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Flow Measurement
Muhajir Ab. Rahim
School of Mechatronic Engineering
Universiti Malaysia Perlis
Introduction
• In the process or
manufacturing industries,
the movement of raw
materials, product and
waste are very important.
• Optimum performance of
some equipment and
operations require specific
flow rates.
• Generally, there are two
types of flow; solid-flow and
fluid-flow.
Solid-Flow Measurement
• A common solid-flow measurement
occurs when material in the form of
particles, such as crushed material or
powder, is carried by a conveyor belt
system.
• The flow is usually described by a
specification of mass or weight per unit
time that is being transported by the
conveyor system. (kg/min or lb/min)
• Flow rate can be calculated by;
Q = WR
L
Q = flow (kg/min or lb/min)
W = weight of material on section of length L
R = conveyor speed (m/min or ft/min)
L = length of weighing platform (m or ft)
Exercise
• A coal conveyor system moves at 100
m/min. A weighing platform is 5m in length,
and a particular weighing shows that 75kg
coal are on the platform.
a) Find the coal delivery.
b) What kind of flow sensor is used?
a) Q= (WR)/ L = [75kg][100m/min]/[5m] = 1500kg/min
b) Load cell (strain gauge) is used to measure the weight of the coal
Fluid-Flow Measurement
• The determination of fluid physical properties like pressure,
density, viscosity and velocity are very important in liquid flow
measurement.
• The viscosity of a fluid refers to its physical resistance to flow. The
most widely used viscosity unit is centipoise.
• The velocity of a flowing fluid is its speed in the direction of flow.
The behavior of a fluid is determined by its velocity.
• Reynolds Number is used to describe the nature of flow.
R = vDρ
μ
v = velocity (ft/s)
D = inside diameter of pipe (ft)
ρ = fluid density (lb/ft3)
μ = viscosity (centipoise)
Flow Characteristics
1.
2.
Laminar flow of a liquid occurs when its
average velocity is comparatively low
and the fluid particles tend to move
smoothly in layers, with the fastest
moving layers toward the center and
the slowest moving layers on the outer
edges of the stream. (parabolic shape)
Turbulent flow occurs when the flow
velocity is high and the particles no
longer flow smoothly in layers and
turbulent or rolling effect occurs. (the
layers disappear and the velocity
across the stream is more uniform)
*R = Reynolds Number
R < 2000
R > 4000
Flow Measurement
•
•
•
•
Flow measurements are normally indirect
measurements using differential pressures to measure
flow rate.
One of the most common methods for measuring the
flow of liquids in pipes is by introducing a restriction in
the pipe, and measuring the pressure drop that result
across the restriction.
When such a restriction is placed in the pipe, the
velocity of the fluid through the restriction increases,
and the pressure in the restriction decreases.
As the flow increases, the pressure drops.
Flow Measurement
Q = K(∆p)1/2
Q = volume flow rate (gal/min or m3/h)
K = a constant for the pipe and liquid type
∆p = drop in pressure across the restriction
V = Q/A
F = ρQ
V = flow velocity (m/min or ft/min)
F = mass or weight flow rate (kg/h or lb/h)
Q = volume flow rate
ρ = mass density or weight density
A = cross-sectional area of flow carrier/ pipe
Q = volume flow rate
Flow Measuring Instrument
The most common instruments used for
measuring fluid flow are;
1) Restriction Flow Sensors
2) Pitot Tube
3) Obstruction Flow Sensor
4) Electromagnetic Flow Meter
Restrictions Flow Sensors
Venturi Tube- the most expensive
and the most accurate, difficult to
install
Venturi
Flow Nozzle- simple and cheap, but
less accurate
Nozzle
Orifice plate- simple and
inexpensive, the easiest to install but
less accurate
Orifice Plate
Pitot Tube
• It is used to measure flow rate at a particular point in a
flowing fluid (liquid or gas)
• The Pitot tube consists of a tube with a small opening
at the measuring end. This small hole faces the
flowing fluid.
• When the fluid contacts the Pitot tube, the fluid velocity
is zero and the pressure is at the maximum.
• This small hole or impact opening, provide the higher
pressure for pressure measurement. While the
ordinary pressure tap provides the lower pressure
reading.
Obstruction Flow Sensor
Moving vane flow meter
Rotameter
Turbine flow meter
Obstruction Flow Sensor
•
•
•
•
In Rotameter, the obstruction is a float that rises in a vertical tapered
column. The lifting force and thus the distance to which the float rises in the
column is proportional to the flow rate. The lifting force is produced by the
differential pressure that exists across the float, because it is a restriction in
the flow. This type of sensor is used for both liquids and gases.
A moving vane flow meter has a vane target immersed in the flow region,
which is rotated out of the flow as the flow velocity increases. The angle of
the vane is a measure of the flow rate. If the rotating vane shaft is attached
to an angle-measuring sensor, the flow rate can be measured for use in a
process-control application.
A turbine type of flow meter is composed of a freely spinning turbine blade
assembly in the flow path. The rate of rotation of the turbine is proportional
to the flow rate. If the turbine is attached to a tachometer, a convenient
electrical signal can be produced.
In all of these methods of flow measurement, it is necessary to present a
substantial obstruction into the flow path to measure the flow. For this
reason, these devices are used only when an obstruction does not cause
any unwanted reaction on the flow system
Electromagnetic Flow Meter
• This technique is only valid for electrical conducting
fluids, by measuring the emf induced across the fluid
stream when it passes through magnetic field.
How to Select?
• Before you start measuring a flow rate,
you need to know, what type of fluid that
you are dealing with. What is the fluid
properties like viscosity, conductivity, etc.
• The higher accuracy of instrument, the
expensive it will be, and more difficult to
be installed.