Page 443 - The Mechatronics Handbook

0066_Frame_C19 Page 65 Wednesday, January 9, 2002 5:27 PM

Orifice Plate
Pressure drop across flowmeter
is a measure of fluid flowrate

p p
1 2

Flow Direction d D

FIGURE 19.53 Flowrate measurement using an orifice plate.

documents known as standards. These standards not only specify C and ε, but also the geometry and
installation conditions for the square-edged orifice plate, and two other common types of differential
pressure flowmeters, the Venturi tube and nozzle. Installation recommendations are intended to ensure
that fully developed turbulent flow conditions exist within the measurement section of the flowmeter.
The most commonly used standard in Europe is ISO 5167-1 [7], while in the USA, API 2530 is the
most popular [8]. Thus, one of the major reasons for the continued use of the orifice plate flowmeter
is that measurement uncertainty (typically ±2% of reading) can be predicted without the need for
calibration, as long as it is manufactured and installed in accordance with one of these international
standards.
The major disadvantages of the orifice plate are its limited range and sensitivity to changes in the
flow’s velocity profile. The fact that fluid flow rate is proportional to the square root of the measured
differential pressure limits the range of a one plate/one differential pressure transmitter combination to
about 3:1. The required diameter ratio (also known as beta ratio) of the plate depends on the maximum
flow rate to be measured and the range of the differential pressure transducer available.
Equation (19.66) assumes a fully developed and stable flow velocity profile, and so installation of the
device is critical, particularly the need for sufficient straight pipework, upstream and downstream of the
meter. Wear of the leading edge of the orifice plate can also severely alter measurement accuracy and so
this device is normally only used with clean fluids.
The other two differential pressure flowmeters covered by international standards are the Venturi tube
and nozzle. The Venturi tube has a lower permanent pressure less than the orifice plate, and is less sensitive
to erosion and upstream disturbances. Major disadvantages are its size and cost. It is more difficult, and
therefore more expensive, to manufacture than the orifice plate.
Nozzles have pressure losses similar to orifice plates but because of their smooth design they retain
their calibration over a long period. However, these devices are more expensive to manufacture than the
orifice plate but cheaper than the Venturi tube. The two most common nozzle designs of nozzle are
covered by international Standards, with the ISA-1932 nozzle being preferred in Europe and the ASME
long radius nozzle being preferred in the U.S.
There are many other types of differential pressure flowmeter, such as the segmental wedge, V-cone,
elbow, and Dall tube. Each of these has advantages over the orifice plate, Venturi tube, and nozzle for
specific applications. For example, the segmental wedge can be used with flows having a low Reynolds
number, and a Dall tube has a lower permanent pressure loss than a Venturi tube. However, none of
these instruments are yet covered by international standards and so calibration is needed to determine
their accuracy.

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