Measuring up: Papermaking puts optical fibre to the test

The thought ran through Frank Haran’s mind more than once–”We’re breaking a whole lot of rules.”

Haran, a sensor physicist with Honeywell’s process solutions business in Vancouver, B.C., wanted to use optical fibre sensor technology in one of industry’s most demanding environments: papermaking.

“You’re not supposed to use optical fibre in high temperatures and high humidity. You’re not meant to drag an optical fibre over pulleys for millions and millions of cycles. You’re also not meant to use a fibre optic cable to drive a sensor head back and forth,” says Haran.

With the help of OFS Specialty Photonics Division, Honeywell developed ExPress Moisture, a compact, robust, high-speed system to remotely measure moisture throughout the papermaking machine.

Simply put, the papermaking process starts with a slurry composed of wood fibre, water and other additives such as dyes. This slurry passes through a series of presses and dryers that gradually remove the water and ensure that the resulting paper meets exacting specifications for thickness and other properties.

According to Ross MacHattie, a Honeywell product manager based in Mississauga, Ont., “Our quality control systems measure different characteristics of paper, such as moisture content, so they can be controlled during the manufacturing process. We design and manufacture sensors and the equipment that is used to position those sensors over the sheet of paper. Some sensors are in fixed positions while others are mounted so that they move across the sheet and provide measurements at specific intervals, such as one centimetre across a 10-metre wide sheet.”

Honeywell’s ExPress Moisture cross-directional sensors are guided by carbon fibre rails and are moved across the width of the paper by a reinforced fibre optic cable. The sensor measures moisture by selective infrared absorption and transmits this data to a computer system that calculates control information and directs the paper machine to make minute adjustments to keep the paper consistently within specifications. By maintaining uniform moisture distribution and removing water at optimal rates along the length of the machine, paper is produced more uniformly and efficiently.

In a typical papermaking machine, the area where the process of removing water from the slurry begins–the so-called “wet end”–is characterized by a very harsh environment.

“With temperatures in excess of 70ºC and nearly 100 per cent humidity, we couldn’t use traditional light sources, active optics, electronics or complex mechanical systems,” says Haran.

The objective was to place only the passive sensor components in the machinery environment. To do this, Haran and the Honeywell development team wanted to use fibre optics to deliver light to the sensor and to return the data to the computer.

“Some fibre manufacturers told us this wouldn’t work, and one that did think it could was unwilling to produce the fibre in the quantities we needed,” recalls Haran.

Honeywell turned to OFS Specialty Photonics Division, in Avon, Conn.

“We said, ‘Here’s what we need, can you make it for us?’ That’s when I thought about breaking all the rules about using optical fibre in this environment,” Haran explains.

ExPress Moisture uses a hard clad silica (HCS) fibre. This type of fibre is designed to resist heat and humidity, and withstand tight bend radii and repeated mechanical cycling over extended periods of time. In HCS fibre, the hard polymer that surrounds the core not only acts as a cladding to guide light, but it also acts as a strength enhancer. This polymeric material forms “bridge bonds” that heal small flaws on the surface of the silica. Fibre strength is greatly enhanced, static fatigue significantly retarded, and tighter bend radii are possible. HCS fibre can withstand less than an inch bend for 20 years.

In order to physically push and pull the sensor along its tracks, the optical fibre was reinforced using proprietary fibre sheathing. The company also worked with Honeywell on solutions to ensure that water would not get in through the connection.

Honeywell has put the OFS solution to the test. “We’ve run over 10 million cycles at 70 C and 100 per cent humidity,” said Haran. “There has never been a breakage of fibre.”

Mike Conroy is the market manager for industrial products at OFS Specialty Photonics Division.