Instrumentation Monthly Test | Measurement | Control
When you need to take a level reading in a tank full of molten sulphur or some other low dielectric material, guided wave radar can offer significant benefits over other types of level measurement solutions. Alan Hunt of ABB Measurement & Analytics 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 devices with most digital communication protocols to obtain data on the contents of tanks, silos, hoppers, bins, mixing basins and vessels.
This article looks at how guided sensors compare against other time-of-flight technologies including through-the-air radar, radar and ultrasonic.
Guided waves
Radar works by measuring the time of a transmitted signal. Through-air radar technology was one of the pioneers in non-contact level measurement, but false echoes remain a significant problem.
Simply pointing a radar transmitter toward the bottom of a silo allows unguided waves to bounce off the sides of the vessel, creating spurious return signals that must be cancelled out at the receiving end. Moreover, there are often internal obstructions such as piping, nozzles and ladders, that can produce unwanted signals.
A similar problem presents itself in ultrasonic measurements, where divergent angles of up to 20 degrees are common.
In guided wave technology, the radar beam is focused by a probe or ‘waveguide’ in the form of a specially designed metal rod or cable that is inserted into the product to be measured.
This guide serves to concentrate the radar signal into a smaller-diameter cylindrical pattern along the probe length and so prevent dispersion in the vessel. The results are better performance and reliability. Furthermore, it is not necessary to program a unit to ignore spurious readings from the sides of the vessel.
Signal strength
In addition to problems with pulses returned from the vessel walls and objects inside the tank, non-contact radar units are very sensitive to changes in process conditions such as product build-up, foam, turbulence and condensation.
Similarly, ultrasound can be adversely affected by tank conditions and vapour phases that affect the speed of sound.
While it might seem that the signal-to-noise ratio (SNR) could be improved by increasing the strength of the transmitted radar signal, it isn’t that simple. Loop powered devices by their very nature operate on a tight energy budget.
Through-air level sensors also use more energy because of the wide-beam spread of the microwaves, whereas guided-wave radar uses energy much more efficiently by focusing it along a probe. It can therefore achieve an optimum SNR even with the limited energy available.
Response time
To weed out spurious signals, through-air radar and ultrasonic technologies use fuzzy logic to assign each target a probability level. The requisite signal processing slows both the response time and the update rate. Guided-wave radar can take up to 10 readings per second; with no additional filtering necessary, the update rate can be similarly fast.
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 cooling molten sulphur and paraffin wax in petrochemical processes.
Guided-wave radar level transmitters that rely on coaxial, as well as dual-rod or dual cable waveguides, are at a particular disadvantage here. Problems arise when product build-up bridges the gap between the two rods, rendering the unit inoperable because of the modified impedance between the two wires. The preferred solution calls for the use of a single cable or rod hung in the tank; build-up on a single guide will have minimal effect on transmitter operation.
High-turbulence vessels
Both guided wave and through-air radar can be configured to operate in highly turbulent environments. Although the former, with its narrower beam, provides better performance under these conditions, installing a stilling well around the probe or the through-air signal can help maintain a more constant level reading.
The stilling well should have holes drilled along its length to allow the product to remain in full contact with the radar signal.
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, so turbulence has little effect.
Summary
Guided-wave radar level sensors can accurately measure a broad range of challenging products under adverse conditions such as low-dielectric materials, caustic chemical environments, and high operating temperatures and pressures.
The inherent benefits of the technology are seeing it become the preferred method for applications measuring 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. Due to their straightforward design and operating frequency of < I GHz, the sensors are often less expensive than comparable through-air devices.
ABB manufactures and supplies a wide range of equipment for level measurement applications. For more information, call 0870 600 6122 or email enquiries.mp.uk@gb.abb.com ref. ‘level’.