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SUB SONIC METERS:
For Accurate Flow Measurement
and Control of Gases
Subsonic Primary Elements
Venturis, ASME Flow Nozzles and Orifice Meters
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Subsonic
Primary Elements (SPEs) have long been used to
provide quality flow measurements of both
gases and liquids. FLOW SYSTEMS designs,
manufactures and integrates VENTURIS, ASME
FLOW NOZZLES and ORIFICE METERS for use in
many applications. The mass flow through an
SPE is proportional to the square root of the
density times the metering differential
pressure. Thus these devices can service a
range of mass flow rates at a constant
density. This is an attractive feature in
low-pressure systems.
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Figure 1 Venturi Type Meter |
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FLOW SYSTEMS designs and manufactures SPE's in accordance with both ASME and ISO standards.
SPEs
create a restriction in the flow, which causes static pressure to be reduced as fluid velocity is increased. If the fluid state, area restriction, and differential pressure are known, the flow rate can be calculated accurately. These devices have different performance characteristics, the most significant being the overall pressure loss.
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ADVANTAGES
»
Internationally Recognized
» Variable Mass Flow with Constant
Density
» Long Term
Accuracy
» Good to Excellent
Repeatability
» No Moving
Parts |
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APPLICATIONS
»
Calibration of Flowmeters
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PTC Applications
»
Flow Metering
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Calibration of Turbine Engine Component Passages
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Calibration of Automotive Component Passages
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Measurement of Automotive Induction Air
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Automotive Emissions Testing
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Compressor or Pump Discharge Capacity Tests
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Valve CV Tests
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Flow Limiting both Choking and Cavitation
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Over Speed Protection of Gas Flowmeters |
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Application Notes are available from FLOW
SYSTEMS that discuss in detail many success
stories with SPEs.
Application engineering services are offered
to assist you with any questions.
VENTURIS offer the best repeatability and pressure recovery of our SPEs, due to the continuous geometry of the contraction, throat, and divergent sections. The low overall pressure loss characteristic of a Venturi, reduces operating costs and head requirements of prime movers in systems. Experience has shown that Venturis have less stringent up- and downstream piping requirements than other SPEs offered by FLOW SYSTEMS.
FLOW SYSTEMS offers Venturis with several geometries. A particular geometry may be recommended by our Application Engineers based on the user's specific application, compliance requirements, beta ratio (throat diameter to inlet pipe internal diameter) and line size, in order to determine the best combination of performance and economy. Classical Herschell-Type Venturis consist of a conical inlet section, cylindrical throat section, and conical divergent section. Meters with line sizes less than 8 inches are machined, while larger line sizes are fabricated. Beta ratios are offered from 0.4 to 0.75. Circular-Arc Venturis have a radial converging section, cylindrical throat section, and a conical divergent section. This geometry is applicable for line sizes below 8 inches and for low beta ratio meters. An ASME Flow Nozzle which has an elliptical converging section and cylindrical throat section can be combined with a conical divergent section to produce a hybrid Venturi. This geometry works well for any beta ratio and is currently available in line sizes up to 20 inches.
ASME FLOW NOZZLES consist of an elliptical converging section and cylindrical throat section.
This device has a greater overall pressure loss or operating cost in terms of head pressure than a Venturi but offers lower installation costs.
Repeatability is better than an Orifice Meter but can be lower than that of a Venturi. Installation requirements are more stringent than for a Venturi but less than for an Orifice Meter. These meters are offered with either throat taps or pipe wall taps for pressure measurement.
ORIFICE METERS offer the lowest installation cost of our SPEs but do have the highest overall pressure loss. They are the SPE that is most affected by noise in piping networks and therefore exhibit the lowest repeatability. The installation requirements are more stringent that for a Venturi or ASME Flow Nozzle.
A complete metering system includes a SPE, Meter Run, and both pressure and temperature sensors. Flow calculations can be performed by a Rosemount
3095MV™ transmitter or with most any computational
device such as a FLOW SYSTEMS FC200 or FC500
Flow Computer as discussed later in this
document.
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Click
to enlarge this chart
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DESIGN OPTIONS
Sizing - FLOW SYSTEMS will size each SPE to meet the requirements of each application. Throat or bore diameters are offered from 0.02 inches and up, with a standard resolution of +/-0.001 inches. An optional resolution of -0.000 +0.001 inches is available for sizes less than 0.75 inches upon request. Presently, the largest Venturi meter we have produced had a throat diameter of 32 inches and was installed in a 54-inch diameter duct. For assistance with sizing, contact our Sales Department via email at:
info@flowsystemsinc.com. Charts showing
estimated flow performance for standard sizes of Venturis and ASME Flow Nozzles are
also available from FLOW SYSTEMS.
When sizing SPE's, the ASME and ISO standards limit the metering differential pressure
(Dp) to less than or equal to 25% of the absolute inlet pressure (P1). Thus,
Dp/P1 should not exceed 0.1 to avoid increased uncertainty and decreased repeatability.
The turndown, in terms of flow rate, for an SPE is proportional to the square root of the metering differential pressure, assuming a constant density.
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If a 3 to 1 turndown in flow rate is desired, then the metering differential pressure must be reduced by a factor of 9. When determining the turndown, two issues related to metering differential pressure must be considered. Noise or disturbance in the piping network may bias the differential pressure. Small differentials are more likely to be affected by piping noise and a minimum Dp/P1 of 0.005 is recommended for good repeatability. Piping noise least affects Venturis; while Orifice Meters are the most affected, and ASME Flow Nozzles fall in between. The second issue concerns increased uncertainty in flow measurement due to differential pressure turndown. The Technical Note on "Uncertainty When Using SPEs" details the effect of differential turndown on flow measurement uncertainty. However, assuming constant density, and a variation Dp/P1 from 0.1 to 0.005 (20:1), the uncertainty in flow rate at the minimum differential pressure will be 10 times the uncertainty at the maximum differential pressure.
Bi-Directional Venturis
- For applications requiring measurement in both directions, FLOW SYSTEMS has expertise in designing, manufacturing and testing of BI-DIRECTIONAL VENTURIS that can be used to accurately measure the flow rate of both liquids and gasses.
Cavitating Venturis - FLOW SYSTEMS has expertise in designing, manufacturing and testing CAVITATING VENTURIS
used to limit the flow rate of liquids that protect system components while providing low pressure loss at normal flows.
Line
Size - Line sizes vary from 1/2 inch diameter tubing to the largest pipes or ducts.
End Connections
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SPEs can be configured with almost any desired connections such as "AN" Flare, NPT, ANSI Flange,
Swagelok™, VCO™, VCR™, CPV™, Butt Weld or Weld In. Custom flanges with O-Ring Seals (with or without V-Clamps) are also available upon request. Insert type meters are designed to be held between flanges and offer lower initial cost.
Material of Construction - Generally 300 series stainless steel is recommended, however other materials such as aluminum, carbon steel and machinable plastics may be used. For economy, users can specify different materials for the element, body and piping. For Venturis with lines sizes greater than 6 inches, it may be more economical to combine a stainless steel throat with a carbon steel body and piping. ASME Flow Nozzles and Orifice Meters generally combine a stainless steel plate or element with carbon steel flanges and piping.
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Venturi Divergent Options - The divergent section of a Venturi may be designed for maximum pressure recovery and economy. Our standard divergent section is a conic with a 15-degree total angle and is commonly referred to as a Short Form Venturi. Optionally, a Long Form
Venturi utilizes a 7-degree total angle, conic divergent section. Please see the "SPE Overall Pressure Loss" Chart
to determine the estimated loss for both Long and Short form Venturis.
Meter Tubes - INLET and EXIT SECTIONs are available to provide users with a complete meter run. Inlet Sections designed to meet the proper upstream straight piping requirements also contain taps for pressure and temperature measurements. Multiple or additional taps, pressure averaging piezometer rings, and flow conditioners are also available.
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MULTIPLE ELEMENT ARRAYS
Multiple SPEs can be fixtured between a common inlet and outlet plenum. This will increase rangeability and minimize installation requirements. |
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ACCURACY
Without calibration, ASME predictions for flow rate accuracy are at best +/-
1.5% of reading. Flow calibrations traceable to NIST in conjunction with standard instrumentation packages allow
for +/- 0.77% or +/-0.50% of reading accuracy levels.
Calibration of SPEs installed in actual piping networks that contain fittings and valves gives extended confidence to flow measurements, especially when design constraints prevent less than optimal installations.
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FLOW COMPUTER / INSTRUMENTATION
In order to calculate mass flow through an
SPE, the differential pressure, inlet absolute pressure, and temperature must be measured. For liquids, the absolute pressure measurement is not required as compressibility is minimal in most applications. If the service is humid air, then a relative humidity (or
dewpoint) measurement may be required to minimize the uncertainty.
Applications for volumetric measurement only require differential pressure instrumentation.
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FLOW
TRANSMITTER
Upon
request, FLOW SYSTEMS can provide all instrumentation
required for each application with consideration for
accuracy and economy.
The Rosemount 3095MV® flow transmitter
combines an RTD (Resistive Temperature Device), a
Temperature Transmitter, a Differential Pressure
Transmitter, and a Static Pressure Transmitter in a
single, compact package.
The 3095MV® transmitter performs the flow
calculations for all SPEs utilizing real gas
properties and calibrated discharge coefficients.
More information about this product can be
found by visiting: http://www.rosemount.com/products/flow/m3095mv.html
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FLOW
COMPUTERS
FLOW SYSTEMS offers two Flow Computer models with various levels of instrumentation accuracy to give users a choice when considering both accuracy and economy.
MODEL
FC175
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FEATURES:
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» Combines the Performance of
a Control Valve and a Flowmeter in One Package.
» For use with single,
manually interchangeable Sub-Sonic Venturis or
Sonic Nozzles.
» MS-WindowsTM-based
Setup Software and Interface allow Easy
Configuration and Monitoring
Capabilities.
» USB
Communication.
» PC
Connection Required.
(No Stand-alone Operation.)
» Optional
External Pressure Sensors or simply connect an
existing Absolute / Differential,
2-wire, Loop-Powered,
4-20ma output transmitters.
» Includes
2252 kΩ Thermister Probe for temperature
measurement.
» Custom
Software and additional, User-Scaleable, I/O
channels available.
(3)
Thermister (1-2)
scaleable 4-20 MA DC.
» Calibration
Software to Maintain Internal Instruments.
» Test
Set-up Screen.
» Flowmeter Configuration.
» Data Logging Capabilities
via USB Interface.
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Optional Internet Support and Diagnostics.
» Calculations per ASME MFC-3M
/ ISO 5167.
» Real Gas Properties for most
gasses.
» Iterative
Solution for Cd-based on Actual Calibration Data
or Theoretical Models.
» Best Economy.
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FC175-CFV Flow Computer
Shown with interchangeable Sonic Nozzle Meter Run
and
Customer-Supplied Laptop Computer. |
Below is a summary of instrumentation configurations and
accuracies for the FC175.
Options include a humidity sensor for moist air applications, required fittings, lead wire and tubing.
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Options
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Configuration
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Mass Flow Accuracy
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Comments
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Level 1
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+/-0.61% of Reading
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Assumes: Uncalibrated Venturi, 4:1 pressure range (+/-0.05% full scale), 10:1 Dp range
(+/-0.02% of full scale) and 40° to 100°F gas temperature (+/-0.2°C).
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Level 2
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+/-0.43% of Reading
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Assumes:
a Venturi with +/-0.5%
CEESI calibration,
4:1 pressure range (+/-0.02% full scale), 10:1 Dp range
(+/-0.01% of full scale and 40° to 100°F gas
temperature (+/-0.2°C).
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MODEL
FC500
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FEATURES
»
For use with
Venturi Meter Runs, single Sonic Nozzles or Multiple Sonic Nozzle Arrays.
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Advanced Functionality for the High-End User.
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Self-Contained Package including Instrumentation.
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MS
Windows™-based Configuration and Monitoring Software.
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Ethernet, Analog (2), and Configurable Relay (2) Outputs.
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Stand-Alone Operation with Internal or Remote Data Logging Capabilities.
» Combines the Performance of a
Control Valve and a Flowmeter in One Package.
» Calculations per
ASME
MFC-3M / ISO
5167.
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Real Gas Properties for most gasses.
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Iterative Solution for Cd based on Actual Calibration Data or Theoretical Models.
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Rack-mount or Bench-Top.
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Internet Support and Diagnostics.
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Calibration Software to Maintain Internal Instruments.
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FC500 Flow Computer
shown with Optional Venturi Meter Run |
Below is a summary of instrumentation configurations and
accuracies for the FC500-VEN.
Options include a humidity sensor for moist air applications, required fittings, lead wire and tubing.
| Configuration
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Mass Flow Accuracy
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Comments
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| Level 1
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+/-1.41% of Reading
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Assumes: Uncalibrated Venturi, 6:1 pressure range(+/-0.10% full scale), 10:1 Dp range
(+/-0.30% of reading) and 40° to 100°F gas temperature.
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| Level 2
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+/-0.61% of Reading
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Assumes:
a Venturi with +/-0.50%
CEESI calibration, 6:1 pressure range (+/-0.04% full scale), 10:1 Dp range
(+/-0.30% of reading) and 40° to 100°F gas temperature.
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| Level 3
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+/-0.43%
of Reading
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Assumes:
a Venturi with +/-0.25% CEESI calibration, 6:1 pressure range (+/-0.02% full scale), 10:1 Dp range
(+/-0.30% of reading) and 40° to 100°F gas temperature.
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FLOW CALCULATION
SOFTWARE / EQUATION REPORT
For those users with instrumentation, FLOW SYSTEMS offers software to calculate flow rate through
SPEs. The available format is Microsoft
Excel™. An equation report will allow users to program the flow equations directly into existing software/data acquisition systems.
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SYSTEMS
FLOW SYSTEMS combines SPEs with instrumentation, data acquisition, controls, computational products, prime movers and material handling to provide our customers with turnkey solutions. These systems may be used for gas flow measurement, calibration of other
flowmeters, or product performance testing. Users may select from one of our standard systems or we can design a system that meets your specifications. Our CONSULTING SERVICES can even help clients prepare their system specifications. Contact our sales department for more information on our SYSTEM INTEGRATION
SERVICES.
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Swagelok®, VCO® and VCR® are registered trademarks of Cajon Company, Macedonia, OH.
CPV® is a registered trademark of CPV Manufacturing Inc., Philadelphia, PA.
Excel® and Visual Basic® are registered trademarks of Microsoft Corporation, Seattle, WA.
3095MV® is a registered trademark of Fisher-Rosemount, Eden Prairie, MN.
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