1. Gujarat Technological University
L.D. College of Engineering
Year: 2016-17
Subject: Fluid Flow operation(2130502)
Topic: Orifice meter & Pitot tube
Name of the Students:
Sindhav Jaydrath 160283105011
Vasava Yogesh 160283105012
2. ORIFICE METER
The venturi meter is a reliable flow measuring device.
Furthermore, it causes little pressure loss.
For these reasons it is widely used, particularly for large-volume liquid
and gas flows.
However this meter is relatively complex to construct and hence
expensive.
Especially for small pipelines, its cost seems prohibitive, so simpler
devices such as orifice meters are used.
3. PRINCIPAL OF ORIFIC METER
The principle of the orifice meter is identical with that of the venturi meter.
The reduction of the cross section of the flowing stream in passing through
the orifice increases the velocity head at the expense of the pressure head,
and the reduction in pressure between the taps is measured by a manometer.
Bernoulli's equation provides a basis for correlating the increase in velocity
head with the decrease in pressure head.
One important complication appears in the orifice meter that is not found in
the venturi. Because of the sharpness of the orifice, the fluid stream
separates from the downstream side of the orifice plate and forms a free-
flowing jet in the downstream fluid and vena contracta forms.
5. Continue…
It is also known as Pipe orifice & Orifice plate.
It may be installed in pipeline with a minimum of trouble and expense.
Pipe orifice is a device used for measuring the rate of flow of a fluid through
a pipe.
It consists of a thin, circular plate with a hole in it. The plate is held in the
pipeline between two flanges.
Pipe orifice is a cheaper device as compared to venturimeter.
The diameter of orifice is generally 0.5 times the diameter of the pipe (D),
although it may vary from 0.4 to 0.8 times the pipe diameter.
6. Orifice plates can be a surprisingly good way of measuring small gas
flows. To be able to use orifice plates of very small orifice diameter in an
existing installation in a declining gas field may be much more economical
than to replace the metering or shut the field.
From the beginning of the flow measurement to its conclusion a 4″ orifice
meter with different orifice plates (including those described in this
appendix) might measure a range of around 3000:1 in terms of mass flow
rate.
The orifice fitting, which enables the operator to change or remove an
orifice plate easily.
Continue…
7. APPLICATION
Orifice plates are most commonly used to measure flow rates in pipes, when
the fluid is single-phase (rather than being a mixture of gases and liquids, or
of liquids and solids) and well-mixed, the flow is continuous rather than
pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas),
the flow profile is even and well-developed and the fluid and flow rate meet
certain other conditions.
Under these circumstances and when the orifice plate is constructed and
installed according to appropriate standards, the flow rate can easily be
determined using published formulae based on substantial research and
published in industry, national and international standards.
Plates are commonly made with sharp-edged circular orifices and installed
concentric with the pipe and with pressure tapping's at one of three standard
pairs of distances upstream and downstream of the plate.
8. The edges may be rounded or conical, the plate may have an orifice the same
size as the pipe except for a segment at top or bottom which is obstructed,
the orifice may be installed eccentric to the pipe, and the pressure tapping's
may be at other positions. Variations on these possibilities are covered in
various standards and handbooks. Each combination gives rise to different
coefficients of discharge which can be predicted so long as various
conditions are met, conditions which differ from one type to another.
Once the orifice plate is designed and installed, the flow rate can often be
indicated with an acceptably low uncertainty simply by taking the square
root of the differential pressure across the orifice's pressure tapping's and
applying an appropriate constant. Even compressible flows of gases that
vary in pressure and temperature may be measured with acceptable
uncertainty by merely taking the square roots of the absolute pressure and/or
temperature, depending on the purpose of the measurement and the costs of
ancillary instrumentation.
Continue…
9. Pitot tube
A Pitot tube is a pressure measurement instrument 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 velocities in industrial
applications. The pitot tube is used to measure the local velocity at a given
point in the flow stream and not the average velocity in the pipe or
conduit.
12. Working
The basic pitot tube consists of a tube pointing directly into the fluid flow.
As this tube contains fluid, a pressure can be measured; the moving fluid is
brought to rest (stagnates) as there is no outlet to allow flow to continue.
This pressure is the stagnation pressure of the fluid, also known as the total
pressure or (particularly in aviation) the pitot pressure.
The liquid flows up the tube and when equilibrium is attained, the liquid
reaches a height above the free surface of the water stream.
13. Measurement of static pressure in this case is made at the boundary of the
wall (Fig.-1). The axis of the tube measuring the static pressure must be
perpendicular to the boundary and free from burrs, so that the boundary is
smooth and hence the streamlines adjacent to it are not curved. This is done
to sense the static pressure only without any part of the dynamic pressure.
A Pitot tube is also inserted as shown (Fig.-2) to sense the stagnation
pressure. The ends of the Pitot tube, measuring the stagnation pressure, and
the piezometric tube, measuring the static pressure, may be connected to a
suitable differential manometer for the determination of flow velocity and
hence the flow rate.
15. Operations
In industry, the velocities being measured are often those flowing in ducts and
tubing where measurements by an anemometer would be difficult to obtain. In
these kinds of measurements, the most practical instrument to use is the pitot
tube.
The pitot tube can be inserted through a small hole in the duct with the pitot
connected to a U-tube water gauge or some other differential pressure gauge
for determining the velocity inside the ducted wind tunnel. One use of this
technique is to determine the volume of air that is being delivered to a
conditioned space.
