It contents all type of Flow Measuring devices like Orificemeter, Venturimeter, Rotameter Etc. There are several types of flowmeter that rely on Bernoulli's principle. The pressure is measured either by using laminar plates, an orifice, a nozzle, or a Venturi tube to create an artificial constriction and then measure the pressure loss of fluids as they pass that constriction or by measuring static and stagnation pressures to derive the dynamic pressure.

1. FLOW MEASUREMENT
Prof. S D Yadav
sdyadav@jnec.ac.in,
MGM’s JNEC.

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3. Classification of Flow-meters
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4. VARIABLE HEAD OR DIFFERENTIAL METER
❖ Orifice Plates
❖ Venturi Tubes
❖ Flow Nozzle
❖ Pitot Tube
❖ Rotameter
❖ Weir
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5. VARIABLE HEAD OR DIFFERENTIAL METER
❖ Differential pressure meters work on the principle of partially obstructing the flow in a pipe.
This creates a difference in the static pressure between the upstream and downstream side of the
device. This difference in the static pressure (referred to as the differential pressure) is measured
and used to determine the flow rate.
❖ Head flow meters operate on the principle of placing a restriction in the line to cause a
differential pressure head. The differential pressure, which is caused by the head, is
measured and converted to a flow measurement.
❖ The devices in general, can therefore be termed as “obstruction type” flowmeters.
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6. VARIABLE HEAD OR DIFFERENTIAL METER
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7. MERITS AND DEMERITS – DIFFERENTIAL FLOW METER
❖ Merits
1. They are simple to make, containing no moving parts
2. Their performance is well understood
3. They are cheap – especially in larger pipes when compared with other meters
4. They can be used in any orientation
5. They can be used for most gases and liquids
6. Some types do not require calibration for certain applications
❖ The Demerits
1. Rangeability (turndown) is less than for most other types of flow meter
2. Significant pressure losses may occur
3. The output signal is non-linear with flow
4. The discharge coefficient and accuracy may be affected by pipe layout or nature of flow
5. They may suffer from ageing effects, e.g. the build-up of deposits or erosion of sharp edgesProf. S D YADAV
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8. ORIFICE METER
❖ An Orifice Meter is a conduit and a restriction to create a pressure drop. An hour glass
is a form of orifice.
❖ In order to use any of these devices for measurement it is necessary to empirically
calibrate them. That is, pass a known volume through the meter and note the reading in
order to provide a standard for measuring other quantities.
❖ An orifice in a pipeline is shown in figure with a manometer for measuring the drop in
pressure (differential) as the fluid passes through the orifice. The minimum cross
sectional area of the jet is known as the “vena-contracta.”
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9. ORIFICE WORKING
❖ As the fluid approaches the orifice the pressure increases slightly and then drops suddenly as the
orifice is passed. It continues to drop until the “vena contracta” is reached and then gradually
increases until at approximately 5 to 8 diameters downstream a maximum pressure point is
reached that will be lower than the pressure upstream of the orifice.
❖ The decrease in pressure as the fluid passes thru the orifice is a result of the increased velocity of
the gas passing thru the reduced area of the orifice.
❖ When the velocity decreases as the fluid leaves the orifice the pressure increases and tends to
return to its original level.
❖ The pressure drop across the orifice increases when the rate of flow increases. When there is no
flow there is no differential. The differential pressure is proportional to the square of the velocity.
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10. ORIFICE PLATE
1.Concentric – commonly used for general applications (gas, liquid & vapour).
2. Eccentric – recommended for fluids with extraneous matter to a degree that would clog
up concentric type.
3. Segmental – recommended for fluids combine with vapour or vapour with fluids.
a) Square Edge – applicable for higher pipe Reynolds Number; typical Re 500 to10,000
b) Quadrant – for lower pipe Reynolds Number; typically ranges from Re 250 to3300.
c) Conical – for Reynolds Number typically range from Re 25 to 75.
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11. ADVANTAGES AND DISADVANTAGES OF ORIFICE
Advantages
❖ Simple construction.
❖ Inexpensive.
❖ Easily fitted between flanges.
❖ No moving parts.
❖ Large range of sizes and opening ratios.
❖ Suitable for most gases and liquids.
❖ Well understood and proven.
❖ Price does not increase dramatically with
size.
Disadvantages
❖ Inaccuracy, typically 1%.
❖ Low rangeability, typically 4:1.
❖ Accuracy is affected by density, pressure
and viscosity fluctuations.
❖ Erosion and physical damage to the
restriction affects measurement accuracy.
❖ Cause some unrecoverable pressure loss.
❖ Viscosity limits measuring range.
❖ Require straight pipe runs to ensure
accuracy ismaintained.
❖ Pipeline must be full (typically for liquids).
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12. VENTURI METER
❖ A venturi meter, or venturi flow meter, is a device used to measure the velocity, or flow rate, of fluid
flowing through a pipeline. The venturi meter constricts the flow using a Herschel venturi tube.
❖ As the liquid flows through the pipeline, the device measures the pressure of the liquid before it enters the
venturi tube and as it exits the constricted area. These measurements are then compared to figure the
volumetric flow rate of the fluid. The flow meter is commonly used in plumbing applications to determine
the flow of fluids such as water, liquid propane, and oil.
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13. COMPARISON ORIFICE AND VENTURI METERS
1. Orifice reducing element is sharp edged while venturi is taperedtube.
2. Permanent pressure loss of orifice is 65% of measured d/p while venturi is only10%.
3. Venturi tubeis less sensitiveto Reynolds Number and gives more
accurate measurement when the process flow varies over a wide range.
4. Venturi tube is less affected by dirty fluid which build up deposits at orifice plates and
pressure tap connections.
5. Venturi tube meter is more costly compared to orifice plate costly compared to orifice
plate and requires greater length of pipeline.
6. Orifice plate is relatively easy to change for new range.
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14. PITOT TUBE
❖ A pitot tube, also known as pitot probe, is a flow measurement device used to measure fluid flow
velocity.
❖ The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was
modified to its modern form in the mid-19th century by French scientist Henry Darcy.
❖ It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure
liquid, air and gas flow velocities in certain industrial applications
Working:
❖ Pitot or Pitot static tube is an open ended tube where moving fluid flows in order to measure the
stagnation pressure . It is usually mounted under the wings of an aircraft . The Pitot tube is then
connected to a Diaphragm which expands and contracts depending on the speed of the air
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15. PITOT TUBE
Advantages of Pitot tube
❖ Low cost.
❖ Low permanent pressure loss.
❖ Ease of installation into existing systems.
Disadvantages of Pitot tube
❖ Low accuracy.
❖ Low Rangeability.
❖ Requires clean liquid, gas or vapour as
holes are easilyclogged.
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16. VARIABLE AREA - ROTAMETER
❖ A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.
❖ It belongs to a class of meters called variable flow meter, which measure flow rate by allowing the cross-sectional area the fluid
travels through to vary, causing a measurable effect
❖ The first variable area meter with rotating float was invented by Karl Kueppers in Aachen in 1908
❖ A rotameter consists of a tapered tube, typically made of glass with a 'float', inside that is pushed up by the drag force of the
flow and pulled down by gravity.
❖ A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards.
However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows
increases.
❖ Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally
grooved and partially colored so that it rotates axially as the fluid passes.
❖ This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the
float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.
❖ The "float" must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there
is no flow.
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17. VARIABLE AREA - ROTAMETER
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18. VARIABLE AREA - ROTAMETER
Advantages
❖ A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along
with gravity, to measure flow rate.
❖ A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials,
allowing for its widespread use.
❖ Since the area of the flow passage increases as the float moves up the tube, the scale is approximately
linear.
❖ Clear glass is used which is highly resistant to thermal shock and chemical action.
Dis-Advantages
❖ a rotameter must be oriented vertically. Significant error can result if the orientation deviates
significantly from the vertical.
❖ Due to the direct flow indication the resolution is relatively poor compared to other measurement
principles. Readout uncertainty gets worse near the bottom of the scale.
❖ Since the float must be read through the flowing medium, some fluids may obscure the reading.
❖ Rotameters are not easily adapted for reading by machine.
❖ Rotameters are not generally manufactured in sizes greater than 6 inches/150 mm, but bypass designs
are sometimes used on very large pipes.
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19. FLOW NOZZLE
❖ Flow nozzle is a restriction consisting of an elliptical contoured inlet and a cylindrical throat
section. Pressure taps used to measure the difference in static pressure created by flow nozzle are
commonly located one pipe diameter (1D) upstream and ½ pipe diameter (1/2D) downstream from
the inlet face of the nozzle.
❖ The Flow Nozzle is similar to the venturi but are in the shape of an ellipse. They have a higher flow
capacity than orifice plates.
❖ Another main difference between the flow nozzle and the venturi is that although they have similar
inlet nozzles, the flow nozzle has no exit section.
❖ These devices are more cost effective, but as such they provide less accuracy than venturis, and have
a higher unrecoverable pressureloss.
❖ Flow Nozzles can handle larger solids and be used for higher velocities, greater turbulence
and high temperature applications.
❖ They are often used with fluid or steam applications containing some suspended solids, and in
applications where the product is being discharged fromservice.
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20. FLOW NOZZLE
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21. FLOW NOZZLE
Advantages of Flow nozzle
❖ High velocity applications.
❖ Operate in higher turbulence.
❖ Used with fluids containing suspended solids.
❖ More cost effective than venturis.
❖ Physically smaller than the venturi.
Disadvantages of Flow nozzle
❖ More expensive than orifice plates.
❖ Higher unrecoverable pressure loss.
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22. OPEN CHANNEL FLOW-METER - WEIR
❖ The "open channel" refers to any conduit in which liquid flows with a freesurface.
❖ Included are tunnels, non-pressurized sewers, partially filled pipes, canals, streams, and
rivers.
❖ Of the many techniques available for monitoring open-channel flows, depth-related
methods are the most common.
❖ These techniques presume that the instantaneous flow rate may be determined from a
measurement of the water depth, or head.
❖ Weirs and flumes are the oldest and most widely used primary devices for measuring
open-channel flows.
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23. OPEN CHANNEL FLOW-METER - WEIR
❖ The flow rate over a weir is a function of the weir geometry and of the weir head (the
weir head is defined as the vertical distance between the weir crest and the liquid
surface in the undisturbed region of the upstreamflow).
❖ Weirs are variable head, variable area flow meters employed for measuring large
volumes of liquids in open channels.
❖ The device operates on the principle that if a restriction of specified shape and form is
introduced in flow path, a rise in the upstream liquid occurs which is a function of the
flow rate through the restriction.
❖ Weirs operate on the principle that an obstruction in a channel will cause water to back
up, creating a high level (head) behind the barrier.
❖ The head is a function of flow velocity, and, therefore, the flow rate through the device.
Weirs consist of vertical plates with sharpcrests.
❖ The top of the plate can be straight or notched. Weirs are classified in accordance with
the shape of the notch. The basic types are V-notch, rectangular, andtrapezoidal.
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