Flow measurement
Flow Measurement
Definition:
The amount of material passing through a specific point in a specific time is called the flow of that material
Flowmeter
What is a flow meter?
Flowmeter is a device that is used to measures the rate of flow or total quantity of fluid moved in an open or closed conduit.
Units of flow
Cubic meter/sec
Liter / sec
Liter/minute
Liter/ hour
Barrel per sec
Ton/hour
Flow measurement methods
Industrial flow measurements include measuring the flow rate of solids, liquids, and gases.
There are two basic methods used to measure flow in industrial instrumentation.
1. On a volumetric basis
2. On a weight basis.
As for as Gas flow is concerned it is usually measured on a volumetric base.
Solid materials flow is measured either by weight per unit of time or mass per unit of time.
Very rarely solid material flow quantity is measured in terms of volume.
On the other hand as for as Liquids are concerned the flow is measured either in volume rate or in weight rate.
Classification of Fluids
Two main classifications of Fluids are known,
1. Incompressible fluids
2. Compressible fluids.
The liquid is considered as incompressible fluids,
The gaseous form of fluid is usually known as compressible fluids.
Because Gases occupy different volumes at different pressures so considered compressible fluids.
In contrast fluid in Liquid form occupy the same volume at different pressures.
Before Calibrating the flow meters this property of fluids must be considered.
The measurements taken at actual conditions must be converted into Standard temperature 0°C and pressure 760 mm Hg or to Normal temperature 20°C.
Types of flow meter
There are many types of flow meters used in instrumentation and industrial measurement systems.
Different types of flow meters are discussed here, like;
3. Velocity flow meter
4. Mass flowmeters
5. Open channel flow meter
6. Variable area flowmeter
1. Inferential Flow Meters
Inferential type flow meter works on the following principle;
The flow rate is inferred from a characteristic effect of a related phenomenon or other measured values e.g differential pressure.
1. Fixed restriction variable head
DP Transmitter
In the variable head type flow meters, a restriction of known dimensions is generally introduced into the pipeline, so a pressure drop occurs at the restriction point. Restriction increases the velocity of the fluid.
Pressure drop produced due to these restrictions is an indication of the flow rate.
This type of flow meters are using different sensors to produce restrictions in the flow, some restrictions sensors are listed below.
a. Orifice plate
b. Venturi tube
c. Flow nozzle
d. Pitot tube
e. Dall tube
a. Orifice plate
The orifice plate inserted in the pipeline causes a restriction in the flow and an increase in flow velocity and it causes a decrease in pressure in the cross-section beyond the orifice plate, with a maximum velocity and minimum pressure at the vena contracta where an orifice plate is placed in the pipeline,
The flow rate can be obtained by measuring the pressure difference (p1-p2).
An orifice tab is welded on the orifice plate which extends outer of the line indicating the orifice plate.
Orifice construction material
Any rigid material is chosen for the orifice plate. the material should be non-rusting and non-corrodible.
The material mustn't corrode in the fluid being metered otherwise the edge of the orifice will get damaged to a sufficient extent to interfere with the character of the flow and the accuracy of the measurement will be affected. A material of known thermal expansion coefficient should be chosen.
The common materials used for orifice plate maybe
Stainless steel,
Monel,
Phosphor bronze,
Glass,
Ceramics,
Plastics,
Brass,
Copper,
Aluminum and
Tantalum.
Types of the orifice plate
There are four types of orifice plates namely.
I. Concentric
II. Eccentric
III. Segmental
IV. Quadrant edge
I. Concentric
The concentric orifice plate is ideal for liquid as well as gases and steam service flow measurement.
The beta ratio is the ratio of the orifice bore to the internal diameter of the pipe. Concentric orifice plate beta ratio is 0.15 to 0.75 for liquids
The concentric orifice plate beta ratio for gases and steam is 0.20 to 0.70. Best results occur for the beta ratio of 0.4 and 0.6.
The concentric type is used for clean fluids, dirty fluids are not good to use for this type because dirt will be contaminated on the base of the orifice.
II. Eccentric
The eccentric orifice plate has an eccentric hole. This is useful for measuring fluids that contain solids, oil, water, and wet steam.
Eccentric plates use either flange taps or vena contracta taps,
the only condition for taps that the angle must be at 180º or 90º to the eccentric opening.
Eccentric orifices have the bore away from the center of plate solids containing materials less contaminate in this type of orifice plate.
III. Segmental type
This type of orifice has a segmented circle hole so we call it a segmental orifice. This is used to measure the flow of slurry.
Segmental orifice tap location is better at 180º from the center.
IV. Quadrant Edge.
Quadrant edge orifice is used for pipe sizes less than 50 mm.
For low Reynolds numbers in the range ( 100,000 down to 5,000) quadrant edge orifices produce a constant Coefficient of discharge.
Eccentric or segmental type orifice is used in a location that its lower edge coincides with the inside bottom of the pipe, so these can be used in the measurement of dirty fluids, slurries, and fluids containing solids. This allows the solids to flow through the orifice without any obstruction so no contaminated particles on the orifice.
Types of pressure taps
There are five common locations for the differential pressure taps :
(i) Flange taps
(ii) Venacontracta taps
(iii) Radius taps
(iv) Full flow or pipe taps and
(v) Corner taps.
(i) Flange taps.
Flange taps are used in a system where the diameter of pipes is more than 50 mm. Flange pressure taps have a specification that center lines are 25 mm from the orifice plate surface. They cannot be used for pipe sizes of less than 35 mm diameter. Since the venacontracta may be closer than 25 mm from the orifice plate and this may cause an error in the measurement of flow.
(ii) Venacontracta taps.
Venacontracta taps are normally used to pipe size above 150 mm of diameter.
Venacontracta taps are used in a location of an upstream tap one pipe diameter upstream of the orifice plate, and a downstream tap located at the point of minimum pressure.
d1 = d
d2 = minimum pressure
(iii) Radius taps.
Radius taps same as venacontracta taps except that the downstream tap is located at one-half pipe diameter (while in venacontracta taps it is at a point of minimum pressure).
Radius taps simplify the pressure taps location dimensions so these taps are generally considered superior to the venacontracta taps.
Radius taps do not vary with changes in orifice ratio so easy calculation.
d1 = d
d2 = 1/2 d
(iv) Pipe taps.
Pipe taps are located 2.5 pipe diameters upstream (d1 = 2.5D) and 8 diameters downstream (d2 = 8D) from the orifice plate. Because of the distance from the orifice, the exact location is not critical, but the effects of pipe roughness, dimensional inconsistencies, and so on are more severe.
d1 = 2.5 D
d2 = 8D
(v) Corner taps.
These are called corner taps because the pressure is measured at the Corner of the orifice plate and the pipe wall.
These taps are similar in many respects to flange taps. These are used for pipe sizes having smaller diameters of less than 50 mm.
Installation of the orifice plate
The orifice plate is inserted into the main pipeline between adjacent flanges, the outside diameters of the plate being turned to fit within the flange bolts. Flanges are either screwed or welded to the pipes
Advantages of orifice plates
Following are the properties of orifice plate;
High differential pressure is generated due to restrictions so easy to measure and exhaustive data is available.
purchasing and installation cost of orifice plate is low as compare to others sensors.
One advantage of rice plate is that its easy to replace.
b. Flow Nozzle
Flow nozzle is consists of a smooth and convergent section of pipe that discharges the flow of fluid parallel to the wall of the downstream pipe.
The diameter ratio of a flow nozzle is smaller and the flow coefficient of a flow nozzle is larger.
The downstream end of the flow nozzle is a short tube and has the diameter of the venacontracta of an orifice of the same capacity.
Flow nozzle advantage over an orifice is that its Pressure recovery is better than an orifice.
Pitot tube
Working principle
If a blunt object is placed in the flow channel its will reduce the velocity of the fluid to zero, at the point just before the blunt object, this point is called the stagnation point.
By measuring the difference between normal pressure and stagnation point we can measure the flow of fluid.
So the flow is equal to the difference between total pressure and static pressure.
Pitot tubes are very simple and don't have any moving parts.
Pitot tubes are used in air crafts.
d. Venturi tube
Venturi tubes are used to create a difference of pressure and by measuring that difference in pressure we can calculate the flow of fluid the same as the orifice.
Here in the figure below construction of the venturi tube is shown.
e. Dall tube
Dall tube is a modification of venturi tube. The difference between venturi tube and dall tube is that dall tube produces a large differential pressure
Target flowmeters
If we place a target in the path of fluid flow it results in annular orifice configuration
A target (which may be a circular disk) is supported concentrically in the pipe.
The target meter is a combination of a sensor and force measuring mechanism with a flow transmitter.
Pressure difference applies a force on target proportional to the square of the flow rate.
a rod passing through the diaphragm seal carries out this force and measurement done by an electronic force balancing system
Advantages
Its sensor and the responsive mechanism are in a single casing. This eliminates the differential pressure fluid connections.
Applications
2. Positive displacement flow meter
Positive displacment meters are also known as quantity flow Meters
Quantity meters or positive displacement flow Meters the flow of fluids by dividing the fluid into a separate container of known capacity increments and then pass the fluid downstream just by calculating the separate increments we can calculate the fluid flow. The number of times the container is filled and emptied gives the quantity of flow.
In industrial measurement systems, these positive displacement flow meters are used to measure low flow rates.
There are different types like
A. Piston flow meter
B. Oval gear flow meter
C. Nutating disk flow meter
D. Rotary vane flow meter
A. Piston flow meter
Several piston-type operations on a center crank are generally incorporated in this type of meter so it is called a piston flow meter.
The reciprocating piston passes the liquid alternately through each end of the cylinder from the inlet to the outlet and the slide valve is used to control the inlet and outlet ports.
A counter is connected to measure the total amount of flow
The differential pressure is small for a low flow so large diameter pistons with small strokes are used in this system. These meters are designed to have minimum friction. The accuracy of the piston flow meter is within 0.5%.
B. Oval gear
the oval gear flow meter is a special variety of the rotating tube flow meter.
In an oval gear flow meter, a precise volume of liquid is captured by the gap formed between the housing and the gear. so it is very precise.
These are best for the measurement of viscous liquid, which is difficult to measure using other types of flow meters.
C. Nutating disk
Nutating disk flow meters are also called disk meters.
Its made up of radially slotted disk moving assembly, which separates the fluid into increments. the assembly contains a ball bearing and an axial pin. Fig. below shows the nutating disk type meter.
Nutating disk meters are used extensively for residential water service billings.
D. Rotary vane
The rotary vane flow meter is a type of rotary positive displacement meter. a rotor sI contained by a housing which is machined accurately. The rotor is carrying evenly spaced blades, the rotor revolves on ball bearings when fluid passes through the meter, the rotor and vanes or blades revolve about a fixed cam causing the blades to move outward.
The movement of the blades forms a measuring chamber of precise volume between two blades. so by calculating the rotation of the chamber we can measure flow.
Used for oil and gas flow measurement
3. Velocity flow meter
some of the types of velocity flow meters are:
a. Turbine flow meters
b. electromagnetic flowmeter
c. sonic and ultrasonic flow meter
d. Vortex shedding flow meter
a. Turbine flow meter
A turbine flow meter is a simpler device for measuring flow velocity.
A rotating shaft with turbine-type angular blades is placed inside the flow pipe. The fluid flowing through the pipe causes a rotation of the turbine blades, speed of rotation is measured that is directly proportional to the rate of flow.
In units of a turbine flow meter, a magnetic pick-up system senses the rotation of the rotor through the tube wall.
The output is usually in the form of electrical pulses from the magnetic pick-up sensor with a frequency proportional to the flow rate.
Turbine flow meters are used to measure the flow of clean and non-corrosive fluids.
b. Electromagnetic
Electromagnetic flowmeter is unique as compared to all other flow meters.
It works on the principle of electromagnetic induction. when a conductor is moved in a magnetic field, voltages are induced in the coil, electromagnetic flowmeter work on the same principle. so measurement of voltage is proportional to the velocity of a fluid.
The advantages of the electromagnetic type flow meter are;
There is no obstruction to the fluid flow path, linear voltage output.
If flow direction changes inversely it can be measured.
Any changes in pressure, temperature, and viscosity of the fluid do not affect the output of the meter.
Limitation
Only measure the velocity of conductor liquids like mercury or week conductor like water.
c. Ultrasonic
Ultrasonic meters are velocity meters. They measure the velocity of the gas in the pipe. the volume of the gas can be calculated by measuring the velocity and the cross-sectional area.
The transit time T1-2 of an ultrasonic signal traveling with the flow is measured from Transducer 1 to Transducer 2.
Then transit time T2-1 of an ultrasonic signal traveling against the flow is measured from Transducer 2 to Transducer 1.
The transit time of the signal traveling with the flow will be less than that of the signal traveling against the flow due to the velocity of the gas within the pipe.
The measurement of the transit time is proportional to the flow velocity.
Speed Of Sound
Speed of sound is the sum of the transit times divided by their product, all then multiplied by the path length.
SOS =(T1+T2 / T1*T2)L
Modern ultrasonic flowmeters use high-frequency clocks to accurately perform transit time measurements.
In a 12-inch flow meter, the average transit time may be on the order of 1 millisecond. Accurate meter performance needs consistent and repeatable transit time measurements.
d. Vortex shedding
Vortex meters work on the principle of the Karman vortex street.
When fluid flows past a bluff body, vortices are alternately formed on both sides with opposite directions of rotation.
These vortices generate a local low pressure so a fluctuating pressure is generated. The pressure fluctuations are recorded by the sensor and converted to an electrical pulse signal. The vortices develop very regularly within the permitted application limits of the device. Volumetric flow is proportional to the frequency of vortex shedding.
The vortex flowmeter has excellent performance, robustness and is very reliable.
4. Mass flow meter
2. corolios mass flow meter
a. Angular-Momentum-Type Mass Flow Meters
Newton’s second law of the angular motion is the definition of angular momentum,
when an impeller is rotating if a material hits over the impeller (or passes through the impeller because it is designed in vanes to pass material through it, usually solid) it produces torque and by measuring this torque we can calculate the mass flow.
So angular momentum is introduced into the fluid stream and by measuring torque made by angular momentum.
b. Coriolis mass meter
Thermal dispersion meter
A thermal heater is used to produce heat and with the flow thermal dispersion varies as we want to maintain the temperature at a specific point, so by this method, the mass of fluids is measured.
It is a flow meter using the heat transfer principle.
This flow meter has many limitations. The temperature sensors and the heater must place into the fluid stream, so more damage to these parts and increases maintenance and cost.
Heated Tube-Type Mass Flow Meter
5. Open channel flow meter
V-notch
Weirs
Flumes
6. Variable area flowmeters
In this category are rota flow meter
ROTA METER
Rotameter consists of a vertical pipe this pipe is tapered downward. The flow passes from the bottom side to the top side of the vertical tube. cylindrical float is used inside the tube. The fluid flows upward through the gap between the tube and the cylindrical float.
As the float moves up or down there is a change in the gap, as a result changing the area of the orifice. So by measuring this change on a scaled flow can be measured.
The float position is calibrated with the rate of flow.
The float settles down at a position, where the pressure drop across the orifice will create an upward thrust so it balances the downward force due to gravity. This is the design of a rota flow meter.
Laser doppler Anemometers
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