of Primary Elements
Primary elements are an
essential component of differential pressure (DP) flow measurement.
Primary elements place a constriction in the flow line that creates
a pressure drop in the line. The
DP transmitter uses the difference between upstream pressure and
downstream pressure in the line as a basis for computing flow.
The primary element types explained here are:
- Orifice measuring points
- Venturi tubes
- Averaging Pitot tubes
- Flow nozzles
- Wedge elements
Orifice Measuring Points
Orifice plates are the most common type
of primary element. An orifice
plate is a flat, usually round piece of metal, often steel, with an
opening or “orifice” in it. The
orifice plate needs to be positioned at a correct position in the
flowstream for it to function as a primary element for the purpose of
making a differential pressure flow measurement.
For it to be so positioned, it must be held in place.
This is typically done by an orifice assembly, an orifice flange,
or a holding element.
In addition to an orifice
plate and assembly or flange, most orifice plate installations require the
presence of a valve manifold, which serves to isolate the pressure
transmitter from the process. DP
flow transmitters use either a three valve or a five valve manifold.
Since an orifice plate
cannot serve as a functioning primary element unless it is held in proper
position, and since valve manifolds are required for most DP flowmeter measurements, an orifice measuring
point is defined here as having the following three components:
orifice assembly, flange, or holding element
orifice measuring point includes an orifice plate, but it also includes an
orifice assembly, flange, or holding element, and also a valve manifold in
Orifice plates are
classified according to the shape and position of the hole or opening they
contain. The following are the
main types of orifice plates:
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The Pitot tube is named for Henri Pitot, who invented it in 1732.
Henry Philibert Gaspard Darcy, another Frenchman, published a paper
in 1858 that made improvements on Pitot’s invention.
The first patent for the use of a Pitot tube to measure velocity in
pipes was given to Henry Fladd of St. Louis, Missouri, in 1889.
Pitot tubes are of two types:
A single port Pitot tube
includes an L-shaped tube measuring impact pressure.
This tube is inserted into the flowstream, with the opening facing
directly into the flow. Another
tube measuring static pressure has an opening parallel to the direction of
flow. Flowrate is proportional
to the difference between impact pressure and static pressure.
A multiport averaging Pitot tube has multiple ports to measure impact
pressure and static pressure at different points.
The DP transmitter computes flowrate by taking the average of the
differences in pressure readings at different points.
companies such as Emerson Rosemount and Veris have introduced proprietary
versions of the averaging Pitot tube.
Emerson Rosemount’s proprietary version is called the Annubar,
and it was formerly sold by Dieterich Standard, now part of Emerson
Process Management. Veris’
averaging Pitot tube is called the Verabar.
The Venturi tube was
invented by an Italian physicist named Giovanni Battista Venturi in 1797.
In 1887, Clemens Herschel used Venturi’s work to develop the first
commercial flowmeter based on it. His
version of the Venturi flowmeter became known as the Herschel Standard
Venturi. Herschel published
his paper called “The Venturi Water Meter” in 1898.
In 1970, a company called BIF introduced the Universal Venturi Tube™.
A Venturi tube is a flow tube that has a tapered inlet and a diverging
exit. The DP transmitter
measures pressure drop and uses this value to calculate flowrate.
A flow nozzle is a flow tube with a smooth entry and a sharp exist.
The DP transmitter computes flowrate based on the difference
between upstream pressure and downstream pressure.
Flow nozzles are mainly used for high-velocity, erosive,
non-viscous flows. Flow
nozzles are sometimes used as an alternative to orifice plates when
erosion or cavitation would damage an orifice plate.
They offer excellent long-term accuracy.
A wedge element is a flow tube that has a V-shaped flow
restriction protruding into the flowstream from at least one side of the
pipe. Wedge elements are
designed to measure fluids with a high solids content.
They are also well-suited for air, viscous flows, and for slurries.
Other Primary Elements
Other primary elements include low loss
flow tubes, Dall tubes, and the V-Cone primary element.
Low loss flow tubes are designed to produce a minimum amount of
permanent pressure loss. The
Dall tube was invented by an ABB hydraulics engineer named Horace E. Dall.
It is an adaptation of the Venturi tube.
The V-Cone is a proprietary device that is designed for flow
measurement with minimal upstream piping.
It is manufactured and sold by McCrometer in Hemet, California.
Spirax Sarco’s Gilflo
product is also classified as a primary element.
One other category that deserves mention here is laminar flow elements.
They are often used for air and gas flow measurement.
Laminar flow elements are used with mass flow controllers to create
a pressure drop and a flow measurement.
They are also used to measure air flow to internal combustion
engines. They are lower in
cost than most other primary elements.
Two companies in this market are Meriam Process Technologies and
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