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01 April 2004

Aloha to dated levels

By Adrian Totten, Arnold Dumas, and Gerald Boteilho

Magnetostriction is the most accurate measurement technology available with accuracy to 1/16 inch—nonlinearity to 0.008% full scale.

The Gas Company is Hawaii's only full-service gas energy company, making gas products and services available to Hawaii's 1.3 million full-time residents and more than 7 million visitors a year.

The Gas Company (TGC) offers regulated utility service on Maui and Molokai islands and nonregulated tank and bottled liquefied petroleum gas (LPG) on five of the six major islands. From local residents to major hotels and restaurants, TGC's operations are crucial to the livelihood of those on the islands.

TGC (www.hawaiigas.com) operates three LPG storage and distribution facilities on three separate islands in the state—Maui, Molokai, and Lanai. The company had specific requirements in choosing a liquid level sensor with regard to existing tank installations, maintenance, monitoring convenience, and atmospheric conditions. TGC chose magnetostrictive technology liquid level gauges.

Gallon missed is money lost

TGC's storage yards reside on the island of Maui. Its old yards consist of five 30,000-gallon tanks and three 2,000-gallon tanks. The new yard holds two 5,000-barrel spheres and four 30,000-gallon tanks. The newer spherical tanks were the choice for storage because they hold more LPG than typical horizontal tanks, yet they presented some challenges for liquid level measurement. Measurement is required on a continuous basis, but is particularly crucial upon delivery. Because propane goes through processing at a remote location, it ships to TGC by barge. The propane then pump transfers into storage tanks for future distribution on the island. It is important at this point to confirm the amount delivered and stored. At about $2.00 per gallon, every gallon missed is money lost.

The first challenge was installation of the sensors. The company wanted to use existing openings in the tanks rather than cutting or drilling newer, bigger holes. This required a sensor that could easily fit through the existing opening. TGC was also looking for a gauge that would require minimum maintenance with convenient and easy-to-read monitoring. And because the tanks are outdoors, the gauges needed to be resistant to atmospheric corrosion and wear from wind, water, salt, and other elements.

In addition to standard level measurement, the company also welcomed temperature measurement, quick display monitoring, and electronics that were easily fixable or replaceable without disturbing the tanks. However, because LPG is a high-value product, measurement accuracy was the most important requirement. The sensors would have to reliably and accurately measure monitored stock and usage and detect leaks.

Viable retrofit technology

Historically, The Gas Company used sonar-based level gauges as its main sensor type with 10% fixed level gauges for backup and added measurement accuracy. However the sonar gauges were unreliable, and as a result, operations relied upon the fixed level gauges for all their level data. Oftentimes operations workers would simply eyeball the level, which ultimately resulted in low accuracy and lost revenues. It was while searching for a viable retrofit technology that magnetostrictive sensors became an option.

Magnetostrictive level gauges provided many of the features TGC required. The sensors consist of a flexible pipe and electronics head. The pipe acts as a protective sheath for the magnetostrictive materials that run the length of the pipe. The depth of the tank determines the pipe length.

The technology works by sending an electrical pulse along the length of a magnetostrictive waveguide (inside the pipe). A float installed with a magnet rests on the liquid level's surface. The magnet distorts the electrical signal, and this distortion signal then travels back to the electronic sensor head, which resides outside of the tank and calculates the liquid level inside the tank.

The float uses the specific gravity of the liquid to determine its floatability. The circular float easily moves up and down the pipe as liquid rises or lowers in the tank.

We leveraged magnetostriction sensing technology, which is the most accurate measurement technology available with accuracy to 1/16 inch—nonlinearity to 0.008% full scale. It is also reliable and repeatable. Magnetostrictive sensors leverage standard power sources and do not need calibration, even if the power fails.

Installation of the gauges was simple, because TGC could fit the sensor pipe through the existing tank opening. The flex pipe was easy to transport and install, because its flexible nature allowed it to be snaked into the opening (this is often a problem for indoor tanks with little overhead room). Through the tank manholes, the company then mounted the pipe to the bottom of the tank and installed a float. Because the sensor monitors temperature data as well as level, there was no need to purchase or install separate sensors or create additional openings.

Diagnostics to troubleshoot

Monitoring the data collected by the gauges was another benefit to TGC. Each tank sensor has its own digital liquid-crystal-display operator interface terminal at eye level with the meter. This provided "at-a-glance" monitoring of all data. The installation technicians enter tank data manually during installation with a simple keypad, and because the American Petroleum Institute tables are part of the sensor electronics, the volume of spherical tanks is automatically calculated and ready to display on the monitor. Wireless data transmission capabilities are available and an option for the future.

In addition to standard liquid level data, these electronics enable technicians to troubleshoot problems in the field thorough diagnostics. The operator interface terminal provides gradient, gain, temperature, and current level data, all at the touch of a keypad.

Because TGC's electronics reside above the tanks and the magnetostrictive materials live in the outer pipe, the sensors are ideal for repairing or replacing the electronics. The electronics can easily uninstall form from the sensor head without disturbing the tank or its contents. In the past, if there was a lightening strike or unforeseen surge that caused an electronics breakdown, the company would have had to evacuate the tank to replace the gauge. Now maintenance can take place without losing product or time.

TGC was searching for a more accurate, reliable sensor that could readily move into its existing tanks and replace its sonar system. The sensor it was seeking required quick retrofit capabilities using the old tank openings, and TGC wanted quick and easy monitoring options. The magnetostrictive sensors rewarded the utility with improved liquid level measurement accuracy and reliability in all its tank yards. DT

Behind the byline

Gerald Boteilho (gboteilho@hawaiigas.com) is the operation supervisor at The Gas Company in Maui, Hawaii.

Adrian Totten (adrian.totten@mts.com) has a B.S. in electrical engineering and an MBA. He is MTS Sensors product marketing manager for liquid level products.

Arnold Dumas (arnold.dumas@mts.com) has more than twenty years of experience in liquid level management and measurement. He is liquid level sales and marketing manager for MTS Sensors.