Flow Module Page 1
College of Technology
Instrumentation and Control
Module # 4 Flow Measurement
Document Intent:
The intent of this document is to provide an example of how a subject matter expert might teach Flow Measurement. This approach is what Idaho State University College of Technology is using to teach its Energy Systems Instrumentation and Control curriculum for Flow
Measurement. The approach is based on a Systematic Approach to Training where training is developed and delivered in a two step process. This document depicts the two step approach with knowledge objectives being presented first followed by skill objectives. Step one teaches essential knowledge objectives to prepare students for the application of that knowledge. Step two is to let students apply what they have learned with actual hands on experiences in a controlled laboratory setting.
Examples used are equivalent to equipment and resources available to instructional staff members at Idaho State University.
Flow Measurement Introduction:
This module covers aspects of Flow measurement as used in process instrumentation and control. Flow measurement addresses essential knowledge and skill elements associated with measuring Flow. Students will be taught the fundamentals of Flow measurement using classroom
instruction, demonstration, and laboratory exercises to demonstrate knowledge and skill mastery of Flow measurement. Completion of this module will allow students to demonstrate mastery of knowledge and skill objectives by completing a series of tasks using calibration/test equipment, Flow indicating, and Flow transmitting devices.
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References
This document includes knowledge and skill sections with objectives, information, and examples of how pressure measurement could be taught in a vocational or industry setting. This document has been developed by Idaho State University’s College of Technology. Reference material used includes information from:
American Technical Publication – Instrumentation, Fourth Edition, by Franklyn W. Kirk, Thomas A Weedon, and Philip Kirk, ISBN 979-0-8269-3423-9 (Chapter 5)
Department of Energy Fundamentals Handbook, Instrumentation and Control, DOE-HDBK-1013/1-92 JUNE 1992, Re-Distributed by http://www.tpub.com
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STEP ONE
Flow Measurement Course Knowledge Objectives
Knowledge Terminal Objective (KTO)
KTO 4. Given examples, EVALUATE Flow measurement fundamentals as they apply to measuring Flow process variables to determine advantages and disadvantages associated with different types of devices used to indicate, measure, and transmit Flow.
Knowledge Enabling Objectives (KEO)
DEFINE FLUID FLOW and its importance as a process variable.
KEO 4.1.
DESCRIBE FLOW RATE as it applies to flow measurement.
KEO 4.2.
DESCRIBE TOTAL FLOW as it applies to flow measurement.
KEO 4.3.
DESCRIBE the characteristics of FLUID FLOW to include Physical Properties, KEO 4.4.
Reynolds Number, and Compressibility.
DESCRIBE how pressure, temperature, and volume define GAS LAWS for
KEO 4.5.
Boyle’s Law, Charles’ Law, Gay-Lussac’s Law, and the Combined Law.
DESCRIBE the concept associated with DIFFERENTIAL PRESSURE KEO 4.6.
FLOWMETERS.
DEFINE what a PRIMARY FLOW ELEMENT is.
KEO 4.7.
DESCRIBE what an ORIFICE PLATE is and how it used to measure flow.
KEO 4.8.
DESCRIBE what a FLOW NOZZLE is and how it used to measure flow.
KEO 4.9.
DESCRIBE what a VENTURI TUBE is and how it used to measure flow.
KEO 4.10.
DESCRIBE what a LOW-LOSS FLOW TUBE is and how it used to measure
KEO 4.11.
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DESCRIBE what a PITOT TUBE is and how it used to measure flow.
KEO 4.12.
DESCRIBE OPERATING PRINCIPLES associated with DIFFERENTIAL KEO 4.13.
PRESSURE FLOWMETERS to include the Bernoulli Equation and the Vena Contracta Point.
DESCRIBE why locations for DIFFERENTIAL PRESSURE KEO 4.14.
CONNECTIONS of FLOWMETERS vary.
DESCRIBE how DIFFEENTIAL INSTRUMENT LOCATIONS are
KEO 4.15.
determined for Liquid, Gas, and Steam flow applications.
DESCRIBE how BLOCKING VALVES AND MANIFOLDS are used for
KEO 4.16.
measuring differential measurements associated with flow.
DESCRIBE how VARIABLE-AREA FLOWMETERS maintain a constant
KEO 4.17.
differential pressure and allows the flow area to change with flow rate.
DESCRIBE how ROTAMETERS are used and how they measure flow.
KEO 4.18.
DESCRIBE how MODIFIED ROTAMETERS are used as PURGE OR KEO 4.19.
BYPASS METERS.
DESCRIBE how METERING-CONE and SHAPTE-FLOAT & ORIFICE KEO 4.20.
VARIABLE-AREA METERS measure flow.
EXPLAIN operating principles associated with VARIABLE-AREA KEO 4.21.
FLOWMETERS.
DESCRIBE how MECHANICAL FLOWMETERS measure flow to include KEO 4.22.
the following Positive-Displacement Flometers: Nutating Disc, Rotating-Impeller, and Sliding Vane.
DESCRIBE how TURBINE METERS and PADDLE WHEEL METERS KEO 4.23.
measure flow.
DESCRIBE how MAGNETIC METERS measure flow.
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DESCRIBE how MAGNETIC VORTEX SHEDDING METERS measure
KEO 4.25.
flow.
DESCRIBE how ULTRASONIC FLOWMETERS measure flow.
KEO 4.26.
DESCRIBE how MASS FLOWMETERS measure flow to include a
KEO 4.27.
CORIOLIS METER and a THERMAL MASS METER.
DESCRIBE how ACCESSORY FLOW DEVICES function andhow they are
KEO 4.28.
used.
EXPLAIN how different FLOW SWITCHS function and how theyare used to
KEO 4.29.
include: DIFFERENTIAL PRESSURE SWITCHES, BLADE SWITCHES, THEREMAL SWITCHES, and ROTAMETER SWITCHES.
EXPLAIN how OPEN-CHANEL WEIRS and PARSHALL FLUME FLOW KEO 4.30.
MEASUREMTNS function and how they are used.
EXPLAIN how a BELT WEIGHING SYSTEM is used to measure a solids
KEO 4.31.
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FLOW MEASURMENT
DEFINE FLUID FLOW and its importance as a process variable.
KEO 4.1.
FLUID FLOW is the movement of liquids in pipes or channels, and gases or vapors in pipes or ducts. A fluid is a material that flows and takes the shape of its container. All liquids and gases are fluids.
Measuring flow is an important process variable which requires the use of many types of instruments and scientific principles. It is often more convenient to measure the flow of a fluid by measuring some other characteristic that varies in a predictable and reliable way with the rate of flow, such as a drop in pressure caused by restriction in a pipeline. This is drop in pressure is commonly used as well as a host of other methods.
FLUID FLOW is an important process variable that needs to be monitored and controlled not only in our homes and communities, but throughout all aspects of industry in the world. Fluids can be harmless, toxic, caustic, acidic, or volatile and to measuring them requires not only accuracy, but constant control at all times.
DESCRIBE FLOW RATE as it applies to flow measurement.
KEO 4.2.
FLOW RATE is the quantity of fluid passing a point at a particular moment. Flow rate is expressed in volumetric or mass units. The common volumetric units used in the United States are Gallons per Minute (gpm) or Gallons per Hour (gph). Also in the United States metric units used are Liters per Minute, Cubic Meters per Hour, and Cubic Centimeters per Minute. The unit of Mass in the United States is Pounds per Hour and the Metric unit of Mass is Kilograms per Hour.
DESCRIBE TOTAL FLOW as it applies to flow measurement.
KEO 4.3.
TOTAL FLOW is the quantity of fluid that passes a point during a specific time interval. An example would be the Flow Rate of pumping a fluid may be given in gallons per hour and the
Flow Module Page 7 Flow is measured in many units. Conversion tables like the table below are used to convert from one unit to another:
Figure 5-1 page 167
SUMMARY
Fluid flowis the movement of liquids in pipes or channels, and gases or vapors in pipes or ducts.
A fluid is a material that flows and takes the shape of its container.
All liquids and gases are fluids.
Flow rateis the quantity of fluid passing a point at a particular moment.
Total flowis the quantity of fluid that passes a point during a specific time interval.
DESCRIBE the characteristics of FLUID FLOW to include Physical Properties, KEO 4.4.
Flow Module Page 8 A FLUID FLOW’s most important characteristic that affects its flow is whether the fluid is a liquid, gas, or vapor. This is because at certain temperatures and pressures, most fluids can change phase between vapor, liquid, or solid. An example would be water that when heated becomes steam and when cooled becomes ice. Gases can also be condensed to a liquid like liquid nitrogen or liquid oxygen, or a solid like dry ice.
A number of Physical Properties common to most fluids that influence the selection of the method chosen to measure fluid flow include: Pressure, Velocity, Density, Viscosity,
Compressibility, Electrical Capacitance and Conductance, Thermal Conductivity, and the response to Sonic Impulses, Light, or Mechanical Vibration. All of these properties allow for the measurement of these fluids to determine Flow Rate and Total Flow. The fact that so many
properties and characteristics can be measured account for the wide variety of flowmeters. In addition to Physical Properties of fluids, there are other factors that affect flow. They include configuration o the pipes or ducts; the location, style, and number of valves; and changes in elevation of the fluid. The most important factors affecting fluid flow are the properties of the fluid, the Reynolds Number describing the type of flow, and the Compressibility of the fluid.
Physical Properties greatly affecting the measurement of flow include Density, Specific Gravity, and Viscosity.
Density is a measurement of Mass per Volume with common units of density being pounds per cubic foot (lb/ft3 or lb/cu ft) and grams per cubic centimeter (g/cm3 or g/cu cm). Density varies with changes in temperature.
Specific Gravity is the ratio of density of a fluid to the density of a reference fluid. For liquids this reference is usually water. For gases, the reference fluid is dry air. When two liquids that do not mix are in a container, the one with the lowest specific gravity will float on top of the one with the greater specific gravity. An example would be most oils having a specific gravity of from 0.75 to 0.85 at ambient temperature mixed with a fluid like water having a specific gravity of 0.998 the oil will rise to the top and float on the surface of the water. Gasoline would also float on top of water. Oils and fuel are examples of fluids called organic fluids and solutions containing water are called aqueous fluids.
Absolute Viscosity is the resistance to flow of a fluid and has units of Centipoise (cp).
Kinematic Viscosity is the ration of absolute viscosity to fluid density and has units of
Flow Module Page 9 The following picture illustrates how Viscosity is affected by temperature and other factors which normally decreases with increasing temperatures:
Figure 5-2 page 168
Many fluids must be preheated before being pumped. A property of fluid flow that describes the type of flow is the Reynolds Number.
The Reynolds Number of a fluid is the ratio between the inertial forces moving a fluid and viscous forces resisting that movement. The Reynolds Number describes the nature of the fluid flow. This number has no units of measure and is calculated from velocity or flow rate, density, viscosity, and the inside diameter of the pipe. Reynolds Numbers commonly range from 100 to 1,000,000. However, they can be higher or lower than these values.
Flow Module Page 10
Figure 5-3 page 169
Velocity is the speed of fluid in the direction of flow and typically is expressed in ft/sec. A
Streamline is a line that shows the direction and magnitude of smooth flow at every point across a pipe profile. A Flow Profile is a representation of the Velocity of a fluid at different points across the pipe or duct as depicted in the above picture.
Laminar Flow is the smooth fluid flow that has a Flow Profile that is parabolic in shape with no mixing between the stream lines. Laminar Flow in pipes occurs at Reynolds Numbers below
Flow Module Page 11 about 2100. A cross section of a Laminar Flow is a parabolic Flow Profile, with the maximum
Velocity in the center and the minimum Velocity at the pipe walls.
Turbulent Flow is fluid flow in which the Flow Profile is a flattened parabola, the streamlines are not present, and the fluid is freely intermixing. Turbulent Flow in pipes typically occurs at
Reynolds Numbers above about 4000. The exact shape of the flattened profile depends on the
Reynolds Number.
There is a sudden transition between Laminar Flow and Turbulent Flow as the flow rate
increases and normally occurs at Reynolds Numbers between 2100 and 4000. Many Flowmeters require Turbulent Flow and specify Reynolds Numbers above 10,000 to ensure that Turbulent Flow is the prevailing condition.
Compressibility is a determination as to whether or not a fluid can be compressed. An
incompressible fluid is a liquid fluid where there is very little change in pressure. Liquids are essentially incompressible. As an example, fluid power systems transmit power through an impressible hydraulic fluid. A compressible fluid is a fluid where the volume and density change when subjected to a change in pressure. Gases and Vapors are examples of
compressible fluids.
A Flowing Condition is the pressure and temperature of the gas or vapor at the point of measurement. A Standard Condition is when an acceptable set of temperature and pressure condition is used as a basis for measurement.
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SUMMARY
A fluid flow’s most important characteristic that affects its flow is whether the fluid is a liquid, gas, or vapor.
Physical properties common to most fluids that influence the selection of the method chosen to measure fluid flow include:
o Pressure, Velocity, Density, Viscosity, Compressibility, Electrical Capacitance and Conductance, Thermal Conductivity, and the response to Sonic Impulses, Light, or Mechanical Vibration.
The most important factors affecting fluid flow are the properties of the fluid, the
Reynolds Number describing the type of flow, and the Compressibility of the fluid.
The Reynolds Number of a fluid is the ratio between the inertial forces moving a fluid and viscous forces resisting that movement and describes the nature of the fluid flow.
Compressibility is a determination as to whether or not a fluid can be compressed.
An incompressible fluid is a liquid fluid where there is very little change in pressure.
Liquids are essentially incompressible.
A compressible fluid is a fluid where the volume and density change when subjected to a change in pressure.
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DESCRIBE how pressure, temperature, and volume define GAS LAWS for
KEO 4.5.
Boyle’s Law, Charles’ Law, Gay-Lussac’s Law, and the Combined Law.
Gas Laws show how gases behave with changes in temperature, pressure and volume. Gas Laws are used to determine the volume of gas at one set of pressure and temperature conditions when data from another set of conditions are known. The following figure depicts three gas laws, Boyle’s, Charles’, and Gay-Lussac’s with their corresponding calculations:
Flow Module Page 14
Boyle’s Law is a gas law that states that the absolute pressure of a given quantity of gas varies inversely with its volume provided the temperature remains constant.
Boyle’s Law P2 = Final Pressure (in psia)
P1 = Initial Pressure (in psia) V2 = Final Volume (in cubic units) V1 = Initial Volume (in cubic units)
Charles’ Law is a gas law that states the volume of a given quantity of gas varies directly with its absolute temperature provided the pressure remains constant.
Charles’ Law T2 = Final Temperature (in oR)
T1 = Initial Temperature (in oR) V2 = Final Volume (in cubic units) V1 = Initial Volume (in cubic units)
Gay-Lussac’s Law is a gas law that states that the absolute pressure of a given quantity of a gas varies directly with its absolute temperature provided the volume remains constant.
Flow Module Page 15 T2 = Final Temperature (in oR)
T1 = Initial Temperature (in oR) V2 = Final Volume (in psia) V1 = Initial Volume (in psia)
The three gas laws can be combined into one equation, called the Combined Gas Law in order to simplify calculations as depicted below:
Figure 5-5 page 172
Flow Module Page 16 V = Volume (in ft3 or other volumetric terms)
P = Pressure (in psia or other absolute pressure terms) T = Temperature (in oR or oK)
All Subscripts refer to different sets of conditions.
SUMMARY
Gas Laws show how gases behave with changes in temperature, pressure and volume.
Gas Laws are used to determine the volume of gas at one set of pressure and temperature conditions when data from another set of conditions are known.
Boyle’s Law is a gas law that states that the absolute pressure of a given quantity of gas varies inversely with its volume provided the temperature remains constant.
Charles’ Law is a gas law that states the volume of a given quantity of gas varies directly with its absolute temperature provided the pressure remains constant.
Gay-Lussac’s Law is a gas law that states that the absolute pressure of a given quantity of a gas varies directly with its absolute temperature provided the volume remains constant.
The three gas laws can be combined into one equation, called the Combined Gas Law in order to simplify calculations.
DESCRIBE the concept associated with DIFFERENTIAL PRESSURE KEO 4.6.
FLOWMETERS.
A pressure difference is created when a fluid passes through a restriction in a pipe. The point of maximum developed differential pressure is between the upstream of the restriction and the pressure downstream of the restriction, at the point of highest velocity. The shape and
configuration of the restriction affects the magnitude of the differential pressure and how much of the differential is recoverable.
Differential Pressure flowmeters are commonly used throughout industry and are called Differential Pressure Transmitters or DP Cells. Devices that restrict a flow and measure its differential pressure are called primary flow elements and they work together with the DP Devices to provide critical measurement and control of fluids.
DEFINE what a PRIMARY FLOW ELEMENT is.
Flow Module Page 17
Primary Flow Elements are devices that create or detect a pressure drop as fluid flows through a pipeline. Primary Flow Elements are designed to provide accuracy, low cost, ease of use, and pressure recovery, but not necessarily all in the same element. Examples of Primary Flow elements include: Orifice Plates, Flow Nozzles, Venturi Tubes, Low-Loss Flow Tubes, and
Pitot Tubes.
DESCRIBE what an ORIFICE PLATE is and how it used to measure flow.
KEO 4.8.
An ORIFICE PLATE is a primary flow element consisting of a thin circular metal plate with a sharp-edged round hole in it and a tab that protrudes from the flanges. The tab has orifice plate information stamped onto it. This information usually includes: Pipe Size, Bore Size, Material, and Type of Orifice.
Orifice plates are not always reversible so the stamping information is on the upstream face of the plate. The Orifice is held in place between two special pipe flanges called orifice flanges. The below picture illustrates flow being straightened after going through pipe 90o fittings to allow a smooth non turbulent flow upstream of the orifice plate (Straightening Vanes remove flow disturbances upstream of an orifice plate):
Figure 5-6 page 173
.Flow Module Page 18
Orifice Plates are simple, inexpensive, and replaceable. The hole in the plate is generally in the center (concentric) by may be off-center (eccentric). Eccentric plates are usually used to prevent excessive build up of foreign material or gases on the inlet side of the orifice. Some orifice plates will have a smaller hole near the top of the plate to release any build up of gases that may be present.
Orifice Plates have a straight run requirement of about 20 times the pipe diameter before the orifice and 6 times the pipe diameter after the orifice plate to provide the most accurate
differential pressure. Orifice plates have the poorest recovery of differential pressure (50%) of any of the primary flow elements.
DESCRIBE what a FLOW NOZZLE is and how it used to measure flow.
KEO 4.9.
A similar primary flow element to the orifice plate is the Flow Nozzle. A Flow Nozzle is a primary flow element consisting of a restriction shaped like a curved funnel that allows a little more flow than an orifice plate and reduces the straight run pipe requirements associated with orifice plates.
The Flow Nozzle is mounted between a pair of flanges like an orifice plate. The pressure sensing taps are located in the piping a fixed distance upstream and downstream of the flow nozzle. The following picture depicts a typical Flow Nozzle:
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DESCRIBE what a VENTURI TUBE is and how it used to measure flow.
KEO 4.10.
A VENTURI TUBE is a primary flow element consisting of a fabricated pipe section with a converging inlet section, a straight throat, and a diverging outlet section. The static pressure connection is located at the entrance to the inlet section. The reduced pressure connection is in the throat. Venturi tubes are much more expensive than orifice plates, but are more accurate and recover 90% or more of the differential pressure. This recovery reduces the burden on pumps and the cost of power to run them. Venturi Tubes are frequently used to measure large flows of water. A Venturi Tube is depicted below:
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DESCRIBE what a LOW-LOSS FLOW TUBE is and how it used to measure
KEO 4.11.
flow.
A LOW-LOSS FLOW TUBE is a primary flow element consisting of an aerodynamic internal cross section with the low-pressure at the throat as depicted below:
Figure 5-7 page 174
Low-Loss Flow Tubes are used for a higher energy efficiency of up to 97%, but are very expensive. Low-Loss Flow Tubes are often used in applications where the line pressure is low and therefore the pressure recovery must be high. Low-Loss Flow Tubes can often pay for themselves in energy savings in a short time as the following picture illustrates:
Figure 5-8 page 174
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DESCRIBE what a PITOT TUBE is and how it used to measure flow.
KEO 4.12.
A PITOT TUBE is a tube inserted into the piping or water flow to measure the impact pressure. A Pitot Tube is a flow element consisting of a small bent tube with a nozzle opening facing into the flow stream.
NOTE: Pitot Tubes are also used in other applications to measure water flow of a river or stream and on air craft to measure air flow to determine the speed of an aircraft is flying. The Pitot Tube Nozzle is called the impact opening and senses the velocity pressure plus the static pressure. The Static Pressure is sensed at the pipe wall perpendicular to the fluid stream. Pitot tubes are commonly used to measure air velocity in ducts and for measuring air speed of planes in flight.
A Standard Pitot Tube is depicted below:
Figure 5-9 page 175
A standard Pitot Tube senses the impact pressure at only one point in the center of the flow path. This is the high pressure tap as the pressure is greater in the center as it is on the walls of the pipe due to pipe resistance to the flow. Even though the velocity varies across the whole stream, it is greater in the middle of the flow stream. For example if a Pitot Tube were used to measure river water flow, the impact opening would be inserted the middle of the river as the water moves faster there than at the sides of the river banks due to the resistance of the river banks.
Flow Module Page 22 To overcome the disadvantage of only having one impact point to measure the flow, an
Averaging Pitot Tube was developed as depicted below:
Figure 5-9 page 175
The advantage of the Averaging Pitot Tube is that you have several sensing points to average the flow reading for a more accurate flow rate reading; therefore they are improved devices for measuring flow.
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SUMMARY
A pressure difference is created when a fluid passes through a restriction in a pipe.
The point of maximum developed differential pressure is between the upstream of the restriction and the pressure downstream of the restriction, at the point of highest velocity.
Primary Flow Elements are devices that create or detect a pressure drop as fluid flows through a pipeline.
An Orifice Plate is a primary flow element consisting of a thin circular metal plate with a sharp-edged round hole in it and a tab that protrudes from the flanges and this tab has orifice plate information stamped onto it.
Orifice plates are not always reversible so the stamping information is on the upstream face of the plate.
Orifice plates have the poorest recovery of differential pressure (50%) of any of the primary flow elements.
A Flow Nozzle is a primary flow element consisting of a restriction shaped like a curved funnel that allows a little more flow than an orifice plate.
A Venturi Tube is a primary flow element consisting of a fabricated pipe section with a converging inlet section, a straight throat, and a diverging outlet section.
Venturi tubes are much more expensive than orifice plates, but are more accurate and recover 90% or more of the differential pressure.
Venturi Tubes are frequently used to measure large flows of water.
Low-Loss Flow Tubes are used for a higher energy efficiency of up to 97%, but are very expensive.
Low-Loss Flow Tubes are often used in applications where the line pressure is low and therefore the pressure recovery must be high.
A Pitot Tube is a tube inserted into the piping or water flow to measure the impact pressure.
A Pitot Tube is a flow element consisting of a small bent tube with a nozzle opening facing into the flow stream.
There are two types of Pitot Tubes: A Standard Pitot Tube and an Averaging Pitot Tube.
Flow Module Page 24
DESCRIBE OPERATING PRINCIPLES associated with DIFFERENTIAL KEO 4.13.
PRESSURE FLOWMETERS to include the Bernoulli Equation and the Vena Contracta Point.
OPERATING PRINCIPLES of all differential pressure flowmeters are based on equations developed by Daniel Bernoulli, a late 18th century Swiss Scientist. His experiments related to the pressure and velocity of flowing water. He determined that at any point in a closed pipe there were three types of head pressure present:
1. Static Head Pressure due to elevation
2. Static Head Pressure due to applied pressure 3. Velocity Head Pressure.
The different types of head pressure can be converted to each other by changes in flow. The
Bernoulli Equation states that the sum of the heads of an enclosed flowing fluid is the same at any two locations.
Differential Pressure Flowmeters primary flow elements, have pressure measured upstream and downstream of the flow element. The flow steam contracts slightly before it passes through the flow element and continues to do so until it reaches maximum contraction, and then slowly expands until it again fills the pipe. This concept is depicted below:
Figure 5-10 page 176
The Vena Contracta is the point of lowest pressure and the highest velocity downstream frfom a primary flow element. According to the Bernoulli Equation, the velocity increases and the
Flow Module Page 25 pressure decreases as fluid flows through the restriction. The actual location of the Vena
Contracta point varies with flow rate and design of the flow element.
Differential Pressure Measurements: For turbulent flow, the flow rate is proportional to the square root of the differential pressure. This square root relationship affects the Rangeability of the flow metering system. Rangeability or Turndown, is the ratio of the maximum flow to the minimum measurable flow at the desired measurement accuracy.
This is a Characteristic of the Instrument and is not adjustable. For example, if the maximum measurable flow rate of a flowmeter were 100 gpm of water and the minimum rate were 20 gpm of water, the Rangeability or Turndown, is 5 to 1 (5:1) as depicted in the below picture:
Figure 5-11 page 178
The above picture depicts how flow varies with the square root of pressure drop, restricting the meter from full flow to 20% if flow.
Flow measurement is only accurate as long as the flowing conditions remain the same as when the system was designed. Changes in pressures and temperature are common in gas and vapor flow measurements. Liquid flow measurements are usually more consistent.
Flowing conditions that differ from the original flowmeter design calculation can result in significant errors. When the original design conditions and the actual flowing conditions are known, the flowmeters displayed flow rate can be changed to the correct value.
To obtain the correct flow, multiply the corrections factors for PC (Pressure Correction) and TC (Temperature Correction) times the displayed flow to obtain the correct flow as depicted in the below picture:
Flow Module Page 26
Figure 5-12 page 179
The above picture illustrates how to use the correction formulas to correct Gas and Vapor flows from measured conditions to design flow conditions.
Flow Module Page 27
Daniel Bernoulli determined that at any point in a closed pipe there were three types of head pressure present:
1. Static Head Pressure due to elevation
2. Static Head Pressure due to applied pressure 3. Velocity Head Pressure.
Bernoulli Equation states that the sum of the heads of an enclosed flowing fluid is the same at any two locations.
The Vena Contracta is the point of lowest pressure and the highest velocity downstream from a primary flow element. According to the Bernoulli Equation, the velocity
increases and the pressure decreases as fluid flows through the restriction.
Differential Pressure Measurements: For turbulent flow, the flow rate is proportional to the square root of the differential pressure.
Flow measurement is only accurate as long as the flowing conditions remain the same as when the system was designed.
To obtain the correct flow, multiply the corrections factors for PC (Pressure Correction) and TC (Temperature Correction) times the displayed flow to obtain the correct flow
DESCRIBE why locations for DIFFERENTIAL PRESSURE KEO 4.14.
CONNECTIONS of FLOWMETERS vary.
DIFFERENTIAL PRESSURE CONNECTIONS of FLOWMETERS vary as there are two locations selected depending of the application. There are two different connections used to measure the high pressure (Static Pressure in the pipe) and the low pressure (to measure the reduced pressure developed by the flow through the flow element). Not all connection locations are the same and are based on the type of flow element used and manufacture specifications. Pitot Tubes vary from the standard Pitot Tube to the Averaging Pitot Tube. The standard Pitot Tube uses two taps and the averaging Pitot Tube uses just one. When dealing with Orifice Tap Locations, there are Flange Taps, Vena Contracta Taps, and Pipe Taps located at different positions for measuring pressure drops.
Flange Taps are in the two flanges between the Orifice Plate. These tap connections requires the distance to be 1 inch upstream of the Orifice Plate, and 1 inch downstream of the Orifice Plate.
Vena Contracta Taps are located at 1 pipe diameter upstream of the Orifice Plate and ½ the pipe diameter downstream of the Orifice Plate.
Pipe Taps are located at 2 ½ times the pipe diameter upstream of the Orifice Plate and 8 times the pipe diameter downstream of the Orifice Plate.
Flow Module Page 28
NOTE
Pipe Tap Locations are generally specified by the manufacture for Flow
Nozzles and Low-Loss Flow tubes, whereas pipe taps for Venturi Tubes are
manufactured with the tubes purchased.
The following picture illustrates Orifice Plate Tap Locations:
Figure 5-13 page 179
DESCRIBE how DIFFEENTIAL INSTRUMENT LOCATIONS are
KEO 4.15.
Flow Module Page 29 When installing Differential Pressure Instruments/Transmitters, there are different requirements for Liquid, Gas and Steam flow applications. The following picture illustrates how they are to be connected to the process flow in order to accurately measure flow:
Figure 5-14 page 181
Location of a flow transmitter varies with the type of flowing fluid
Liquid Flow Transmitter location must be mounted below the elevation of the flow element, and the impulse lines must be filled with the liquid being measured as depicted below:
Flow Module Page 30 Care must be exercised to ensure that no bubbles are trapped in the instrument or impulse lines. This is accomplished via special valves for venting and releasing any air that may have entered the transmitter or impulse lines.
A discuss later on will address how transmitter manifold assemblies can accomplish this or the use of special vent release fittings on a transmitter.
The length of the transmitter impulse lines has no effect on the measurement accuracy as long as the two impulse lines start and end at equal elevations as indicated in the above picture.
Gas Flow Transmitter location must be mounted above the elevation of the flow element and the diameter of the impulse lines must be large enough and routed so that any liquids which may condense in the impulse lines drain freely into the main piping as depicted below:
Flow Module Page 31
Steam Flow Transmitter location must be located below the elevation of the flow element even though the fluid is a vapor. This is because steam condenses to water very easy, and mounting it below the flow element allows the instrument and impulse lines to fill with condensate.
Flow Module Page 32 The steam condensate protects the instrument from coming in contact with the hot steam. The best way to set up this transmitter is to manually backfill the impulse lines with water. The following picture illustrates how to correctly locate the Steam Flow Transmitter:
Flow Module Page 33
SUMMARY
The standard Pitot Tube uses two taps and the averaging Pitot Tube uses just one.
When dealing with Orifice Tap Locations, there are Flange Taps, Vena Contracta Taps, and Pipe Taps located at different positions for measuring pressure drops.
Flange Taps are in the two flanges between the Orifice Plate. These tap connections requires the distance to be 1 inch upstream of the Orifice Plate, and 1 inch downstream of the Orifice Plate.
Vena Contracta Taps are located at 1 pipe diameter upstream of the Orifice Plate and ½ the pipe diameter downstream of the Orifice Plate.
Pipe Taps are located at 2 ½ times the pipe diameter upstream of the Orifice Plate and 8 times the pipe diameter downstream of the Orifice Plate.
Pipe Tap Locations are generally specified by the manufacture for Flow Nozzles and Low-Loss Flow tubes, whereas pipe taps for Venturi Tubes are manufactured with the tubes purchased.
When installing Differential Pressure Instruments/Transmitters, there are different requirements for Liquid, Gas and Steam flow applications.
Liquid Flow Transmitter location must be mounted below the elevation of the flow element, and the impulse lines must be filled with the liquid being measured.
Gas Flow Transmitter location must be mounted above the elevation of the flow element and the diameter of the impulse lines must be large enough and routed so that any liquids which may condense in the impulse lines drain freely into the main piping.
Steam Flow Transmitter location must be located below the elevation of the flow element even though the fluid is a vapor. This is because steam condenses to water very easy, and mounting it below the flow element allows the instrument and impulse lines to fill with condensate.
The steam condensate protects the instrument from coming in contact with the hot steam.
DESCRIBE how BLOCKING VALVES AND MANIFOLDS are used for
KEO 4.16.
measuring differential measurements associated with flow.
A Blocking Valve is a valve used at the differential measuring instrument (transmitter, gauge, or sensor) to provide a convenient location to isolate the instrument from the impulse, equalizing, or venting lines and to provide a method to equalize the high and low pressure sides of the
Flow Module Page 34
Equalizing instrument pressure is necessary so that the instrument can be periodically calibrated and its zero status checked. Blocking valves may be connected with individual pipe fittings and valves or can be part of a manifold assembly in one block that can be attached to a differential pressure device.
Typical blocking valves are single-valve equalizers or three, four, or five valve manifolds. They are essential in setting up and maintaining process flow instrumentation. The below picture illustrates manifold valves:
Figure 5-15 page 182
The One-Valve is for equalizing pressure to perform a static test.The Three-Valve is for equalizing, static testing, and isolating and is most commonly used. The Five-Valve provides the Three-Valve function and adds the ability to vent and test.
Flow Module Page 35 The following picture depicts a typical Differential Pressure Transmitter configured to measure flow with a three valve manifold and an Integral Orifice:
Picture page 188
DESCRIBE how VARIABLE-AREA FLOWMETERS maintain a constant
KEO 4.17.
differential pressure and allows the flow area to change with flow rate.
VARIABLE-AREA FLOWMETERS maintain a constant differential pressure through the flow of some type of flow restriction that repositions with changes in flow. This can be accomplished by a fixed-size plug that moves in a tapered tube, a shaped plug that partially blocks an orifice, or a restriction that moves up and down on a cone. The most common type of
Variable-Area Flowmeter is a ROTAMETER.
DESCRIBE how ROTAMETERS are used and how they measure flow.
KEO 4.18.
A ROTAMETER is a tapered tube and a float with a fixed diameter. The float of the rotameter changes its position in the tube to keep the forces acting on the float in equilibrium. One of forces is gravity and the other force is produced by the velocity of the process fluid.
Flow Module Page 36 The following picture illustrates three types of floats depicting the fact that Rotameter floats have different configurations for different fluids and applications and shows where on the float a reading is taken:
Figure 5-16 page 183
As a rule, when reading floats, most floats have a sharp edge at the point where the reading should be made on a scale.
The exception is a round float and the reading would be directly in the center. In the floats pictured above, the general rule is the widest point on a float is where the reading reference point is. There are also Guided Rod Glass Tube Rotameters.
Flow Module Page 37 There are Clear-Glass Tube Rotameters, Plastic Tube Rotameters, and Metal Tube Rotameters as depicted below:
Figure 5-17 page 184
Glass Tube Rotameters are selected for fluids that are glass compatible. High temperatures water with a high pH will actually soften glass. For non-glass compatible fluids, Plastic Tube Rotameters are selected and are usually used with high temperatures, high pH, wet steam, caustic soda, and hydrofluoric acid.
Metal-Tube tapered Rotameters consist of a metal tapered tube and a rod-guided float. A rod is attached to the float passes through top and bottom guides in the tube. A magnet in the float is coupled to a matching magnetic and indicator located outside the tube. Magnetic-Coupled Rotameters cannot be used in areas where strong magnetic fields are generated.
Variations on Metal-Tube Rotameters include PVC Flanged Bodies and floats or with stainless steel bodies and matching floats. Additionally, an indicating electrical transmitter is often substituted for the visual indicator. Metal Tube Rotameters are selected for applications involving fluids which obscure the float or those too hot, too corrosive, or involving high
Flow Module Page 38 pressures. A disadvantage to using magnetic coupled metal tube rotameters is that high
temperature can diminish the magnetic coupling effect.
SUMMARY
A Blocking Valve is a valve used at the differential measuring instrument (transmitter, gauge, or sensor) to provide a convenient location to isolate the instrument from the impulse, equalizing, or venting lines and to provide a method to equalize the high and low pressure sides of the differential instrument.
Blocking valves may be connected with individual pipe fittings and valves or can be part of a manifold assembly in one block that can be attached to a differential pressure device.
The One-Valve is for equalizing pressure to perform a static test.
The Three-Valve is for equalizing, static testing, and isolating and is most commonly used.
The Five-Valve provides the Three-Valve function and adds the ability to vent and test.
VARIABLE-AREA FLOWMETERS maintain a constant differential pressure through the flow of some type of flow restriction that repositions with changes in flow.
The most common type of Variable-Area Flowmeter is a ROTAMETER.
A ROTAMETER is a tapered tube and a float with a fixed diameter. The float of the rotameter changes its position in the tube to keep the forces acting on the float in equilibrium. One of forces is gravity and the other force is produced by the velocity of the process fluid.
As a rule, when reading floats, most floats have a sharp edge at the point where the reading should be made on a scale. The exception is a round float and the reading would be directly in the center. The general rule is the widest point on a float is where the reading reference point is.
Glass Tube Rotameters are selected for fluids that are glass compatible. High temperature water with a high pH will actually soften glass.
For non-glass compatible fluids, Plastic Tube Rotameters are selected and are usually used with high temperatures, high pH, wet steam, caustic soda, and hydrofluoric acid.
Metal-Tube Rotameters consist of a metal tapered tube and a rod-guided float. A rod is attached to the float passes through top and bottom guides in the tube. A magnet in the float is coupled to a matching magnetic and indicator located outside the tube.
Flow Module Page 39
DESCRIBE how MODIFIED ROTAMETERS are used as PURGE OR KEO 4.19.
BYPASS METERS.
MODIFIED ROTAMETERS combine a standard rotameter with another device or the
rotameter itself is modified to achieve a specific function. Two common modified rotameters are the Purge Meter and the Bypass Meter as depicted below:
Figure 5-18 page 185
A Purge Meter is a small metal or plastic rotameter with an adjustable valve at the inlet or outlet of the meter to control the flow rate of the purge fluid. A Purge Meter is used for purging
applications such as regulating a small flow of nitrogen or air into an enclosure to prevent the buildup of hazardous or noxious gases. Purge Meters are often used in a bubbler level
measuring system. A small bead or ball moves in the fluid stream to indicate the flow rate which is accomplished by adjusting a small needle valve. Purge Meters also keep hazardous fumes and fluids from entering the impulse lines or transmitting devices.
Flow Module Page 40
A Bypass Meter is a combination of a rotameter with an orifice plate used to measure flow rates through large pipes. How this works is the differential pressure across the main pipe line is matched to the differential pressure across the rotameter at the maximum flow rate. The rotameter manufacture must provide a metering orifice plate at the inlet to the rotameter to accomplish this matching of differential pressures.
DESCRIBE how METERING-CONE and SHAPTE-FLOAT & ORIFICE KEO 4.20.
VARIABLE-AREA METERS measure flow.
Other Variable-Area Meters use principles similar to those used in rotameters to measure flow rate. Different from Rotameters, they consist of a straight tube meter body with other types of movable parts. Two Common Other Variable-Area Meters are the Metering-Cone Meter, and the Shaped-Float and Orifice Meter. The Metering Cone Meter is depicted below:
Figure 5-19 page 186
The Metering-Cone Meter is a flowmeter consisting of a straight tube and a tapered cone, instead of a tapered tube, with an indicator that moves up and down the cone with changes in flow. The variable area is the annular space between the flat and the tapered cone. The indicator is often spring-loaded to allow the meter to be mounted at any angle. The indicator is often spring loaded to allow the meter to be mounted at any angle.
Flow Module Page 41 A Shaped-Float and Orifice Meter is a flowmeter consisting of an orifice as part of the float assembly that acts as a guide. Instead of a tapered tube, the float has a shaped profile that provides more open flowing area as the float rises. The variable area is the annular space between the float and the disk. The picture below depicts the Shaped-Float and Orifice Meter:
Figure 5-19 page 186
The Shaped-Float and Orifice Meter can provide external readouts as indicators or transmitters with or without alarms.
SUMMARY
MODIFIED ROTAMETERS combine a standard rotameter with another device or the rotameter itself is modified to achieve a specific function. Two common modified rotameters are the Purge Meter and the Bypass Meter
A Purge Meter is a small metal or plastic rotameter with an adjustable valve at the inlet or outlet of the meter to control the flow rate of the purge fluid.
A Purge Meter is used for purging applications such as regulating a small flow of nitrogen or air into an enclosure to prevent the buildup of hazardous or noxious gases.
Purge Meters are often used in a bubbler level measuring systems.
A Bypass Meter is a combination of a rotameter with an orifice plate used to measure flow rates through large pipes. How this works is the differential pressure across the main pipe line is matched to the differential pressure across the rotameter at the maximum flow rate.
Flow Module Page 42
The Metering-Cone Meter is a flowmeter consisting of a straight tube and a tapered cone, instead of a tapered tube, with an indicator that moves up and down the cone with changes in flow. The indicator is often spring loaded to allow the meter to be mounted at any angle.
A Shaped-Float and Orifice Meter is a flowmeter consisting of an orifice as part of the float assembly that acts as a guide. Instead of a tapered tube, the float has a shaped profile that provides more open flowing area as the float rises.
The Shaped-Float and Orifice Meter can provide external readouts as indicators or transmitters with or without alarms
EXPLAIN operating principles associated with VARIABLE-AREA KEO 4.21.
FLOWMETERS.
The operating principles of a variable area flowmeters are different from the operating principles of a differential pressure flowmeter. A differential pressure flowmeter maintains a constant flow area and measures the differential pressure. A variable area flowmeter maintains a constant differential pressure and allows the area to change with the flow rate.
Rotameters can only provide correct flow rates for compressible gases and vapors when the flowing conditions are the same as the design conditions. When flowing conditions have
changed, the pressure and temperature correction factors described for orifices are also valid for rotameters.
DESCRIBE how MECHANICAL FLOWMETERS measure flow to include
KEO 4.22.
the following Positive-Displacement Flometers: Nutating Disc, Rotating-Impeller, and Sliding Vane.
A MECHANICAL FLOWMETER is a flowmeter that uses the force of the flowing fluid, usually liquid, to drive the meter. Positive DisplacementFlowmeters include: Nutating Disc, Rotating-Impeller, and Sliding Vane.
Positive Displacement Flowmeters separate the flowing stream into equal-volume segments which are then mechanically counted. The velocity of the flowing material drives the propeller, turbine or paddle wheel and the rotational speed can be measured mechanically or electronically. A Positive Displacement Flowmeter is a mechanical flowmeter that admits fluid into a chamber of known volume and then discharges it. The number of times the chamber is filled during a given interval is counted. These types of meters are commonly used for measuring total flow in homes and factories. The chambers are arranged so that as one if filling, the other is being
Flow Module Page 43 emptied. This action is registered by a counting mechanism. The Total Flow is then determined by reading the counters. The following picture illustrates how fluid flows through typical types of Positive Displacement Flowmeters:
Flow Module Page 44 A Nutating Disc Meter is a positive displacement flowmeter for liquids where the liquid flows through the chambers, causing a disk to rotate and wobble (nutate). This wobble momentarily forms a filled chamber.
The rotation of the disc moves the chamber through the meter body. As the chamber is releasing liquid, another chamber is being formed. A counter indicates the number the chamber has released its volume of fluid. The rotation motion resembles that of a spinning coin just before it stops. This type of meter us usually used as a domestic water meter.
A Rotating Impeller Meter is a positive displacement flowmeter for liquids where the liquid flows into the chambers defined by the shape of the impellers. The impellers rotate, allowing fluid to flow into the chambers. The fluid measurement chambers are created in the space between the lobes and the housing. A counter indicates the number of times the fluid fills and discharges the chambers.
A Sliding Vane Meter is a positive displacement flowmeter for liquids where the fluid fills a chamber formed by sliding vanes mounted on a common hub rotated by the fluid. As the first chamber fills, the hub rotates on a fixed cam, moving the chambers around the meter. One revolution of the hub is equal to four times the chamber volume. A counter mechanism registers each revolution.
NOTE
All Positive Displacement Flowmeters have chambers that alternately fill and empty.
DESCRIBE how TURBINE METERS and PADDLE WHEEL METERS KEO 4.23.
measure flow.
A Turbine Meter is another example of a mechanical flowmeter. It consists of turbine blades mounted on a wheel that measures the velocity of a liquid stream by counting pulses produced by the blades as they pass an electromagnetic pickup.
Flow Module Page 45 The Turbine Wheel is suspended between bearings in a tubular body. The electromagnetic pickup is threaded into the tube wall perpendicular to the wheel as illustrated below:
Figure 5-21 page 191
Flow straighteners are added before and after a turbine wheel to ensure that the velocity of steam is the sole cause of its rotation. Turbine Meters are widely used in blending applications.
A Paddle Wheel Meter is another example of a mechanical flowmeter. It consists of a number of paddles mounted on a shaft fastened in a housing, which can be inserted into a straight section of piping. The housing is inserted so that only half of the paddles are exposed to the liquid velocity. The following picture illustrates the Paddle Wheel Meter functionality below:
Flow Module Page 46 The paddles rotate in proportion to the liquid velocity like an old fashioned water wheel. This
rotation can then be detected by two methods. In one method, magnets are imbedded in the tips of plastic paddles and are sensed by an electromagnetic coil housing. In the other method, has a coil mounted in the housing that creates a magnetic field. The passage of the metal tips disrupts the magnetic field. In both methods, the frequency generated by moving paddles is linearly related to the liquid flow rate.
Paddle Wheel Meters are only used for measuring liquid flows and then only for the less critical applications.
SUMMARY
A differential pressure flowmeter maintains a constant flow area and measures the differential pressure. A variable area flowmeter maintains a constant differential pressure and allows the area to change with the flow rate.
A MECHANICAL FLOWMETER is a flowmeter that uses the force of the flowing fluid, usually liquid, to drive the meter.
Positive Displacement Flowmeters separate the flowing stream into equal-volume segments which are then mechanically counted.
A Positive Displacement Flowmeter is a mechanical flowmeter that admits fluid into a chamber of known volume and then discharges it. The number of times the chamber is filled during a given interval is counted.
A Nutating Disc Meter is a positive displacement flowmeter for liquids where the liquid flows through the chambers, causing a disk to rotate and wobble (nutate). This wobble
momentarily forms a filled chamber.
A Rotating Impeller Meter is a positive displacement flowmeter for liquids where the liquid flows into the chambers defined by the shape of the impellers. The impellers rotate, allowing fluid to flow into the chambers.
A Sliding Vane Meter is a positive displacement flowmeter for liquids where the fluid fills a chamber formed by sliding vanes mounted on a common hub rotated by the fluid. As the first chamber fills, the hub rotates on a fixed cam, moving the chambers around the meter. One revolution of the hub is equal to four times the chamber volume.
All Positive Displacement Flowmeters have chambers that alternately fill and empty.
A Turbine Meter is another example of a mechanical flowmeter. It consists of turbine blades mounted on a wheel that measures the velocity of a liquid stream by counting pulses produced by the blades as they pass an electromagnetic pickup.
A Paddle Wheel Meter is another example of a mechanical flowmeter. It consists of a number of paddles mounted on a shaft fastened in a housing. The paddles rotate in proportion to the liquid velocity like an old fashioned water wheel.
Flow Module Page 47
DESCRIBE how MAGNETIC METERS measure flow.
KEO 4.24.
Electrical Flowmeters are Magnetic Meters, Voretx Shedding Meters, and Ultrasonic Flowmeters.
Magnetic Flowmeters are based on the electrical principle of voltage generation by a conductor moving through a magnetic fluid. Magnetic Meters is commonly called a MAGMETER and is an electromagnetic flowmeter consisting of a stainless steel tube lined with a non-conductive material, with two coils mounted on the tube like a saddle.
Two Electrodes in contact with the electrically conductive fluid but insulated from the metal tubes are located opposite one another and at right angles to the flow and magnetic field as depicted below:
Figure 5-22 page 192
As the conductive fluid passes through the magnetic field created by the coils, a voltage is
induced into and detected by the electrodes. When the magnetic field strength, the position of the electrodes, and the liquid’s conductivity remain constant, the generated voltage is linearly related to the velocity of the liquid stream.
Flow Module Page 48 Magnetic Flowmeters (Magmeter) has no moving parts and now flow restricting components, and they are not adversely affected by complicated piping configurations. They are used in many water supply and waste water facilities and are somewhat immune to internal buildups.
DESCRIBE how MAGNETIC VORTEX SHEDDING METERS measure
KEO 4.25.
flow.
A Vortex Shedding Meter is an electrical flowmeter consisting of a pipe section with
symmetrical vertical bluff body (a partial dam) across the flowing stream. A Vortex Shedding Meter uses the formation of vortices as its principle of operation. A Vortex is a fluid moving in a whirlpool or whirlwind motion. A Vortex Shedding Meter is depicted below:
Figure 5-23 page 192
A common way to describe how a Vortex Shedding Meter works is to look at a flag blowing in the wind. The flag ripples faster when the wind is blowing because of the increase in vortices formed along the flag.
A common Bluff Body shape is a triangular block with the broad surface facing upstream. As the fluid in impeded by the bluff body, a vortex forms on one side of the body. It increases in size until it becomes too large to remain attached to the bluff body and breaks away.
Flow Module Page 49 The information of a vortex on one side of the bluff body alters the flowing stream so that
another vortex is created on the other side of the bluff body and acts similarly. The alternating vortices are formed and travel downstream at a frequency that is linearly proportional to the speed of the flowing fluid and is inversely linearly proportional to the width of the body. The frequency of release of the vortices can be measured using Temperature, Pressure, Ultrasonic, Crystal, or Stain Gauge sensor. The Vortex Shedding Meter has been used successfully to measure the flow of a wide variety of fluids such as Steam, Hot Oil, and Liquefied gases such as Chlorine.
DESCRIBE how ULTRASONIC FLOWMETERS measure flow.
KEO 4.26.
Ultrasonic Flowmeters are electronic flowmeters that uses the principle of sound transmission in liquids to measure flow. They use either the change in frequency or a sound reflected from moving elements or measure the change in the speed of sound in a moving liquid.
One major advantage of Ultrasonic Flowmeters is that nothing protrudes into the flowing liquid. There are two types of Ultrasonic Flowmeters commonly used in industry. They are Doppler Ultrasonic Meters and Transit Time Ultrasonic Meters.
A Doppler Ultrasonic Flowmeter is an electronic flowmeter that transmits an ultrasonic pulse diagonally across the flow stream, which reflects off turbulence, bubbles, or suspended particles and is detected by a receiving crystal.
The frequency of the reflected pulses, when compared to the transmitted pulses, results in a
Doppler Frequency shift that is proportional to the velocity of the flowing stream.
This is the same principle as radar used to measure the speed of vehicles on the highway, but with different frequencies. Knowing the pipe size and velocity is sufficient to determine the volumetric flow rate. The success of this meter is dependent on the presence of particles or bubbles in the flowing liquid. Clear liquids or liquids with high solids entrapped cannot be measured with a Doppler Meter.
Flow Module Page 50 A Doppler Ultrasonic Meter is depicted below:
Figure 5-24 page 193
A Transit Time Ultrasonic Flowmeter is an electronic meter consisting of two sets of transmitting and receiving crystals, one set aimed diagonally upstream and the other aimed diagonally downstream. The liquid velocity slows the upstream signal and increases the received frequency while speeding up the downstream signal and decreasing the received frequency. The difference in the measured frequencies is used to calculate the transit time of the ultrasonic beams and thus the liquid velocity. A Transit Time Ultrasonic Flowmeter is depicted below:
Flow Module Page 51 The flowmeter circuitry is able to convert this information to a flow rate by multiplying the velocity by the pipe area. This measurement method has been applied successfully to very large pipes carrying clean, noncorrosive, bubble free liquids.
SUMMARY
Magnetic Flowmeters are based on the electrical principle of voltage generation by a conductor moving through a magnetic fluid. Magnetic Meters is commonly called a
MAGMETER and is an electromagnetic flowmeter consisting of a stainless steel tube lined with a non-conductive material, with two coils mounted on the tube like a saddle.
Magnetic Flowmeters (Magmeter) has no moving parts and now flow restricting components, and they are not adversely affected by complicated piping configurations.
A Vortex Shedding Meter is an electrical flowmeter consisting of a pipe section with symmetrical vertical bluff body (a partial dam) across the flowing stream.
A Vortex Shedding Meter uses the formation of vortices as its principle of operation. A Vortex is a fluid moving in a whirlpool or whirlwind motion.
Ultrasonic Flowmeters are electronic flowmeters that uses the principle of sound transmission in liquids to measure flow. They use either the change in frequency or a sound reflected from moving elements or measure the change in the speed of sound in a moving liquid.
A Doppler Ultrasonic Flowmeter is an electronic flowmeter that transmits an ultrasonic pulse diagonally across the flow stream, which reflects off turbulence, bubbles, or
suspended particles and is detected by a receiving crystal.
A Transit Time Ultrasonic Flowmeter is an electronic meter consisting of two sets of transmitting and receiving crystals, one set aimed diagonally upstream and the other aimed diagonally downstream.
The liquid velocity in the Transit Time Flowmeter slows the upstream signal and increases the received frequency while speeding up the downstream signal and
decreasing the received frequency. The difference in the measured frequencies is used to calculate the transit time of the ultrasonic beams and thus the liquid velocity.
Flow Module Page 52
DESCRIBE how MASS FLOWMETERS measure flow to include a
KEO 4.27.
CORIOLIS METER and a THERMAL MASS METER.
A MASS FLOWMETER is a flowmeter that measures the actual quality of mass of a flowing fluid. Mass Flow measurement is a better way to determine the quantity of material than volumetric flow measurement. Changes in pressure and temperature can affect density, which then introduces errors into calculations that convert volumetric flow to actual quantity of
material. Two common types of mass flowmeters are the Coriolis Meter and the Thermal Mass Meter.
A Coriolis Meter is a mass flowmeter consisting of specially formed tubing that is oscillated at a right angle to the flowing mass of fluid. Coriolis Force is the force generated by the inertia of fluid particles as the fluid moves toward or away from the axis of oscillation. The following picture illustrates how Mass Flow through a meter causes a phase shift between the inlet and outlet velocity sensors:
Flow Module Page 53 A Coriolis Mass Flowmeter uses the vibrations and twist of a tube to measure flow.
Flow is divided and then passes through two tubes of equal length and shape. The tubes are firmly attached to the meter body (a section of pipe). The two tube sections of tubing are made to oscillate at their natural frequency in opposite directions from each other. The fluid accelerates as it is vibrated and causes the tubing to twist back and forth while the tube oscillates.
Two detectors, one on the inlet and one on the outlet, consist of a magnet and a coil mounted on each tubing section at the points of maximum motion. Each of these detectors develops a sine wave current due to the opposite oscillations of the two sections of tubing as depicted in the picture above.
The sine waves are in phase when there is no flow. When flow is present, the tubes twist in opposite directions, resulting in the sine waves being out of phase. The degree of phase shift varies with the mass flow through the meter. A Coriolis Mass Flowmeter accurately measures the flow of either liquids or gasses and can also measure fluid density.
Thermal Mass Meter
A Thermal Mass Meter is a mass flowmeter consisting of two RTD (Resistance Temperature Detector) probes and a heating element that measure the heat loss to the fluid mass. Thermal Mass Meters are predominantly used for measuring gas flow.
Flow Module Page 54 The two RTD’s are immersed in the flow stream. One probe is in an assembly that includes an adjacent heating element that is measured by the RDT. The other probe is spate and it measures the temperature of the flowing fluid as depicted below:
Figure 5-26 page 195
The heated probe looses heat to the stream by convection. The electrical circuitry is designed to maintain a constant difference in temperature between the two probes by varying the power to the heating element. The power becomes the measured variable of the system and variations in power are proportional to the variations in mass flow. Its circuitry includes corrections for thermal conductivity, viscosity, and density.
