Nuclear sensors nixed in tissue mills

Mark Willliamson

Scanning measurements of fiber weight and moisture are now combined in a single infrared sensor. Nuclear material licensing, handling and staff training are eliminated.

Nuclear material handling and safety regulations have been part of tissue making from the beginning of QCS systems. Now, that is changing.

Nuclear basis weight sensors have been the standard for scanning the tissue web for many years. They have been improved regularly to give very precise, repeatable readings even in the notoriously dusty environment of a tissue mill. To provide an oven dry basis weight reading for precise control purposes, infrared moisture sensors are always positioned alongside. Today, that infrared technology is developed to the point where it can take over the task of measuring oven dry basis weight (fiber weight) with the same precision as a nuclear sensor and, furthermore, measure moisture simultaneously at the same spot of paper.

Lower lifecycle costs

These two-in-one sensors are now making nuclear sensors redundant and eliminating the need for them entirely. The single-sensor QCS system for tissue quality control has arrived and is being adopted in many tissue mills. With the large number of previously-installed nuclear sensors, the retrofit market for existing tissue QCS systems is an obvious and promising growth area. In fact, several of these sensors have already been retrofitted to older quality control systems in Canadian tissue mills.

By consolidating both measurements in a single sensor, the lifecycle costs of servicing and spare parts have been reduced considerably and the continuing costs of nuclear safety training, licensing and "hazmat" handling have been eliminated. The new sensors also have some practical maintenance advantages since there is no need for ultra-precise sensor head alignment and cross-direction air gap weight compensation (the so-called air scans) that are needed to maintain the precision of nuclear measurements.

Metso was the first off the mark with the IQFiber sensor and now it supplies this sensor as a standard product with new tissue machine quality control systems. Some retrofits on older systems have already been done. As you might expect in a very competitive market, the other major QCS suppliers – ABB, Honeywell, and Voith - have since introduced their versions of a single fiber weight and moisture sensor.

The measurements are based on the wavelength-specific absorption of infared energy by cellulose and water. See Figure 1. Reference measurement points ensure the readings are absolute. When a semiconductor material is used as an infrared detector the fiber weight and moisture readings are much faster than previous lead sulfide detectors, leading to an improved response to transient weight changes and streaks. Also, the measurement spot size is considerably smaller than a nuclear sensor. Unlike nuclear sensors whose source strength decays and measurement sensitivity is diminished over time the infrared measurement sensitivity is constant.

Figure 1: The online measurement is based on the absorption of infrared energy by the tissue sheet at wavelengths specifically sensitive to the water and fiber content of the sheet. Reference measurement points ensure the readings are absolute.

Figure 2 show the side-by-side sensor comparison trials in a tissue mill have confirmed that the infrared measurement of fiber and the oven dry weight measurement derived from the nuclear sensor minus the moisture measurement were essentially the same. The fiber measurement and the on-control stock flow changes tracked closely during normal production periods and grade changes, and the profiles were equivalent. The online fiber reading correlation to this laboratory reading was excellent as well, with the infrared reading being slightly closer than the nuclear reading.

Figure 2: A side-by-side sensor comparison trial confirmed that the infrared measurement of fiber and the oven dry weight measurement derived from the nuclear sensor minus the moisture measurement were essentially the same, even during a grade change.

Look; something is missing! There are no more radiation warning signs on this tissue mill scanner.

Nuclear-free tissue mill?

There is an obvious trend to single-sensor systems for new and replacement quality control systems since the measurement offers lifecycle cost advantages and the overhead expenses of safe nuclear material use are eliminated. Don't expect the infrared sensors to be more precise than nuclear devices as that was not the point of their development; rather, they are designed to provide an equivalent reading of dry fiber weight without the radioactive source.

If older systems can be retrofitted with the new sensors there is a possibility that tissue mills could be nuclear-free some day. Looking into the future, will this technology be adapted to other paper grades so that nuclear measurement will be eliminated? Not today maybe, but time will tell. Some suppliers may have those ideas under wraps for now.

Data and drawings are courtesy of Metso