Table of Content
Objectives................................................................................................................................... 3
Flow
Measurement..................................................................................................................... 4
The
velocity of the fluid.................................................................................................... 4
Pipe
size........................................................................................................................... 4
Friction
due to contact with the pipe................................................................................ 4
The
viscosity of the fluid................................................................................................... 4
The
specific gravity of the fluid......................................................................................... 4
Fluid
Condition................................................................................................................. 4
Velocity
Profiles................................................................................................................ 4
Laminar
or Streamlined........................................................................................... 5
Turbulent................................................................................................................ 5
Transitional............................................................................................................. 5
Flow-straightening
devices............................................................................................... 5
Rate
of flow....................................................................................................................... 6
Total
flow.......................................................................................................................... 6
Flow
meters with wetted non-moving parts............................................................................ 7
Differential
Pressure Flow meters.................................................................................... 7
Orifice
Plates.......................................................................................................... 8
Orifice
plates and holders........................................................................................ 9
Orifice
taps............................................................................................................. 9
Differential
Pressure Measurement....................................................................................... 13
Flow
indicator and recorders.......................................................................................... 13
Relationship
between Differential pressure and flow.................................................... 14
Segmental
and Eccentric orifice plates.................................................................... 16
Integral
Orifice plate............................................................................................... 17
Venturi
tube.................................................................................................................... 18
Pitot
tube........................................................................................................................ 19
Variable
Area Flow meter (Rotameter).......................................................................... 22
Bluff
body or Vortex shedding flow meters.................................................................... 23
Flow
meters with wetted moving parts.................................................................................. 26
Turbine
Flow meter........................................................................................................ 26
Positive
Displacement Flow meters............................................................................... 28
Reciprocating
piston....................................................................................................... 30
Obstruction
less Flow Meters................................................................................................. 32
Magnetic
flow meter....................................................................................................... 32
Mass
Flow Measurement......................................................................................................... 36
Microprocessor-Based
Volumetric Flow meters............................................................ 36
Coriolis
Flow Meters....................................................................................................... 38
Straight
Tube........................................................................................................ 39
Curved
Tube......................................................................................................... 39
Thermal
Mass Flow meters............................................................................................ 42
Flow
meters with sensors mounted externally..................................................................... 43
Ultrasonic
Flow meters................................................................................................... 43
Doppler-Effect
Flow Meter............................................................................................. 44
FLOW
SWITCHES..................................................................................................................... 46
FLOW
GLASSES....................................................................................................................... 46
EURO
CALTECH OPCO HSE Regulation.............................................................................. 47
Refer
to HSE Regulation No. 7 “Isolation”............................................................... 48
Refer
to HSE Regulation No. 7 “Isolation”............................................................... 48
Refer
to HSE Regulation No. 7 “Isolation”............................................................... 49
Review
Questions.................................................................................................................... 51
FLOW MEASUREMENT
Objectives
At completion of this module, the developee
will have an understanding of:
1. Major factors affecting the flow of fluids through the pipes
2. Classification of flow meters
3. Orifice plates construction
4. The purpose of using orifice plates
5. Different types of orifice fitting
6. Fluid profile when passing through an orifice bore
7. Relationship between fluid flow through an orifice and its
differential pressure
8. Venturi tube construction and principle of operation
9. Difference between orifice meter and Venturi tube meter
10. Pitot tubes (Annubars) construction and function.
11. Principle of operation of Rotameter
12. Turbine flow meter parts, function and maintenance
13. Magnetic flow meters construction and principle of operation
14. Principle of operation of Vortex flow meters
15. Positive displacement meters construction and function
16. PDM advantages and disadvantages
17. Ultrasonic flow meters principle of operation
18. Mass flow metering methods and the instruments used
19. Flow switches types and pre-setting procedure
Flow Measurement
Fluid flow measurements in oil and gas
production operations are used as the basis for revenue payment, determining
well allocations, and controlling the process for certain systems. There are
many types of instruments for measuring liquid and/or gas flow. The accuracy of
flow measurement will vary from instrument to instrument and the desired
accuracy will vary from application to application.
Measuring flow is one of the most important
aspects of process control. It is one of the most frequently measured process
variables. Flow tends to be the most difficult variable to measure. No single
flow meter can cover all flow measurement applications.
The physical properties of fluids are
important factor in flow metering accuracy. The major factors affecting the
flow of fluids through pipes are:
The velocity of the fluid
The velocity of a flowing fluid is its
speed in the direction of flow. Fluid velocity depends on the head pressure
that is forcing the fluid through the pipe. Greater the head pressures, faster
the fluid flow rate.
Pipe size
Pipe size also affects the flow rate.
Larger the pipe the greater the potential flow rate.
Friction due to contact with the
pipe
Pipe friction reduces the flow rate through
the pipe. Because of the friction due to the fluid in contact with the pipe,
flow rate of the fluid is slower near walls of the pipe than at then the
centre.
The viscosity of the fluid
The viscosity of a fluid refers to its
physical resistance to flow. Higher the viscosity the fluid, the slower fluid
flow.
The specific gravity of the fluid
Specific gravity of liquid is the density
of the liquid/density of water. The specific gravity of gas is the density of
the gas / the density of air. At any given operating condition, higher the
fluid's specific gravity, lower the fluid's flow rate.
Fluid Condition
The condition of the fluid (clean or dirty)
also limitations in flow measurement. Some measuring devices become
blocked/plugged or eroded if dirty fluids are used.
Velocity Profiles
Velocity profiles have major effect on the
accuracy and performance of most flow meters. The shape of the velocity profile
inside a pipe depends on:
·
The
momentum or internal forces of the fluid, that moves the fluid through the pipe
·
The
viscous forces of the fluid, that tend to slow the fluid as passes near the
pipe walls.
There are three types of flow profile.
·
Laminar or Streamlined
·
Transition
·
Turbulent
·
Laminar or Streamlined
Laminar
or streamlined flow is described as liquid flowing through a pipeline,
divisible into layers moving parallel to each other.
Turbulent
Turbulent flow is the most common type of flow pattern found in pipes.
Turbulent flow is the flow pattern which has a transverse velocity (swirls,
eddy current).
Transitional
Transitional flow profile exists which is
between the laminar and turbulent flow profiles. Its behaviour is difficult to
predict and it may oscillate between the laminar and turbulent flow profiles.
Flow-straightening devices
These devices are used to improve the
flow-pattern from turbulent to transition or even to laminar.
There are three common elements; tubular
element, radial Vane element and aerodynamic straightening vanes.
There are two kinds of flow measurement:
Rate of flow
The rate of flow of a fluid is defined as
the amount of fluid that passes a given point in a set time.
Total flow
The total flow of a fluid can be defined as
the total amount of fluid that passes a given point over an extended period of
time.
Note
Most flow meters measure volumetric flow,
but some types measure mass flow. Volume is related to mass by the density of
the liquid.
Flow meters operate according to many
different principles of measurement although this can by broadly classified
into four areas:
1. Flow meters with wetted
non-moving parts
2. Flow meters with wetted
moving parts
3. Obstruction less Flow
meters
4.
Flow meters with sensors mounted externally
Flow meters can further classified into
four types:
·
Volumetric flow meters that measure volume directly
Positive
displacement meters
·
Velocity
Magnetic,
turbine and ultrasonic
·
Inferential flow meters
Differential
pressure, target, and variable area flow meters
·
Mass flow meters that measure mass directly
Coriolis
Flow meters with wetted non-moving
parts
These devices with no moving parts that
gives them an advantage. However excessive wear plugged impulse lines and
excessively dirty fluids may cause problems.
Differential Pressure Flow meters
Differential pressure type flow meters
provide the best results where the flow conditions are turbulent. Some of the
most common types of differential pressure flow meters are:
·
Orifice
plate
·
Venture
tube
·
Elbow
·
Pitot
tube
Advantages
·
They are simple to use
·
They have no moving parts
·
They are sturdy
·
They are available in a wide
selection of ranges models
Disadvantages
·
They tend to have low accuracy
due to the wear of the primary element.
·
Some have a high permanent
pressure loss.
Orifice Plates
Orifice plates in various forms are the
most widely used primary elements and consist of a flat piece of metal with a
sized hole bored in to it.
When
fluid through the orifice its velocity increases, resulting a drop in pressure
and an increase in turbulence. After the fluid has passed through the orifice
its velocity decreases again, causing an increase in pressure although only
some of the pressure loss is recovered. The amount of pressure recovery can be
up to 50% of the total pressure drop across the orifice plate.
The flow of liquid through the orifice
plate creates a differential pressure across it, in such a way that the faster
the flow the larger the pressure drop.
The components of a typical orifice plate
installation are:
·
Orifice plate and holder
·
Orifice taps
·
Differential pressure
transmitter
·
Flow indicator / recorder
Orifice plates and holders
Orifice plates are usually installed
between special flanges in a horizontal pipe run. The flanges are thicker than
normal to accommodate two small bore tapings for connection to a DP cell. The
plate is positioned concentrically within the flange bolt circle with the tap
protruding near the top of the flanges. The tab has a hole in to indicate that
it is an orifice plate and not a pipe blank. Orifice plates have information
engraved on them to indicate the correct upstream / downstream orientation.
Orifice taps
There are 4 common arrangements of pressure
taps:
·
Flange taps
Flange taps are
the most popular because their distance from the orifice plate is precisely
controlled.
·
Vena Contracta
Vena contracta
taps are located to obtain the maximum differential pressure across the
orifice.
·
Corner taps
Corner taps are
located at each side of the orifice plate and are good for pressure
measurements in pipes less then 50 mm diameter.
·
Pipe taps
Pipe taps
measure the permanent loss of pressure across an orifice.
Differential Pressure Measurement
This type of measurement uses a
differential pressure transmitter to send a signal to an indicator, recorder or
controller.
Flow indicator and recorders
When a DP (differential pressure) cell is
used to transmit a flow measurement the output of the transmitter is not
linear. To solve this problem some form of signal conditioning is needed to
condition the signal for use with a linear scaled indicator.
Relationship between Differential
pressure and flow
When the differential pressure is obtained
experimentally and plotted against flow, the resulting graph is a square
function.
If the square root of differential pressure
is plotted against flow, a straight line is obtained showing that the rate of
flow is in direct proportion to the square root of differential pressure.
Therefore, in many flow measurement installations a Square Root Extractor is
fitted to the output of a differential pressure transmitter.
Most of the modern electronic transmitters
have the option of integral square root function. Modern control systems using
the DCS, contains the square root function within their computation modules A
simple alternative to this is to use a square root scale on the local indicator
(in the conventional systems).
Orifice plates typically have a drain hole
located at the bottom for steam and gas applications and a vent hole at the top
for liquid applications.
Advantages
·
They are easy to install.
·
One differential pressure
transmitter applies for any pipe size.
·
Many DP sensing materials are
available to meet process requirements. Type 316 stainless steel is the most
common material used in orifice plates unless material of higher quality is
required by the process conditions.
·
Orifice plates have no moving
parts and have been researched extensively; therefore, application data well
documented (compared to other primary differential pressure elements).
Disadvantages
·
The process fluid is in the
impulse lines to the differential transmitter may freeze or block (plug).
·
Their accuracy is affected by
changes in density, viscosity, and temperature.
·
They require frequent
calibration
Segmental and Eccentric orifice
plates
The eccentric orifice plate is typically
used for dirty liquids, gases, liquids containing vapour (bore above pipeline
flow axis) or vapour containing liquid (bore below pipeline flow axis).
The segmental orifice plate is the same as
the square edged orifice plate except that the hole is bored tangentially to a
concentric circle with a diameter equal to 98% that of the pipe inside
diameter. They are used for dirty fluids, in preference to eccentric bore
plates, because it allows more drainage around the circumference of the pipe.
During installation, care must be taken
that no portion of the gasket or flange covers the hole.
Advantages
·
This type of orifice plate is
less subject to wears than the square edged orifice plate, however it is good
for low flows only.
·
For slurry applications where
differential pressure devices are required, segmental orifice plates provide
satisfactory measurements.
Integral Orifice plate
The integral orifice plate is identical to
a square edged orifice plate installation except that the plate, flanges, and
differential pressure transmitter are supplied as one unit.
Advantages
It is used for small lines (typically under
2 inches) and is relatively inexpensive to install since it is part of the
transmitter.
Venturi tube
Venturi tube consists of a section of pipe
with a conical entrance, a short straight throat, and a conical outlet. The
velocity increases and the pressure drops at the throat. The differential
pressure is measured between the inlet (upstream of the conical entrance) and
the throat.
Venture Tube
Advantage
·
It can handle low-pressure
applications
·
It can measure 25 to 50% more
flow than a comparable orifice plate
·
It is less susceptible to wear
and corrosion compared to orifice plates
·
It is suitable for measurement
in very large water pipes and very large air/Gas ducts.
·
Provides better performance
then the orifice plate when there are solids in Suspension.
Disadvantage
·
It is the most expensive among
the differential pressure meters
·
It is big and heavy for large
sizes
·
Its has considerable length
Pitot tube
Pitot tube consists of two parts that
senses two pressures:
·
The impact pressure (dynamic)
·
The static pressure
The impact pressure is sensed with either
one-impact tube bent towards the flow. Sometimes four or more pressure taps
(averaging type) are used.
The non-averaging type is extremely
sensitive to abnormal velocity distribution profiles (because it does not
sample the full stream) hence the advantage of the averaging.
Advantage
·
Pitot tubes are easy and quick
to install, especially in existing facilities.
·
They can be inserted and
removed from the process without shutting down.
·
They are simple in design and
construction
·
They produce energy savings
when compared to equivalent orifice (low-permanent pressure loss)
·
They are suitable for
measurement in large water pipes and large air/gas ducts
Disadvantages
·
Their low differential pressure
for a given flow rate
·
They tend to block/plug in the
process lines, unless provision is made for purging or flushing
Typical Installation of single and multiple taps Pitot
Tubes
Variable Area Flow meter
(Rotameter)
The variable area flow meter or Rotameter
is the simple, low cost, direct reading indicator for measuring flow of liquids
or gases It is used on clean, low viscosity fluids, such as light hydrocarbons.
In a
Rotameter, a moving body called the float represents a restriction in the line.
Since the float moves freely within the tube, the pressure drop across the
float remains constant as the flow rate changes.
The tube is designed so that the area of
the annulus is proportional to the height of the float in the tube. The scale
can be very nearly linear over the range.
Conventional Rotameters permit flow
measurement as low 0.1 cm3/ min of water or an equivalent gas flow.
For measuring very small flows, down to
0.05 cm3 / min, variable area meters are available with
glass tube having non-circular cross-sections. Metal tube Rotameters are used
for measuring low flows of liquids or gases of high temperatures and pressure.
These instruments can be used to determine
liquid flows as low as 10cm3 /
min. at 500° F and pressure above 300 psi.
Typical
Rotameter
Typical Rotameter floats
Bluff body or Vortex shedding flow
meters
This type flow meter is suitable for
measuring liquid flows at high velocity.
Its
output is linear and maintains accuracy when fluid velocity, temperature or
pressures varies. The vortex-producing meter consists of a smooth bore pipe
across which an obstruction called a bluff body fitted to cause turbulence in
the flow stream. Vortices are produced from alternate edges of the bluff body
at a frequency proportional to the volumetric flow rate without the use of any
moving parts.
This makes this type of flow meter
inherently more reliable than a flow meter with moving parts.
Advantages
·
Since the vortex meter has no
moving parts it can be installed vertically, horizontally, or in any position,
although, when used in a liquid line the pipe must be kept full to avoid gas
bubbles.
·
It does not suffer from zero
drift and requires minimal maintenance.
·
It is suitable for many types
of fluids, has excellent price for performance ratio.
·
Its frequency output is
linearly proportional to the to volumetric flow.
Disadvantages
·
Unfortunately the meter’s bluff
body obstructs the centre of the pipe, and if it wears it may cause a
calibration shift. The meter should not be used where the fluid viscosity may
vary significantly.
Flow meters with wetted moving
parts
Performance of these types of flow meters
depends on the precision machining of its moving parts. These moving parts are
subject to mechanical wear and therefore are best suited to clean fluids only.
Turbine Flow meter
In a turbine flow meter a rotor with a
diameter almost equal to the pipe internal diameter is supported by two
bearings to allow free rotation. A magnetic pickup, mounted on the pipe detects
the passing of the rotor blades generating a frequency output. Each pulse
represents the passage of a calibrated amount of fluid. The angular velocity
(i.e. the speed of rotation) is proportional to the volumetric rate of flow.
There is a minimum flow below which accuracy cannot be guaranteed due to liquid
slippage. When the flow ceases, the liquid itself provides sufficient damping
to stop the rotor rotating.
Advantages
·
The turbine meter is easy to
install and maintain. They:
·
Are bi-directional
·
Have fast response
·
Are compact and light weights
The device is not sensitive changes in
fluid density (but at very low) specific gravity's, range ability may be
affected), and it can have a pulse output signal to directly operate digital
meters.
Disadvantages
·
They generally are not
available for steam measurement (since condensate does not lubricate well.
·
They are sensitive to dirt and
cannot be used for highly viscous fluids or for fluids with varying.
·
Flashing or slugs of vapour or
gas in the liquid produce blade wear and excessive bearing friction that can
result in poor performance and possible turbine damage.
·
They are sensitive to the
velocity profile to the presence of swirls at the inlet; therefore, they
require a uniform velocity profile (i.e. straight upstream run and pipe
straightness may have to be used).
·
Air and gas entrained in the
liquid affect turbine meters (in amounts exceeding 2% by volume: therefore, the
pipe must be full).
·
Strainers may be required
upstream to minimise particle contamination of the bearings (unless special
bearings are used), finely divided solid particles generally pass through the
meter without causing damage.
·
Turbine meters have moving
parts that are sensitive to wear and can be damaged by over speeding. To
prevent sudden hydraulic impact, the flow should increase gradually into the
line.
·
When installed, bypass piping
may be required for maintenance. The transmission cable must be well protected
to avoid the effect of electrical noise. On flanged meters, gaskets must not
protrude into the flow stream.
Positive Displacement Flow meters
Principle of measurement
The positive displacement meter separates
the incoming fluid into a series of known discrete volumes then totalises the
number of volumes in a known length of time. The common types of
positive-displacement flow meters include:
·
Rotary piston
·
Rotary vane
·
Reciprocating piston
·
Nutating disk
·
Oval gear
This figure is a sectional schematic of
Oval gear flowmeter, showing how a crescent-shaped gap captures the precise
volume of liquid and carries it from inlet to outlet.
This figure shows the sliding-vane rotary
meter, vanes are moved radially as cam followers to form the measuring chamber.
This
figure shows another version, the retracting-vane type
Positive displacement meters are selected
mainly according to the type of fluid and the rate of flow to be measured and
are normally used for clean liquids where turbines cannot be used. When
installed, the following should be avoided to prevent damage to the meter:
·
Over speeding
·
Back flow
·
Steam or high-pressure cleaning
Advantages
·
Positive Displacement Meter
(PDM) has many advantages.
·
Simple versions require no
electrical power.
·
They are unaffected by upstream
pipe conditions
·
Direct local readout in
volumetric units is available.
·
The highly engineered versions
are very accurate.
·
The low cost mass produced
versions are commonly used as domestic water meters.
Disadvantages
·
They have many moving parts
·
Clearances are small (and dirt
in the fluid is destructive to the meter).
·
Depending on the application,
seals may have to be replaced regularly since they are subject to mechanical
wear, corrosion, and abrasion.
·
Periodic calibration and
maintenance are required
·
They are they are sensitive to
dirt (and may require upstream filters).
·
PD meters are large in size
(and thus heavy and expensive).
·
They cannot be used for reverse
flow or for steam (since condensate does not lubricate well).
·
Viscosity variations have a
detrimental effect on performance.
·
These meters have a high
maintenance cost
Mechanical failure the meter can block the
flow in the line.
Reciprocating piston
Fluid enters the meter, fills a compartment
of fixed size and then continues on its way to the pipe work system. The number
of compartments filled are counted and registered by means of a gear train and
pointers operating over dials or by a cyclo-meter dial.
The system is very accurate, provided that
the compartment size does not change and that there is on leakage. Fouling of
the mechanism may slow down the meter operation limiting the throughput but the
accuracy remains unaffected.
Obstruction less Flow Meters
These meters allow the fluid to pass
through undisturbed and thus maintain their performance while handling dirty
and abrasive fluids.
Magnetic flow meter
The magnetic flow meter is a volumetric
device used for electrically conductive liquids and slurries.
The magnetic flow meter design is based on
Faraday’s law of magnetic induction, which states that:
"The voltage induced across a
conductor as it moves at right angles through a magnetic field proportional to
the velocity of that conductor." That is, if a wire is moving
perpendicular to its length through a magnetic field, it will generate an
electrical potential between its two ends.
Based on this principle, the magnetic flow
meter generates a magnetic field perpendicular to the flow stream and measures
the voltage produced the fluid passing through the meter. A set of electrodes
detects the voltage.
The voltage produced is proportional to the
average velocity of the volumetric flow rate of the conductive fluid.
The tube is constructed of non-magnetic
material (to allow magnetic field penetration) and is lined with a suitable
material to prevent short-circuiting of the generated voltage between the
electrodes. The tube is used to support the coils and transmitter assembly.
Generally the electrodes are of stainless
steel but other materials are also available. These electrodes have to be
chosen with care to avoid corrosion.
Dirty liquids may foul the electrodes, and
cleaning methods such as ultrasonic may be required.
Theoretically, it can measure flow down to
zero, but in reality its operating velocity should less than 3 ft / s (1 m/s).
A velocity of 6 to 9 ft/s (2 to 3 m/s) is preferred to minimise coating.
It should be noted that at velocities
greater than 15 ft/s (5 m/s) accelerated liner wear could result.
This meter has no moving parts; and is
unaffected by changes in
·
Fluid
·
Viscosity
·
Pressure
Advantages
·
Are bi-directional
·
Have no flow obstruction
·
Are easy to re-span
·
Are available with DC or AC
power
·
It can measure pulsating and
corrosive flow.
·
It can measure multiphase;
however, all components should be moving at the same speed; the meter can
measure the speed of the most conductive component.
·
It can install vertically or
horizontally (the line must be full, however) and can be used with fluids with
conductivity greater than 200 umhos/cm.
·
Changes in conductivity value
do not, affect the instrument performance.
Disadvantages
·
It's above average cost
·
It's large size
·
Its need for a minimum
electrical conductivity of 5 to 20 umhos / cm
·
Its accuracy is affected by
slurries containing magnetic solids (some meters can be provided with
compensated output in this case).
·
Electrical coating may cause
calibration shifts
·
The line must be full and have
no air bubbles (air and gas bubbles entrained in the liquid will be metered as
liquid, causing a measurement error).
·
Vacuum beakers may require in
some applications to prevent the collapse of the liner under certain process
conditions
·
In some applications,
appropriate mechanical protection for the electrodes must be provided.
DC types are unaffected by variations fluid
conductivity and thus are generally preferred. However, AC types are used for.
·
Pulsating flow applications
·
Flow with large amounts of
entrained air
·
Applications with spurious
signals that may be generated from small
Electro-Chemical reactions
·
Slurries with non-uniform
particle size (they may clamp together)
·
Slurries with solids not g well
mixed into the liquid.
·
Quick response.
Mass Flow Measurement
Traditionally fluid flow measurement has
been made in terms of the volume of the moving fluid even though the meter user
may be more interested in the weight (mass) of the fluid. Volumetric flow
meters also are subject to ambient and process changes, such as density, which
changes with temperature and pressure. Viscosity changes also may affect
volumetric flow sensors.
Thus for a number of years there has been
much interest in finding ways to measure mass directly rather than to use
calculating means to convert volume to mass. As of the early 1990s, there are
three ways to determine mass flow:
1. The application of microprocessor
technology to conventional volumetric meters.
2. Use of Coriolis flow meters, which
measure mass flow directly.
3. The use of thermal mass flow meters that
infer mass flow by way of measuring heat dissipation between two points in the
pipeline.
Microprocessor-Based Volumetric
Flow meters
As shown in the figure below, with
microprocessors it is relatively simple to compensate a volumetric flow meter
for temperature and pressure. With reliable composition (density) information,
this factor also can be entered into a microprocessor to obtain mass flow
readout. However, when density changes may occur with some frequency, and
particularly where the flowing fluid is of high monetary value (for example, in
custody transfer), precise density compensation (to achieve mass) can be
expensive.
For example, a gas mass flow meter system
may consist of a vortex gas velocity meter combined with a gas densito-meter.
The densito-meter can be located upstream of the flow device and produce a
pressure difference that is linearly proportional to the density of the flowing
gas at line conditions. This unit will automatically correct for variations in
pressure, temperature, specific gravity, and super-compressibility.
The gas sample from the pipeline passes
across a constant-speed centrifugal blower and returns to the pipeline. The
pressure rise across the blower varies directly with the gas density. A
differential-pressure signal from the densito-meter is combined with a
flow-rate signal from the gas meter. The cost of such instrumentation can be
several times more than an uncompensated meter.
The relatively high cost of this
instrumentation, combined with an increasing need for reliable mass-flow data,
established the opportunity for direct mass-flow instruments of the Coriolis
and thermal types.
Pressure-compensated meter wherein the
differential pressure is measured by an appropriate sensor and the signal is
fed into a combining module, along with a signal representing the pressure
correction. The output from the combining module is used for display and to
regulate the meter integrator.
Temperature-compensated
meter wherein the differential pressure is measured by an appropriate sensor
and the signal is fed into a combining module, along with a signal representing
the temperature correction. The output from the combining module is used for
display and to regulate the meter integrator.
Flow measurement where the flow is
compensated for any change in the operating temperature and pressure.
Coriolis Flow Meters
The complete Coriolis
unit consists of (1) a Coriolis force sensor and (2) an electronic transmitter.
The sensor comprises a tube (or tubes) assembly, which is installed in the
process pipeline. As shown in the below figure, an U-shaped sensor tube is vibrated
at its natural frequency. The angular velocity of the vibrating tube, in
combination with the mass velocity of the flowing fluid, causes the tube to
twist. The amount of twist is measured with magnetic position detectors,
producing a signal, which is linearly proportional to the mass flow rate of
every parcel and particle passing through the sensor tube.
Typical Coriolis Meter
The output is
essentially unaffected by variations in fluid
properties, such as viscosity, pressure, temperature,
pulsation, entrained gases, and suspended solids.
The detectors are not
in contact with the flowing fluid, except the fluid at the inside wall of the
tube. The tube is usually made of stainless steel. In some other application it
is made of corrosion and erosion resistant material. Two magnetic position
detectors, one on each side of the U-shaped tube, generate signals that are
routed to the associated electronics for processing into an output.
There are two common tube types:
·
Straight
·
Curved
Straight Tube
The straight tube is used mainly for
multiphase and for fluids that can coat or clog (since the straight type can be
easily cleaned). In addition, the straight tube requires less room, that can be
drained, has a low-pressure loss. Straight tube reduces the probability of air
and gas entrapment, which would affect meter performance. However, the straight
tube must be perfectly aligned with the pipe.
Curved Tube
Compared to the straight tube, the curved
tube has a wider operating range measures low flow more accurately, is
available in larger sizes, tends to be lower in cost (due to low cost of
materials), and has a higher operating temperature range. However it is more
sensitive to plant vibrations than the straight type.
Advantages
·
It measures mass flow directly.
·
One device that measures flow
and density. Some Coriolis meter also measures temperature.
·
It can handle difficult
applications.
·
It is applicable most fluids
that has no Reynolds number limitation.
·
It is not affected by minor
changes in specific gravity or by viscosity.
·
This type of device requires
low maintenance.
·
It is not sensitive to velocity
profiles
·
It can be used bi-directional
·
It can handle abrasive fluids
Disadvantage
·
Its purchase cost is high
·
Inaccurate measurement when air
and gas pockets in the liquid and by slug flow.
·
The pipe must be full and must
remain full to avoid trapping air gases inside the tube.
·
A high-pressure loss is
generated due to the small tube diameters
·
It needs re-calibration if the
density of the liquid being measured is very different from the one for which
calibration was performed.
·
Coating of the tube affects the
density measurement (since it will affect the measured frequency), but not the
flow measurement (since the degree of tube twist is independent of tube
coating).
Thermal Mass Flow meters
Like the Coriolis flow
meter, after many years of design work and limited applications, the thermal
mass flow meter did not become widely accepted until the late 1970s and early
1980s. In Thermal Mass Flow Meter's thermodynamic operating principle is applied.
As shown in the below
figure, a precision power supply directs heat to the midpoint of a sensor tube
that carries a constant percentage of the flow. On the same tube at equidistant
two temperature elements (RTD) are installed upstream and downstream of the
heat input. With no flow, the heat reaching each temperature element (RTD) is
equal. With increasing flow the flow stream carries heat away from the upstream
element T1 and an increasing amount toward the downstream element T2. An
increasing temperature difference develops between the two elements.
This temperature difference detected by the temperature
elements is proportional to the amount of gas flowing, or the mass flow rate.
A bridge circuit
interprets the temperature difference and an amplifier provides the 0- to
5-volt dc and 4- to 20-mA output signal.
Typical Thermal Mass Flow meter construction
Flow meters with sensors mounted
externally
These offer no obstruction to the fluid and
have no wetted parts. They cannot be used in all applications due to their
inherent limitations.
Ultrasonic Flow meters
Transit
Time, Time-of-Travel, Time-of-flight
In an ultrasonic (transit time) flow meter
two transducers are mounted diametrically opposite, one upstream of the other.
Each transducer sends an ultrasonic beam at approximately
1 MHz generated by a piezoelectric crystal. The difference in transit
time between the two beams is used to determine the average liquid velocity.
The beam that travels in the direction of the flow travels faster then the
opposite one.
Ultrasonic Flow Meter
This figure shows the principle of
transit-time ultrasonic flow meter, clamp-on type. Transducers alternately
transmit and receive bursts of ultrasonic energy.
Each transducer acts as a transmitter and
receiver. Two transducers are used to cancel the effect of temperature and
density changes on the fluid sound transmission properties. The speed of sound
is not a factor since the meter looks at differential values.
The crystals producing the ultrasonic beam
can be in contact with the fluid or mounted outside the piping (clamp-on
transducers).
Advantages
·
It does not cause any flow
obstruction
·
It can be installed
bi-directional
·
It is unaffected by changes in
the process temperature
·
It is suitable to handle
corrosive fluids and pulsating flows.
·
It can be installed by clamping
on the pipe and is generally suited for measurements in very large water pipes.
Disadvantages
·
This
type of meters are highly dependent on the Reynolds number (the velocity
profile)
·
It
requires nonporous pipe
material (cast iron, cement and fibreglass should be avoided)
·
It requires periodic
re-calibration
·
It is generally used where
other metering methods are not practical or applicable.
Doppler-Effect Flow Meter
The configuration shown
utilises separated dual transducers mounted on opposite sides of the pipe.
It is mandatory in a Doppler-Effect Flow Meter the flowing
stream contains sonically reflective materials, such as solid particles or
entrained air bubbles. Without these reflectors, the Doppler system will not
operate. In contrast, the transit-time ultrasonic flow meter does not depend on
the presence of reflectors.
Doppler-effect flow
meters use a transmitter that projects a continuous ultrasonic beam at about
0.5 MHz through the pipe wall into the flowing stream. Particles in the stream
reflect the ultrasonic radiation, which is detected by the receiver. The frequency
reaching the receiver is shifted in proportion to the stream velocity. The
frequency difference is a measure of the flow rate. When the measured fluid
contains a large concentration of particles or air bubbles, it is said to be
sonically opaque. More opaque the liquid, greater the number of reflections
that originate near the pipe wall, a situation exemplified by heavy slurries.
It may be noted from the flow profile, that the fluid velocity is greatest near
the centre of the pipe and lowest near the pipe wall.
The Doppler Flow meter works satisfactorily
for only some applications and is generally used when other metering methods
are not practical or applicable. It should not be treated as a “universal“
portable meter.
Doppler-effect Ultrasonic Flow Meter
This figure shows the principle of
Doppler-effect ultrasonic flow meter with separated opposite-side dual
transducers.
Advantage
·
The common clamps-on versions
are easily installed without process shutdown.
·
It can be installed
bi-directional
·
Flow measurement is not
affected due to change in the viscosity of the process.
·
Generally suitable for
measurements in large water pipes
·
The meter produces no flow
obstruction
·
Its cost is independent of line
size.
Disadvantage
·
The sensor may detect some
sound energy travelling in the causing interference reading errors.
·
Its accuracy depends on the
difference in velocity between the particles, the fluid, the particle size,
concentration, and distribution.
·
The instrument requires
periodic re-calibration.
FLOW SWITCHES
Flow switches are devices, which indicate
either the presence or the absence of flow. Any of the primary flow elements
can have associated switches that may be a part of the controller circuitry or
operated from the pneumatic or electronic output signal.
Dedicated flow switches are available which operate by a paddle or vane inserted into the flow. When flow is present the
paddle or vane is moved and a switch mechanically tripped.
Thermal flow
switches use a heater and a heat sensor. When flow is present the heat sensor
is cooled by the flow and the switch activated a thermal flow switch.
Flow Switch
FLOW GLASSES
Flow
glasses are windows in the pipe, which allow the fluid to be directly observed.
Usually a fitting is provided with a glass on
either side of the pipe so that one can see the flow of the liquid. A paddle
wheel, float or other device is often used so
that movement in the fluid is more readily
observed.