Orifice Meter
Orifice Meter
1. Introduction
The first recorded use of an orifice device for fluid measurement
was in 1797 by Giovanni B. Venturi, an Italian physicist whose work
led to the development of the modern Venturi meter in 1886 by
Clemons Herschel. In 1890, it has been reported that an orifice
meter designed by Professor S.W. Robinson of Ohio State
University was used to measure gas near Columbus, Ohio. In
1903, T.B. Weymouth began a series of tests in Pennsylvania
leading to the publication of coefficients for orifice meters with
flange taps. At the same time, E.O. Hickstein conducted a similar
series of tests at Joplin, Missouri from which he developed data for
orifice meters with integrated pipe taps.
The orifice meter is used widely as a device for measuring the
flow rate of a fluid in a pipeline. Compared with other head meters,
such as the venturi meter and the nozzle, the orifice meter is less
expensive to fabricate and install; however, the permanent energy
loss is relatively high. The orifice plate can have a square edged or
sharp-edged hole. For measurement of the pressure drop across
the orifice, the pressure taps can be at corner, radius, pipe, flange,
or vena contracta locations. The direction of flow through the orifice
can be horizontal, vertical, or inclined. The measured pressure
drop across the orifice is related to the flow rate by means of an
orifice coefficient, which accounts for friction.
A differential pressure meter creates a pressure drop by
combining a conduit and a restriction. A nozzle, Venturi or thin,
sharp-edged orifice can be used as the flow restriction. Prior to
1
Orifice Meter
Orifice Meter
using any of these devices for measurement, it is necessary to
empirically calibrate them by passing a known volume through the
meter and noting the reading to provide a measurement standard
for other quantities. Due to the ease of duplication and the simple
construction, the thin, sharp-edged orifice has been adopted as a
measurement standard. Extensive calibration work has also been
performed on the device, making it widely accepted as a standard
means for measuring fluids. Provided the standard mechanics of
construction are followed, no calibration is required. An orifice
installed in a pipeline along with a manometer for measuring the
drop in pressure (differential) as the fluid passes through the orifice
is shown in Figure 1. The minimum cross-sectional area of the jet
immediately after the orifice is known as the “vena contracta”.
Fig.1: Typical Orifice Flow Pattern Flange Tap Diagram
2
Orifice Meter
Orifice Meter
2. How an Orifice Meter Works
As fluid approaches the orifice, the pressure increases slightly
and then drops suddenly as the fluid passes through the orifice.
The pressure continues to drop until it reaches the “vena contracta”
and then it gradually increases until it is approximately 5D to 8D. At
this point, it reaches maximum downstream pressure which is
lower than the pressure upstream of the orifice. The pressure
decrease as fluid passes through the orifice is due to the increased
velocity of the natural gas passing through 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 loss is not fully recovered due to loss of friction and
turbulence in the stream. The pressure drop across the orifice
(Figure 2) increases when the rate of flow increases. When there is
no flow, there is no differential pressure. The differential pressure is
proportional to the square root of the velocity. Therefore, it follows
that if all other factors remain constant the differential is
proportional to the square root of the flow rate.
Fig.2 : Orifice plate showing vena contracta.
3
Orifice Meter
Orifice Meter
3. Shape & Size of Orifice meter
Orifice meters are built in different forms depending upon the
application specific requirement, The shape, size and location of
holes on the Orifice Plate describes the Orifice Meter Specifications
as per the following:
 Concentric Orifice Plate
 Eccentric Orifice Plate
 Segment Orifice Plate
 Quadrant Edge Orifice Plate
Fig.3 : Holes on the Orifice Plate.
3.1
Concentric Orifice Plate
It is made up of SS and its thickness varies from 3.175 to 12.70
mm. The plate thickness at the orifice edge should not be exceeded
by any of following parameters:
1. D/50 where, D = The pipe inside diameter
2. d/8 where, d = orifice bore diameter
3. (D-d)/8
4
Orifice Meter
3.2
Orifice Meter
Eccentric Orifice Plate
It is similar to Concentric Orifice plate other than the offset hole
which is bored tangential to a circle, concentric with the pipe and of
a diameter equal to 98% of that of the pipe. It is generally
employed for measuring fluids containing

Media having Solid particles

Oils containing water

Wet steam
3.3 Segment Orifice Plate
It has a hole which is a semi circle or a segment of circle. The
diameter is customarily 98% of the diameter of the pipe.
3.4
Quadrant Edge Orifice Plate
This type of orifice plate is used for flow such as crude oil, high
viscosity syrups or slurries etc. It is conceivably used when the line
Reynolds Numbers* range from 100,000 or above or in between to
3,000 to 5,000 with a accuracy coefficient of roughly 0.5%.
4. Flow Measurement By Orifice Meter
An orifice meter is essentially a cylindrical tube that contains a
plate with a thin hole in the middle of it. The thin hole essentially
forces the fluid to flow faster through the hole in order to maintain
flow rate. The point of maximum convergence (vena contracta)
usually occurs slightly downstream from the actual physical orifice.
This is the reason why orifice meters are less accurate than venturi
meters, as we cannot use the exact location and diameter of the
5
Orifice Meter
Orifice Meter
point of maximum convergence in calculations. Beyond the vena
contracta point, the fluid expands again and velocity decreases as
pressure increases.
Fig. 4 : Orifice meter
Figure 4 shows an orifice meter with the variable position of vena
contracta with respect to the orifice plate. By employing the
continuity equation and Bernoulli’s principle, the volumetric flow
rate through the orifice meter can be calculated as described
previously for venturi meter.
where Co is the orifice discharge coefficient, S2 is the area of crosssection of the orifice, V2 is the flow velocity through the orifice, β is
the ratio of the diameter of orifice to that of the diameter of pipe, ΔH
is the manometric height difference × (specific gravity of
manometric fluid – specific gravity of water), and g is the
acceleration due to gravity.
6
Orifice Meter
Orifice Meter
Rather than use this or any other unwieldy equations and because
assumptions would have to be made, the simple solution is to use
the supersonic formula and allow for a reduction in flow when
making the initial guess.
7