A primary element physically interacts with the flowing fluid to create a predictable, repeatable effect that can be related to flowrate. It operates by placing a constriction in the flow that creates a pressure drop in the fluid. A differential pressure (DP) transmitter measures the upstream and downstream pressure of the fluid to compute flowrate. A primary element is not the sensor, the transmitter, or the electronics. It is the flow-shaping element that makes measurement possible.
The concept comes from differential pressure (DP) flow measurement, where the separation is very explicit. For example, an orifice plate constricts the flow by forcing it through an opening in a (usually) steel plate. This results in a pressure drop that is proportional to flow. A secondary element, the DP transmitter, calculates flow based on the amount of pressure drop created by the orifice plate.
Another example is a flow nozzle or Venturi flow element. These primary flow elements accelerate the flow smoothly and create a predictable pressure difference. A DP transmitter is used to calculate the flow based on this pressure difference.
In these systems you can replace the transmitter and you can upgrade the electronics, but the primary element defines the measurement.
Orifice measuring point
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. To be in position, 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, we define an orifice measuring point as having the following three components:
- An orifice plate
- An orifice assembly, flange, or holding element
- A valve manifold
Orifice plates are classified according to the shape and position of the hole or opening they contain:
- Concentric
- Conical
- Eccentric
- Integral
- Quadrant
- Segmental
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 and Veris have introduced proprietary versions of the averaging Pitot tube. Emerson’s proprietary version is called the Annubar, and it was formerly sold by Dieterich Standard, now part of Emerson. 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.
Cone elements
Cut-away view of a Wafer Cone®
llustration courtesy of McCrometer
Cone meters have been around since McCrometer, Inc. developed and patented the first successful model, its V-Cone®, in 1985.
Cone meters consist of a specially tapered element positioned within the flowstream to create a restriction to the flow through the pipe. Some cones are held in place by a tube that also connects it to the outside of the pipe. In the even more compact Wafer Cone® design, the element is fastened in place with an attached bar that spans the pipe diameter. The cone element creates a difference in speed and pressure as the flow is forced around it, then allowed to resume unobstructed flow beyond the cone. DP transmitters get measurements via a port in the pipe upstream from the cone (the high side) and also downstream (the low side) via another port either in the pipe wall or in the blunt end of the cone.
Flowrate calculations are based on the same physical principles as other differential pressure-type flowmeters, including Venturi meters. However, cone elements and meters are distinct from Venturi ones in design and are called just “cone” elements and meters. Perhaps the confusion arose from the “V-” in the name people became familiar with from the first and still successful cone meters, McCrometer’s proprietary V-Cone? But McCrometer says that “V-” actually refers to the shape.
Cone meters can be used to measure gas, steam and liquids in wide variety of industries. Extremely robust, accurate, economical, low-maintenance, and useful in tight fits, they can be found from subsea to processing plants to Navy ships to satellites.
Cut-away view of a cone element
Illustration courtesy of Samil Industry
Flow nozzles
A flow nozzle is a flow tube with a smooth entry and a sharp exist. With, it might be said, elegant simplicity, the restriction caused by the reductions in diameter within the nozzle creates measureable changes in the flow through it, but the smooth, tapered shape causes less permanent pressure loss than an orifice plate.
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 — the “wedge” — protruding into the flowstream from at least one side of the pipe. The wedge might be attached and contained inside the pipe or it might be created by heavy sections of metal welded into a notch cut in the pipe. In the latter case, there are often reinforcing bars across the notch (parallel to the length of the pipe). In any case, the wedge is solidly built in, making these elements extremely robust, with no moving parts, no critical areas to wear or shift. They can handle any type of flow profile, whether laminar, transitional, or turbulent, and they are less prone to clogging or build up than some other element types. The wedge shape is usually symmetrical – presenting the same on the upstream and downstream sides – thus allowing for bi-directional flow. Or, in the case of highly erosive fluids, should there eventually be significant-enough wear on one side of the wedge, the flow tube can be turned around to present the unworn side to the flow thus prolonging the life of the meter.
Wedge elements are easy to install and easy to use. There are variations of wedge elements designed to handle air and gases, steam, and all sorts of liquids – clean, dirty, high-solids, slurries, viscous, corrosive or erosive.
Combination Elements
A cross section view of the Veris Accelabar® flowmeter that combines
a specially designed flow nozzle and an averaging Pitot tube
Photo courtesy of Armstrong – Veris Flow Measurement
An example of a flowmeter design that uses a combination of primary elements is Armstrong’s Veris Accelabar® flowmeter. In the illustration above, it can be seen how the flow enters the first element, a specially designed flow nozzle, and then encounters a second element, an averaging Pitot tube. The Veris Accelabar® nozzle’s design provides a “settling distance” to accelerate, linearize and stabilize the velocity profile of the flow. The Veris Verabar® averaging Pitot tube significantly increases the differential pressure output and provides accurate measurement. The Accelabar’s design makes possible a tremendous operating range (turndown) and eliminates the need for straight or upstream runs. It is able to cover an extensive range of applications.
Other primary elements
Other primary elements include low loss flow tubes, Dall tubes, and laminar flow elements.
Low loss flow tubes are designed to produce a minimum amount of permanent pressure loss. Going back to the early 1960s, the first “Lo-Loss” flow tubes were designed and marketed by the Penn Meter Company, bought by Badger Meter a few years later. In 2001, Wyatt Engineering purchased Badger Meter’s differential-producing flow element division. Both companies in turn continued to further refine the “PMT” (Peter Meter Tube) “Lo-Loss” flow tube. At the time, a radical departure from the traditional Venturi flow tube, this type of flow tube has a continuously curved transition between the inlet section and the recovery cone rather than a long, cylindrical throat, and placement of the high- and low-pressure taps is different. They can be made in many sizes and materials for a variety of applications handling water, wastewater, sludge, slurries, clean fluids, and gases in full pipe conditions, and are ideally suited to applications where minimum permanent pressure loss is important, such as in gravity-fed systems, or where saving on pumping costs is desirable. Both Wyatt and Badger still offer low loss flow tube designs today and, over the years, other companies also began to make and offer their own variations of low loss flow tubes.
The Dall tube was invented by an ABB hydraulics engineer named Horace E. Dall. It is an adaptation of the Venturi tube, but with a flow path design that gives a shorter overall length, much higher differential pressure, and very low permanent pressure loss. Drawbacks are that it is complex to manufacture, sensitive to turbulence, and not suitable for hot feed water or fluids with suspended solids. Two of the companies in this market today are Solartron ISA (in the UK, owned by AMETEK) and MATTECH, s.r.o. (in the Czech Republic).
Laminar flow elements can be used for a wide variety of clean, non-condensing gas and air flows. (Note: In fluid dynamics, laminar refers to flowing in streamlines without turbulence.) Laminar flow elements can be made in sizes and materials to handle a wide range of flowrates and gases. They are used with mass flow controllers to create a pressure drop and a flow measurement. Their accuracy, stability, response time, and repeatability make them excellent for critical gas flow measurement applications as well as for calibration of various process instruments including some flowmeters, flow regulators, and thermal anemometers. They are also used to measure air flow to internal combustion engines. Other applications include leak detection, quantification, and testing. They are lower in cost than most other primary elements and somewhat difficult to quantify, therefore, laminar flow elements are included in the “Other” category.