 Special on : FLOW |
 (article:FLOW-E-1 ) |
Special on : FLOW |
(article:FLOW-E-1 ) |
Waste Management
Accurate open channel Flow Measurement using Bubble Flow Meters
Isco Inc. Environmental Group- USA
There are many methods of
measuring open channel flow, and each method has advantages and
disadvantages. The most common is a system consisting of a weir
or flume along with an open channel flow meter. The weir or flume
controls the shape and velocity of the flow. The flow meter then
determines the flow rate by measuring the level at one point in
the flow stream.
There are many technologies available today to measure level,
including bubblers, ultrasonics, and submerged pressure
transducers. No single level measurement technology is suitable
for all applications.
This article explains the operation of a bubbler level
measurement system and how it provides accurate measurement of
open channel flows.
Principle of Operation
A bubbler flow meter supplies a constant flow of pressurized air
through a small diameter tube anchored in the flow stream. A
transducer in the flow meter measures the pressure required to
force bubbles from the end of the tube. This pressure is
proportional to the liquid level.
Figure 1 is a schematic diagram of an Isco 4230 Bubbler Flow
Meter. A small, high-efficiency diaphragm pump in the flow meter
generates the pressurized air supply for a bubbler flow meter.
The intake side of the pump is vented to the outside of the flow
meter. A hydrophobic filter prevents liquid from entering the air
system should the pump intake become submerged. The exhaust side
of the pump connects to the air tank, which stores the
pressurized air.
A pressure switch with a fixed set-point monitors air pressure in
the tank. The reference side of the differential pressure switch
connects to the bubble line. When the pressure in the tank
decreases to less than a fixed amount above the water pressure
being measured, the pressure switch turns the pump on for a fixed
period of time. This maintains the pressure in the tank at a
fixed amount above the current water pressure. For example, the
Isco 4230 uses a pressure switch with a 3.5 psig (24 kPa)
set-point, which maintains the pressure in the tank at
approximately 3.5 psig above the water pressure.
During normal operation, air flows from the air tank through the
bubble rate adjust valve before exiting the end of the bubble
line anchored in the flow stream. This adjustable, fine-metering
valve controls the flow rate of air escaping from the bubble
line, thus establishing the bubble rate. The adjustment knob of
the valve extends through the flow meter case, allowing the
bubble rate to be externally adjusted. It is recommended that the
bubble rate be set at around one bubble per second.
A pressure transducer located inside the flow meter measures the
air pressure in the bubble line, which is proportional to the
liquid level. The transducer is temperature-compensated to
provide maximum accuracy across a wide range of operating
conditions. The automatic drift compensation valve connects the
pressure transducer to the bubble line. The reference side of the
differential pressure transducer is vented to the outside of the
flow meter. This automatically compensates for changes in
atmospheric pressure that would otherwise affect accuracy. The
flow meter processes the output from the pressure transducer into
the flow stream level and corresponding flow rate.
Automatic Drift
Compensation
Automatic drift compensation is the key to the accuracy of a
bubbler flow meter. An automatic drift compensation valve
connects directly across both ports of the pressure transducer.
The flow meter uses this valve to periodically connect the input
and reference ports of the pressure transducer together. Ideally,
the output of the transducer under this condition should be zero.
However, the flow meter may measure a non-zero pressure reading
due to transducer drift or other effects. The flow meter then
takes any error into account in subsequent readings, providing
the most accurate level measurement possible.
The Isco 4230 conducts automatic drift compensation upon power-up
and at intervals from two to 15 minutes thereafter. Software in
the flow meter controls this function and requires no programming
by the operator.
Not all bubbler flow meters feature automatic drift compensation.
However, it is a recommended feature due to the high accuracy
that it enables the bubbler flow meter to achieve.
Bubble Line Purge
The end of the bubble line may become plugged or clogged due to
suspended solids in the flow stream, such as domestic raw sewage
or streams with high grease content. However, modern bubbler flow
meters feature automatic bubble line purge to combat this
situation and ensure measurement accuracy.
The purge valve provides a second path by which pressurized air
in the tank can reach the bubble line. When opened, this
electrically controlled solenoid valve bypasses the bubble rate
adjust valve and releases a large amount of air to purge the
bubble line.
Bubble line purges can occur automatically at periodic intervals,
or can be manually initiated.
Responding to Rapidly
Rising Liquid Levels
The purge valve also allows the flow meter to accurately respond
to a rapidly rising liquid level. If the flow meter detects a
rapidly increasing level, it opens the purge valve momentarily to
release small amounts of air, effectively increasing the bubble
rate. This allows the flow meter to operate normally with a
power-conserving low bubble rate, but also to respond quickly to
a rapidly rising liquid level.
The purge valve is another feature that is not offered on all
bubbler flow meters. However, it allows the bubbler flow meter to
combat plugging and respond to rapidly rising liquid levels,
which would otherwise affect its accuracy.
Installing the Bubble
Line
A variety of methods are available for mounting the bubble line
in the flow stream. In many cases, the bubble line is simply
anchored to the wall or floor of the channel.
In sewer flow monitoring applications, the bubble line is often
secured to a stainless steel mounting ring or mounting strap,
which is then inserted into the sewer pipe or invert.
In addition, most flumes are available with an integral bubble
tube at the appropriate level measurement point.
Bubbler vs. Other
Technologies
Bubbler flow meters
have many advantages in comparison with other level measurement
technologies.
Unlike ultrasonic level measurement, a bubbler flow meter is not
affected by foam and turbulence on the surface of the flow
stream, nor by strong wind and steam above the flow. It can also
be used in pipes and channels of almost any size, including
extremely small sizes such as 4 inch (100 mm) diameter pipes.
Unlike submerged pressure transducers, a bubbler flow meter is
immune to damage by lightning and flow stream debris. Also, it is
not affected by flow stream chemicals or changing flow stream
temperature.
Because it can automatically compensate for transducer drift, a
bubbler flow meter is the most accurate level measurement
technology. Automatic drift compensation also allows it to
maintain its level calibration indefinitely, making a bubbler
flow meter ideal for standby applications such as storm water
runoff monitoring.
As with any level measurement technology, bubbler flow meters
also have disadvantages. A bubbler flow meter requires mounting
the bubble line in the flow stream, which can be difficult in
some situations. The bubble line may be subject to plugging,
although automatic purges usually alleviate this problem.
Due to its air pump, a bubbler flow meter requires higher power
consumption than ultrasonics and submerged pressure transducers,
although many bubbler flow meters are used in battery-powered
applications.
Finally, a bubbler flow meter requires periodic maintenance to
regenerate the desiccant on the air pump inlet. The desiccant
cartridge adsorbs moisture from the air being drawn in by the
pump, ensuring reliable long-term operation.
The versatility and accuracy of bubbler flow meters make them
useful in many open channel flow measurement applications,
including wastewater treatment plants, industrial discharge
monitoring, storm water runoff monitoring, sewer flow
measurement, irrigation, and stream gauging.
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