UNIT IV�Flow Measurement

FLOW MEASUREMENT�

• The measurement of the flow rate and flow quantity of materials is made primarily for the purpose of determining the proportions of materials introduced to a manufacturing process and the amount of materials evolved by the process.

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Applications of flow measurement�

• The flow rates of utilities for cost accounting of the plant.
• The flow rate is highly essential for material balancing of the plant process.
• The quantity of product in continuous processing plant mainly depends upon the correct flow rate of raw materials.

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Types of flow measuring instruments�

• Differential-pressure meters-(head flow meters)
• Orifice meter
• Venturi meter
• Pitot tube
• Flow nozzle

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Types of flow measuring instruments�

• Variable-area flow meters
• Rotameter
• Piston type or valve type area meter
• Open Channel Meters
• Weirs and flumes
• Electromagnetic flow meter

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Types of flow measuring instruments�

• Total flowmeters
• Metering pumps

(a) Reciprocating piston pumps

(b) Peristaltic pumps

(c) Diaphragm pumps

• Nutating disc meter

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Orifice meter�

• It is a variable head meter used for measuring the discharge / flow rate through pipe.
• In this meter, the fluid is accelerated by causing it to flow through the orifice
• The kinetic energy of the fluid increases and the pressure energy therefore decreases.

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Orifice meter

• With this meter the overall pressure drop is high but is relatively cheap and reliable instrument and its installation requires a small length as compared with venturi meter.
• As such where the space is limited, the orifice meter may be used for the measurement of flow rate through pipes.

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Orifice meter

Principle

• The basic principle on which a orifice meter works is that by reducing the cross-sectional area of the flow passage, the fluid is accelerated and a pressure difference is developed, and measurement of the pressure difference enables the determination of the discharge flow rate through the pipe.

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Orifice meter

• The underlying principle behind an orifice meter is Bernoulli’s energy equation for streamline flow which states that when a flow is contracted or expanded, the total energy of the fluid remains constant. During contraction, kinetic energy increases and potential energy decreases, whereas potential energy increases at the cost of kinetic energy in expansion. A change in potential energy in reflected in a change of static pressure.

Orifice meter

Construction

• A stainless steel orifice plate which is held between flanges of a pipe carrying the fluid whose flow rate is being measured.
• It should be noted that for a certain distance before the orifice plate fitted between the flanges, the pipe carrying the fluid should be straight in order to maintain laminar flow conditions.

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Orifice meter

• The orifice diameter may vary from 0.2 to 0.85 times the pipe diameter, but generally the orifice diameter of 0.5 times the diameter of pipe is used.
• The pressure taps are made directly in the side of the pipe: the upstream tap located 8 times pipe diameter from the orifice, and the downstream tap located 2.5 times pipe diameter from the orifice.
• The pressure difference is measured by connecting the manometer or other pressure measuring device.

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Orifice meter

Orifice meter

Working

• Orifice meter of known coefficient of discharge is installed in a pipe line and pressure taps are connected to pressure measuring devices.
• The fluid having uniform cross section of flow converges into the orifice plate opening in its upstream.
• When the fluid comes out of the orifice plate opening, its cross section is minimum and uniform for a particular distance and then the cross section of the fluid starts diverging in the downstream.

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Orifice meter

• At the upstream, before converging takes place pressure P₁ is maximum.
• As the fluid starts converging to enter the opening, its pressure drops.
• When it comes out, its pressure is minimum P₂ at the downstream.
• This minimum cross section area of the fluid obtained at the downstream from the orifice edge is called ‘VENA-CONTRACTA’.

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Orifice meter

• In the meter the fluid is first accelerated and then retarded so that the pressure drop across the meter is created.
• Once the drop across the meter is attained then the pressure drop across the meter is noted. This pressure difference (p₁-p₂) is then used to calculate volumetric flow rate using the mathematical flow equation for the meter.

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Orifice meter

• Applications
• Used to measure flow rates of pure liquids, water and gases.
• Used to measure flow rates of suspended materials (solids, oil mixed with water).
• Used to measure flow rates in large pipes.

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Orifice meter

• Advantages
• Simple in construction
• It can be installed and replaced easily
• It is relatively cheap
• Occupies less space

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Orifice meter

• Disadvantages
• Pressure recovery at downstream is poor.
• Gets clogged when slurries flow.
• The orifice plates gets corroded.
• Co-efficient of discharge is low (0.61).
• Large power consumption.

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Orifice meter

• Different types of orifice

The orifice are classified as

• Sharp edge orifice plates.
• Concentric orifice plates.
• Eccentric orifice plates.
• Segmental orifice plates.

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Orifice meter

• Sharp edge orifice plates
• It has a sharp edge.
• The angle of the opening is 45^o
• The plate is used as an ordinary orifice plate.

Orifice meter

• Concentric orifice plates
• The most widely used orifice is concentric orifice.
• It is used for measuring flow of fluids containing solids.
• The Concentric orifice plate is made of flat metal sheet with a circular hole.
• The plate is fitted in such a way that the orifice centre and pipe centre matches.

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Orifice meter

Orifice meter

• Eccentric orifice plates
• The opening of the plate is not in the axis of pipe.
• It placed just below the axis of pipe.
• It is used for measuring flow of fluids containing solids.
• It requires special calibration.

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Orifice meter

• Segmental orifice plates
• It has a half circle opening in the plate.
• The axis of the opening is on the axis of the pipe.
• It requires special calibration.

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Orifice meter

• Materials of construction of orifice plates

The orifice plates are made from the following materials.

• Steel,
• Stainless steel,
• Monel,
• Phosphor bronze or almost all metal that will withstand corrosive effects of the fluids.

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Venturi meter�

• A venturi meter is a variable head meter which is used for measuring the flow rate of fluid through pipe.
• In venturi meter, the fluid is gradually accelerated to a throat and then gradually retarded in the diverging section where flow channel expands to the pipe size. Most of the kinetic energy is recovered.

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Venturi meter

Principle

• The basic principle on which a venturi meter works is that by reducing the cross-sectional area of the flow passage, a pressure difference is created and the measurement of the pressure difference enables the estimation of the flow rate through the pipe.

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Venturi meter

Construction

• A venturi meter consists of inlet section followed by convergent section.
• The inlet section of the venturi meter is of same diameter as that of pipe which is followed by a short convergent section with converging cone angle of 19^o to 23^o and its length parallel to the axis.
• In the convergent section the fluid is accelerated.

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Venturi meter

• A cylindrical throat has the section of constant cross-section with its length equal to diameter.
• The flow area is minimum at the throat.
• More commonly the diameter of throat is 0.5 the pipe diameter.
• A large diverging section with the cone angle of about 5 -15^o wherein the fluid is retarded and large portion of kinetic energy is converted back to pressure energy.
• Pressure taps are provided at high and low pressure taps respectively.

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Venturi meter

Venturi meter

Working

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• A venturi meter of known coefficient is installed in a pipeline and the pressure taps are connected to pressure measuring device.
• In the meter, the fluid is accelerated in converging cone and then retarded in the diverging cone gradually.
• The increase in flow velocity at throat results in decrease in the pressure at the throat.

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Venturi meter

• As such the pressure difference is developed between the inlet section and the throat section which is measured with the help of manometer or pressure gauges after the attainment of steady state.
• This pressure difference is then related to the flow rate by mathematical flow equation for the meter.

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Venturi meter

Applications

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• Used where the high pressure recovery is required.
• Used for measuring flow rates of water, gases, suspended solids, slurries and dirty gases.

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Venturi meter

Advantages

• Coefficient of discharge is high (0.98)
• Can be installed vertically, horizontally or inclined.
• Low permanent pressure loss and hence high pressure recovery.
• High accuracy over wide flow range.
• High reproducibility.

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Venturi meter

Disadvantages

• Large in size and expensive.
• It occupies considerable space.
• Relatively complex in construction.
• Not suitable for measurement of highly viscous slurries.

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Comparison

Pitot tube�

• The pitot tube is a device used to measure local or point velocity along a streamline.

Principle

• The basic principle is that if the velocity of flow at a particular point is reduced to zero, the pressure there increases due to conversion of kinetic energy into pressure energy, and by measuring the increase in the pressure energy at this point the velocity may be determined.

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Pitot tube

Pitot tube

Construction and Working

• It consists simply of a tube supported in the pipe with the impact opening of about 0.125 to 0.25 inch diameter arranged to point directly toward the oncoming fluid.
• This is called the impact opening and is used to measure the stagnation pressure.
• The static pressure is measured through the ordinary pressure tap.

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Pitot tube

• The pitot tube consists of an impact tube, the opening of which is perpendicular to the direction of flow and static tube.
• The static tube opening is parallel to the direction of flow.
• The two tubes are connected to the legs of a manometer for measuring small pressure differences.
• The impact tube measures impact pressure and the static tube measures the static pressure.

Pitot tube

• With the help of pitot tube, maximum velocity which is at the centre of pipe is measured and based upon laminar and turbulent flow conditions, the average velocity is estimated which in turn used for calculation of flow rate.

Pitot tube

Applications

• Used in wind tunnel experiments.
• Used to measure fluid flow velocity.
• To measure air speed in racing cars and air force fighter.

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Pitot tube

Advantages

• Installation is relatively simple.
• Flow measurement made in very large pipes or ducts.
• They can be employed in either open-channel streams or pipes.
• Error not more than 1%.

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Pitot tube

Disadvantages

• Do not give average velocity directly.
• Readings for gases are extremely small.
• It cannot be used in fluids containing solid particles.

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Rotameter�

• In orifice meter and venturi meter, the area of flow is constant and pressure drop across meter varies with the flow rate.
• In variable area meters, the pressure drop across the meter is constant and the flow rate is a function of the area of flow.
• Thus, any change in flow rate can be measured in terms of change in area of flow, hence, the name variable area meter.

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Rotameter

Principle

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• Rotameter operates on the principle that the pressure drop across the meter is constant and the area through which the fluid flows varies with flow rate. The area is related to the flow rate.

Rotameter

Construction

• It consists of a tapered glass tube mounted vertically in frame with the large end up.
• It contains a freely moving solid float.
• The diameter of the float is smaller than the diameter of the bottom portion of tapered tube.
• The density of the float is higher than that of the liquid.

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Rotameter

• A perforated plate or similar device is provided at both the ends of the tapered tube for arresting the float in the tube.
• The flow scale is marked on the glass tube or it is mounted close to the tube so that the position of the float can be marked and the flow rate is then obtained.

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Rotameter

Rotameter

Working

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• In case of rotameter as the flow varies, the float rises or falls, thus altering the area of the annular space between float and the tube.
• The flow area is the annular opening between the float and the tube.

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Rotameter

• When no fluid flows through the meter, rotameter float rests at the bottom of the tube.
• But as the fluid begins to flow to the lower side of tube, float rises until its weight is balanced by the up thrust of the fluid and fluid flows through the meter through annular space.

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Rotameter

• As the flow rate increases, the float rises in the tube moves upward, thus increasing the area of the opening keeping differential pressure across it constant.
• On the other hand, as the flow rates decreases, the float falls in the tube, thus decreasing the flow area with constant pressure drop across it.

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Rotameter

• At a given flow rate, float stabilizes at certain fixed position in the tube and at steady state it is recorded as rotameter reading on the scale provided.
• The variation of flow area with flow rate can be measured in terms of change in float position. The rotating motion of float helps to keep it steady.

Rotameter

Materials of construction for float are

• Silver,
• Stainless steel,
• Lead,
• Tantalum,
• Aluminum,
• Brass,
• Bronze,
• Monel, and
• Nickel

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Rotameter

Applications

• Used to measure the flow rates of corrosive fluids.
• Particularly useful to measure low flow rates.
• Used for measuring of flow of liquids having variable viscosity.
• Used in chemical industry for measurement of flow rate of compressible and incompressible fluids.

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Rotameter

Advantages

• Flow conditions are visible.
• It has relatively low and constant pressure drop.
• Good accuracy at low flow rates.
• Relatively cheap.
• They provide direct reading of the flow rate.

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Rotameter

Disadvantages

• They should be installed vertically.
• Limited to relatively low temperature fluids.
• The float is not visible when colored liquid is used.

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Open Channel Meters�

• Head-area meters such as weirs, nozzles, and flumes are used in open channels for liquid flow measurement.
• They are used in large and open streams.
• Open-channel meters are employed in central power stations, hydraulic power generation stations, municipal water works, sewage-disposal units, and water irrigation projects for measuring the flow of water, sewage sludge, chemical wastes, and other semi fluids.

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Open Channel Meters

• The common method of measuring flow through an open channel is to measure the height or head of the liquid as it passes over an obstruction (a flume or weir) in the channel.
• For any open channel that is free flowing through a specific controlled metering structure, there is a specific relationship between inlet height of inlet water and the flow rate.

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Open Channel Meters

• Whenever a given inlet height occurs, there will always be the corresponding flow.
• Therefore, if we know the flow corresponding to each inlet height, we can construct an inlet height-to-flow relationship.
• The water level or "Head" is accurately measured using a level sensor.
• The height or level sensor outputs an electrical signal that corresponds with the height of the liquid.
• This signal indirectly relates to the flow rate.

Weirs�

• Weirs are used to measure flow rate primarily in open channels such as water including irrigation, waste and sewage systems, and in pipes and conduits that are generally not completely filled with liquid.
• Weirs are sharp-crested, overflow structures that are built across open canals.
• They are easy to construct and can measure the discharge accurately when correctly installed.

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Weirs

• However, it is important that the water level downstream is always below the weir crest; otherwise the discharge reading will be incorrect.
• It is an obstruction in a flowing stream over which the liquid is made to pass.
• There are three types of weirs such as rectangular, V-notch and trapezoidal.

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Weirs

Weirs

• The familiar rectangular notch is a rectangular opening, most often employed for measuring large flow rates. Since the velocity is proportional to depth at the weir,
• V = (2gz)^0.5

where

• V= velocity
• z = depth from surface

Weirs

• Q = 0.67 C_RW b (2gh³)^0.5

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where

• Q = flow rate
• b = width of rectangular notch
• h = head over weir crest

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Weirs

• The trapezoidal notch is shown in figure (b).
• This weir in fact an improved rectangular weir, with a slightly higher capacity for the same crest length.
• Its opening is trapezoidal has side slopes in the vertical to horizontal ratio of 4 to 1.

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Weirs

• The V-notch weir has a triangular opening, and this type is well suited to measuring small flows with high accuracy.
• In addition, a weir should be preceded by a straight, uniform cross-section channel of sufficient length to insure uniform velocity distribution.

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Weirs

• Baffle plates and screens extending across the channel are often necessary to reduce turbulence.
• The upstream edge of the weir plate should be square.
• The downstream edge of the weir plate should be beveled, so that the stream will break free of the edge and not fall down the face of the weir plate.

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Weirs

Advantages

• Its cost is low.
• It is constructed on location where being used.
• It is not easily damaged.

Disadvantages

• It is applicable only to open-channel measurements.
• Its field calibration is required.
• Its accuracy is poor (generally not over 2 to 3%).

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Flumes�

• Other well-known structures for discharge measurement are flumes.
• Flumes consist of a narrowed canal section with a particular, well-defined shape.
• Hydraulic flumes are used in open streams where a sufficient fall or head cannot be obtained to use a weir.
• The most common type of measuring flume is Parshall flume.

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Flumes

• A Parshall flume consists of three principal sections:

(i) a converging section at the upstream end,

(ii) a constricted section or throat in the middle

(iii) a diverging section at downstream.

• The floor of the throat slopes downwards and the diverging section has slopes upwards.
• Parshall flumes have standard dimensions which must be followed closely in order to obtain accurate measurements.

Flumes

Flumes

• It is considered more practical than weirs because its loss of head is about one-fourth that of a weir.
• Its throat sizes from about 6 in. to 8 ft. for flows ranging from 1 to 70 million gallons per day.
• The head is measured at a single point about one third the distances downstream from the inlet in the entrance section.

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Flumes

• The head may vary from 6 to 30 in.
• Like measurements with weirs, the water level upstream of the flume is a measure of the discharge through the flume, and when the head has been measured the discharge can be obtained.
• The discharge is determined by an empirical formula or from test data.

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Flumes

Advantages

• It handles greater flow than weir.
• It can be constructed on location.
• It is easy to construct as all its sections are plane surface.
• Its dimensions are not easily altered to cause incorrect measurements.

Disadvantages

• It is more expensive than weir.
• It requires calibration on location.
• Its accuracy not over 2 to 3%.

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Nutating disc meter�

Construction

• The most common type of displacement flow meter is the nutating disk, or wobble plate meter.
• A typical nutating disk is shown in Figure.
• The movable element is a circular disk which is attached to a central ball.

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Nutating disc meter

• A shaft is fastened to the ball and held in an inclined position by a cam or roller.
• The disk is mounted in a chamber which has spherical side walls and conical top and bottom surfaces.
• The fluid enters an opening in the spherical wall on one side of the partition and leaves through the other side.

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Nutating disc meter

Nutating disc meter

Working

• As the fluid flows through the chamber, the disk wobbles, or executes a nutating motion.
• Liquid enters a precision-machined chamber containing a disc which nutates (wobbles).
• The position of the disc divides the chamber into compartments containing an exact volume.
• Liquid pressure drives the disc to wobble and a roller cam causes the nutating disc to make a complete cycle.

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Nutating disc meter

• This motion is translated into rotary motion by means of a ball and shaft, which is attached to the disc.
• The movements of the disc are transmitted by gear train to an indicator.
• Since the volume of fluid required to make the disc complete one revolution is known, the total flow through a nutating disc can be calculated by multiplying the number of disc rotations by the known volume of fluid.

Nutating disc meter

• To measure this flow, the motion of the shaft generates a cone with the point, or apex, down.
• The top of the shaft operates a revolution counter, through a crank and set of gears, which is calibrated to indicate total system flow.
• Maximum viscosity allowed: 11,000 centipoise.

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Nutating disc meter

Applications

• It is used in residential water meter.
• It is used to measure the flow rates of chemicals with proper materials of construction.
• Higher accuracy between ± 1% to ± 2%.
• Used to measure flow rates from about 15 to 500 gpm.

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Nutating disc meter

Advantages

• Simple and compact in construction.
• Cost is low.

Disadvantages

• Accuracy depends upon density and viscosity.
• Can be used only for clean liquids.

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Magnetic flow meter�

Principle

• Magnetic flow meter works on the principle of Faraday’s law of electromagnetic induction which states that, When a current carrying conductor moves through stationary transverse magnetic field, then emf is induced between the ends of the conductor and the emf is proportional to the conductor length and the magnetic field. This emf induced is given by,

E=BLV

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Magnetic flow meter

• Where,

E = emf

B = magnetic induction

L = length of conductor

V = velocity of the conductor

• Thus E α V, . It can be related with flow rate by continuity equation, Q =VA

Magnetic flow meter

Construction

• The magnetic flow meter consists of flow tube with electrodes and the source of magnetic field.
• The flow tube is a separate unit with flanged ends that can be bolted into main pipe carrying liquid to be metered.
• The tube is made of a non-conducting, non-magnetic alloy and is insulated by glass lining from flowing liquid so as to prevent the short circuiting of emf between the electrodes.
• The tube lining can be Polyurethane, Polytetra-fluro ethylene.

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Magnetic flow meter

• Stainless Steel or Platinum electrodes are located opposite to each other with their axes perpendicular to both the magnetic field and the tube axis.
• Electrodes are placed in such a way that they do not disturb the flow pattern but they are in contact with the fluid.
• For functioning of electromagnetic flow meter a steady magnetic field must be generated around the flow tube in the direction perpendicular to fluid flow.
• These electromagnets are energized by an A.C supply so as to produce steady magnetic field around the pipe.

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Magnetic flow meter

Working

• When the conducting fluid flows through the pipe which is subjected to magnetic field, the conducting fluid cuts the magnetic field, and due to this a voltage is induced.
• By steady magnetic field around the pipe, then by magnetic induction emf is induced between the electrodes.

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Magnetic flow meter

• The emf is given by, E=BLV
• Thus for stationary magnetic field emf produced is proportional to fluid velocity. (V ) which in turn varies with flowrate. The actual relation between emf and fluid flow rate can be derived as follows:
• Q = πd²E/4BL

Magnetic flow meter

Applications

• Used to measure flow rate of conductivity fluids.
• Used for metering slurries, sanitary, dirty, corrosive fluids, abrasive liquids and greasy materials.

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Magnetic flow meter

Advantages

• No obstruction of flow and hence no pressure drop.
• The flow may be laminar or turbulent.
• It gives accurate results.
• Independent of viscosity, density, temperature and pressure.

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Magnetic flow meter

Disadvantages

• The fluid must satisfy conduction condition.
• The fluid should be full in the pipe to get accurate results.
• Air and gas bubbles in the pipe cause errors.
• Output voltage is low and hence requires amplification.
• Hydrocarbons and gases cannot be measured with this technology due to their non-conductive nature and gaseous state.

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