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Feb 20, 2025

Process and Control Today | The benefits of using Guided Wave Radar for level sensing in harsh environments

20/08/2024 ABB Ltd - ABB Measurement and Analytics Guided wave radar (GWR) is a leading technology for measuring levels in tanks, silos, and vessels, especially for challenging substances. Measuring

20/08/2024 ABB Ltd - ABB Measurement and Analytics

Guided wave radar (GWR) is a leading technology for measuring levels in tanks, silos, and vessels, especially for challenging substances. Measuring the time taken for an electromagnetic pulse to reflect back from the material surface to determine the level, the technology is favoured for its reliability, energy efficiency, and ability to handle harsh conditions. In this article Jon Davison, ABB’s Level, Temperature and Pressure Product Manager explains why.

Guided wave radar has established itself as a leading technology for measuring the level of bulk solids, liquids and everything in between. With no moving parts, it works well in harsh chemical environments, under widely varying operating temperatures and on low dielectric materials.

Process engineers who work with molten sulphur, liquid ammonia, petrochemicals and other hard to measure media have welcomed the simplicity of integrating guided wave radar to obtain data on the contents of tanks, silos, hoppers, bins, mixing basins and vessels. In particular, its inherent benefits enable it to offer numerous advantages over other technologies such as “free space” radar or ultrasonic that cannot offer the same degree of performance.

Guided wave technology theory

Guided wave radar works by calculating the time between the transmission and receipt of an electromagnetic pulse, a principle more accurately known as Time Domain Reflectometry or TDR. The pulse is transmitted along the length of a guide probe into the vessel to the product being measured, at which point part of the pulse energy is reflected back to the receiving electronics. By measuring the elapsed time between the initial pulse and reflection, the distance can be accurately calculated to derive the tank level in units such as metres, feet, inches or mm.

Non-contacting “free space” radar technology is a leading technology in level measurement; however, some applications still prove to be problematic for non-contact level technology due to the measured fluid properties, the vessel layout, the available mounting point or internal obstructions. There are similar challenges for ultrasonic level technology, but here the operating conditions often just do not suit the technology, for example the product is foaming, or operates at elevated temperature or pressure.

In guided wave technology, the electromagnetic pulse is transmitted along a probe or “waveguide” which typically takes the form of a metal rod or cable, although for some applications a coaxial style probe is also available. As the waveguide concentrates the signal pulse around the probe this prevents dispersion in the vessel. The results are better performance and reliability without the need to “tune” the device for false echoes or spurious readings. Furthermore, it is not necessary to program a unit to ignore spurious readings from the sides of the vessel.

A significant advantage of guided wave radar technology is its ability to measure the interface between two liquids, in addition to the top fluid level. A typical application is oil and water, where the upper liquid has a lower density and dielectric constant compared to water. In this case the top level is the oil, with water measured as the interface level.

Guided Wave vs Through-air Radar

Signal Strength

In addition to problems with pulses returned from the vessel walls and objects inside the tank, non-contact level technology can be very sensitive to changes in process conditions such as product build up, foam, turbulence, and condensation. While it might seem that the signal-to-noise ratio could be improved by increasing the strength of the transmitted signal, it isn’t that simple. Loop powered devices by their very nature operate on a tight energy budget. Guided wave radar uses energy much more efficiently by focusing it along a probe and can therefore achieve an optimum signal-to-noise ratio even when operating as a low power loop powered device.

Dielectrics

Free space radar level sensors don't work well on low-dielectric media because the pulses pass partially through, rather than reflecting off them, producing only a very small signal return that results in an unreliable or inconsistent level measurement. Whilst solutions are available to improve the measurement on low dielectric products, such as using a stilling chamber, or even a target float, it can be more efficient to consider alternative technology that doesn’t need a “work around”.

For guided wave radar, the coaxial probe version is housed in a stainless-steel tube that acts as a ground plane to help channel the microwave energy and give the entire assembly a coaxial structure. This ensures a constant impedance along the entire waveguide, allowing the sensor to detect more subtle dielectric changes and correctly indicate product level down to dielectric constants as low as 1.4. Free space radar can also use a similar arrangement via the use of a stilling tube to measure low dielectric fluids, but this mode of operation is often harder to install, is not as accurate as a guided wave solution and still typically requires a higher dielectric constant.

Application Guidelines:

Product Caking and Build-up

Obtaining accurate levels has long presented difficulties for industries dealing with products that cling to everything they touch, such as drying molten sulphur, bitumen or the paraffin wax common in petrochemical processes. Here the solution is to use a single probe design, either rod or cable type. In this instance, any product build-up on the probe has minimal impact as the signal return from the actual fluid level is always significantly greater than any build-up. Due consideration should be made to the weight impact of any build-up, but in general most coating liquids dissolve back into solution as the level rises and then layer as a thin coating as the level drops again.

High-Turbulence Vessels

Both guided wave and through-air radar can be configured to operate in highly turbulent environments. While guided wave, with its focused pulse, provides better performance under these conditions, there can still be special requirements.

Installing a stilling well around the probe or the through-air signal can help maintain a more constant level reading. The stilling well, often less than four inches in diameter, should have holes drilled along its length to allow the product to remain in full contact with the signal and ensure the level in the stilling well is representative of the true liquid level.

The same technique is used for through-air radar but requires precise stilling well construction with tight tolerances and special welding. Installation and alignment are also critical, and improper setup can sometimes cause erroneous readings.

The restrictions are not so severe for guided-wave radar. The waveguide causes the signals confined within it to glide past any cut-outs and head straight down to the product, minimizing the effects of any turbulence.

Ensuring Optimal Configuration

Easily configured systems surely head the wish list of process engineers as nothing is more frustrating than attempting to shoehorn a universal-purpose transmitter into a particular application that comes with built-in headaches.

Manufacturers of guided-wave radar level sensors offer a full range of transmitters, mountings, couplers, and probes to satisfy nearly any special requirements. For example, the variety of rigid and flexible probes that house either a single or a coaxial cable, some of which can be bent to accommodate unusual tanks, bins, or silos, means the radar units can arrive from the factory already configured. Some probes can even be cut to length in the field. In Secunda, South Africa, the ability to send a radar beam around a 90° corner by using a guided-wave cable allowed accurate measurement of levels in a cooling tower.

Mounting a guided-wave radar transmitter is less critical than mounting a through-air device, which can rely on precise positioning of the antenna. Guided wave can work with apertures as small as a 3/4-inch NPT opening. In some cases, radar units must be installed in external chambers, for example, when they are replacing mechanical units. Special instrument enclosures are sometimes required. Stainless steel is particularly suited to offshore uses, where saltwater will corrode even powder-coated aluminium. Some transmitters are housed in explosion proof enclosures. Some manufacturers will even create custom, one-off designs.

Monitoring bromine levels nearly always requires a special mounting and probe with hermetic seals, and tanks of corrosive fluids may demand probes made of speciality materials such as Monel, Hastelloy or even Tantalum rather than stainless steel. Many radar units display results locally with a built-in scrolling LCD that offers measurements in a field -selected choice of inches, feet, millimetres, centimetres, meters, or even volume through a tank linearisation or “strapping” table.

Configuration concerns should also include the transmitter's communication capability, as a device's ability to seamlessly communicate the data throughout a processing plant is equally important. Guided-wave units offer not only 4-20 mA output but also a variety of standard digital communications capabilities such as HART, Modbus and other common fieldbus protocols.

Summary

Guided-wave radar level sensors can accurately measure a broad range of challenging products under adverse conditions such as low-dielectric materials, aggressive chemical environments, and high operating temperatures and pressures. Its many benefits compared to other level measurement technologies makes it the preferred method for use with harsh media, including crude oil, butane, propane, molten sulfur, liquid ammonia, plastic pellets, powders, fly ash, slurries, sludges, acids, and chlorine. The sensors are easily configured to both new and existing applications, and a wide choice of communication protocols is available. With their straightforward design, the sensors are often less expensive to install than comparable through-air devices. Moreover, their robust construction and advanced signal processing capabilities ensure reliable operation and precise level measurements even in the most challenging industrial environments.

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For more information, please contact:ABB Ltd - ABB Measurement and AnalyticsOldends LaneStonehouseGloucestershireGL10 3TAUKTel: +44 (0)1453 82661Email: [email protected] Web: https://www.abb.com/measurement

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