About DP Meters
pressure (DP) flowmeters rely on a constriction placed in the flow line that
creates reduced pressure in the line after the constriction.
A DP flowmeter requires a means to detect the difference in upstream vs.
downstream pressure in the flow line. While
this can be done with a manometer, today’s DP flowmeters use differential
pressure transmitters that sense the difference in pressure, and then use this
value to compute flowrate. Differential
pressure (DP) flowmeters are among the most traditional of flowmeters.
Their large installed base means that they still exert a powerful force
on the flowmeter market.
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More info on DP Flow:
The history of
DP flow measurement goes back to at least the 17th century, though
the measurement of flow using nozzles goes back to Roman times.
At the beginning of the 17th century, Torricelli and Castelli
arrived at the concepts that underlie differential pressure measurement today:
that flowrate equals velocity times pipe area, and that the flow through an
orifice varies with the square root of the head.
In 1738, Bernoulli developed his famous equation for flowrate
of primary elements for use in measuring DP flow also began about this time.
Pitot presented a paper on the use of the “Pitot tube” in 1732.
Venturi published his work on the Venturi principle for measuring flow in
1797. However, Venturi’s work was
not developed for commercial application until 1887 when Clemens Herschel used
Venturi’s work to develop the first commercial flowmeter based on it.
In 1898, Herschel published his paper, “The Venturi Water Meter.”
Max Gehre received one of the first patents on orifice flowmeters in
1896. The first commercial orifice
plate flowmeter appeared in 1909, and was used to measure steam flow.
Shortly thereafter, the oil and gas industries began using orifice plate
flowmeters due to ease of standardization and low maintenance.
a DP Flowmeter Is
A DP flowmeter
consists of a differential pressure transmitter integrated with a primary
element. In the past, the pressure
transmitter companies sold DP transmitters and users ordered their primary
elements separately. Now companies
such as Emerson Rosemount are selling their DP transmitters already integrated
with a primary element, such as an Annubar or an orifice plate.
When a DP pressure transmitter is integrated with a primary element and
has the capability of calculating flowrate based on difference in pressure, it
is a DP flowmeter.
pressure (DP) meters excel at measuring clean liquids, steams, and gases when
pressure drop is not a major issue, the application requires low to medium
accuracy, and price is a consideration. DP
flowmeters cause varying amounts of pressure drop, depending on the type of
primary element used. Orifice
plates cause substantial loss of line pressure, while averaging Pitot tubes
cause less. DP flowmeters are
considerably less expensive to buy than most Coriolis and ultrasonic meters.
of differential pressure (DP) transmitters today are facing a dilemma.
Should they upgrade to new-technology flowmeters, or stick with the tried
and true DP meters? Of course, some
end-users at large plants may choose to upgrade some of their DP meters, and
stick with DP technology elsewhere in the plant.
Instead of replacing all their DP meters, they may make this decision on
a case-by-case basis.
Researching the DP Market
and Manufacturers of DP Transmitters
in Pressure Transmitters
Over the past
several years, pressure transmitter suppliers have released a number of new
products with advanced features. These
features promise higher accuracy, greater reliability, enhanced
self-diagnostics, and more advanced communication protocols.
The promise of greater reliability is perhaps the strongest driving force
behind the pressure transmitter market. While
some products may have a higher initial purchase price, end-users cite a number
of reasons for shifting to higher performing products.
These include the need to conform to regulatory requirements, the need
for reliability, a desire to standardize pressure products, and the need to do
in Primary Elements
elements suppliers have made a number of technological improvements in primary
elements in the past five years. Emerson
Rosemount has introduced a conditioning orifice plate that reduces the upstream
requirements for a DP flow measurement. Veris
has introduced the Accelabar that combines elements of a flow nozzle with
elements of an averaging Pitot tube. Emerson
Daniel has upgraded its Senior Orifice Fitting with the release of the new
Senior Model 2000 Orifice Fitting. The
Model 2000 is designed for longer life and lower cost of ownership.
It has check valves and O-Rings that are designed to prevent the escape
of any potentially harmful content through the valve.
Both Emerson’s Senior and Junior Orifice Fittings are widely used in
custody transfer gas flow measurement.
on Primary Elements...