Windows, floats, displacers, magnets, sonics, radar, lasers and nuclear devices have met or at least partly satisfied each new level measurement challenge over the years, and most continue to be refined even now. However, new problems are always arriving, prompting new ways to look into tanks without opening them.
Tank Vessel Meets Ship Vessel
For example, BP Exploration in Houston, Texas, recently replaced unreliable level transmitters on a floating, production, storage and off-loading (FPSO) ship with guided-wave radar (GWR) transmitters from Emerson Process Management. Operating about 100 miles off Africa's west coast, BP's FPSO processes and stores oil for export. The ship is 310 meters long, can store 1.77 million barrels of oil, and can process up to 240,000 barrels per day.
Changing process conditions, foam and vapor, and dirty, sticky fluids had made it difficult to measure level on the FPSO. Its original GWR transmitters weren't compatible with the FPSO's Foundation fieldbus (FF) network, and their limited ability to detect low-dielectric hydrocarbons required coaxial probes to increase surface signal strength. However, these probes were prone to sticky build-up, leading to unplanned downtime.
As a result, BP Exploration replaced the existing GWRs with Rosemount 5300 GWRs with signal-processing that ensures detection of low-dielectric fluids, and can send and receive cleaner, stronger signals (Figure 1). This allows use of single-lead probes that increase tolerance to solids build-up and coating, and eliminate trips due to false readings. Also, the Rosemount 5300's FF interface made installation and configuration quicker and easier. After the Rosemount 5300 GWRs were installed, the FPSO's process data confirmed the accuracy and reliability of the instruments and their suitability for its widely varying process conditions.
Reining in Reactivity
While its tank isn't out on the ocean, U.K.-based Robinson Brothers probably has an even more difficult level measurement challenge—securing level indications for highly reactive carbon disulfide (CS2). The company uses CS2 at its Midlands specialty chemicals plant, but it must store the CS2 under a layer of water to prevent it from igniting. This means the level of the interface between the water and CS2 needs constant monitoring, so any related instruments are safety-critical.
Robinson previously used a simple, magnetic, float-based device to measure the CS2 and water level, but it didn't link to any wider control system, So, Robinson sought help from ICA Services, an instrumentation specialist in Manchester, U.K. ICA recommended using ABB's AT100 magnetostrictive level transmitter, which provides continuous level indication, transmits analog and/or digital signals for monitoring or control, and can boost its resolution to more than 100 times greater than a conventional reed switch-type device (Figure 2). Most importantly, AT100 also meets the most-extreme ATEX Exd IIC T6 protection standard and toughest SIL1 performance standards. "Our new system provides process signals that output to both our local and site monitoring systems, and it meets our internal requirement for SIL1-capable instrumentation," says Tom Rutter, Robinson's E&I manager.
Low-Power, FDT and FCC Aid Level
While ongoing technical advances get the main spotlight in level measurement, organizational efforts have helped, too. Boyce Carsella, consultant at Magnetrol, reports level measurement's migration to lower-power sources has enabled it to serve in new and hazardous applications, while electronic device descriptions (EDDs) standardized by the FDT Group are bringing level instruments closer to plug and play.
"Radar and guided-wave radar are the most successful level measurement technologies today because they're non-contact, unaffected by atmosphere, and can handle the widest range of applications," says Carsella. "However, radar's popularity will be helped even more by the FCC's adding to its Part 15 rules on ‘level probing radar,' which will allow it to be applied outside or on open tanks [see sidebar]. This will open up many applications, such as water/wastewater or other plants with outdoor or open vessels."