According to Kajaani Process Measurements Ltd., Kajaani, Finland, reliable break detection is the key aid for paper mill operators in optimizing production and machine up-time by getting accurate and reliable break information to paper machine control and monitoring.

With the increased speed of modern paper machines, open draws are more often replaced by closed draws where the paper web runs on top of wires or felts. These fabrics come in different colors, the contaminants from the paper change the surface color of the fabric and the optical reflectance characteristics of the fabrics change with time. This is a big challenge for conventional optical web break detectors which use a single wavelength for the detection.

Traditionally, photo cells have been used for web-break detection. They are for limited applications and require continuous cleaning and maintenance. IR-based fiber optic break detectors have been used where the web is supported by fabrics but these detectors cannot differentiate between the web and a red wire or felt.

To overcome these issues, Kajaani has developed a new fiber-optic webbreak detector which uses an RGB (Red-Green-Blue) light source for detection. The best color signal red, green, blue or a combination of them can be selected for reliable break detection even with different fabric colors.

The detector is designed to handle all break detection applications. The unique air purge keeps the sensor's fiber optics clean in dusty and steamy situations, eliminating maintenance. The small sensor head allows for installations in very tight spots inside machines.

The KB detector is inside a stainless steel rod and does not have any active electronics inside the probe. An internal air purge keeps its window openings clean and no regular cleaning is required. The detector system's selfdiagnostics continuously monitor the raw break detection signals. In case of a problem, the KB detector gives a maintenance alarm and thus avoids false breaks.

The KB web-break detector utilizes proven high-temperature-resistant fiber optics so it is usable in hot environments such as the dryer sections of a machine. The air purge arrangement expands applications to very humid, hot and dirty locations.

Applications. The most common application is in an open draw where there is only the paper web with air behind. The much more demanding applications are where there is a need to detect the paper web on the felt. Traditional IR web break detectors only work for some applications because of their limited detection performance. The RGB-based KB webbreak detector can detect different colors and then separate wire/felt reflections easily from the paper web signals.

Features and benefits. Modern RGB-based KB web-break detectors are used in many paper and board machines to detect web breaks. The key features and benefits of Kajaani's KB web-break detectors include:

• LED light-source RGB color detection
• detect in open-draw situations
• detect color changes in web, felt or roll
• easy and quick to start-up and maintain
• reliable sensing head for harsh conditions
• fast web-break signal processing
• applicable for hot applications (up to 180oC)
• small, modular construction
• self-cleaning sensor head
• safe working at remote displays adjacent to rotating cylinders or where hot steam flows exist
• low-cost installation
• analog output signals to DCS trending
• optional computer software.

Kajaani Process Measurements Ltd., www.peokajanni.com

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