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01 February 2004

Intrinsically safe fiber sensing

Low-power fiber-optic sensors can work in refineries, chemical plants, power plants, oil production facilities, or in any hostile environment, because the sensors pose no danger even in hazardous areas where explosive vapors may exist.

John Berthold and Richard Lopushansky of Davidson Instruments in The Woodlands, Texas, enlightened us last month in this department as to the efficacy of fiber-optic sensing technology in hazardous environments and for industrial applications (www.isa.org/intech/200401/sensors).

Instrumentation installation and maintenance costs are significantly less, because low-power fiber-optic sensing systems do not require explosionproof conduit and containment. These fiber-optic sensors can operate at elevated temperatures in hydrogen-rich environments.

OPTIC SENSORS REMAIN ACCURATE

Temperature—Fiber-optic temperature sensors can be designed to measure temperatures that range from –100°F to 1400°F. The sensors can be packaged in stainless steel sheaths that range from 0.050-inch to 0.25-inch diameter. Single point sensors can be a direct substitute for conventional thermocouples and resistance temperature detectors. Multipoint temperature sensor probes can have up to 32 sensors. Multipoint temperature sensor probes are ideal for monitoring the temperature profile of catalyst in tube reactors or along the length of furnace tubes. The probes can range in length from several inches to more than 50 feet and can be packaged in probes as small as 0.125 inch in diameter. The individual temperature sensors can be spaced 1 inch apart. The multipoint sensor probes can connect directly to the signal conditioner or via a rugged multipoint connector.

Pressure sensors—Fiber-optic pressure sensors can operate at temperatures up to 1000°F with maximum pressures to 20,000 pounds per square inch (psi) and be designed as gage or absolute sensors. Inconel-718 typically serves for use in diaphragms because of its high strength and corrosion resistance, but other materials can work if conditions warrant. Each sensor has stainless-steel armor jacketing to protect the fiber where it exits the transducer body. If conditions warrant, one can integrate a temperature sensor into the transducer to provide thermal compensation for changes in the pressure sensor gap due to changes in temperature over the operating temperature range of the transducer. Miniature pressure sensors can be as small as 0.180 inch in diameter. Because the fiber-optic gage pressure sensors do not contain any fill fluid, they are immune to drift and failure due to service at high temperatures in hydrogen-rich environments.

Differential pressure/density/level/flow sensors—Fiber-optic differential pressure (dP) sensors can operate at temperatures up to 500°F and measure 0–400 inches of water with static pressure of 3000 psi. These fiber-optic dP sensors are fluid free and can integrate with a temperature sensor. The differential pressure sensors can measure density or level if defined as such in the signal conditioner set routine. In many cases, the fiber-optic dP sensor can eliminate capillary tubes and impulse lines.

Linear/rotary position sensors—Fiber-optic linear position sensors have an operating range of up to 4 inches and can resolve 0.001 of an inch. These sensors are designed to operate at temperatures up to 500°F. Rotary position sensors have an operating range of 360° and can resolve 0.5°. These sensors can operate at temperatures up to 500°F.

Strain sensors—Fiber-optic temperature-compensated strain sensors are designed to operate at temperatures up to 1000°F with a range of 0–5000 microstrain. These high-temperature strain sensors can configure with a stainless-steel shim that allows one to weld the sensor to a vessel or pipe. These fiber-optic sensors remain accurate through the rated temperature range because of the innovative design that provides automatic temperature compensation. As with all of the other sensors, the temperature-compensated strain sensor has two reflective surfaces, one of which is the end of the fiber. The other reflector is a material that matches the thermal expansion of the work piece. For example, if the work piece is stainless steel, then a stainless-steel wire serves as the second reflector. Because both the work piece and the wire are the same material, the Fabry-Perot gap is not sensitive to temperature changes and is only sensitive to changes in strain.

Accelerometers and vibration/acoustic sensors—Fiber-optic accelerometers and vibration/acoustic sensors are designed to operate at temperatures up to 500°F with a wide range of sensitivity. Because the sensors contain no electronic components, one can quickly weld them to a vessel or pipe using conventional stud welding equipment. Two variations of the vibration/acoustic sensors exist. The first is a microelectromechanical systems–based design with an "H" shaped accelerometer beam. The second is a resonant sensor that is tuned for high sensitivity to a narrow band of acoustic frequencies. The range of frequency response for these sensors is 2 hertz to 100 kilohertz, and performance is comparable piezoelectric accelerometers.

INTEGRATED FIBER FAMILY

The advantages of fiber optics for industrial process control instrumentation are great. The technology is mature; the infrastructure to support industrial fiber applications is in place; and the cost of critical components continues to fall while technical performance improves. Fiber-optic sensing systems offer a number of benefits over conventional electronic sensing systems:

• Lower installed cost than conventional sensors used in hazardous locations, because no explosionproof containers or conduit are necessary
• Inherent safety and suitability for use in Class I, Division 1 explosion hazardous environments
• Tolerance to high temperatures, e.g., 1000°F and hydrogen-rich, corrosive environments
• Immunity to electromagnetic interference, grounding, and lightning problems and isolation from high-voltage circuits

Fiber-optic sensing systems are being designed and packaged to address the harsh environments of industrial process control. Sensors and signal conditioners tested well under field conditions and have demonstrated seamless interface with existing distributed control systems. Integrated families of fiber-optic sensors and signal conditioners are available to measure most physical parameters and systems are being used in refineries, chemical plants, power plants, and in oil and gas production facilities.

Over the next few years, the reliability and economic advantage of fiber-optic sensing will prove out in a wide variety of industrial applications. IT

Nicholas Sheble writes and edits the Sensors and Technology Advances department. Write him at nsheble@isa.org.