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Overview 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
  • Other

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:

  1. An orifice plate

  2. An orifice assembly, flange, or holding element

  3. A valve manifold

An orifice measuring point includes an orifice plate, but it also includes an orifice assembly, flange, or holding element, and also a valve manifold in most cases.

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: 

·        Concentric
·        Conical
·        Eccentric
·        Integral
·        Quadrant
·        Segmental

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Pitot Tubes 

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:

  • Single port

  •  Multiport averaging Pitot tubes

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.

Some 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.

Venturi Tubes

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.

Flow Nozzles 

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. 

Wedge Elements

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 Furness Controls.

For further information on differential pressure, including studies and articles, see www.FlowDP.com.

Flow Research, Inc. | 27 Water Street | Wakefield, MA 01880 | (781) 245-3200 | (781) 224-7552 (fax) | (800) 245-1799 (from the USA) | info@flowresearch.com

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