
for bubbler system:
As shown, a bubbler
tube is immersed to the bottom of the vessel in which the liquid level is to be
measured. A gas (called purge gas) is allowed to pass through the bubbler tube.
Consider that the tank
is empty. In this case, the gas will escape freely at the end of the tube and
therefore the gas pressure inside the bubbler tube (called back pressure) will
be at atmospheric pressure. However, as the liquid level inside the tank
increases, pressure exerted by the liquid at the base of the tank (and at the
opening of the bubbler tube) increases.
The hydrostatic
pressure of the liquid in effect acts as a seal, which restricts the escape of,
purge gas from the bubbler tube.
As a result, the gas pressure in the bubbler tube will continue
to increase until it just balances the hydrostatic pressure (P=S.H) of the
liquid. At this point the backpressure in the bubbler tube is exactly the same
as the hydrostatic pressure of the liquid and it will remain constant until any
changes in the liquid level occurs. Any excess supply will escape as bubbles
through the liquid. As the liquid level rises, the backpressure in the bubbler
tube increases proportionally, since the density of the liquid is constant. A
level transmitter (DP cell) can be used to monitor this backpressure. In an
open tank installation, the bubbler tube is connected to the high-pressure side
of the transmitter, while the low pressure side is vented to atmosphere. The
output of the transmitter will be proportional to the tank level.
A constant
differential pressure relay is often used in the purge gas line to ensure that
constant bubbling action occurs at all tank levels. The relay maintains a
constant flow rate of purge gas in the bubbler tube regardless of tank
variations or supply fluctuations. This ensures that bubbling will occur to
maximum tank level and the flow rate does not increase at low tank level in
such a way as to cause excessive disturbances at the surface of the liquid.
Note that the bubbling action has to be continuous or the measurement signal
will not be accurate.
An additional
advantage of the bubbler system is that, since it measures only the
backpressure of the purge gas, the exact location of the level transmitter is
not important. The transmitter can be mounted some distance from the process.
Closed tank
application for bubbler system:
If the bubbler system
is to be applied to measure level in a closed tank, some pressure-regulating
scheme must be provided for the gas space in the tank. Otherwise, the gas
bubbling through the liquid will pressurize the gas space to a point where
bubbler supply pressure cannot overcome the static pressure it acts against.
The result would be no bubble flow and, therefore, inaccurate measurement
signal. Also, as in the case of a closed tank inferential level measurement
system, the low-pressure side of the level transmitter has to be connected to
the gas space in order to compensate for the effect of gas pressure.
Article Source: Dr. Rosdiazli Ibrahim,Universiti Teknologi Petronas (EEB5223/EAB4223 Industrial Automation & Control Systems)
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