3.Flow.ppt

<Document Number>
Copyright © Yokogawa Electric Corporation
<date/time>
Flow Measuring Techniques
Yokowaga India Ltd.

<Document Number>
<date/time> 2
Proprietary info goes here…
The Various methods for flow measurement
1] Measuring fluid flow is one of the most important aspects of
process control.
2] In fact, it may well be the most frequently measured process variable.
3] Flow is generally measured inferentially by measuring flow velocity
through a known area.
Q = A * V
Where Q = Volume flow rate,
A = cross-sectional area of the pipe,
V = fluid velocity.

<Document Number>
<date/time> 3
The major factors affecting the flow of fluids through pipes
are:
Velocity of the fluid.
Friction of the fluid in contact with the pipe.
Viscosity of the fluid.
Density of the fluid.
The Factors Affecting Flow Rates In Pipes

<Document Number>
<date/time> 4
Velocity of the fluid.
Fluid velocity depends on the head pressure which is forcing the
fluid through the pipe. The greater the head pressure, the faster the fluid
flow rate ( all other factors remaining constant), and consequently, the
greater the volume of flow. Pipe size also affects the flow rate. For
example, doubling the diameter of a pipe increases the potential flow
rate by a factor of four times.
Pipe Friction :
Pipe friction reduces the flow rate of fluids through pipes and is,
therefore, considered a negative factor. Because of the friction of a fluid
in contact with a pipe, the flow rate of the fluid is slower near the walls
of the pipe than at the center. The smoother, cleaner, and larger a pipe is,
the less effect pipe friction has on the overall fluid flow rate.

<Document Number>
<date/time> 5
Viscosity(m) :
Viscosity (m) or the molecular friction within a fluid, negatively affects
the flow rate of fluids. Viscosity and pipe friction decrease the flow rate
of a fluid near the walls of a pipe. Viscosity increases or decreases with
changing temperature, Generally, the higher a fluid’s viscosity, the lower
the fluid flow rate (other factors remaining constant). Viscosity is
measured in units of centipoise.
Density :
Density of a fluid affects flow rates in that a more dense fluid
requires more head pressure to maintain a desired flow rate. Also, the
fact that gases are compressible, whereas liquids essentially are not,
often requires that different methods be used for measuring the flow
rates of liquids, gases, or liquids with gases in them.

<Document Number>
<date/time> 6
REYNOLDS NUMBER
Fluid
Velocity V
D
REYNOLDS NUMBER :
Reynolds number is defined as the relationship between fluid
velocity, viscosity and density and illustrate the state of flow.
Re = D * v *r / m
D : Diameter of pipe (m)
v : Velocity (m/sec)
r : Density (kg/m3)
m: Viscosity

<Document Number>
<date/time> 7
Laminar vs. Turbulent

<Document Number>
<date/time> 8
Flow Measurement Basics - Bernoulli’s Theorem
“In a flowing Stream the sum of Pressure head, the velocity Head and
elevation head at one point is equal to their sum at another point moved in
the direction of the flow from the first Point plus the loss due to friction
between the two points.”
Bernoulli’s Theorem

<Document Number>
<date/time>
Head Type Flow Meter
Head meters are generally simple, reliable, and offer more
flexibility than other flow measurement methods. The head-type
flowmeter almost always consists of two components:
1] Primary Device :The primary devices is placed in the pipe to
restrict the flow and develop a differential pressure. The primary
device can be selected for compatibility with the specific fluid or
application
2] Secondary Device :The secondary device measures the
differential pressure and provides a readout or signal for
transmission to a control measuring device is not required in the
field. The secondary device can be selected for the type or readout
of signal transmission desired.

<Document Number>
<date/time>
Differential Pressure head type
– This is most widely used method of industrial flow
measurement .
– A restriction is introduced into the pipe
– The resulting pressure decrease is proportional to
flow rate in accordance with the formula
)
(
)
(
)
(
ity
fluiddesns
d
alhead
differenti
h
K
flow
Q 
Where K is constant

<Document Number>
<date/time>
Advantages
1] Differential Pressure meter has advantage that they are
most familiar of any meter type.
2] Suitable for gas, viscous and corrosive liquids.
3] They have no moving parts.
4] There are no significant pipe size or flow rate limitation

<Document Number>
<date/time>
The common types to head primary elements include
1] Venturi
2] Orifice plates
3] Flow nozzles, and
4] Pitot tubes.
These elements all work on the theory (Bernoulli's)
That the total energy at any point in a pipeline or conduit
is equal to the total energy at any other point, if friction
losses between the two points are ignored.
Head Type Primary Element

<Document Number>
<date/time>
7
1] The Venturi tube produces a relatively large differential with a
relatively small head loss. This element is often used where the process
contains large amounts of suspended solids or if large head losses are
unacceptable.
2] Orifice plates are widely used in industrial applications. They
are effectively utilized for "clean" fluid flow measurement and where line
pressure losses or pumping costs are not critical.

<Document Number>
<date/time>
7
3] A flow nozzle is in a sense an orifice with a flared approach section.
Line pressure loss is between that of an orifice and a Venturi, as is
generally the cost. Often flow nozzles are used at the end of a pipe,
discharging directly into the air, a tank, etc.
4] Pitot tubes are used when fluid velocity is of prime concern. Very small
pressure losses are incurred, and they are relatively inexpensive, but they
are very susceptible to plugging with processes containing solids.

<Document Number>
<date/time> 16
7
Orifice Plates

<Document Number>
<date/time> 17
Orifice Plates
1] A concentric plate is the simplest and least expensive of the head
meters.
2] Acting as a primary device, the orifice plate constricts the flow of a fluid
to produce a differential pressure across the plate.
3] The result is a high pressure at upstream side and a low pressure at
downstream side the difference of this is proportional to the square of the
flow.
4] An orifice plate usually produces a greater overall pressure loss than
other primary devices.

<Document Number>
<date/time> 18
Orifice Plates
One advantage is that even when the orifice plate is badly worn,
damaged, it will provide a reasonably repeatable output.
The standard orifice plate itself is a circular plate; usually a stainless
steel from 1/8 to ½ inch thick depending upon size and flow velocity.

<Document Number>
<date/time> 19
Orifice Plates
Concentric Orifice Plate :
It is most widely used. It is usually made of stainless steel and its
thickness varies from 3.175 to 12.70mm (1/8 to 1 /2 inch.) depending
on pipe line size and flow velocity. It has a circular hole (orifice) in the
middle, and is installed in the pipe line with the hole concentric to the
pipe.
Eccentric Orifice Plate :
It is similar to the concentric plate except for the offset. It is useful for
measuring fluids containing solids, oils containing water and wet
steam. The eccentric orifice plate is used where liquid fluid contains a
relatively high percentage of dissolved gases.
Segmental Orifice Plate :
This orifice plate is used for the same type of services as the eccentric
orifice plate. It has a hole which is a segment of a circle.

<Document Number>
<date/time> 20
Orifice Plates

<Document Number>
<date/time> 21
Orifice Plates
Vena contracta
The main flow stream takes the shape of the venturi tube with the
narrowest path slightly downstream from the plate. This point is called
the “vena contracta”. ”. At this point, the pressure is at its minimum.
From this point on, the fluid again begins to fill the pipe and the pressure
rises. The pressure, however, does not recover completely. There is a
loss of pressure across the plate.
Beta ratio
The principal consideration in selecting an orifice plate is the ratio
of its opening (d) to the internal diameter of the pipe (D). This is often
called the “beta ratio”. If the d/D ratio is too small, the loss of pressure
becomes too great. If he ratio is too great the loss of pressure becomes
too small to detect and too unstable. Ratios from 0.2 to 0.6 generally
provide best accuracy

<Document Number>
<date/time> 22
Typical Transmitter Installation

<Document Number>
<date/time> 23
Orifice Plates
The taps are used for sensing the differential pressure
due to orifice. The commonly used flanges in case of orifice are
1] Flange taps 2] Vena contracta taps

<Document Number>
<date/time>
Orifice Plates
24

<Document Number>
<date/time> 25
Advantages & Disadvantages of Orifice
Advantages of Orifice Plates
(i) Low cost
(ii) Can be used in a wide range of pipe sizes (3.175 to 1828.8 mm.)
(iii) Can be used with differential pressure devices
(iv) Well-known and predictable characteristics
(v) Available in many materials
Disadvantages and Limitations of Orifice Plates
(i) Cause relatively high permanent pressure loss,
(ii) Tend to clog, thus reducing use in slurry services
(iii) Have a square root characteristics
(iv) Accuracy dependant on care during installation
(v) Changing characteristics because of erosion, corrosion and scaling

<Document Number>
<date/time> 26
Venturi Tubes

<Document Number>
<date/time> 27
Venturi tubes
Venturi tubes exhibit a very low pressure loss compared to other
differential pressure head meters, but they are also the largest and most
costly. They operate by gradually narrowing the diameter of the pipe, and
measuring the resultant drop in pressure. An expanding section of the
meter then returns the flow to very neat its original pressure. As with the
orifice plate, the differential pressure measurement is converted into a
corresponding flow rate.

<Document Number>
<date/time> 28
Advantages And Disadvantages
Advantages
(i) causes low permanent pressure loss
(ii) widely used for high flow rates
(iii) available in very large pipe sizes
(iv) has well known characteristics
(v) more accurate over wide flow ranges than orifice plates or
nozzles
(vi) can be used at low and high beta ratios
Disadvantages
(i) high cost,
(ii) generally not useful below 76.2 mm pipe size
(iii) more difficult to inspect due to its construction

<Document Number>
<date/time> 29
Flow Nozzle
 Flow nozzles have a smooth entry
and sharp exit.
 The permanent pressure loss of a
nozzle is of the same order as that of
an orifice, but it can handle dirty and
abrasive fluid better than and orifice
can.
 It often used in steam service
because of their rigidity, which makes
them more stable at high temperature
and velocities than orifice.

<Document Number>
<date/time> 30
Flow Nozzle
1]Flow nozzles may be thought of as a variation on the venturi
tube.
2] The nozzle opening is an elliptical restriction in the flow but
with no outlet area for pressure recovery.
3] Pressure taps are located approximately 1/2 pipe diameter
downstream and 1 pipe diameter upstream.
4] The flow nozzle is a high velocity flowmeter used where
turbulence is high (Reynolds numbers above 50,000) such as in
steam flow at high temperatures.
5] The pressure drop of a flow nozzle falls between that of the
venturi tube and the orifice plate (30 to 95 percent).

<Document Number>
<date/time> 31
Advantages And Disadvantages
Advantages
permanent pressure loss lower than that for an orifice plate
available in numerous materials
for fluids containing solids that settle
widely accepted for high-pressure and temperature steam flow
Disadvantages
cost is higher than orifice plate & limited to moderate pipe sizes
requires more maintenance (it is necessary to remove a section of pipe
to inspect or install it).

<Document Number>
<date/time> 32
Pitot Tubes

<Document Number>
<date/time> 33
Pitot Tubes
1] Pitot tubes sense two pressures simultaneously, impact and
static. The impact unit consists of a tube with one end bent at
right angles toward the flow direction. The static pressure is the
operating pressure in the pipe, duct. The tubes can be mounted
separately in a pipe or combined in a single casing.
2] The main difference is that, while an orifice measures the
full flowstream, the pitot tube detects the flow velocity at only
one point in the flowstream.
3] An advantage of the pitot tube is that it can be inserted into
existing and pressurized pipelines (called hot-tapping) without
requiring a shutdown.
4] In industrial applications, pitot tubes are used to measure
air flow in pipes, ducts, and stacks, and liquid flow in pipes,
weirs, and open channels. While accuracy and rangeability are
relatively low, pitot tubes are simple, reliable, inexpensive, and
suited for a variety of environmental conditions, including
extremely high temperatures and a wide range of pressures.

<Document Number>
<date/time> 34
Turbine Meters

<Document Number>
<date/time> 35
Turbine Meters
1] It works on the basic principle of turbine
2]A turbine meter uses a multi-bladed rotor that is supported
by bearings within a pipe section perpendicular to the flow.
3] Fluid drives the rotor at a velocity and, consequently, to the
overall volume flow rate.
4] A magnetic pickup coil outside the meter consist of a
permanent magnet with coil windings
5] As each rotor blade passes the magnetic pickup coil, it
generates a voltage pulse which is a measure of flow rate and
total number of pulses give a measure of the total flow
6]Since the rotor is usually made of stainless steel, it is
compatible with many fluids. However, the bearings, which
are necessary to support the rotor and which must allow it to
spin freely at high speeds, require a fairly clean process. They
have fast response and good accuracy.

<Document Number>
<date/time> 36
Turbine Meters
Because the output signal is proportional to the rotational
velocity of the turbines—which, in turn, is proportional to the
liquid flow—the signal is easily scaled and calibrated to read
flow rate and flow totalization.
Turbine flow sensors generally have accuracies in the range
of ±0.25-1% full-scale

<Document Number>
<date/time> 37
Advantages and Disadvantages
The main advantages of the turbine meter are its high
accuracy (±0.25% accuracy) and repeatability, fast response
rate (down to a few milliseconds), high pressure and
temperature capabilities (i.e., up to 5,000 psi and 800°F with
high-temperature pick coils), and compact rugged
construction.
The disadvantage of the turbine meter is that is relatively
expensive and has rotating parts that could clog from larger
suspended solids in the liquid stream
Another disadvantage in some designs is a loss of linearity at the
low-flow end.

<Document Number>
<date/time> 38
Variable-Area Flowmeters
 Rotameter is variable area flow meter and it is simple
and versatile devices that operate at a relatively constant
pressure drop and measure the flow of liquids, gases, and
steam. The position of their float or piston changes as the
flow rate changes it is constructed of a tapered tube
(usually plastic or glass) and a metal or glass float.
Fluid moving through the tube form bottom to top
causes a pressure drop across the float, which produces
an upward force that causes the float to move up the
tube and which is balanced by the weight of float.
Every float position corresponds to a particular flowrate
for a particular fluid's density and viscosity. For this
reason, it is necessary to size the rotameter for each
application.
Because the variable-area flowmeter relies on gravity, it
must be installed vertically

<Document Number>
<date/time> 39
Advantages
The major advantage of the variable-area flow meter is its relative
low cost and ease of installation.
Maintenance-free and, hence, tends to have a long operating life.
Another advantage is its flexibility in handling a wide range of
chemicals.
Today, all-Teflon meters are available to resist corrosive damage
by aggressive chemicals.
The advantage of a Teflon flow meter with a built-in valve is that
you can not only monitor the fluid flow rate, but you can control it,
as well, by opening and closing the valve.
 If the application requires an all-Teflon meter, chances are the
fluid is pretty corrosive, and many users would like the option of
controlling the flow rate by simply turning a valve that is built into
the flow meter itself.

<Document Number>
<date/time> 40
 One potential disadvantage of a variable-area flow meter occurs
when the fluid temperature and pressure deviate from the calibration
temperature and pressure.
 Because temperature and pressure variations will cause a gas to
expand and contract, thereby changing density and viscosity, the
calibration of a particular variable-area flow meter will no longer be
valid as these conditions fluctuate.
 Manufacturers typically calibrate their gas flow meters to a standard
temperature and pressure (usually 70°F with the flow meter outlet
open to the atmosphere, i.e., with no backpressure).
Disadvantages:
During operation, the flow meter accuracy can quickly degrade
once the temperatures and pressures start fluctuating from the
standard calibration temperature and pressure

<Document Number>
<date/time> 41
Variable-area flowmeters are well suited for a wide variety of liquid and gas applications,
including the following:
 Measuring water and gas flow in plants or labs
 Monitoring chemical lines
 Monitoring filtration loading
 Monitoring flow in material-blending applications
(i.e., lines that use a valved meter)
 Monitoring hydraulic oils (although this may require special
calibration)
 Monitor makeup water for food & beverage plants
Applications:

<Document Number>
<date/time> 42
Thermal Mass Flow meters
Mass flow meters are one of the most popular gas-measurement
technologies in use today
Design principle

<Document Number>
<date/time> 43
Thermal Mass Flow meters
 A gas stream moves into the flow meter chamber and is immediately
split into two distinct flow paths. Most of the gas will go through a bypass
tube, but a fraction of it goes through a special capillary sensor tube,
which contains two temperature coils.
 Heat flux is introduced at two sections of the capillary tube by means of
these two wound coils. When gas flows through the device, it carries heat
from the coils upstream to the coils downstream. The resulting
temperature differential creates a proportional resistance change in the
sensor windings.
Special circuits, known as Wheatstone bridges, are used to monitor the
instantaneous resistance of each of the sensor windings. The resistance
change, created by the temperature differential, is amplified and
calibrated to give a digital readout of the flow.

<Document Number>
<date/time> 44
Advantages:
The main advantage of a mass flowmeter for gas streams is its
ability (within limitations) to "ignore" fluctuating and changing line
temperatures and pressures.
Mass flowmeters measure the mass or molecular flow, as
opposed to the volumetric flow.

<Document Number>
<date/time> 45
Disadvantages:
Aside from the fact that the gas going through the mass flow
meter should be dry and free from particulate matter, there are
no major disadvantage to the mass flow technology.
Mass flowmeters must be calibrated for a given gas or gas
blend.

<Document Number>
<date/time> 46
Applications:
Applications for mass flowmeters are diverse, but here are
some typical uses:
o · Monitoring CO2 for food packaging
o · Gas delivery and control for Leak testing
Hydrogen flow monitoring (e.g., in the utility industry)
o · Control of methane or argon to gas burners
o · Blending of air into dairy products
o · Regulating CO2 injected into bottles during beverage
production
o · Nitrogen delivery and control for tank blanketing

<Document Number>
<date/time> 47
ULTRASONIC FLOW METERS
Ultrasonic flowmeters uses two distinct measurement principles
(i) Transit time flowmeters (ii) doppler flowmeter
Transmitter A
Receiver A
Receiver B
Transit time flowmeters:
In transit time flow meters, an ultrasonic
transducer is mounted at an angle to the pipe.
The ultrasonic waves are transmitted across the
fluid, The velocity of the ultrasonic waves is
increased or decreased by the fluid velocity
depending upon the direction of the fluid flow.
For A, the fluid velocity V aiding the
transmission, the velocity of ultrasonic signal
from transmitter A to the receiver A is increased
For B, It is reduced by the fluid velocity.
From both of this fluid velocity is calculated.
Transmitter B

<Document Number>
<date/time> 48
Doppler flowmeter
Doppler meters measure the frequency shifts
caused by liquid flow.
Two transducers are mounted in a case
attached to one side of the pipe.
A signal of known frequency is sent into the
liquid to be measured. Solids, bubbles, or any
discontinuity in the liquid, cause the pulse to be
reflected to the receiver element.
Because the liquid causing the reflection is
moving, the frequency of the returned pulse is
shifted. The frequency shift is proportional to
the liquid's velocity.

<Document Number>
<date/time> 49
Advantages and Disadvantages
Advantages
 As this technique is not dependent upon pressure, temp, viscosity. So
these factor does not affect the measurement.
 It gives very accurate result without any pressure loss
 Linear relation
Disadvantages
 Cost is very high

3.Flow.ppt

Recommended

Recommended

More Related Content

Similar to 3.Flow.ppt

Similar to 3.Flow.ppt (20)

Recently uploaded

Recently uploaded (20)

3.Flow.ppt