info about all flow meters
Oct 28, 2008, 10:20 AM
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info about all flow meters
found some good info for everyone to gain knowledge about flow meters.
5.2.1 Differential Pressure
Historically, differential pressure measurements have been the most common flow rate meters. Differential pressure flowmeters employ the Bernoulli Equation that describes the pressure difference that results when a restriction is placed in a pipe. At the restriction, the flow velocity increases which in turn decreases the static pressure downstream. The pressure difference generated is a measure of the fluid flow rate through the restriction and the pipe. The two key components found in differential pressure flowmeters are a restriction to cause a pressure drop in the flow (differential producer) and a method of measuring the pressure drop across the obstruction (differential pressure transducer).
The basic principle on which differential pressure flow meters operate is the conversion of energy from one form to another. For liquid flows, only kinetic energy and the energy due to static pressure are considered. For gases or vapors, the internal energy of the compressed fluid is also involved.
The equation for mass flow rate of fluids thru the orifice, venture, flow nozzle can be calculated by the following equation:
m is the mass flow rate (lbm/s)
C0 is the DP device coefficient
A0 is the cross sectional area of the instrument (ft2)
Y is a correction factor supplied by the vendor (Y=0 for liquids)
p1 and p2 are upstream and downstream pressures (psi)
ρ is the density of the fluid (lb/ft3)
D0 and D1 (ft)
is a constant 4633.24 (in2*lbm)/(ft*lbf*s2)
A. Orifice Plates
One of the most common primary flow devices is the orifice plate. For pipes above 2 inches in diameter, orifice plates are mounted between bolted flanges. The flanges are threaded or welded to the pipe depending on pipe size and operating line pressures.
Most orifice plates have sharp, square or rounded upstream edges. Concentric plates are the most common design used since the accuracy is highly predictable and extensive data is available for a large range of flows, pressure differentials and pipe sizes.
Eccentric and segmental orifice plates may be used when the measured fluid contains suspended material or has both liquid and gaseous phases. Use of concentric plates in this application may lead to accumulations behind the plates and cause false readings.
Figure 1 – Orifice Plate Types
Beta ratios should be between 0.15 and 0.70 for flange taps. Beta ratios should be between 0.20 and 0.67 for pipe taps. Orifice plates with small Beta ratios and high pressure drops often function as restriction orifices. These plates may have a thickness up to ½” to withstand the energy that is dissipated.
Permanent pressure drop loss can be from 75% to 90% of the up stream pressure depending on the Beta.
Pressure tap locations are discussed in Guideline EG-19-222 (Flow Meter Installation).
B. Flow Nozzle
Flow nozzles are widely used for flow measurements at high flow velocities. The flow nozzle is more rugged and erosion resistant than the orifice plate. For a given diameter and a given differential pressure, the flow nozzle will pass approximately 65% more flow than the orifice plate.
Permanent pressure drop of a flow nozzle is greater than the Venturi flow element and less than the orifice flow element.
The flow nozzle, because of its streamlined contour, tends to sweep solids through the throat. For non-homogeneous fluids, the flow nozzle is preferable to the orifice plate. The flow nozzle should not be used if large percentages of solids are present.
Typical pressure measurements are at the radius tap locations of 1 pipe diameter upstream and 0-.5 pipe diameters downstream of the flow nozzle. The downstream tap should never be located beyond the end of the nozzle. As in all differential producers, its output varies as the square root of the flow rate.
C. Venturi
The Venturi tube can measure 25 to 50% more flow than orifice for comparable line size and head loss. The flow range for satisfactory measurement is usually considered to extend upward from Reynolds numbers of about 200,000.
Some advantages of Venturi tubes are:
• high flow rates
• minimal piping straight run requirements (typically 10 pipe diameters)
• good accuracy with Beta ratios greater than .75
• integral pressure connections
The purchase cost of the Venturi tube is greater than most other primary flow elements. However, the greater pressure recovery can result in significant energy savings in large pipelines.
The Venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. A standard Venturi tube is shown in Figure 3.
A number of taps are often employed around the circumference of the high and low pressure areas which are connected together in what is known as a piezometer ring. This allows multiple pressure measurements in order to get a better average of the pressures. As with all differential pressure measurements, the flow rate varies as the square root of the differential pressure.
D. Flow Tubes
Flow tubes are primary elements with converging and diverging sections, similar to Venturi, whose design is usually proprietary.
Some advantages of flow tubes are:
• high differential pressure with high recovery
• low cost
Limitations include:
• inapplicability to low flows and small pipes
• sensitivity to viscosity variations
• erroneous readings in highly viscous or dirty liquids
Flow tubes require approximately the same piping runs as an orifice plate.
E. Pitot Tubes
Pitot tubes are the simplest velocity head sensors. Models can be specified for a variety of difficult fluid services that include high temperature and high pressure. The sensor probes are often designed to be inserted into conduits without process shutdown.
One fundamental problem with the pitot tube is that velocity measurement is made at only one point in the flow stream rather than providing integrated volumetric flow measurement. The probes must be traversed across the pipes or the velocity profiles known in advance to calculate the average flow rate. For high accuracy and consistent results, the pitot should be preceded by 50 or more diameters of straight smooth pipe. If a sufficient straight run of pipe is installed ahead of the pitot tube, an approximate average velocity reading will be obtained at a location approximately 30% of the pipe radius from the pipe wall. The basic pitot tube configuration is shown below in Figure 5.
Figure 5 – Basic Pitot Tube Configuration
One port is placed in the flowing fluid, facing upstream, and is connected through internal tubing to one side of the secondary element. This port registers the total dynamic head in the stream since the velocity is zero at the sensing tip. The static pressure is obtained from a port which faces perpendicular to the flow and is usually located in the pipe wall.
Errors are introduced in pitot tube measurements because the total and static pressures are not measured at the same point in the flow profile. This problem is eliminated in pitot static tubes, shown below in Figure 6.
Figure 6 – Pitot Static Tube
A static pitot has dual coaxial tubes. One terminates in a port facing upstream to register the total dynamic pressure at a stagnation point and the other tube is located away from the tip and faces normal to the flow or downstream to measure the static pressure at approximately the same streamline.
Combined-reversed pitot static tubes circumvent many of the problems associated with flow profiles across the channels. One design is the multi-port or annular averaging element. This element senses dynamic pressure at multiple sensing ports distributed along the diameter to provide a single indication of the average flow through the channel without a transverse. Static pressure is measured by a tube terminating in a port which faces downstream at the centerline of the conduit.
The Annubar™ manufactured by Dieterich Standard Corporation is a specialized pitot tube of this type. The ProBar Flowmeter™ manufactured by Rosemount is also a specialized pitot tube with pressure and temperature compensation. Both these flow elements are very commonly used in low pressure applications.
F. Wedge Meters
The segmental wedge is a proprietary design flow meter similar to the segmental orifice plate but with the upstream and downstream surfaces of the measuring point at a 45 degree angle to the flow stream. The wedge meter is located at the top of the pipe or conduit allowing the bottom of the pipe to be unrestricted. Wedge meters are highly linear from Reynolds numbers as low as 500 (laminar flow) to Reynolds numbers in the millions (highly turbulent flows). The wedge meter is useful when measuring slurry flow.
Figure 7 – Segmental Wedge
Some advantages of segmental wedge flow elements are:
• permanent pressure loss is approximately 50% of a similar orifice
installation
• viscous fluids measure accurately over a wide range of Reynolds
numbers
• changing viscosities can be measured
• bi-directional flow
G. Elbow Meter
The elbow meter is simply a pipe elbow that is inserted into the pipe and a pressure differential is created by the centrifugal force between the inside diameter and the outside walls of the pipe. The only pressure losses are from the elbow itself.
One limitation of the elbow is that the pressure differential produced is very small. Elbow flow meters are usually used for balancing loads for multiple compressors or pumps in multi-unit unit service. They have also been used in the nuclear industry to detect the extremely high velocities associated with pipe breaks.
H. Variable Area Meters
Variable area meters, more commonly known as rotameters, are available as indicators, transmitters, recorders, and controllers in any combination.
Variable area meters measure a wide range in of flow rates and are suitable for most fluids including high viscosity liquids and low concentration slurries. They are frequently used in low flow services such as purges where the requirements are below the range of an orifice plate.
A rotameter consists of a plummet or float within a tapered vertical tube. The force of the flowing fluid causes the float to rise until the force is balanced by gravity. Rotameter calibration suffers if the variation in viscosity or density is greater than 15%.
Rotameters can be configured to either measure liquid or gas flow. Measuring the flow of liquids and gases is a critical need in many industrial plants. Important parameters to consider when specifying rotameters include liquid volumetric flow rate, gas volumetric flow rate, operating pressure, and fluid temperature.
• Liquid volumetric flow rate applies only to those rotameters that are liquid volumetric flow sensors or meters. It is expressed as the range of flow in volume/time.
• Gas volumetric flow rate applies only to those rotameters that are gas volumetric flow sensors or meters. It is the range of flow in volume/time.
• Operating pressure is the maximum head pressure of the process media the meter can withstand.
• The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials. Pipe diameter is also important to consider, especially when specifying specific mounting options.
Mounting options for rotameters include insertion types, in-line flanged, in-line threaded, and in-line clamp.
• Insertion flow meters are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access.
• In-line flanged flow meters are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes.
• In-line threaded flow meters are inserted parallel to the flow path, and threaded into two existing process pipes.
Rotameters are often easy to install and offer a low cost solution to flow measurement.
I. Target Flowmeter
Target elements are impact devices that are coupled to restoring mechanisms such as springs or servomotors to maintain equilibrium. Target meters are located directly in the fluid flow. The deflection of the target and force bar is proportional to the square of the flow rate (according to Bernoulli’s equation).
Advantages of target flowmeters are:
• high accuracy
• long term repeatability
• long life
• low cost installation
• no pressure ports to plug
• dirty or clean fluids can be measured
• can be used for any type of liquid, gas, or steam cryogenics
• no moving parts such as bearings, to wear out causing failures
• high reliability where life tests have been made to 20,000,000
cycles
• can be used for any line size from 0.5 inches and up with any
type of mounting
• range/fluid changes accomplished by simply changing targets
• turndowns aprox.15:1
• can accept bi-directional flow where signal polarity indicates
direction
Limitations of target flowmeters are:
• accuracy depends on temperature (approx. 0.5% error/100C)
• much data is needed to determine the optimum size of the target
• calibration must be field verified.
5.2.2 Velocity Meters
These instruments operate linearly with respect to the volume flow rate. Because
there is no square-root relationship (as with differential pressure devices), their
rangeability is greater. Velocity meters have minimum sensitivity to viscosity
changes when used at Reynolds numbers above 10,000. Most velocity-type
meter housings are equipped with flanges or fittings to permit them to be
connected directly into pipelines.
A. Magnetic Flowmeter
The electromagnetic flowmeter approaches the ideal flow measurement device for liquids because it has no restriction in the flow line, can accurately measure liquids almost impossible to handle with other meters, has a linear output that is directly proportional to flow, and has the ability to measure bi-directional flow.
The only limitation on liquid measurement is that it must meet a minimum standard of electrical conductivity.
The principle of operation for magnetic flowmeters is Michael Faraday’s Law of Electromagnetic Induction. This law states that a conductor, when moving across lines of force in a magnetic field, will induce a voltage within the conductor and the magnetic field. (For a more detailed description of this operating principle, refer to the ISA book Flow Measurement: Practical Guides for Measurement and Control, 2nd edition, D.W. Spitzer, editor).
The process fluid serves as the conductor in the flow tube. If the tube is metal it must have a lining (flourcarbon resin, polyurethane, neoprene, etc) that serves as an electrical insulator on the inside of the tube wall. A pair of electrodes, extending through the wall of the tube are flush with the inside surface of the lining. The tube end connections are usually flanged to simplify mounting in a pipeline.
Magmeter sizes range from 0.01 to 96 inches and can measure flows from 0.01 GPM to 500,000 GPM. Measurement accuracy is better than 1 percent of the flow rate.
Magmeters can measure flow rates of clean fluids, dirty fluids and slurries. The meter is sensitive to changes in density and viscosity. The meter can not be used with most hydrocarbons because of their low conductivity.
Magmeter sizing
To properly size a magmeter, the following formula should be used:
Where: = velocity in Feet/sec.
= nominal diameter of the flowmeter (inches)
The following table lists typical sizing guidelines. These guidelines are based on Rosemount Magnetic flow meters and may vary slightly by manufacturer.
Application Velocity Range (ft/s) Velocity Range (m/s)
Normal Service 2-20 0.6-6.1
Abrasive Slurries 3-10 0.9-3.1
Non-Abrasive Slurries 5-15 1.5-4.6
B. Turbine Flowmeter
A turbine flowmeter uses the moving fluid or gas to turn a turbine rotor (Figure 8). The rotational speed of the rotor varies with the flow rate. When a steady rotational speed is obtained, the speed is proportional to the fluid velocity. Flow quantity data is supplied via a precisely known number of pulses for a given volume for fluid displaced between two adjacent rotor blades. The relationship is linear within given limits for flow rate and fluid viscosity.
Figure 8 – Cut-Away of a Turbine Flowmeter
Advantages of turbine meters include:
• high accuracy and repeatability
• wide flow ranges
• many materials of construction available
• low pressure drop
Turbine flowmeters are primarily used for flow totalizing for inventory control and custody transfer, precision automatic batching for loading and batch mixing.
Turbine meters should be selected with 30% to 50% excess capacity above the maximum flow rate. Turbine meters operating below the maximum capacity provide greater reliability.
C. Vortex Flowmeter
Vortex flowmeters measure flow via a natural phenomenon known as vortex shedding (Figure 9).
Figure 9 – Principle of Vortex Shedding
When a fluid or gas flows past an obstruction, boundary layers of slow moving fluids or gases are formed along the outer surfaces. If the obstacle is streamlined, the flow cannot follow the obstacle contours on the downstream side. The separated layers become detached and roll themselves into vortices in the low pressure area behind the body. Vortices are shed from alternate sides. The frequency at which they are shed is directly proportional to velocity. The signal output from the flowmeter is generated from the action of the fluid itself, so it belongs in the “fluidic” class of flowmeters.
Advantages of vortex shedding meters are:
• high accuracy
• long term repeatability
• good rangeability
• measure liquid, gas, and two-phase flow
• calibration is independent of viscosity, density, pressure and
temperature and can be maintained for long periods of time
Limitations of vortex shedding meters are:
• not suitable for dirty or abrasive fluids
• not suitable for viscous liquids
• limited choice of materials of construction
• limited size range
• limited maximum pressure and temperature capability
• no measured flow below the low flow cut-off velocity
D. Fluidic Flowmeter (Coanda Effect)
Another type of fluidic flowmeter utilizes the Coanda effect. The Coanda effect is basically a hydraulic feed back circuit. A chamber is designed with two feedback channels on opposite sides that produce a continuous, self induced oscillation. The frequency of the oscillation relates to the fluid velocity.
Advantages of fluidic flowmeters are:
• low cost
• high accuracy
• insensitive to temperature changes
Fluidic flowmeters can be used to measure clean fluids only. They will not measure gas or slurry flows.
5.2.1 Differential Pressure
Historically, differential pressure measurements have been the most common flow rate meters. Differential pressure flowmeters employ the Bernoulli Equation that describes the pressure difference that results when a restriction is placed in a pipe. At the restriction, the flow velocity increases which in turn decreases the static pressure downstream. The pressure difference generated is a measure of the fluid flow rate through the restriction and the pipe. The two key components found in differential pressure flowmeters are a restriction to cause a pressure drop in the flow (differential producer) and a method of measuring the pressure drop across the obstruction (differential pressure transducer).
The basic principle on which differential pressure flow meters operate is the conversion of energy from one form to another. For liquid flows, only kinetic energy and the energy due to static pressure are considered. For gases or vapors, the internal energy of the compressed fluid is also involved.
The equation for mass flow rate of fluids thru the orifice, venture, flow nozzle can be calculated by the following equation:
m is the mass flow rate (lbm/s)
C0 is the DP device coefficient
A0 is the cross sectional area of the instrument (ft2)
Y is a correction factor supplied by the vendor (Y=0 for liquids)
p1 and p2 are upstream and downstream pressures (psi)
ρ is the density of the fluid (lb/ft3)
D0 and D1 (ft)
is a constant 4633.24 (in2*lbm)/(ft*lbf*s2)
A. Orifice Plates
One of the most common primary flow devices is the orifice plate. For pipes above 2 inches in diameter, orifice plates are mounted between bolted flanges. The flanges are threaded or welded to the pipe depending on pipe size and operating line pressures.
Most orifice plates have sharp, square or rounded upstream edges. Concentric plates are the most common design used since the accuracy is highly predictable and extensive data is available for a large range of flows, pressure differentials and pipe sizes.
Eccentric and segmental orifice plates may be used when the measured fluid contains suspended material or has both liquid and gaseous phases. Use of concentric plates in this application may lead to accumulations behind the plates and cause false readings.
Figure 1 – Orifice Plate Types
Beta ratios should be between 0.15 and 0.70 for flange taps. Beta ratios should be between 0.20 and 0.67 for pipe taps. Orifice plates with small Beta ratios and high pressure drops often function as restriction orifices. These plates may have a thickness up to ½” to withstand the energy that is dissipated.
Permanent pressure drop loss can be from 75% to 90% of the up stream pressure depending on the Beta.
Pressure tap locations are discussed in Guideline EG-19-222 (Flow Meter Installation).
B. Flow Nozzle
Flow nozzles are widely used for flow measurements at high flow velocities. The flow nozzle is more rugged and erosion resistant than the orifice plate. For a given diameter and a given differential pressure, the flow nozzle will pass approximately 65% more flow than the orifice plate.
Permanent pressure drop of a flow nozzle is greater than the Venturi flow element and less than the orifice flow element.
The flow nozzle, because of its streamlined contour, tends to sweep solids through the throat. For non-homogeneous fluids, the flow nozzle is preferable to the orifice plate. The flow nozzle should not be used if large percentages of solids are present.
Typical pressure measurements are at the radius tap locations of 1 pipe diameter upstream and 0-.5 pipe diameters downstream of the flow nozzle. The downstream tap should never be located beyond the end of the nozzle. As in all differential producers, its output varies as the square root of the flow rate.
C. Venturi
The Venturi tube can measure 25 to 50% more flow than orifice for comparable line size and head loss. The flow range for satisfactory measurement is usually considered to extend upward from Reynolds numbers of about 200,000.
Some advantages of Venturi tubes are:
• high flow rates
• minimal piping straight run requirements (typically 10 pipe diameters)
• good accuracy with Beta ratios greater than .75
• integral pressure connections
The purchase cost of the Venturi tube is greater than most other primary flow elements. However, the greater pressure recovery can result in significant energy savings in large pipelines.
The Venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. A standard Venturi tube is shown in Figure 3.
A number of taps are often employed around the circumference of the high and low pressure areas which are connected together in what is known as a piezometer ring. This allows multiple pressure measurements in order to get a better average of the pressures. As with all differential pressure measurements, the flow rate varies as the square root of the differential pressure.
D. Flow Tubes
Flow tubes are primary elements with converging and diverging sections, similar to Venturi, whose design is usually proprietary.
Some advantages of flow tubes are:
• high differential pressure with high recovery
• low cost
Limitations include:
• inapplicability to low flows and small pipes
• sensitivity to viscosity variations
• erroneous readings in highly viscous or dirty liquids
Flow tubes require approximately the same piping runs as an orifice plate.
E. Pitot Tubes
Pitot tubes are the simplest velocity head sensors. Models can be specified for a variety of difficult fluid services that include high temperature and high pressure. The sensor probes are often designed to be inserted into conduits without process shutdown.
One fundamental problem with the pitot tube is that velocity measurement is made at only one point in the flow stream rather than providing integrated volumetric flow measurement. The probes must be traversed across the pipes or the velocity profiles known in advance to calculate the average flow rate. For high accuracy and consistent results, the pitot should be preceded by 50 or more diameters of straight smooth pipe. If a sufficient straight run of pipe is installed ahead of the pitot tube, an approximate average velocity reading will be obtained at a location approximately 30% of the pipe radius from the pipe wall. The basic pitot tube configuration is shown below in Figure 5.
Figure 5 – Basic Pitot Tube Configuration
One port is placed in the flowing fluid, facing upstream, and is connected through internal tubing to one side of the secondary element. This port registers the total dynamic head in the stream since the velocity is zero at the sensing tip. The static pressure is obtained from a port which faces perpendicular to the flow and is usually located in the pipe wall.
Errors are introduced in pitot tube measurements because the total and static pressures are not measured at the same point in the flow profile. This problem is eliminated in pitot static tubes, shown below in Figure 6.
Figure 6 – Pitot Static Tube
A static pitot has dual coaxial tubes. One terminates in a port facing upstream to register the total dynamic pressure at a stagnation point and the other tube is located away from the tip and faces normal to the flow or downstream to measure the static pressure at approximately the same streamline.
Combined-reversed pitot static tubes circumvent many of the problems associated with flow profiles across the channels. One design is the multi-port or annular averaging element. This element senses dynamic pressure at multiple sensing ports distributed along the diameter to provide a single indication of the average flow through the channel without a transverse. Static pressure is measured by a tube terminating in a port which faces downstream at the centerline of the conduit.
The Annubar™ manufactured by Dieterich Standard Corporation is a specialized pitot tube of this type. The ProBar Flowmeter™ manufactured by Rosemount is also a specialized pitot tube with pressure and temperature compensation. Both these flow elements are very commonly used in low pressure applications.
F. Wedge Meters
The segmental wedge is a proprietary design flow meter similar to the segmental orifice plate but with the upstream and downstream surfaces of the measuring point at a 45 degree angle to the flow stream. The wedge meter is located at the top of the pipe or conduit allowing the bottom of the pipe to be unrestricted. Wedge meters are highly linear from Reynolds numbers as low as 500 (laminar flow) to Reynolds numbers in the millions (highly turbulent flows). The wedge meter is useful when measuring slurry flow.
Figure 7 – Segmental Wedge
Some advantages of segmental wedge flow elements are:
• permanent pressure loss is approximately 50% of a similar orifice
installation
• viscous fluids measure accurately over a wide range of Reynolds
numbers
• changing viscosities can be measured
• bi-directional flow
G. Elbow Meter
The elbow meter is simply a pipe elbow that is inserted into the pipe and a pressure differential is created by the centrifugal force between the inside diameter and the outside walls of the pipe. The only pressure losses are from the elbow itself.
One limitation of the elbow is that the pressure differential produced is very small. Elbow flow meters are usually used for balancing loads for multiple compressors or pumps in multi-unit unit service. They have also been used in the nuclear industry to detect the extremely high velocities associated with pipe breaks.
H. Variable Area Meters
Variable area meters, more commonly known as rotameters, are available as indicators, transmitters, recorders, and controllers in any combination.
Variable area meters measure a wide range in of flow rates and are suitable for most fluids including high viscosity liquids and low concentration slurries. They are frequently used in low flow services such as purges where the requirements are below the range of an orifice plate.
A rotameter consists of a plummet or float within a tapered vertical tube. The force of the flowing fluid causes the float to rise until the force is balanced by gravity. Rotameter calibration suffers if the variation in viscosity or density is greater than 15%.
Rotameters can be configured to either measure liquid or gas flow. Measuring the flow of liquids and gases is a critical need in many industrial plants. Important parameters to consider when specifying rotameters include liquid volumetric flow rate, gas volumetric flow rate, operating pressure, and fluid temperature.
• Liquid volumetric flow rate applies only to those rotameters that are liquid volumetric flow sensors or meters. It is expressed as the range of flow in volume/time.
• Gas volumetric flow rate applies only to those rotameters that are gas volumetric flow sensors or meters. It is the range of flow in volume/time.
• Operating pressure is the maximum head pressure of the process media the meter can withstand.
• The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials. Pipe diameter is also important to consider, especially when specifying specific mounting options.
Mounting options for rotameters include insertion types, in-line flanged, in-line threaded, and in-line clamp.
• Insertion flow meters are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access.
• In-line flanged flow meters are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes.
• In-line threaded flow meters are inserted parallel to the flow path, and threaded into two existing process pipes.
Rotameters are often easy to install and offer a low cost solution to flow measurement.
I. Target Flowmeter
Target elements are impact devices that are coupled to restoring mechanisms such as springs or servomotors to maintain equilibrium. Target meters are located directly in the fluid flow. The deflection of the target and force bar is proportional to the square of the flow rate (according to Bernoulli’s equation).
Advantages of target flowmeters are:
• high accuracy
• long term repeatability
• long life
• low cost installation
• no pressure ports to plug
• dirty or clean fluids can be measured
• can be used for any type of liquid, gas, or steam cryogenics
• no moving parts such as bearings, to wear out causing failures
• high reliability where life tests have been made to 20,000,000
cycles
• can be used for any line size from 0.5 inches and up with any
type of mounting
• range/fluid changes accomplished by simply changing targets
• turndowns aprox.15:1
• can accept bi-directional flow where signal polarity indicates
direction
Limitations of target flowmeters are:
• accuracy depends on temperature (approx. 0.5% error/100C)
• much data is needed to determine the optimum size of the target
• calibration must be field verified.
5.2.2 Velocity Meters
These instruments operate linearly with respect to the volume flow rate. Because
there is no square-root relationship (as with differential pressure devices), their
rangeability is greater. Velocity meters have minimum sensitivity to viscosity
changes when used at Reynolds numbers above 10,000. Most velocity-type
meter housings are equipped with flanges or fittings to permit them to be
connected directly into pipelines.
A. Magnetic Flowmeter
The electromagnetic flowmeter approaches the ideal flow measurement device for liquids because it has no restriction in the flow line, can accurately measure liquids almost impossible to handle with other meters, has a linear output that is directly proportional to flow, and has the ability to measure bi-directional flow.
The only limitation on liquid measurement is that it must meet a minimum standard of electrical conductivity.
The principle of operation for magnetic flowmeters is Michael Faraday’s Law of Electromagnetic Induction. This law states that a conductor, when moving across lines of force in a magnetic field, will induce a voltage within the conductor and the magnetic field. (For a more detailed description of this operating principle, refer to the ISA book Flow Measurement: Practical Guides for Measurement and Control, 2nd edition, D.W. Spitzer, editor).
The process fluid serves as the conductor in the flow tube. If the tube is metal it must have a lining (flourcarbon resin, polyurethane, neoprene, etc) that serves as an electrical insulator on the inside of the tube wall. A pair of electrodes, extending through the wall of the tube are flush with the inside surface of the lining. The tube end connections are usually flanged to simplify mounting in a pipeline.
Magmeter sizes range from 0.01 to 96 inches and can measure flows from 0.01 GPM to 500,000 GPM. Measurement accuracy is better than 1 percent of the flow rate.
Magmeters can measure flow rates of clean fluids, dirty fluids and slurries. The meter is sensitive to changes in density and viscosity. The meter can not be used with most hydrocarbons because of their low conductivity.
Magmeter sizing
To properly size a magmeter, the following formula should be used:
Where: = velocity in Feet/sec.
= nominal diameter of the flowmeter (inches)
The following table lists typical sizing guidelines. These guidelines are based on Rosemount Magnetic flow meters and may vary slightly by manufacturer.
Application Velocity Range (ft/s) Velocity Range (m/s)
Normal Service 2-20 0.6-6.1
Abrasive Slurries 3-10 0.9-3.1
Non-Abrasive Slurries 5-15 1.5-4.6
B. Turbine Flowmeter
A turbine flowmeter uses the moving fluid or gas to turn a turbine rotor (Figure 8). The rotational speed of the rotor varies with the flow rate. When a steady rotational speed is obtained, the speed is proportional to the fluid velocity. Flow quantity data is supplied via a precisely known number of pulses for a given volume for fluid displaced between two adjacent rotor blades. The relationship is linear within given limits for flow rate and fluid viscosity.
Figure 8 – Cut-Away of a Turbine Flowmeter
Advantages of turbine meters include:
• high accuracy and repeatability
• wide flow ranges
• many materials of construction available
• low pressure drop
Turbine flowmeters are primarily used for flow totalizing for inventory control and custody transfer, precision automatic batching for loading and batch mixing.
Turbine meters should be selected with 30% to 50% excess capacity above the maximum flow rate. Turbine meters operating below the maximum capacity provide greater reliability.
C. Vortex Flowmeter
Vortex flowmeters measure flow via a natural phenomenon known as vortex shedding (Figure 9).
Figure 9 – Principle of Vortex Shedding
When a fluid or gas flows past an obstruction, boundary layers of slow moving fluids or gases are formed along the outer surfaces. If the obstacle is streamlined, the flow cannot follow the obstacle contours on the downstream side. The separated layers become detached and roll themselves into vortices in the low pressure area behind the body. Vortices are shed from alternate sides. The frequency at which they are shed is directly proportional to velocity. The signal output from the flowmeter is generated from the action of the fluid itself, so it belongs in the “fluidic” class of flowmeters.
Advantages of vortex shedding meters are:
• high accuracy
• long term repeatability
• good rangeability
• measure liquid, gas, and two-phase flow
• calibration is independent of viscosity, density, pressure and
temperature and can be maintained for long periods of time
Limitations of vortex shedding meters are:
• not suitable for dirty or abrasive fluids
• not suitable for viscous liquids
• limited choice of materials of construction
• limited size range
• limited maximum pressure and temperature capability
• no measured flow below the low flow cut-off velocity
D. Fluidic Flowmeter (Coanda Effect)
Another type of fluidic flowmeter utilizes the Coanda effect. The Coanda effect is basically a hydraulic feed back circuit. A chamber is designed with two feedback channels on opposite sides that produce a continuous, self induced oscillation. The frequency of the oscillation relates to the fluid velocity.
Advantages of fluidic flowmeters are:
• low cost
• high accuracy
• insensitive to temperature changes
Fluidic flowmeters can be used to measure clean fluids only. They will not measure gas or slurry flows.
Oct 28, 2008, 10:20 AM
#2
Account Disabled
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E. Ultrasonic Flowmeter
The principle behind an ultrasonic meter is that the speed of a sound wave is decreased if directed against a flowing fluid and increased if directed with the flow. In its simplest form, one sound transducer, mounted in the pipe wall, generates a sound which is received by another transducer located a known distance away. Flow is mathematically inferred from the velocity measured.
1. Transit Time
Principle of Operation: Transit time decreases in the downstream
direction and increases in the upstream direction. Flow is proportional to
Where values of are the measured transit times.
Acceptable fluids must:
• support the passage of sound from the transmitting transducer to
the receiving
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall
Advantages include:
• highest accuracy ultrasonic meter
• fast response time
Limitations:
• usually limited to single-phase fluids
2. Reflection or Doppler
Principle of Operation: Sound waves are reflected from moving particles
in the flow path. These reflections undergo frequency shifts proportional
to the Mach number (V/C where V is the fluid speed and C is the speed
of sound).
Acceptable fluids must:
• support the passage of sound
• contain sufficient scatterers or other disturbances to provide a
Doppler reflection but not contain so many scatterers that the sound cannot penetrate into the flow
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall
Advantages include:
• suitable for liquids with entrained gases or undissolved solids
• easy to install
• low price
Limitations:
• not recommended for single-phase fluids
• accuracies between 3% & 5% when installed properly
3. Beam Drift
Principle of Operation: The ultrasonic transmitter utilizes 2 acoustic
beams separated by a short distance to send sonic signals downstream
with the fluid. When turbulent flow occurs, the movement of the eddy
causes a change to the acoustic signal which produces a unique
signature. The transmitter looks for the second identical acoustic signal.
When found, the difference in time is calculated to determine the
velocity.
Advantages include:
• suitable for highly turbulent flow
Limitations:
• will not work in laminar flow and some swirl flows
4. Surface Height In Open or Partially Filled Channels
Principle of Operation: The height of a liquid in an open weir or flume or
a partially filled duct is a function of the flow velocity. The height can be
found using ultrasonic air-or-liquid path time-of-flight measurements.
Advantages include:
• Widely used for water flow applications
• Accuracy about 3%
Limitations:
• Compensation usually needed for the acoustic velocity in the
beam path
5. Noise
Principle of Operation: Noise increases with flow rate.
Advantages include:
• Suitable for low cost flow switches, leak detectors, and boundary
layer acoustic monitors to detect the transition from laminar to
turbulent flow
Limitations:
• Not accurate for flow measurements
• Highly non-linear
• Sensitive to errors caused by ambient vibration or sound
5.2.3 Mass Flowmeters
The continuing need for more accurate flow measurements in mass-related
processes (chemical reactions, heat transfer, etc.) has resulted in the
development of mass flowmeters. Various designs are available, but the one
most commonly used for liquid flow applications is the Coriolis meter. Its
operation is based on the natural phenomenon called the Coriolis force,
hence the name.
A. Coriolis Mass Flowmeter
Coriolis meters come in many shapes and sizes, but all function basically the same way. Each coriolis meter consists of one or more flow tubes. As fluid enters a vibrating coriolis tube, the particles accelerate (due to the vibration) exerting a force on the inlet side of the tube. As fluid leaves the vibrating coriolis tube, the particles decelerate and exert a force in the opposite direction from the inlet. The resulting forces angularly deflect the tube(s) an amount that is inversely proportional to the mass flow rate within the tube. The angular deflection is optically measured. Coriolis mass flow measurement is not affected by changing process conditions.
The Coriolis meter is insensitive to operating conditions of viscosity, density, type of fluid, and slurries.
B. Thermal Mass Flowmeter
Thermal flowmeters can be divided into two types. The first type measures the current required to maintain a fixed temperature across a heated element. The greater the flow, the more current required to maintain a constant temperature. The current required is proportional to the mass flow rate.
The second type of thermal flowmeter is the “hot wire”. The hot wire method measures the temperature at two points on an element or “hot wire”. As the fluid or gas flows over the element, heat is dissipated. The upstream side of the hot wire will be hotter than the downstream side. The change in temperature is proportional to the mass flow.
5.2.4 Open Channel Flow
The "open channel" refers to any conduit in which liquid flows with a free surface.
Included are tunnels, non-pressurized sewers, partially filled pipes, canals,
streams, and rivers. Of the many techniques available for monitoring open-
channel flows, depth-related methods are the most common. These techniques
presume that the instantaneous flow rate may be determined from a
measurement of the water depth, or head. Weirs and flumes are the oldest and
most widely used primary devices for measuring open-channel flows.
A. Weirs
Weirs and flumes are distinguished as “rate meters” in that they are used in open pipe or channels that do not flow full. They fall into the general category of “head-area” meters and are used extensively in the measurement of irrigation water as well as the primary device for municipal and industrial wastewater applications.
Weirs are dam-like structures placed across the flowing stream. A notch of predetermined size and shape, cut out of the upper edge, creates a path for the flow. The sheet of liquid falling over the weir through the notch is called the nappe. When air has free access beneath the nappe, the flow is considered free; otherwise, it is considered submerged. The degree of submergence can significantly reduce the flow over the weir. Most weirs are sharp crested. Common weir profiles are V-Notch (triangular), Rectangular, or trapezoidal (Cipolletti).
The V-Notch Weir is especially recommended for metering flows less than 1 ft3 /s (cfs) equivalent to 0.65 million gal/day (mgd) and is suitable for measuring slowly changing flows up to 10 cfs. Extensive experiments have been made to determine the calibration data for v-notch weirs with 60 and 90 degree angles. Accuracy is limited to about 3%.
The Rectangular Weir is capable of high capacity metering and is simple and inexpensive to construct. Accuracy is limited to about 3%.
The Trapezoidal Weir is similar to the rectangular weir except for sloping sizes (1 horizontal to 4 vertical) of the notch. An advantage of this design is a simplified discharge formula that is more convenient to work with. Accuracy is limited to about 5%.
Weirs are capable of wide ranges. Triangular devices can operate over ranges of 50:1 and rectangular models offer ranges of 15:1. One shortcoming of weirs is that they introduce substantial head loss and may increase pumping cost. Cleanouts are needed in wastewater applications because debris and solids may become trapped at the upstream face of the weir.
B. Flumes
Flumes are primary elements that restrict the width of the channel. These are more expensive than weirs but have advantages of minimal head loss and self-cleaning design.
Venturi flumes are the forerunners of current practice. Parshall flumes are now the most common. A number of specialized configurations are also used. Leopold- lagco Lagco and palmerPalmer-bowless Bowlus designs are suited for partially filled circular pipe often used in sewer systems.
Figure 10 – Parshall Flume
Parshall Flumes consist of a converging upstream section, a downward sloping throat and an upward sloping, and a diverging downstream section. It is usually constructed of concrete, but may be constructed of wood. Stainless steel and fiberglass reinforced plastic liners have been used for metering corrosive solutions. Parshall flumes have been constructed in sizes with throat widths ranging from 3 in. to 40 ft. for measuring flows up to 1500 mgd.
FREE-FLOW CONDITION
Free flow discharge, the condition under which the rate of discharge is dependent only on the width of the throat and the depth of water at gage point Ha in the converging section, can occur at two different stages:
1. Where the liquid moves at high velocity in a thin sheet conforming
closely to the dip at the lower end of the throat
2. Where the back water raises the water surface to elevation Hb, causing
a ripple or standing wave to form at or just downstream from the end of
the throat.
Under the latter condition, the flume operates under partial submergence, but the free flow rate of discharge is not impeded as long as the ratio of Hb to Ha does not exceed the values given below.
SUBMERGED FLOW
When the ratio of Hb to Ha exceeds the values given below, the flume is operating under submerged flow and the rate of discharge is reduced. Operation of flumes under submerged flow conditions is not recommended since two gage points are required to determine the negative correction factor to apply to the free flow calibration data. There are no instruments available for direct and accurate measurement of submerged flow.
FLUME THROAT Hb / Ha
3 – 9 in. 0.6
1 – 8 ft. 0.7
10 –50 ft. 0.8
SECONDARY ELEMENTS
Float operated devices are widely used with cables or rigid rods to convert vertical float motion into rotation in the transducer.
Bubbler or purge systems are also popular as secondary elements in open channel flowmeters. A constant regulated flow of air or water is forced through a dip tube. The back pressure on the tube is monitored. This is proportional to the immersed depth of the tube, and therefore to the level. Signal conditioners are needed to obtain linear flow output.
Capacitance and other electronic probes are available for level measurement in open-channel flow applications. These are used almost exclusively for transmission and incorporate signal conditioning circuits for calibration and linearization.
Sonic or radar measurements are also being applied to level sensing for open-channel flow in non foaming applications. The transducers are mounted above the stream where they are not subject to fouling or solids accumulation.
Secondary elements, especially bubblers and ball type sensors, are often used with stilling wells to nullify effects of stream velocity. The wells also provide protection from accumulations of solids and plugging the bubbler tubes. Cleanout provisions or purging should be specified to prevent particles or debris from settling on the bottom and interfering with the measurement.
Figure 11 – Representative Secondary Elements
5.2.5 Positive Displacement Meters
Positive Displacement (PD) meters measure flow directly in quantity terms
instead of indirectly or inferentially. PD meters operate by trapping a known
volume of fluid, passing it from inlet and to outlet and counting the number of fluid
“packets” that pass. An output shaft drives through gearing to a local counter.
By selection of suitable gearing, readout in the required volumetric units can be
obtained. A pulse generator (either optical or electromagnetic) may be fitted for
transmission to a remote device. Because of production tolerances, meters must
be individually calibrated. Temperature compensation can be provided to
convert the measured flow to standard units.
PD meters are extremely accurate and repeatable if adequately maintained. PD
meters offer unequalled accuracy and flow range capabilities on high viscous
liquids. As liquid viscosity increases, the slippage and hence the error is
reduced. Measurement accuracies of 0.001 gallon per pulse are available.
PD meters are widely used for flow measurement of fuel oils and other chemicals and hydrocarbon products in small pipe sizes. The basic limitation of a PD meter is that it has moving parts with close tolerances and clearances. This limits its use to clean liquids and necessitates regular maintenance. High temperatures and pressures also can distort the output signal.
For large size PD meters, physical size and weight will require special mounting pads. Also a device to eliminate air and vapor from the liquid is required, since the meter will measure air along with the liquid. PD meters must not be left dry kept flooded at all times. If PD meters are left dry, the meter will be “hammered” every time the pump is turned on.
The principle behind an ultrasonic meter is that the speed of a sound wave is decreased if directed against a flowing fluid and increased if directed with the flow. In its simplest form, one sound transducer, mounted in the pipe wall, generates a sound which is received by another transducer located a known distance away. Flow is mathematically inferred from the velocity measured.
1. Transit Time
Principle of Operation: Transit time decreases in the downstream
direction and increases in the upstream direction. Flow is proportional to
Where values of are the measured transit times.
Acceptable fluids must:
• support the passage of sound from the transmitting transducer to
the receiving
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall
Advantages include:
• highest accuracy ultrasonic meter
• fast response time
Limitations:
• usually limited to single-phase fluids
2. Reflection or Doppler
Principle of Operation: Sound waves are reflected from moving particles
in the flow path. These reflections undergo frequency shifts proportional
to the Mach number (V/C where V is the fluid speed and C is the speed
of sound).
Acceptable fluids must:
• support the passage of sound
• contain sufficient scatterers or other disturbances to provide a
Doppler reflection but not contain so many scatterers that the sound cannot penetrate into the flow
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall
Advantages include:
• suitable for liquids with entrained gases or undissolved solids
• easy to install
• low price
Limitations:
• not recommended for single-phase fluids
• accuracies between 3% & 5% when installed properly
3. Beam Drift
Principle of Operation: The ultrasonic transmitter utilizes 2 acoustic
beams separated by a short distance to send sonic signals downstream
with the fluid. When turbulent flow occurs, the movement of the eddy
causes a change to the acoustic signal which produces a unique
signature. The transmitter looks for the second identical acoustic signal.
When found, the difference in time is calculated to determine the
velocity.
Advantages include:
• suitable for highly turbulent flow
Limitations:
• will not work in laminar flow and some swirl flows
4. Surface Height In Open or Partially Filled Channels
Principle of Operation: The height of a liquid in an open weir or flume or
a partially filled duct is a function of the flow velocity. The height can be
found using ultrasonic air-or-liquid path time-of-flight measurements.
Advantages include:
• Widely used for water flow applications
• Accuracy about 3%
Limitations:
• Compensation usually needed for the acoustic velocity in the
beam path
5. Noise
Principle of Operation: Noise increases with flow rate.
Advantages include:
• Suitable for low cost flow switches, leak detectors, and boundary
layer acoustic monitors to detect the transition from laminar to
turbulent flow
Limitations:
• Not accurate for flow measurements
• Highly non-linear
• Sensitive to errors caused by ambient vibration or sound
5.2.3 Mass Flowmeters
The continuing need for more accurate flow measurements in mass-related
processes (chemical reactions, heat transfer, etc.) has resulted in the
development of mass flowmeters. Various designs are available, but the one
most commonly used for liquid flow applications is the Coriolis meter. Its
operation is based on the natural phenomenon called the Coriolis force,
hence the name.
A. Coriolis Mass Flowmeter
Coriolis meters come in many shapes and sizes, but all function basically the same way. Each coriolis meter consists of one or more flow tubes. As fluid enters a vibrating coriolis tube, the particles accelerate (due to the vibration) exerting a force on the inlet side of the tube. As fluid leaves the vibrating coriolis tube, the particles decelerate and exert a force in the opposite direction from the inlet. The resulting forces angularly deflect the tube(s) an amount that is inversely proportional to the mass flow rate within the tube. The angular deflection is optically measured. Coriolis mass flow measurement is not affected by changing process conditions.
The Coriolis meter is insensitive to operating conditions of viscosity, density, type of fluid, and slurries.
B. Thermal Mass Flowmeter
Thermal flowmeters can be divided into two types. The first type measures the current required to maintain a fixed temperature across a heated element. The greater the flow, the more current required to maintain a constant temperature. The current required is proportional to the mass flow rate.
The second type of thermal flowmeter is the “hot wire”. The hot wire method measures the temperature at two points on an element or “hot wire”. As the fluid or gas flows over the element, heat is dissipated. The upstream side of the hot wire will be hotter than the downstream side. The change in temperature is proportional to the mass flow.
5.2.4 Open Channel Flow
The "open channel" refers to any conduit in which liquid flows with a free surface.
Included are tunnels, non-pressurized sewers, partially filled pipes, canals,
streams, and rivers. Of the many techniques available for monitoring open-
channel flows, depth-related methods are the most common. These techniques
presume that the instantaneous flow rate may be determined from a
measurement of the water depth, or head. Weirs and flumes are the oldest and
most widely used primary devices for measuring open-channel flows.
A. Weirs
Weirs and flumes are distinguished as “rate meters” in that they are used in open pipe or channels that do not flow full. They fall into the general category of “head-area” meters and are used extensively in the measurement of irrigation water as well as the primary device for municipal and industrial wastewater applications.
Weirs are dam-like structures placed across the flowing stream. A notch of predetermined size and shape, cut out of the upper edge, creates a path for the flow. The sheet of liquid falling over the weir through the notch is called the nappe. When air has free access beneath the nappe, the flow is considered free; otherwise, it is considered submerged. The degree of submergence can significantly reduce the flow over the weir. Most weirs are sharp crested. Common weir profiles are V-Notch (triangular), Rectangular, or trapezoidal (Cipolletti).
The V-Notch Weir is especially recommended for metering flows less than 1 ft3 /s (cfs) equivalent to 0.65 million gal/day (mgd) and is suitable for measuring slowly changing flows up to 10 cfs. Extensive experiments have been made to determine the calibration data for v-notch weirs with 60 and 90 degree angles. Accuracy is limited to about 3%.
The Rectangular Weir is capable of high capacity metering and is simple and inexpensive to construct. Accuracy is limited to about 3%.
The Trapezoidal Weir is similar to the rectangular weir except for sloping sizes (1 horizontal to 4 vertical) of the notch. An advantage of this design is a simplified discharge formula that is more convenient to work with. Accuracy is limited to about 5%.
Weirs are capable of wide ranges. Triangular devices can operate over ranges of 50:1 and rectangular models offer ranges of 15:1. One shortcoming of weirs is that they introduce substantial head loss and may increase pumping cost. Cleanouts are needed in wastewater applications because debris and solids may become trapped at the upstream face of the weir.
B. Flumes
Flumes are primary elements that restrict the width of the channel. These are more expensive than weirs but have advantages of minimal head loss and self-cleaning design.
Venturi flumes are the forerunners of current practice. Parshall flumes are now the most common. A number of specialized configurations are also used. Leopold- lagco Lagco and palmerPalmer-bowless Bowlus designs are suited for partially filled circular pipe often used in sewer systems.
Figure 10 – Parshall Flume
Parshall Flumes consist of a converging upstream section, a downward sloping throat and an upward sloping, and a diverging downstream section. It is usually constructed of concrete, but may be constructed of wood. Stainless steel and fiberglass reinforced plastic liners have been used for metering corrosive solutions. Parshall flumes have been constructed in sizes with throat widths ranging from 3 in. to 40 ft. for measuring flows up to 1500 mgd.
FREE-FLOW CONDITION
Free flow discharge, the condition under which the rate of discharge is dependent only on the width of the throat and the depth of water at gage point Ha in the converging section, can occur at two different stages:
1. Where the liquid moves at high velocity in a thin sheet conforming
closely to the dip at the lower end of the throat
2. Where the back water raises the water surface to elevation Hb, causing
a ripple or standing wave to form at or just downstream from the end of
the throat.
Under the latter condition, the flume operates under partial submergence, but the free flow rate of discharge is not impeded as long as the ratio of Hb to Ha does not exceed the values given below.
SUBMERGED FLOW
When the ratio of Hb to Ha exceeds the values given below, the flume is operating under submerged flow and the rate of discharge is reduced. Operation of flumes under submerged flow conditions is not recommended since two gage points are required to determine the negative correction factor to apply to the free flow calibration data. There are no instruments available for direct and accurate measurement of submerged flow.
FLUME THROAT Hb / Ha
3 – 9 in. 0.6
1 – 8 ft. 0.7
10 –50 ft. 0.8
SECONDARY ELEMENTS
Float operated devices are widely used with cables or rigid rods to convert vertical float motion into rotation in the transducer.
Bubbler or purge systems are also popular as secondary elements in open channel flowmeters. A constant regulated flow of air or water is forced through a dip tube. The back pressure on the tube is monitored. This is proportional to the immersed depth of the tube, and therefore to the level. Signal conditioners are needed to obtain linear flow output.
Capacitance and other electronic probes are available for level measurement in open-channel flow applications. These are used almost exclusively for transmission and incorporate signal conditioning circuits for calibration and linearization.
Sonic or radar measurements are also being applied to level sensing for open-channel flow in non foaming applications. The transducers are mounted above the stream where they are not subject to fouling or solids accumulation.
Secondary elements, especially bubblers and ball type sensors, are often used with stilling wells to nullify effects of stream velocity. The wells also provide protection from accumulations of solids and plugging the bubbler tubes. Cleanout provisions or purging should be specified to prevent particles or debris from settling on the bottom and interfering with the measurement.
Figure 11 – Representative Secondary Elements
5.2.5 Positive Displacement Meters
Positive Displacement (PD) meters measure flow directly in quantity terms
instead of indirectly or inferentially. PD meters operate by trapping a known
volume of fluid, passing it from inlet and to outlet and counting the number of fluid
“packets” that pass. An output shaft drives through gearing to a local counter.
By selection of suitable gearing, readout in the required volumetric units can be
obtained. A pulse generator (either optical or electromagnetic) may be fitted for
transmission to a remote device. Because of production tolerances, meters must
be individually calibrated. Temperature compensation can be provided to
convert the measured flow to standard units.
PD meters are extremely accurate and repeatable if adequately maintained. PD
meters offer unequalled accuracy and flow range capabilities on high viscous
liquids. As liquid viscosity increases, the slippage and hence the error is
reduced. Measurement accuracies of 0.001 gallon per pulse are available.
PD meters are widely used for flow measurement of fuel oils and other chemicals and hydrocarbon products in small pipe sizes. The basic limitation of a PD meter is that it has moving parts with close tolerances and clearances. This limits its use to clean liquids and necessitates regular maintenance. High temperatures and pressures also can distort the output signal.
For large size PD meters, physical size and weight will require special mounting pads. Also a device to eliminate air and vapor from the liquid is required, since the meter will measure air along with the liquid. PD meters must not be left dry kept flooded at all times. If PD meters are left dry, the meter will be “hammered” every time the pump is turned on.
Oct 29, 2008, 12:59 AM
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This is a great technical thread on flow sensors. Should be a sticky.
The Aquamist flow sensor is a cross between a positive displacement and turbine based flow sensor. The aim of the design was mainly trying to catch every molecule flow through the sensor at the tip of the turbine.
The latest sensor is now user serviceable.
The Aquamist flow sensor is a cross between a positive displacement and turbine based flow sensor. The aim of the design was mainly trying to catch every molecule flow through the sensor at the tip of the turbine.
The latest sensor is now user serviceable.
Last edited by Richard L; Oct 29, 2008 at 01:02 AM.
Oct 30, 2008, 11:54 AM
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Import Junky - Hey I know your trying to get in touch with Richard but you gotta be patient. Sometime it may take a day or two or three for him to respond to a PM, but he always does. No need to remind him to check them.
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Oct 31, 2008, 11:47 AM
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Can you not just click attach and then select a pdf when you go to post?
What size is the PDF?
If you can't get it to work I can host it for you, although I suspect Richard would be happy to host it as well and his server is probably more reliable than mine, especially since mine is hosted by my old school and they always send me emails telling me to clear my stuff off otherwise they are going to delete it, lol. I never do though.
What size is the PDF?
If you can't get it to work I can host it for you, although I suspect Richard would be happy to host it as well and his server is probably more reliable than mine, especially since mine is hosted by my old school and they always send me emails telling me to clear my stuff off otherwise they are going to delete it, lol. I never do though.
