01 October 2003
A level playing field
Radar scores points in the industry.
By Ellen Fussell
The level technology market might not change for years; then a new development like radar or microwave level comes along, and new markets just open up.
One of the big areas is discharging effluent from municipalities in industrial plants back into the natural environment, said Tim Chettle, marketing manager at Solartron Mobrey in Flough, U.K. Companies are monitoring how much they are discharging as well as the quality of it. They must remain in consent levels or there is a charge from the government. "In Europe we're quite heavily driven by environmental legislation," Chettle said. "We've spent a lot of money taking products into areas where legislation is requiring instrumentation where there isn't any and better instrumentation where there's only basic stuff.
"We've spent [research and development] money in accurate open-channel flow measurement—where you've got a flow structure [a partial flume where the liquid flowing out of the work is measured in a flume]," he said. "You have to make a good quality measurement, because the customer will be charged if he exceeds his limits. The instrumentation has to be capable of regular certification and inspection." That is one area where Chettle thinks developing ultrasonic and level measurement devices will be useful.
Ultrasonic transmitters are being continually updated and improved, offering better performance, often at lower costs than ever before, Chettle said. Manufacturers are using the latest microprocessor and electronic component technologies, "with smart instruments having onboard calculation facilities increasing the application base beyond simple level measurement duties.
"We also use ultrasonics to measure flow in closed pipes—where the customer is discharging liquids into the environment," said Chettle. It could be the biggest markets are municipalities—water treatment companies—and any large industrial plants that have a process—be they making food or steel or converting timber. They all use vast amounts of liquid that has to be treated before it is returned back to nature.
The biggest benefit of ultrasonics is it is compact—when you're measuring effluents. There are no moving parts, and it is maintenance free. The solid state electronics today are microprocess-powered and can do all calculations to convert level measurement to flow measurement that customers need to record. "So in one instrument you can make a nice package for the customer," he said.
|Ultrasonic noncontact flow measurement through a venturi.|
OLD VS. NEW
While AMETEK Drexelbrook dealt more with ultrasonics in the 1990s, more recently the trend the company sees is a greater use of target discrimination radar (TDR), pressure, and magnetostrictive, said Bill Sholette, point level product manager in Horsham, Penn. Sholette said the company has been working toward low-maintenance products that are much easier for the user to commission and to operate—automatic calibration products, auto-check features that allow the product to check itself to make sure it is operating. "What we recognized over the last decade or so is that companies have less resources in their technical area, and it's much more difficult for them to learn calibration procedures. So our trend is making the products easier to use," he said.
"In the radio frequency capacitance level—our older point-level switches—you'd have to install it in the tank and calibrate via a variable capacitor," Sholette said. "The newer products are microprocessor based, you put them in the tank, power them up, and they calibrate themselves." The auto-calibrated products also adjust themselves for changes in process condition, "so there's no need to tweak the calibration," he said. It also allows you to use the switch in non-dedicated service—that means one day it could measure conductive material, another day insulating materials."
Sholette said self-check features ensure the integrity of the switch to make sure it is operating when it has to. One example is the auto-calibration of the radio frequency (RF) point-level switch. "The biggest benefit for manufacturers is it's very versatile," he said. They can use it in liquids, slurries, granular materials, or interface applications. It is easier to stock products; it reduces inventory and is also low maintenance with no moving parts. Some of the big markets using the RF point-level switch are chemical and petrochemical, "but we sell to food and beverage, pulp and paper, municipal, wastewater, agriculture, and mining—anyone who stores or handles dry or liquid materials," he said.
Ultrasonic noncontact flow measurement over a weir.
Radar in general is a noncontact technology that has a lot of promise—it does all the same things as ultrasonics but with no weaknesses; water temperature and pressure range are not affected by vapors, said Boyce Carsella, product manager for through-air radar technology at Magnetrol in Downers Grove, Ill.
Twenty years ago radar transmitters were line-powered, expensive, and complex. "You needed a technician to set them up, and that limited its usability in the general market," Carsella said. But now loop-powered (two-wire) devices have become a standard for the industry, "because they're so easy to install and to make safe in hazardous areas," he said. They use less power, are less expensive, and are more competitive with standard technologies. "The biggest challenge for us as manufacturers is to make them easy to use," he said.
Contact or guided-wave radar (as opposed to through-air—noncontact—radar) uses a probe or wave guide. A guided-wave radar device would use an eight-foot probe to measure material in an eight-foot tank. Noncontact radar uses an antenna mounted at the top of a tank—whether 10 feet or 50 feet.
While guided-wave radar has been around for the past four or five years, new interface applications are hitting the market, said Bob Botwinski, Magnetrol's guided wave product manager. "Where you have light fluid on top of heavy—oil on water—is a typical application," he said. Guided-wave radar lends itself nicely to helping manufacturers keep water out of their process. "The theory is similar to through-air but we're sending our energy down the probe, instead of shooting it through air," he said. Guiding the wave, sending it down a probe, offers an advantage because, "when you send energy through air, it bounces off and gets spread out—affected by wind or waves," Carsella said. "With guided wave, we're focusing our energy down a probe so we don't lose energy when sending it down a vessel. It hits liquid and bounces back, and we measure time. With interface units we send energy down a probe, it continues through oil level, and we get a second reflection from the water. So the ability to do interface applications is what's new in this guided-wave radar field."
Guided-wave radar—foam and coating have little effect on measurement performance.
PULSE AND CONTINUOUS WAVE
Two of the oldest types of radar are pulse and frequency modulated continuous wave (FMCW). "A pulse of energy has a rise time—certain peak—and comes back to zero, but never goes negative," Carsella said. "You don't get a sine wave; a pulse of energy is its own form of energy versus a burst—say, 6 gigahertz—which is a high-frequency burst." (Magnetrol uses the term "pulse burst" to differentiate from the company's eclipse guided wave—a true pulse device.)
"In a perfect world most people would rather use regular through-air radar. But guided wave is becoming a superior technology, especially in the more difficult applications like turbulent surfaces, and super low dielectrics, and installations in still wells," Carsella said. "But that leaves plenty of applications for regular through-air. There's room in the industry for both." IT
By Kevin Hambrice and Tim Chettle
While manufacturers continue to plough development dollars into new instruments, they are always looking to improve and extend the application of existing products.
Of the two level markets—liquids and bulk solids—the latter includes plastic pellets, powders, lime, sawdust, sugar, cement, plaster, grain seeds, stone, and sand and thus requires a more robust level technology. Laser and ultrasonics are noncontact (non-invasive) technologies. Guided-wave radar works well in high-dust applications. High dust tends to create a problem for noncontact devices. Companies such as ChevronTexaco, Dow Chemical, and ExxonMobil deal with plastics and pellets. They have used older mechanical technologies—yo-yos or plumb bobs—but they want to get away from contact technology.
If users want relatively decent accuracy they can go to ultrasonics. But pinpoint accuracy comes with laser technology. The 4–20 mA output takes the output and feeds it into a programmable logic controller or distributed control system for continuous output. This means manufacturers save money on inventory, because they know they have a real-time measurement of inventory at any given time. No more dipsticks and climbing on top of silos.
Installed above a flow structure such as a weir or a Parshall flume, being noncontact is a great advantage, as it drastically reduces maintenance. Onboard processing and conversion of the level measurement to a volumetric flow measurement results in a reliable and accurate measurement, which inspectors and operators alike can easily verify, eliminating costly penalties for exceeding the limits.
Behind the byline:
Kevin Hambrice is director of marketing at K-TEK in Prairieville, La. Tim Chettle is a marketing manager at Solartron Mobrey in Flough, U.K.
In the past, most manufacturers of tank gauging equipment have only had one bus alternative for communication between the control room and the gauging equipment on the tank—specific for one manufacturer. They could not connect equipment such as level gauges and temperature devices from another manufacturer to this bus. If the user wanted a system extension, partial upgrades, or exchanges, the only practical alternative was to purchase from the same supplier as before.
To install equipment from another supplier required separate cabling for another fieldbus, a second operator display in the control room, and a second interface to the existing distributed control system. Most old tank gauging equipment in service today consists of mechanical level gauges as float gauges or servo gauges. It is not uncommon that users accept very high maintenance costs, poor performance, and unreliable measurement for these instead of exchanging them to modern radar-based level gauges. The reason? The cost is too high for exchanging the whole system, and a partial exchange creates bus compatibility problems.
Today, manufacturers using gauging systems have the option to mix different types of level gauges from different suppliers without having bus incompatibility problems. There's no reason to accept high maintenance costs and poor performance from old mechanical level gauges due to bus incompatibility. It could be far more economical to spend the maintenance money on an emulating radar level gauge. Emulation means you can exchange an existing level gauge installed on a tank with another totally different type of level gauge. Afterward, the existing tank gauge system will not see any difference between the new emulation level gauge and the old system.
You should, however, consider a number of aspects in the exchange. The emulating level gauge should be electrically compatible with the existing system bus. It should be software compatible with the existing system. The software compatibility can often be on different levels for different emulation applications. Emulation most commonly is supported for measurement data only. Yet there's still many different old float and servo gauges being used today—easy and suitable to emulate, because they have used the same fieldbus hardware for years, and software protocol has changed very little.
It is easier to implement a project with little production disturbance by exchanging an existing level gauge system based on emulation. With a successful exchange, you can select the type of fieldbus that you prefer with minimum costs. The open fieldbus communication architecture is also available today for level gauge systems.
—Lennart Hägg, technical manager, Saab Tank Control