Basic of Flow Measurement Technique

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 Turn-Down 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: m3/s, m3/hr, Nm3/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 (m3 /s)
  • Other common units  for volume flow rate
    • Litre per minute LPM): 1L/s = 103 cm3 /s
    • Cubic centimetre per minute: 103 cm3 /s = 10-3 m3 /s
    • Gallons per minute (GPM): 1gal/s = 3.788 L/s
    • Cubic feet per minute: 1 cf/min = 4.719×10-4 m3 /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

  1. Piston Meters
  2. Variable Area Meter
  3. Turbine Flow Meter
  4. Single Jet Meter
  5. Woltmann Meter
  6. Paddle Wheel Meter
  7. Current Meter
  8. Nutating Disc Meter
  9. Pelton Meter
  10. Oval Gear Meter
  11. Inferential Meter
  12. Thermal mass flow meter
  13. Turbine Flow meter: turbine motion is used to calibrate flow rate
  14. Electro-Magnetic: electro-magnetic field is related flow measurement
  15. Coriolis flow meter
  16. Positive Displacement
  17. Vortex Flow meter
  18. Ultrasonic Doppler Flow Tub
  19. Reciprocating Piston
  20. Rotary Vane Swirl
  21. Target
  22. Thermal Dispersion
  23. 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

flow rate equation

           A1 and A2 are cross sectional area at inlet and throat of venturi. ρ is the mass density of fluid.

  • Actual flow rate measured by venturi meter

Practical 2 - Flow Measurement

      Where, H is the difference in static pressure head (P1– P2) measured across the venturi meter, D1 is the diameter of upstream pipe and D2 is the diameter in the throat section (lowest cross area),Cd 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 Kv-value presents the volume flow rate of fluid in m3/hour in a valve with a specified pressure drop at ambient temperature (1 bar and 20°C).
  • If K-v volume flow rate is presented in m3/hour, then the kv-value can be expressed in per unit time (l/min)
  • The volume flow rate (Q) is calculated using the volume coefficient (K-v), the density of the fluid (ρ), and pressure difference between inlet and outlet of pipe (ΔP = P1 -P2 )

                       

 

Q = volume flow rate of fluid (m3/h)

    P1 = Inlet gauge pressure (bar)
   P2 = Outlet gauge pressure (bar)

     Qn = the normal flow rate (m3/h) for 20° temperature and 760 mmHg of pressure

  t = Inlet fluid temperature (°C)
 V1 = Inlet specific volume of fluid (m3/Kg )
 V2 = Outlet specific volume of fluid (m3/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 the potential energy of the fluid
  • The use of the pitot tube is limited  to point measuring
  • It can be an annular or multi-orifice type. The dynamic pressure (1/ρ*V^2) is measured, and the annular is used to get the 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

  • The 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) parts. There are many designs of turbine flow meters. But simple design is shown below.
  • The principle of the 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 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 low-pressure zones are generated downstream. Turbulent flow is created due to strong vortices with vertex shedding frequency. Turbulent flow is unsteady and possesses 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 Flowmeter or Rotameter

  • The rotameter is a variable area flow meter. It comprises a vertically oriented glass or transparent tapered tube.
  • The smaller portion of the tapered tube is at kept the bottom and the larger portion at the top. Water enters through the bottom and leaves from the top section of the 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 the float is calibrated in appropriate flow units (Litre/min).

Electromagnetic Flow Meter

  • The working principle of magnetic flowmeters 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 differences in transmission times.
  • The fluid velocity is calculated based on the difference between the up and down transmission time
  • The average velocity is calculated considering the cross-sectional 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 the ultrasonic signal with respect to flow direction. ΔT is the difference between for upstream (Tup) and downstream (Tdown) 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

  • The heat generated by the coil is transferred to the fluid by convective mode. The mass flow rate is a function of the 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 U-shaped vibrating tube that cause angular harmonic oscillation.
  • The U-shaped 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 non-Newtonian 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

em-tec Flow Measurement System - BioProTT - BPES UK Supplier

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,
  • V-Notch 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 ( V-Notch 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 V-notch 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 fixed-size 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*Hn

Where Q is flow rate, C is the free-flow 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

You Tube Video on Flow Meters

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