FLOW & LEVEL CONTROL FEATURE GUIDING YOU TO ACCURACY


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. 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. Obtaining accurate levels has long


When you need to take a level reading in a tank full of a low dielectric material, guided wave radar can offer benefits over other level measurement solutions. Alan Hunt, of ABB Measurement & Analytics, looks at how guided sensors compare against time-of- flight devices such as through-the-air radar, radar and ultrasonic


G


uided 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. 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. There are often internal obstructions too, 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˚ are common. In guided wave technology, the radar beam is focused by a probe or ‘waveguide’


/ PROCESS&CONTROL


in the form of a 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. In addition to the problems described, 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


An exploded view of a Guided Wave Radar level transmitter


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. 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. Both guided wave and through-air radar can be configured to operate in 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 restrictions are not so severe for


The diagram below shows how Guided Wave Radar technology operates


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. Guided-wave radar level sensors can accurately measure a broad range of challenging products under adverse conditions. The inherent benefits of the technology are seeing it become the preferred method for applications measuring harsh media, including crude oil, butane, liquid ammonia, sludges and acids. The sensors are easily configured to both new and existing applications. Due to their straightforward design and operating frequency of < I GHz, the sensors are often less expensive than comparable through- air devices.


ABB E: enquiries.mp.uk@gb.abb.com


PROCESS & CONTROL | MAY 2017 31


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53