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What are Gravitometers ?
Gravitometers measure the
Specific Gravity (SG) of a gas, relative to air. If the test gas is an
LPG/Air mixture, the SG is an indicator for the quality of the produced
gas. Maintaining a consistent SG value of the LPG/air mixture is important
in applications where variations in the heat content of the gas would
change the heat output of a burner or furnace, and where this change in
heat output would have a negative effect on the process.
Ideally, the signal from the Gravitometer
is looped back to a positioner/actuator at the mixing valve. This
positioner/actuator would then change the gas/air ratio to correct any
variations in the SG of the mixed gas. Alternate Energy Systems offers
these Gravitometer/Actuator Combinations as our AccuBlend™ option for all
Piston Operated Mixers (POM-10 to POM-60). How do Alternate Energy Systems’
Gravitometers work ?
GraviBlend®-3 is a
relatively new
device that was patented in 2001. It is specifically designed for use in
LPG/air mixing systems.
It
measures the relative density of the mixed gas and operates on the
principle that the travel of sound in a specially designed and
temperature-controlled chamber is affected by the density of the test gas. GraviBlend®-3 units have actually two test chambers, one filled with
ambient air at atmospheric pressure, the other filled with the test gas at
near-ambient pressure.
A membrane between the two
chambers oscillates at a controlled frequency and with a controlled
amplitude. The resulting dynamic pressures in the two chambers are
measured by sensors. The output signals from these pressure sensors are
processed by the integrated electronics. The signal from the gas-filled
chamber is then compared against the signal from the air-filled reference
chamber, and a linear output signal is created. This output signal is
directly proportional to the relative Specific Gravity of the Propane/Air
mixture, with air in the reference chamber having an assigned SG of 1. To minimize temperature effects, the sensor is
kept at a constant temperature of 113°F (45°C).
Features
GraviBlend®-3 is a
precision on-line instrument, designed for continuous, unattended
measurement of gasses with specific gravities between 0.2 and 2.2. This
allows the use of the instrument in straight propane/air systems, as well
as in systems where the LPG is a mixture of propane and butane. The
instrument measures the relative density of propane-air mixtures. From
this data, Heating Value (CV) and Wobbe Index Number (WI) are calculated and
displayed.
Measurement data is
provided at a front panel LCD display. Standard and explosion proof
GraviBlend®-3 have a 6-inch color LCD display with touch screen operator
interface; portable GraviBlend®-3 offer a 3-inch monochrome display.
A scaled, linear DC 4-20 mA signal (tied to SG; 4-20 mA = 0.4 to
2.0 SG) and a 27hour trend chart (GB-3 and GB-3E) are standard.
GraviBlend®-3 provides
the user with rapid time response and superb accuracy for signaling
controllers in blending plants and for reporting gas properties. It is
used in propane applications such as standby and peak shaving equipment or
within gas distribution systems.
GraviBlend®-3 is
practically solid-state; the only moving part is the oscillating membrane
between the two sensor chambers. The unit is small, durable, lightweight,
rugged and robust. All components in the GraviBlend®-3 have been
field-proven with other products for several years. Reliability is built
in from the start.
Indicators
and Output Signals
A 6-inch color LCD display
with touch screen operator interface (portable version: 3-inch monochrome)
in the front panel of the electronics enclosure displays the measurement results. Specific Gravity,
Wobbe Index, and Calorific Value are displayed simultaneously. During
initial setup, the display unit is also used for the input of setpoint
data and the input of the properties of the liquid LPG.
The user can select from
one of three methods to enter the properties of the liquid LPG: select one
of 20 pre-defined LPG mixtures (GB-3P: 9 pre-defined LPG); enter the
propane-percentage in the LPG; or enter Calorific Value and Specific
Gravity of the “raw” LPG vapor. The GraviBlend®-3 then
accurately measures the SG of the vapor/air mixture, and calculates
Calorific Value and Wobbe Index Number.
Alarm
Signals
The GraviBlend®-3 is
constantly monitoring 12 separate setpoints. Out-of-range conditions
activate alarm outputs with “dry” contact ratings of 2A. All alarm
setpoints can be configured for various process conditions. Monitored are
the Low/High “Pre-Alarm” points and the Low/High “Cutoff-Alarm”
points of SG, Wobbe Index, and Calorific Value.
Tie-In
with AccuBlend™
The GraviBlend®-3A
and GraviBlend®-3EA versions are equipped to directly drive the electric motor of the AccuBlend™
actuator on an AES POM LPG/Air blender. The two units together will
maintain constant mixed gas properties, compensating for any fluctuations
in the ratio of the LPG/Air mixture.
AutoCalibration
Feature
The standard range of the
density cell used in the GraviBlend®-3 gravitometers is 0.2 … 2.2.
Since the main interest in an application may be concentrated on a much
narrower range (for example 1.0 to 1.4 in LPG/Air Mixing Systems), AES has
developed a method that provides a simple-yet-powerful routine that
recalibrates the entire system in only a few steps.
Because the calibration
procedure includes the entire system (from density cell to signal
converter to PLC input), the overall system accuracy will be greatly
improved.
Communication
Options
Standard versions (GB-3)
and explosion-proof versions (GB-3E) of the GraviBlend®-3 offer the
Allen-Bradley DH485 network interface for remote monitoring. They can also
be equipped with dial-up modems or Ethernet interfaces. Models with
Ethernet interface can also be equipped with built-in web-servers, which
allows remote monitoring over the Internet without the need for any
specialized software, using a standard web browser.
Available
Versions
The GraviBlend®-3 is
currently available in three main versions:
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Standard Configuration with
common enclosure for all system components;
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Ex-Proof Configuration with
separate "Electronics Enclosure" for the signal processing
components (to be installed in a non-hazardous area), "Piping
Enclosure" with all sample-gas distributing elements (to be installed
close to the source of the sample gas), and explosion-proof Sensor
Enclosure, which can be installed in hazardous or non-hazardous areas;
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Portable Version for field service personnel.
All three versions share
the same design concept and operating features and differ mainly in their
"packaging" and the resulting typical installations.
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