Table of Contents
What are Flow Measurement Techniques used in Industries?
Introduction to Flow Measurement
Why Flow measurement is essential?
 Flow meters are widely used to quantify the amount of fluid flowing through pipe in combustion chamber, boiler, oil and gas, process and Chemical industries, thermal power plant, milk and dairy. Flow measurement of water in open canal and sewage flow is also essential for preservation and storage of clean water. In every application, different type of flow meters are used.
 In Fluid mechanics, we learn basic fluid properties like density, conservation of mass flow rate. How mass flow rate is determined that should be known to every CFD engineer and process engineers in power or chemical industries. Mass flow rate is an essential input for forced flow CFD Modeling. Incorrect flow rate can lead to wrong results in both experiment well as CFD analysis.
 Flow Measurement is the experimental technique of measuring the amount fluid flowing through duct or open channel
Laminar and turbulent Flow
 The performance of most of flow measurement devices is also affected by the Reynolds Number. It is a dimensionless number for ratio of inertia to viscous forces.
 For liquid flow, Reynolds number can defined as the ratio of the inertial forces to its viscous drag forces.It is useful to determine whether a flow is laminar or turbulent.
 Lamianr flow rate can be determined using velocity profile. But turbulent flow rate can not determined easily. Turbulent flow is visualized with higher number of vortices if dye or smoke is injected in fluid flow.
 For internal flow through duct, when the Reynolds number (ReD) is lower than 2300. Turbulent flow is noted when the Reynolds number is greater than 2300 for internal flow. The crictical Reynolds number for open channel is 5,00,000. The critical Reynolds number depends on velocity of fluid (V), size of duct (L), density (ρ) and viscosity (η) of fluid, roughness of wall, external material (dirt particles) and practical conditions.
 A certain range around 2300 is considered the transition flow region between laminar and turbulent flow.
Selection of Flow Measurement Devices
Flow measurement devices should be designed by following important factors
 Devices should consider Fluctuations in fluid flow
 Easy Integration with Piping System
 High Accuracy of device is recommended to reduce errors in measurements
 High TurnDown Ratio of flow rate
 Low capital and maintenance cost
 Sensitivity to Dirt Particles should be minimum
 Minimum Pressure Loss due to fitting of flow measurement devices
 Low use of mechanical or moving Parts
 Devices must be resistant to corrosion and Erosion
Essential Quantities for Flow Measurement
 The volume flow rate (Q) is defined as the volume of fluid that flows past a given cross sectional area per unit time
Q = Cross sectional area*Average Velocity = A*V (m3/hr)
 common volume units of volume flow rate: m^{3}/s, m^{3}/hr, Nm^{3}/hr, Gallons Per Minute (GPM), Standard Litre Per Minute (SLPM)
 Mass flow rate is defined as
m = density * volume flow rate = ρ*Q =ρ*A*V (kg/hr)
 Some devices both pressure and temperature along with volume flow rate. Using these measured values, we can find out the density of fluid using the property table or ideal gas equation
 Coefficient of Discharge (Cd) is an important parameter for flow meter to consider pressure loss. It is defined as the ratio of actual mass flow rate to ideal (ρ*A*V ) mass flow rate. After measurement of actual mass flow rate, the coefficient of discharge can be determined.
Units for Flow Measurement
 SI Unit for volume flow rate (volume/time) : Cubic meters per second (m^{3 }/s)
 Other common units for volume flow rate
 Litre per minute LPM): 1L/s = 10^{3} cm^{3} /s
 Cubic centimetre per minute: 10^{3} cm^{3} /s = 10^{3} m^{3} /s
 Gallons per minute (GPM): 1gal/s = 3.788 L/s
 Cubic feet per minute: 1 cf/min = 4.719×104 m^{3} /s
 Mass flow rate can be calculated by multiplying flow rate the density (ρ) of measuring fluid
Classification of Flow measurement devices
The list of commonly used flow meters in industry are given below
Mechanical Type Flow Meters
 Piston Meters
 Variable Area Meter
 Turbine Flow Meter
 Single Jet Meter
 Woltmann Meter
 Paddle Wheel Meter
 Current Meter
 Nutating Disc Meter
 Pelton Meter
 Oval Gear Meter
 Inferential Meter
 Thermal mass flow meter
 Turbine Flow meter: turbine motion is used to calibrate flow rate
 ElectroMagnetic: electromagnetic field is related flow measurement
 Coriolis flow meter
 Positive Displacement
 Vortex Flow meter
 Ultrasonic Doppler Flow Tub
 Reciprocating Piston
 Rotary Vane Swirl
 Target
 Thermal Dispersion
 Ultrasonic Transit Time
Differential Pressure Based Flow Meters
 Orifice flow meter
 Pitot Tube
 Venturi Tube
 Vortex Weir & Flume
 Differential Pressure Transmitters
 Correlation Method
 Elbow Tap – Elbow flow Meter
 Flow Nozzles
 Variable Area
 Flow measurement techniques are further classified based on closed duct (pipe) and open channel flow
 Flow measurement can be carried out for volume flow rate (Q) and mass flow rate (m). Majority of devices of mechanical and pressure differential techniques measures volume flow flow rate after determination of the velocity of fluid flow.
 For determination of mass flow rate, the fluid to be isothermal or fluid density needs to be known.
Direct Volume Flow Rate Measurement
 It needs a large device when the volume flow rate of fluid are high
 For a smaller device, the measured values may not be accurate
 During start or end of measurement, fluctuations in the measuring values are observed because of the disturbances in opening or closing of valves
 The measurement the mass (volume) flow of the fluid and the time may not be consistent.
Venturi Flow Meter
 The principle of Ventury effect is used to measure flow rate for ventury flow meter
 A reduction in fluid pressure occurs when a fluid flows through a constricted section of pipe. Pressure decreases as flow velocity increases across reduced cross section
 The following formula derived from Bernoulli’s equation
A_{1} and A_{2} are cross sectional area at inlet and throat of venturi. ρ is the mass density of fluid.
 Actual flow rate measured by venturi meter
Where, H is the difference in static pressure head (P_{1}– P_{2}) measured across the venturi meter, D_{1} is the diameter of upstream pipe and D_{2} is the diameter in the throat section (lowest cross area),C_{d }is coefficient of discharge for the venturi device
Orifice Flow Meter
 The volume flow rate liquid or gas is determined using the orifice flow meter
 This device creates a pressure drop across the orifice plate which varies with the flow rate
 The formula for orifice meter is similar to that used for Venturi flow meter
Solenoid Valve Flow Meter
 The amount of flow of a fluid through the solenoid valve is generally calculated with the flow coefficient (Kv)
 User has to note that for gases (like air, methane and oxygen etc.), the formula is different with correction factor
 The Kvvalue presents the volume flow rate of fluid in m^{3}/hour in a valve with a specified pressure drop at ambient temperature (1 bar and 20°C).
 If Kv volume flow rate is presented in m^{3}/hour, then the kvvalue can be expressed in per unit time (l/min)
 The volume flow rate (Q) is calculated using the volume coefficient (Kv), density of fluid (ρ), and pressure different between inlet and outlet of pipe (ΔP = P_{1} P_{2} )
 The calculation of volume flow rate for liquid, gases, air and steam is given below. In the following table, the variables are described as:
Q = volume flow rate of fluid (m3/h)
P1 = Inlet gauge pressure (bar)
P2 = Outlet gauge pressure (bar)
Qn = the normal flow rate (m^{3}/h) for 20° temperature and 760 mmHg of pressure
t = Inlet fluid temperature (°C)
V_{1} = Inlet specific volume of fluid (m^{3}/Kg )
V_{2 } = Outlet specific volume of fluid (m^{3}/Kg ) for outlet pressure (P2) and temperature (t)
G = mass flow rate for steam (Kg/h)
Pitot Tubes
 The pitot tubes are widely used to measure air velocity in many applications like air ventilation and airplanes
 The pitot tube is used to find the fluid flow velocity by converting the kinetic energy (dynamic pressure) to potential energy of fluid
 The use of the pitot tube is limited to point measuring
 It can be annubar or multiorifice type. The dynamic pressure (1/ρ*V^2) is measured, and the annubar is used to get average velocity.
 The pitot tube is used to measure the air velocity around the aero plane. It is mounted frontier of outer surface.
Turbine Flow meter
 Turbine flow meter is widely used for flow measurement in aerospace, pulp and paper, water and waste water treatment, power plant, food and processing and chemical industries
 It has mainly mechanical (rotor) part and electrical (frequency to voltage converter) part. There are many designs of turbine flow meters. But simple design is shown below.
 The principle of turbine flow meter is that as fluid moves through a pipe, the rotor rotates. The rate of rotor is measured to determine the volume flow rate.
 In actual design, a digital display is placed above the rotor wheel to provide flow rate directly instead of frequency.
 The turndown ratios fur turbine flow meter can be more than 100:1
 For the turbine flow meter, volumetric flow rate (Q) is proportional to output frequency (f) of the pickup coil. It is expressed as: Q = k*f
Where Q is volume flow rate, f is the measured frequency and K is the specific factor (pulses per unit volume) of turbine blade.
Vortex Flow Meter
 This flow meter created strong vortices in a downstream of flow using an obstruction. Every obstruction body has a vortex shedding frequency at a critical fluid flow speed. Vortex shedding is the instance where alternating lowpressure zones are generated in the downstream. Turbulent flow is created due to strong vortices with vertex shedding frequency. Turbulent flow is unsteady and posses more rotating fluid masses (eddies) behind the body of vertex generator.
 Vortex shedding frequency (f) in fluid flow is directly proportional to the velocity of fluid (V) in the pipe or volume flow rate of fluid (Q)
 The shedding frequency (f) is independent of fluid properties such as mass density, viscosity and thermal conductivity. The volume flow rate is calculated based on vortex shedding frequency
St = f (D/v)
Q = A*V = (A*f*D*B)/St,
Q = f*K
where St is the Strouhal Number, f is the Vortex Shedding Frequency, D is Width of the Bluff Body, A is Cross Sectional Area of duct, V is the average Velocity, of fluid B is Blockage Factor, K is the flow meter Coefficient
 A display is provided to get a direct reading of flow rate in Liter per minute (LPM)
Variable Area Flow meter or Rotameter
 The rotameter is a variable area flow meter. It comprises a vertically oriented glass or transparent tapered tube.
 The smaller portion of tapered tube is at kept the bottom and larger portion at the top. Water enters through bottom and leaves from top section of tube.
 A metering float is free to move up and down within the tapered tube as per the net fluid force acting on it
 Fluid flow pushes the float to move up in the tapered tube as the upward pressure difference and buoyancy force by overcoming the gravity force
 The float rises till the state of dynamic equilibrium of forces due to upward differential pressure and buoyancy, and downward gravity
 The height of the float presents the flow rate. The scale (vertical movement) of float is calibrated in appropriate flow units (Litre/min).
Electromagnetic Flow Meter
 The working principle of magnetic flow meters is based on Faraday’s law of electromagnetic induction (EMI). Magnetic flow meters is used to detect the flow of conductive fluids only.
 Electromagnetic flow meter has coil housing, transmitter and display unit at the top. The inner surface has coating or Linear (PTPE)

 The electric field generated is a function of fluid velocity
E= B*D*C*V
where E is the Induced Voltage, B is the Magnetic Field Strength, D is the Inner Diameter of Pipe, Vis the average velocity of fluid , C is the Constant of flow meter
Ultrasonic Flow Meter
 Ultrasonic wave is passed through across the fluid flow using the transmitter and a receiver
 Ultrasonic waves are also called Elastic waves which an propagates through man substances of solid, liquid and gases
 Based on properties of ultrasonic waves, clamp on flowmeters with unique feature of being can be used to measure flow rate of fluid in the
 In general, there are two types of ultrasonic flowmeters as per the following working principles:
Doppler Effect Ultrasonic Flow meter
 This Flow meter uses based on Doppler effect. The reflected ultrasonic sound is used to calibrate the fluid velocity.
 After measuring the frequency shift between the source of ultrasonic frequency source and the receiver and the fluid carrier
 The resulting frequency shift is called the Doppler Frequency. It is proportional to velocity
 The Instantaneous flow rate, Q (t/h) is calculated using the following formula:
Q = K*H^n
where K is the flow rate constant, H is height of liquid level and n is the power value
Transit Time Difference Ultrasonic (TTDU) Flow meter
 The measurement principle of this flow meter is based on a time difference method using the ultrasound wave
 A sensor fitted at one side of pipe emit the ultrasonic wave and it is propagated in the flowing fluid. The speed of sound wave propagation will increase in the downstream direction but will decrease in the backward direction
 During the same propagation distance, there will be different in transmission times.
 The fluid velocity is calculated based on difference between the up and down transmission time
 The average velocity is calculated considering the crosssectional velocity distribution of the velocity across the pipe
 Velocity is calculated based on diameter of pipe and mass and difference between signal transmission time
In above equation, D is the diameter of pipe diameter. θ represents the angle of ultrasonic signal with respect to flow direction. ΔT is the difference between for upstream (T_{up}) and downstream (T_{down}) transducers signal transmission times.
 The disadvantage of this flow meter, the flow rate depends on a cross sectional velocity profile
Thermal Mass Flow Meter
 This flow meter is used to measure mass flow rate of fluid through the pipe
 This flow meter is also called thermal dispersion or immersible mass flow meters
 Heat generated by the coil is transferred to fluid by convective mode. The mass flow rate is function of volume flow rate
 The actual mass flow rate is shown below. The display shows volume flow rate in Normal Meter Cubed per Hour (N M3/hr)
Coriolis Flow Meter
 Coriolis mass flow meter is used when the measured mass flow rate is affected due to changes in temperature, pressure, viscosity and density of fluid. In industries, this meter can measure mass flow rate of gases, liquids, and slurries
 The mass flow measurement is based on the Coriolis forces. When fluid passes through the Ushaped vibrating tube that cause angular harmonic oscillation.
 The Ushaped tube is energized with a fixed level of vibration. As a fluid medium (gas or liquid) passes through the Coriolis tube, the momentum of fluid will change vibrations in tube, the tube will twist resulting in a phase shift. This phase shift is measured as a linear output Which is proportional to volume flow rate of fluid
 The phenomenon of vibrating tube is presented below
 Coriolis meters are considered the one of the most accurate flow meters compared to other flow meters. This meter has excellent accuracy over wide conditions and minimum maintenance cost
Advantages Coriolis Flow Meter
 A wide range of applications from adhesive, liquid nitrogen, Newtonian or nonNewtonian liquids, slurries and dense gases
 This flow meter can also be used to measure liquid density
 No restriction for Reynolds number
 Flow rate is not affected due to changes in inlet velocity profile.
Piezoelectric Ceramics Flow Meter
 Piezo cermaic platea are placed around the tube. As fluid passes through the tube, the piezo electric properties change. These changed properties can be related to flow rate
Open Channel Flow Meter
Open channel flow meters are widely used for sewage pipes near the manholes, water canal or streams and rivers. Flumes and weirs are common in many applications
 Rectangular Weirs,
 VNotch Weirs,
 Parshall flumes and
 Palmer Bowlus flumes.
The size of flume or weir is decided based on size of channel, flow rate, and amount of solids content in water flow
Triangular Wier ( VNotch Flow Meter)
 Triangular weirs consist of a V shaped opening (or notch). These notches are fitted over an open canal to measure real time flow rate of seepage water
 Measurement is generally carried out manually by directly fixing the Vnotch plate or on a staff gauge fitted to the basin wall
 Notch is specified with height, width and angle of openings. Flow rate is a function of geometric parameter of notches as shown in the following figures
 Weirs acts as a barrier plate which can be installed across the water streams or open channel to measure volume flow rate of water.
 A weir plate constraint the flow in an open channel with a fixedsize opening. Three shapes of weirs such as rectangular, trapezoidal and triangular are commonly used
 Based on measurement of height and width of water flow, volume flow rate can be calculated as follows.
Parshall Flume Flow Meter
 Normally in power plant, this type of arrangement is installed. River/reservoir water coming to plant is measured.
 The amount of fluid flow through Parshall flume depends on geometric parameters such as converging or diverging angle and dimensions of throat section
Q = C*H^{n}
Where Q is flow rate, C is the freeflow coefficient and H is the head at throat section. The constant, n depends on the flume size.
 Ultrasonic flow meter can used to measure flow rate of open canal flow
Summary
 Flow meters are mainly tw0 types mechanical and differential pressure type. Most of flow meter measure volume flow rate.
 Ideal flow meter should have high turn down ratio, minimum pressure loss, low initial and maintenance cost and high accuracy for a wide range
 Electromagnegtic, corilolis and soleniod flow meters provide better accuaracy
References

Flow Measurement, 1st edition, CRC Press (1993)

Fluid Flow Measurement: A Practical Guide to Accurate Flow Measurement, 3rd edition, ButterworthHeinemann (2014)

Industrial Flow Measurement, 3rd edition, Instrument Society of America (2005)
Great work!. Everything is explained in detail. It is worthful to mechanical, electrical and civil engineers!