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Transcript of Module 10 Flow Measurement
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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 teachesessential 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 measuringFlow. 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 documenthas 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 forKEO 4.5.
Boyle’s Law, Charles’ Law, Gay- Lussac’s Law, and the Combined Law .
DESCRIBE the concept associated with DIFFERENTIAL PRESSUREKEO 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 measureKEO 4.11.
flow.
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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 DIFFERENTIALKEO 4.13.
PRESSURE FLOWMETERS to include the Bernoull i Equation and the Vena
Contracta Point .
DESCRIBE why locations for DIFFERENTIAL PRESSUREKEO 4.14.
CONNECTIONS of FLOWMETERS vary.
DESCRIBE how DIFFEENTIAL INSTRUMENT LOCATIONS areKEO 4.15.
determined for L iquid, Gas, and Steam flow applications.
DESCRIBE how BLOCKING VALVES AND MANIFOLDS are used forKEO 4.16.measuring differential measurements associated with flow.
DESCRIBE how VARIABLE-AREA FLOWMETERS maintain a constantKEO 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 ORKEO 4.19.
BYPASS METERS.
DESCRIBE how METERING-CONE and SHAPTE-FLOAT & ORIFICEKEO 4.20.
VARIABLE-AREA METERS measure flow.
EXPLAIN operating principles associated with VARIABLE-AREAKEO 4.21.
FLOWMETERS.
DESCRIBE how MECHANICAL FLOWMETERS measure flow to includeKEO 4.22.
the following Posit ive-Displacement Flometers : Nutating Disc, Rotating-
Impeller, and Sliding Vane.
DESCRIBE how TURBINE METERS and PADDLE WHEEL METERSKEO 4.23.
measure flow.
DESCRIBE how MAGNETIC METERS measure flow.KEO 4.24.
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DESCRIBE how MAGNETIC VORTEX SHEDDING METERS measureKEO 4.25.
flow.
DESCRIBE how ULTRASONIC FLOWMETERS measure flow.KEO 4.26.
DESCRIBE how MASS FLOWMETERS measure flow to include aKEO 4.27.
CORIOLIS METER and a THERMAL MASS METER .
DESCRIBE how ACCESSORY FLOW DEVICES function and how they areKEO 4.28.
used.
EXPLAIN how different FLOW SWITCHS function and how they are used toKEO 4.29.
include: DIFFERENTIAL PRESSURE SWITCHES, BLADE SWITCHES,
THEREMAL SWITCHES, and ROTAMETER SWITCHES.
EXPLAIN how OPEN-CHANEL WEIRS and PARSHALL FLUME FLOWKEO 4.30.
MEASUREMTNS function and how they are used.
EXPLAIN how a BELT WEIGHING SYSTEM is used to measure a solidsKEO 4.31.
flow.
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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
Total Flow is the total gallons pumped.
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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 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.
Flow rate is the quantity of fluid passing a point at a particular moment.
Total flow is 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.
Reynolds Number, and Compressibility.
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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 includeconfiguration 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 Propert ies 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/cm
3 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 fl uids and solutions
containing water are called aqueous fl uids .
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
Centi stokes (cS) .
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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.
The following picture illustrates the relationship of Reynolds Number and F low Profiles :
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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 F low Profile that is parabolic in shape with no
mixing between the stream lines. Laminar F low in pipes occurs at Reynolds Numbers below
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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 Fl ow as the flow rate
increases and normally occurs at Reynolds Numbers between 2100 and 4000. Many Flowmeters
require Turbulent Fl ow 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 areessentially 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 F lowing Condition is the pressure and temperature of the gas or vapor at the point of
measurement. A Standard Conditi on 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 methodchosen to measure fluid flow include:
o Pressur e, Velocity, Density, Viscosity, Compressibi l ity, El ectr ical Capacitance
and Conductance, Thermal Conductivity, and the response to Soni c Impul ses,
L ight, or M echanical 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 f lu id is a liquid fluid where there is very little change in pressure.
L iquids are essentially incompressible.
A compressible fl uid is a fluid where the volume and density change when subjected to a
change in pressure.
Gases and Vapor s are examples of compressible fluids.
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DESCRIBE how pressure, temperature, and volume define GAS LAWS forKEO 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:
Figure 5-4 Page 171
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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 (ino
R)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.
Gay-Lussac’s Law
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T2 = Final Temperature (inoR)
T1 = Initial Temperature (inoR)
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
Combined Law
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V = Volume (in ft3 or other volumetric terms)
P = Pressure (in psia or other absolute pressure terms)
T = Temperature (inoR 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 PRESSUREKEO 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 itsdifferential 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. KEO 4.7.
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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 ofthe 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.
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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:
Figure 5-7 page 174
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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:
Figure 5-7 page 174
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DESCRIBE what a LOW-LOSS FLOW TUBE is and how it used to measureKEO 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
Venturi Tube and Low-Loss Flow Tubes are the most efficient Primary Flow Element
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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.
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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 dif ference is created when a fluid passes through a restriction in a pipe.
The point of maximum developed dif ferential pressure is between the upstream of therestriction 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 Or if ice 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.
Ori f ice plates are not always reversible so the stamping information is on the upstream
face of the plate.
Ori f ice plates have the poorest recovery of differential pressure (50%) of any of the
primary flow elements.
A F low 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.
Ventur i tubes are much more expensive than orifice plates, but are more accurate and
recover 90% or more of the differential pressure.
Ventur i Tubes are frequently used to measure large flows of water.
Low-Loss F low Tubes are used for a higher energy efficiency of up to 97%, but are very
expensive. Low-Loss F low 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 Pi tot Tubes : A Standard Pitot Tube and an Averaging Pitot
Tube .
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DESCRIBE OPERATING PRINCIPLES associated with DIFFERENTIALKEO 4.13.
PRESSURE FLOWMETERS to include the Bernoull i 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
Bernoull i Equation states that the sum of the heads of an enclosed flowing fluid is the same atany 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 Bernoull i Equation , the velocity increases and the
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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.
Di ff erential 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 Character istic of the I nstrument 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:
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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.
SUMMARY
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Daniel Bernoul li 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. Bernoull i Equation states that the sum of the heads of an enclosed flowing fluid is the
same at any two locations.
The Vena Contr acta is the point of lowest pressure and the highest velocity downstream
from a primary flow element. According to the Bernoull i Equation , the velocity
increases and the pressure decreases as fluid flows through the restriction.
Di ff erential Pressure Measurements: For turbulent flow, the flow rate is proportional to
the square root of the differential pressure.
F low measur ement 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 PRESSUREKEO 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 thedistance 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.
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NOTE
Pipe Tap Locations are generally specified by the manufacture for Flow
Nozzles and Low-Loss Flow tubes, whereas pipe taps for Venturi Tubes aremanufactured with the tubes purchased.
The following picture illustrates Orifice Plate Tap Locations:
Figure 5-13 page 179
DESCRIBE how DIFFEENTIAL INSTRUMENT LOCATIONS areKEO 4.15.
determined for L iquid, Gas, and Steam flow applications.
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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:
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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:
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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.
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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:
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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 veryeasy, 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 forKEO 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
differential instrument.
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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.
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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 constantKEO 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.
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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.
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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
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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 Var iable-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 thereading should be made on a scale. The exception is a round float and the reading would
be directly in the center. The general r ule 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.
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DESCRIBE how MODIFIED ROTAMETERS are used as PURGE ORKEO 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.
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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 & ORIFICEKEO 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.
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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 ofnitrogen 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.
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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 thefloat 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-AREAKEO 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 includeKEO 4.22.
the following Posit ive-Displacement F lometers : 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 Displacement Flowmeters 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
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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:
Figure 5-20 page 189
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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 METERSKEO 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.
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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:
Figure 5-21 page 191
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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 dif ferential 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 Di splacement F lowmeters separate the flowing stream into equal-volume
segments which are then mechanically counted.
A Positive Di splacement F lowmeter 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 Impell er 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.
Al l Positi ve Displacement F lowmeters have chambers that alternately fill and empty.
A Tur bine 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.
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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.
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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 measureKEO 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.
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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.
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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:
Figure 5-24 page 193
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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 oftransmitting 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.
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DESCRIBE how MASS FLOWMETERS measure flow to include aKEO 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:
Figure 5-25 page 194
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A Coriolis Mass Flowmeter uses the vibrations and twist of a tube to measure flow.
F low 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 .
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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.
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Thermal Mass Meters are less accurate than many other types of flow metering devices,
but can be used to measure some low-pressure gases that are not dense enough for a
Coriolis Flow Meter.
DESCRIBE how ACCESSORY FLOW DEVICES function and how they areKEO 4.28.
used.
ACCESSORY FLOW DEVICES are instruments that do not actually measure flow, but use
flow principles to obtain information. Examples are devices that measure total flow and flow
switches that can be configured to trigger an alarm or a switch. An Accessory Flow Integrator is
depicted below:
Figure 5-27 page 197
An Integrator is a calculating device that totalizes the amount of flow during a specified time
period. Integrators are available for use with pulse output as produced by turbine flowmeters or
analog outputs (linear or square root) from orifice meters equipped with analog to digital
converters. When an Integrator is used for flow calculations, they can be either electronic or
pneumatic integrators to convert a differential pressure measurement to a flow rate.
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The principle requirements for Integrators are: an accurate measurement of the differential
pressure, a conversion to flow rate, a constant time input, and an easy to read counter.
EXPLAIN how different FLOW SWITCHS function and how they are used toKEO 4.29.
include: DIFFERENTIAL PRESSURE SWITCHES, BLADE SWITCHES,
THEREMAL SWITCHES, and ROTAMETER SWITCHES.
FLOW SWITCHS are devices used to monitor flowing stream to provide a discrete electrical or
pneumatic output action at a predetermined flow rate. Flow rate switches are used to generate
alarms or shutdown signals for high or low flows. Flow switch functions are dependent on
measurement principles such as an orifice plate or a differential pressure switch as depicted
below:
Figure 5-28 page 198
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A DIFFERENTIAL PRESSURE SWITCH is a flow switch consisting of a pair of pressure
sensing element and an adjustable spring that can be set at a specific value to operate an output
switch. The differential pressure switch measures the pressure drop across a primary flow
element.
A BLADE SWITCH is a flow switch consisting of a thin, flexible blade inserted into a pipeline.
The fluid flow develops a force which presses against the blade. The motion of the blade is
transferred through a sea and is opposed by an adjustable spring which establishes the trip point.
An electrical or pneumatic switch can sense the blade motion.
A THERMAL SWITCH is a flow switch consisting of a heated temperature sensor. The
flowing fluid carries away heat from the heated temperature sensor. The electronic circuits in the
switch can be set to trip at some predetermined flow rate.
A ROTAMETER SWITCH is a flow switch that consists of a shaped float, a fixed orifice, anda magnetic sensing switch outside the tube to activate a flow circuit at a predetermined flow rate.
SUMMARY
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.
A Coriolis Mass Flowmeter uses the vibrations and twist of a tube to measure flow.
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.
ACCESSORY FLOW DEVICES are instruments that do not actually measure flow, but
use flow principles to obtain information.
An Integrator is a calculating device that totalizes the amount of flow during a specified
time period.
The principle requirements for Integrators are: an accurate measurement of the
differential pressure, a conversion to flow rate, a constant time input, and an easy to read
counter.
FLOW SWITCHS are devices used to monitor flowing stream to provide a discrete
electrical or pneumatic output action at a predetermined flow rate.
A DIFFERENTIAL PRESSURE SWITCH is a flow switch consisting of a pair of
pressure sensing element and an adjustable spring that can be set at a specific value to
operate an output switch.
A BLADE SWITCH is a flow switch consisting of a thin, flexible blade inserted into a
pipeline.
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A THERMAL SWITCH is a flow switch consisting of a heated temperature sensor. The
flowing fluid carries away heat from the heated temperature sensor.
A ROTAMETER SWITCH is a flow switch that consists of a shaped float, a fixed
orifice, and a magnetic sensing switch outside the tube to activate a flow circuit at a
predetermined flow rate
EXPLAIN how OPEN-CHANEL WEIRS and PARSHALL FLUME FLOWKEO 4.30.
MEASUREMTNS function and how they are used.
OPEN-CHANEL WEIRS use a restriction to create a head of liquid. A WEIR is an OPEN-
CHANEL device consisting of a flat plate that has a notch cut into the top edge as depicted
below:
Figure 5-29 page 199
The rate of flow is determined by measuring the height of liquid in the stilling basin upstream ofthe Weir. The crest is the bottom of the Weir. A Weir can be notched as a rectangular,
trapezoidal, or triangular and has a sharp upstream edge (similar to an orifice plate). The Weir is
installed in the outlet of a stilling basin. The flow is related to the height of the water above
the bottom of the Weir Notch measured at a point upstream of the Weir where the water has no
draw-down.
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Weir Height Measurements are made a distance upstream equal to four times the height of
the water above the Crest.
A PARSHALL FLUME is a special form of an open-channel flow element that requires much
less channel elevation than a Weir. A PARSHALL FLUME has a horizontal configuration
similar to a Venturi Tube, with converging inlet walls, a parallel throat, and diverging outlet
walls as depicted below:
Figure 5-29 page 199
The bottom profile is specially designed to generate a hydraulic jump in the throat. Flow can be
calculated from a measurement of the elevation of the inlet water at a specific point.
PARSHALL FLUMES are much less subject to problems from dirt or other fouling factors than
a Weir and have the ability to measure much larger flows.
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EXPLAIN how a BELT WEIGHING SYSTEM is used to measure a solidsKEO 4.31.
flow.
A BELT WEIGHING SYSTEM is used to measure the flow of solids like granular (bulk)
solids. Measuring solids is a different task because of the basic properties solids consist of. Bulk
solids vary greatly in flow properties. Some are sticky and do not flow well, and others are so
fine and slippery that they flow like liquids.
Bulk Solids are usually transported by a belt, screw, or a drag conveyor. The most successful
flow measurement is by the use of a BELT WEIGHING SYSTEM.
A BELT WEIGHING SYSTEM is a solids flow meter consisting of a specially constructed
belt conveyer and a section that is support by electronic weight cells as depicted below:
Figure 5-30 page 200
The conveyer belt is designed to minimize the transfer of the weight of the unmeasured section
of the conveyer. Solids are deposited on the conveyer and carried onto the weighing section.
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The weight of the solids on the measured section divided by the length of the measured
section times the conveyer speed results is a pounds per unit time.
SUMMARY
OPEN-CHANEL WEIRS use a restriction to create a head of liquid.
The liquid head pressure is what is measured in Weirs.
A WEIR is an OPEN-CHANEL device consisting of a flat plate that has a notch cut into
the top edge.
Weir Height Measurements are made a distance upstream equal to four times the
height of the water above the Crest.
A PARSHALL FLUME is a special form of an open-channel flow element that requires
much less channel elevation than a Weir.
A PARSHALL FLUME has a horizontal configuration similar to a Venturi Tube, with
converging inlet walls, a parallel throat, and diverging outlet walls.
PARSHALL FLUMES are much less subject to problems from dirt or other fouling
factors than a Weir and have the ability to measure much larger flows.
A BELT WEIGHING SYSTEM is used to measure the flow of solids like granular
(bulk) solids.
A BELT WEIGHING SYSTEM is a solids flow meter consisting of a specially
constructed belt conveyer and a section that is support by electronic weight cells.
The weight of the solids on the measured section of a Belt Weighing System is divided
by the length of the measured section times the conveyer speed results is a pounds perunit time.
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STEP TWO
Flow Measurement Course
Skill/Performance Objectives
Skill Knowledge Introduction:
Below are the skill knowledge objectives. How these objectives are performed depend on
equipment and laboratory resources available. With each skill objective it is assumed that a set
of standard test equipment and tools be provided.
For example, to be able to perform flow calibration tasks, the following tools and equipment will
be required:
1. A measuring device capable of measuring / indicating the output signal such as
meter or smart calibrator
2. Pressure sources to simulate head pressure generated for differential pressure
device/transmitter
3. An appropriate power supply to power the equipment being calibrated
Skill Terminal Objective (STO)
Given a Flow Measurement Task Checklist, under the direction of anSTO 4.1.
instructor, complete a series of tasks using calibration equipment, Flow
indicating devices, and Flow transmitting devices to demonstrate mastery of
both knowledge and skill objectives associated with the measurement ofFlow.