Flow measurement��Flow meter is a device that measures the rate of flow or quantity of a moving fluid in an open�or closed conduit�

• Solid materials are measured in terms of either weight per unit time or mass per unit time

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• Liquids are measured either in volume rate or in weight rate

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• Gases are normally measured in volume rate.

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• Units: kg/h or m³/h or tonnes/h or litre/h

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Flow measuring devices are generally classified into four groups

1. Mechanical type flow meters

Fixed restriction variable head type flow meters

Ex. Orificemeter, Venturimeter

2. Inferential type flow meters.

Variable area flow meters (Rotameters), turbine flow meter, target flow meters

3. Electrical type flow meters.

Electromagnetic flow meter, Ultrasonic flow meter, Laser doppler Anemometers etc. fall under electrical type flow meters

4. Other flow meters.

Purge flow regulators, Flow meters for Solids flow measurement, Cross-correlation flow meter, Vortex shedding flow meters, flow switches

Orifice meter

Advantages of Orifice meter:

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Easy to install/remove.

The orifice meter can be easily maintained.

Measures a wide range of flows.

Simple construction.

Suitable for most gases and liquids.

They are cheap, The price does not increase dramatically with size.

Disadvantages of orifice meter:

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Requires homogeneous/ single phase fluid.

It causes a pressure drop in the fluid.

Its accuracy is affected by the density, pressure and viscosity of the fluid.

The range of measurement of viscosity limits of fluids.

It requires straight conduits to ensure accuracy is maintained.

Venturi meter

Advantages of venturi meter:

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Less chance of getting stuck with sediment.

The discharge coefficient is high.

It can be installed vertically, horizontally, inclined.

They are more precise and can be used for a wide range of flows.

About 90% of the pressure drop can be recovered.

Disadvantages of venturi meter:

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Large in size

Initial costs, installation and expensive maintenance.

Requires a long placement length.

It can not be used in pipes of less than 7.5 cm in diameter.

Maintenance is not easy.

Rotameter

Advantages of rotameter

• Low cost.
• Low pressure drop.
• It is suitable for small flow rates.
• It is easily equipped with alarms and switches or any transmitting devices.
• It also measures the flow rate of corrosive fluids.
• There is an availability of viscosity-immune floats.
• It can be used in some light slurry services.

Disadvantages of rotameter:

• It is difficult to handle the glass tube type.
• It must be mounted vertically.
• It is not suitable for pulsating services.
• Generally it is limited to small pipe services.
• Accuracy is = ± 1/2 to 10%.
• It requires in-line mounting.

• Magnetic flowmeters have been widely used in industry for many years.
• Unlike many other types of flowmeters, they offer true noninvasive measurements.
• Easy to install and use to the extent that existing pipes in a process can be turned into meters simply by adding external electrodes and suitable magnets.
• They can measure reverse flows and are insensitive to viscosity, density, and flow disturbances.
• Electromagnetic flowmeters can rapidly respond to flow changes and they are linear devices for a wide range of measurements.

Electromagnetic Flowmeters

According to this Law, when a conductive medium passes through a magnetic field B, a voltage E is generated which is proportional to the velocity v of the medium, the density of the magnetic field and the length of the conductor.

Principle of the electromagnetic flowmeter is Faraday’s law of electromagnetic induction.

The induced voltages in an electromagnetic flowmeter are linearly proportional to the mean velocity of liquids or to the volumetric flow rates.

Limitations of electromagnetic Flow Meters

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1. The substance being measured must be conductive. Therefore, it can’t be employed for metering the flow rate of gases and steam, petroleum products and similar liquids having very low conductivity.
2. To render the meter insensitive to variations in the resistance of liquid, the effective resistance of the liquid between the electrodes should not exceed 1% of the impedance of the external circuit.
3. It is a very expensive device.
4. As the meter always measures the volume rate, the volume of any suspended matter in the liquid will be included.

5. To avoid any trouble which would be caused by entrained air, when the flow tube is installed in a horizontal pipe-line, the electrodes should be on the horizontal diameter.

6. As a zero check on the installation can be performed only by stopping the flow, isolating valves are required and a bypass may also be necessary through which the flow may be directed during a zero check.

7. The pipe must run full, in case regulating valves are installed upstream of the meter.

Advantages of Electromagnetic Flow Meter

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1. The obstruction to the flow is almost nil and therefore this type of meters can be used for measuring heavy suspensions, including mud, sewage and wood pulp.
2. There is no pressure head loss in this type of flow meter other than that of the length of straight pipe which the meter occupies.
3. They are not very much affected by upstream flow disturbances.
4. They are practically unaffected by variation in density, viscosity, pressure and temperature.
5. Electric power requirements can be low (15 or 20 W), particularly with pulsed DC types.

6. These meters can be used as bidirectional meters.

7. The meters are suitable for most acids, bases, water and aqueous solutions because the lining materials selected are not only good electrical insulators but also are corrosion resistant.

8. The meters are widely used for slurry services not only because they are obstruction less but also because some of the liners such as polyurethane, neoprene and rubber have good abrasion or erosion resistance.

9. They are capable of handling extremely low flows.

Ultrasonic Flowmeters

The term ‘ultrasonic’ refers to the signals (usually are short bursts of sine waves) whose frequency is above the range audible to human hearing which is 20 to 20000 Hz.

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Principle

The ultrasonic flow meter operates on the principle that the velocity of sound in a fluid in motion is the resultant of the velocity of sound in the fluid at rest plus or minus the velocity of the fluid itself.

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Types of Ultrasonic Flow Meters

1. Transit time flow meters
2. Doppler Flow meter.

• (1) Transit time: This is today’s state-of-the-art technology and most widely used type.
• This type of ultrasonic flowmeter makes use of the difference in the time for a sonic pulse to travel a fixed distance.
• First against the flow and then in the direction of flow.
• Transmit time flowmeters are sensitive to suspended solids or air bubbles in the fluid.

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(2) Doppler:

• This type is more popular and less expensive, but is not considered as accurate as the transit time flowmeter.
• The transmitter of a Doppler flow meter projects an ultrasonic beam at a frequency of about 0.5 MHz into the flowing stream and deflects the reflected frequency.
• The difference between transmitted and reflected velocities is called the ‘beat frequency’ and is related to the velocity of the reflecting surfaces (solid particles and gas bubbles) in the process stream.

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Advantages:

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• Ultrasonic meters are made up of no moving parts.
• No pressure loss.
• Maintenance-free operation. It is a key advantage as compared to conventional mechanical meters such as positive displacement meters, turbines etc.
• Consistent, more accurate and reliable than a lot of other metering systems.
• With the emergence of 3-beam ultrasonic, all other flowmeters like mass, vortex, positive displacement and turbine flowmeters which are used to measure non-conductive fluids, have been successfully replaced by ultrasonic meters.

Disadvantages:

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• Still problematic for liquid and gas measurements
• Sound beam must traverse a representative cross section, therefore flow profile dependent. Long inlet and outlet sections required
• Errors due to deposits
• Transit time meters require clean liquids
• Doppler meters only for slight contamination or few gas bubbles
• Doppler meters affected by sound velocity changes due to temperature, density and concentration
• Unsuitable for heavily contaminated liquids
• Gas bubbles cause errors

Hot Wire Anemometer works When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the wire also changes. This change in resistance of the wire becomes a measure of flow rate.

Hot wire anemometers are commonly available in two forms :

(i) Constant current type and

(ii) Constant temperature type.

In the constant—current mode of operation, the current through the hot wire is kept constant at a suitable value.

Constant-Temperature Type

The current through the hot wire filament is adjusted to keep the wire temperature constant. A galvanometer is used to detect the balance conditions.

Advantages of thermal anemometry:

• high accuracy
• easily-automated collection procedure, and
• high frequency response allowing measurement of turbulent flows.
• Very simple to use, because no individual sensor housing (channel required) measure in turbulent and laminar flow profiles
• Heater is also the sensor. It measures by itself
• Reference sensor is on the same chip
• High flow rates possible

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Disadvantages of Constant Temperature Anemometry

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• Flow signal depends on sensor alignment
• There is a thermal coupling between heater and sensor
• Chip contamination possible,
• no flow direction detectable
• Relatively high heating power necessary

Vortex flow meter

a non-streamlined object (also called a bluff body) placed in the path of a fast-flowing stream, causes the fluid to alternately separate from the object on its two downstream sides, and, as the boundary layer becomes detached and curls back on itself, forming vortices (also called whirlpools or eddies)

https://www.youtube.com/watch?v=_Hbbkd2d3H8

Vortex principle

https://www.youtube.com/watch?v=GmTmDM7jHzA

• There are no moving parts that are vulnerable to wear
• Maintenance is not required on a regular basis
• Liquid, gas and stream all can be measured using the vortex flow meter
• Reliability
• Long term accuracy
• Cost of installation is also less for the vortex flow meter
• The flow meter is available in many temperature ranges
• Available in wide variety of pipe sizes
• The volumetric flow rate of the vortex flowmeter is not affected by thermal parameters such as temperature, pressure, density or viscosity of the fluid being measured.
• It causes little pressure loss.

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Advantages

• cannot be used for the fluid with low velocity
• minimum length of straight pipe which is required upstream and downstream
• poor anti-vibration performance. External vibrations can cause measurement errors in the vortex flowmeter and may not even work properly.
• high flow velocity shock of the fluid causes vibrations in the vortex body, which reduces the measurement accuracy.
• Cannot measure dirty media

Disadvantages

https://www.youtube.com/watch?v=XIIViaNITIw

https://www.youtube.com/watch?v=31jYXlnu-hU

Mass flow rate measurements

CORIOLIS MASS FLOW METERS FOR GAS AND LIQUID MEASUREMENT

CORIOLIS MASS FLOW METERS FOR GAS AND LIQUID MEASUREMENT

A mass flowmeter is a system that provides a measurement of fluid flow in units of mass; pounds, tons.

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The Coriolis flowmeter is a type of flowmeter which measures the mass of the fluid flow directly.

The Coriolis mass flowmeter is named after the French mathematician, Gaspar Gustave de Coriolis, who described the apparent forces on a mass moving through a rotational plane. Coriolis flow meters operate on the principle of the Coriolis Effect.

 Coriolis force is an inertial or fictitious force that acts on objects that are in motion

 Deflection of an object due to the Coriolis force is called the Coriolis effect.

Working Principle

• It directly measures the mass of the flowing fluid, liquids and gases.
• The meter indirectly measures liquid density and also the temperature of the containment tube.
• The temperature can be used to approximate the temperature of the fluid.
• The Coriolis flowmeter is bi-directional.
• Need not be calibrated for different types of fluids
• Compositional changes in the fluids does not affect the measurement
• Measurement of high viscosity fluids, such as crude oil, heavy oil, residual oil and other liquids with higher viscosity.
• Coriolis mass flowmeters are used, with good reliability and accurate measurement result.

Advantages

• Poor zero stability which affects the flow meter accuracy.
• It cannot be used to measure fluids with lower density, such as low pressure or low density gas.
• Slightly higher gas content in the liquid may cause a significant increase in measurement error.
• It is sensitive to external vibration interference.
• It cannot be used for larger diameters. Currently max size is 8 inch Coriolis flow meter.
• The pressure loss is large, especially when measuring a liquid with a high saturated vapour pressure, the pressure loss may cause vaporization of the liquid, and cavitation occurs.

Disadvantages

https://automationforum.co/advantages-and-disadvantages-of-orifice-and-venturi-meter/

http://ecoursesonline.iasri.res.in/mod/resource/view.php?id=3609

https://www.youtube.com/watch?v=TSXS4FqzxAQ