Level indicators reach new heights

Aurora combines a float-based MLI with Eclipse.

 

GWR is based on time-domain reflectometry (TDR), in which pulses of electromagnetic energy are transmitted down a probe, reflected when they reach a liquid surface, and measured to indicate level. Aurora combines the operating system of a float-based, magnetic-level indicator (MLI) with Eclipse’s technology. Change in the process tank level corresponds to change within Aurora’s chamber. The float in the chamber actuates flags or a shuttle for visual indication while its Eclipse GWR transmitter provides continuous electronic level measurement.

Tim Lingg, senior process engineer at Oxy (formerly Vulcan Chemicals) in Wichita, Kan., says his facility most recently installed Aurora units, which are working in combination on its liquid chlorine processing vessels. It’s also had Eclipse device on its chlorine storage units and in its50% caustic plant for about four years. These replaced differential pressure level transmitters that had been for the previous decade.                

“Differential pressure needed a lot of maintenance because we had to use nitrogen purges through the differential pressure meters, but it was hard to keep them clean, and we still saw a lot of plugging.” says Lingg. “So, when we put in Eclipse, we put in two transmitters per storage tank on each of five 50-ton tanks to make sure they agree with each other, and they do.

“This is a much more hardy solution, and we now have a lot more time between maintenance tasks. We used to have to clean the differential pressure cells once per month, but the coaxial connection between the probe and the transmitter now only needs to be cleaned once every six months. And with the new permanent, ceramic o-ring from Magnetrol that we just installed two months ago, we’re thinking we may be able to goat least two or three years between cleanings.”

The fact that Aurora incorporates Eclipse in it allows both local, visual level checks, and electronic checks that are sent back to Oxy’s DCS board. Because liquid chlorine boils at –40 °C and is usually stored at 0 °C and 50 psi, level indicators used with it typically frost over. Lingg adds the beauty of Aurora is that its indicator isn’t glass, but rather a magnetic floating inside the vessel, which actuates the indicator on the outside. This means Oxy’s staff no longer has to clean iced-up indicators by blasting them with steam.

Similarly, Louis Tafoya, control analyst at Chevron’s Salt Lake City refinery, says he presently uses Eclipse indicators to measure level on four vessels and one sulfur pit. This constantly moving process also previously used differential pressure transmitters on the tanks and a sonar-based capacitance probe on the pit, but they resulted in many erratic readings and weren’t reliable, according to Tafoya.
“Eclipse is trouble-free, rugged and reliable. The guided-wave radar goes through the solid tube,hits the medium, and reflects back,” says Tafoya. “This means we can’t overfill or over pressure our vessels, which would cause upsets in the system. So, as new applications come up, we’re probably going to look at using Eclipse more.”

Though less hazardous then petroleum and chemical processes, biotech application often have to be far cleaner. Steve Ward, co-owner of AQC Calibrations, in Discovery Bay, Calif., says these processes initially used floats and magneto-restrictive devices, but they were hard to set up, prone to errors, and—worst of all—were typically in sealed genetics-related and/or fermentation vessels from 50 to 20,000 liters that couldn’t be opened once their processes had started. Ward previously worked as a metrologist at Genentech in South San Francisco.

“We had to come up with a way to replace level transmitters once these processes were underway, and be able to calibrate transmitters without pulling the probe out and opening the tank,” says Ward. “These tasks used to mean a half-day shutdown, so we worked with Don Brooks to refine these processes, so they’d be repeatable and have validated calibrations. This was helped by the Eclipse 708 we were using that formerly had a deadband at the top of the probe, which now has been removed so we can read up to the connection point.”

Ward adds the 21-ft probe in these fermentation vessels has targets set on it, which allow readings to be taken using a Leica laser. Once the level probe was set, a tank could be filled via a MicroMotion flowmeter, and measurements could be applied to a strapping table, which Ward adds Magnetrol has since incorporated into its transmitter to linearize inputs and outputs to PLCs.

“Now anytime we have a tank in a clean or hold state, we can remove the calibration transmitter to a test bench, and run a calibration in half an hour, instead of half a day,” says Ward. “Magnetrol’s response has always been to make the improvements we’ve requested. Over the years, they’ve responded tremendously as we’ve redesigned these sanitary systems, and that’s made these technologies very solid.”