Two-Wire Magmeters

In order to fulfill their original promise, two-wire magnetic flowmeters will have to perform better, have different features and cost less. Can this be accomplished? Yes, here”s how and why.

In order to fulfill their original promise, two-wire magnetic flowmeters will have to perform better, have different features and cost less. Can this be accomplished? Yes, here”s how and why.

Magnetic flowmeters are the workhorse flowmeter of many industries, having replaced differential pressure flow transmitters and orifice plates in many applications. In the mid-1990s, several manufacturers of magnetic flowmeters brought out what should have been the Holy Grail of magnetic flowmeters — two-wire magnetic flowmeters.

But two-wire magnetic flowmeters never really took off. In our research updating the Magnetic Flowmeter Competitive Intelligence Report, we studied the reasons why two-wire magnetic flowmeters were not wildly successful and what needed to be done to change that. In addition, investigation done while updating another study, Magnetic Flowmeter Marketing Intelligence Report, showed that in 2002, sales of two-wire magnetic flowmeters from all manufacturers were significantly less than 10,000 units in a market that we estimate to have been approximately $580 million in sales.

Nonetheless every market report published since 1995 has indicated that sales for two-wire magnetic flowmeters will grow. However, the manufacturers whose sales of two-wire magnetic flowmeters will increase will have to resolve some specific issues outlined in this article.

Magnetic Flowmeters: A Quick Review
Magnetic flowmeters operate using Faraday’s Law. Shaped coils are used to set up a magnetic field at right angles to the direction of flow. The passage of conductive fluid through the magnetic field induces a voltage on electrodes set perpendicular to the field.

Most magnetic flowmeters use a switched DC magnetic field that approximates a square wave. It requires power to generate this field, sometimes as much as 20 watts. Typically, AC line voltage is brought to the “converter” or transmitter, and the “converter” produces the switched DC field current to the coils. It then receives and amplifies the small voltage induced on the electrodes. The “converter” typically produces either a pulse or current (4-20 mA DC) output, or both. In some cases, the “converter” is also a Fieldbus transmitter.

Magnetic flowmeters are widely used because of their inherent accuracy and reliability. They are inherently accurate because the output of the flowmeter closely approximates the average velocity in the entire pipeline, and the flowmeter is resistant to many flow perturbations. While other flowmeters often require a minimum of 10 diameters of straight run upstream and five downstream, many magnetic flowmeters can operate accurately with less than five diameters upstream of the electrodes and 3 downstream. Since the electrodes are near the device centerline, this can result in virtually no straight run requirements in many applications involving pipe sizes. Magnetic flowmeters have replaced many orifice plates and turbine meters in conductive fluid applications because they are non-intrusive and have very low wear even in abrasive and corrosive services.

Two-Wire Magnetic Flowmeters: The Promise
Two-wire transmitters are also known as “loop-powered” devices. That is, they receive their operating power from the current output loop.

The first common two-wire transmitters were temperature and pressure transmitters. Two-wire differential pressure transmitters are widely used in flow measurement systems with head-producing primary flow elements. Primary flow elements include orifice plates, venture nozzles, flow tubes, the proprietary V-Cone, and single and multiple port Pitot tubes.

Other two-wire devices developed in the past 10 years include a variety of level devices, including capacitance and ultrasonic level transmitters. A variety of flowmeters other than differential pressure flow transmitters have also been designed for two-wire operation.

Why the push to two-wire? It is a matter of simple economics. Running a single pair of wires with a low-voltage signal to each of many field sensors simplifies wiring and is significantly less expensive (and safer) than providing AC power at each field sensor. In some cases, it can be as much as four or five times less costly to only run two wires. In addition, it is generally easier to design two-wire transmitters to operate safely in confined spaces and classified hazardous areas.

A major problem with magnetic flowmeters is that a significant amount of electrical energy is required to excite the coils while two-wire transmitters are limited to the power than can be obtained from an instrument loop that has limited current (4-20 mA) and voltage (typically 24 VDC). In some installations, higher voltages can be used, but the current is still limited to between 4 and 20 mA. Yet, to perform accurately, magnetic flowmeters use approximately the same amount of power regardless of flow rate. This is because the power used to operate the magnetic field is approximately the same at zero flow and at maximum flow.

In the mid-1990s, several manufacturers of traditional (four-wire) magnetic flowmeters made two-wire magnetic flowmeters available. They solved the power requirements problem by a combination of storing power, operating the coils less frequently and exciting the coils using less power. This permitted a signal to be induced on the magnetic flowmeter’s electrodes that was proportional to the average velocity of the conductive fluid in the primary.

There are seven manufacturers of two-wire magnetic flowmeters today. Three of them are market leaders in the North American market, while the other four have not yet achieved much penetration in North America.

Two-Wire Magnetic Flowmeters: The Problems
The problems begin with size. Because of the limited excitation power available, two-wire magnetic flowmeters are limited in line size. The largest size available is 8 inches (200 mm), however most manufacturers offer two-wire magnetic flowmeters through 4 inches (100 mm). Some do not make a two-wire magnetic flowmeter smaller than 1 inch (25 mm), but one manufacturer produces two-wire magnetic flowmeters as small as 0.1 inch (2.5 mm).

Next, the typically lower field strength of two-wire magnetic flowmeters caused early versions of the devices to be more susceptible to noisy fluids (such as paper and pickling liquors) than traditional four-wire magnetic flowmeters. This limited applications to those where the “background noise” from electrically active fluids was low.

Another problem surfaced when two-wire magnetic flowmeters were used in batching applications. One of the compromises made by early two-wire magnetic flowmeter manufacturers was to slow down the speed of response of the flowmeter, thus conserving power between “readings.” Since many batching operations can be fast, two-wire magnetic flowmeters were slow enough to introduce totalization errors, and they were not widely adopted in these applications.

Still another problem was identified in hazardous areas. The typical two-wire magnetic flowmeter has an integral converter, meaning that the circuitry, including the means of storing power to operate the coils, must be inside the hazardous area with the primary flow tube. Only one vendor offered a remotely mounted converter (or transmitter). The integral converter limited the ability of the device to be used in Class I Div. 1 (Zone Zero and One) applications and even in some Class I Div. 2 (Zone Two) areas. Remotely mounted converters can also be located where they are better protected and more accessible for maintenance.

Another problem was that only one manufacturer offered a sanitary design. Magnetic flowmeters are commonly used in food and beverage applications, such as in the pharmaceuticals and biochem industries, so this limited use of this technology for these applications. Sanitary (3A) primary flow tubes are required for many of these applications.

Additionally, the performance of two-wire magnetic flowmeters was relatively poor. The accuracy of most two-wire magnetic flowmeters placed them far below the median performance for all magnetic flowmeters.

Finally, two-wire magnetic flowmeters were priced at a substantial premium over their traditional four-wire counterparts. In some cases, this premium was as much as 25 percent, and the heavy discounting common in the magnetic flowmeter market (as high as 35 and 40 percent) is typically not practiced for two-wire magnetic flowmeters. In an aggressively discounting market, it became possible for four-wire magnetic flowmeters to be sold cheaply enough to negate the additional cost of running power directly to the flowmeter.

Two-Wire Magnetic Flowmeters: The Solutions
The reasons to develop, market and use two-wire magnetic flowmeters are still valid. Two-wire versions of other devices that require high sensor power inputs, such as ultrasonic level meters and radar level meters, have been developed successfully and are sold competitively with their four-wire counterparts.

It is still getting more expensive to run terminate and maintain wire. As more and more process plants become fully integrated, it is becoming important to reduce the amount of wiring necessary to connect plant floor sensors. Additionally, electricity costs are continuing to increase, and two-wire transmitters reduce energy consumption.

In addition, there is a trend to move as much of the intelligence of the control system to the instrument as possible. Plant floor sensors are no longer seen as being “at the end of a wire” but rather are seen as nodes on the plant-wide control network. Two-wire transmitters that communicate digitally would be able to perform these functions.

Applications that are relatively easily implemented with four-wire magnetic flowmeter technology are the most likely applications to begin using their two-wire counterparts. Therefore, measuring continuously flowing conductive liquid flows would seem to be logical applications to apply two-wire magnetic flowmeters while avoiding the problems of noise and slow speed of response. Batching applications performed at constant flow rates over a relatively long period of time are also suitable candidates for two-wire magnetic flowmeters. Fluids that marginally meet conductivity requirements should be avoided.

Two-wire magnetic flowmeters can also replace existing flowmeters that may be obsolete, nearing the end of their useful life, are problematic or do not meet the needs of the process. Although piping changes will typically be required to accommodate the primary flow tube, the electrical changes needed to install a two-wire magnetic flowmeter are usually trivial when the existing flowmeter is a two-wire device. While there may not be many retrofit installations of two-wire magnetic flowmeters when compared to new installations, two-wire magnetic flowmeters provide an important option for installations where power is not readily available or where conduit and cable runs are long.

Many flow measurements are problematic. They seem to “work;” that is, they increase when the valve is moved more open and decrease when the valve is moved more closed. The measurement may not be accurate, and the process engineers may have data to prove the point. Yet the existing flowmeter may not be replaced with a magnetic flowmeter because it is too expensive to run the conduit and cable to power the instrument. Sometimes, power for additional four-wire transmitters is connected locally, such as to lighting circuits and convenience outlets, to reduce the effort and expenditures necessary to connect the transmitter to a circuit breaker in the instrumentation power distribution panel. These shortcuts may save time and money in the short run, but they can wreak havoc over the life of the facility when seemingly extraneous circuits trip or are intentionally tripped for other reasons, such as to maintain the convenience outlet that (unbeknownst to the electrician) powers the transmitter.

Often, existing flowmeters do not meet the needs of the process. One example is the installation of a differential pressure flow measurement system that can measure accurately from 30 percent to 100 percent of full-scale flow. Magnetic flowmeters offer a larger turndown, so this technology might be applicable if the process requires the measurement of flow from 5 percent to 100 percent of full-scale flow. Two-wire magnetic flowmeter technology may allow this to be achieved with little electrical rework.

Not in the least, suppliers have to make sure that users understand the financial and maintenance benefits of two-wire transmitter technology. Most users understand that the electrical installation costs associated with a two-wire transmitter are lower than those associated with a four-wire transmitter. But this is often as far as the understanding goes because most users have not quantified the cost differential that includes the transformer, distribution panel, circuit breaker, conduit, wire, and terminations, and their associated engineering, documentation, installation and maintenance.

Two-Wire Magnetic Flowmeters: The Future
In order to fulfill their original promise, and the promises that industry analysts keep making on their behalf, two-wire magnetic flowmeters will have to perform better, have different features and cost less. Can this be done? Our answer is emphatically, yes.

Already, there is a shift in performance. Some two-wire magnetic flowmeters are performing as well as many four-wire units.

Also, the speed of response of two-wire magnetic flowmeters is improving. Some manufacturers are successfully selling two-wire magnetic flowmeters in applications monitoring the feed of chemicals in pulsing applications from metering pumps.

Pricing is going to have to be adjusted so that two-wire magnetic flowmeters maintain their installation cost edge over their four-wire competition.

Successful two-wire magnetic flowmeters will need hazardous area and sanitary certification, as well as Fieldbus capability or perhaps even embedded Web server capability.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

Walt Boyes is a is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies.

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