Andy Pye examines a unique tool using ultrasonics for early prediction of the performance of paper and board materials while optimising key production processes.
Lorentzen & Wettre, though founded in 1895 by two Norwegians having their origins in the Norwegian timber, pulp and paper industry, is now headquartered in Sweden and has been part of ABB since 2011.
The company supplied its first automatic paper testing systems in the 1960s and installed at predominantly Swedish companies. They were primitive and crude by today’s standards, but still gave rise to the idea that the future might lie in a combination of on-line measurements for control and off-line measurements by standardised measurement methods.
The L&W TSO Tester measures Tensile Stiffness Index (TSI) and Tensile Stiffness Orientation (TSO) properties on machine- or cross-machine profile paper samples cut out from a jumbo paper reel. The device works by sending an ultrasonic pulse across the paper and measuring velocity in eight different directions. The results predict the performance of paper used in a sheeting process, in a multi-colour printing process, in laser copying machines and when manufacturing corrugated board. By using the new device, a paper machine using the tester can easily achieve specifications for compressive, tensile and bursting strength.
Typical troubleshooting and prevention facilitated by the tool relate to the headbox, press and drying sections of the manufacturing process. In the headbox, for example, the device can help manufacturers to identify quickly whether action is required to manage pressure pulsations or deviating pressure profiles.
The device can also be used to predict runnability in a range of applications. When making corrugated board, for example, the device enables manufacturers to avoid warping by ensuring the TSI/TSO measurement of the single face liner matches that of the double backer. In multicolour printing, where the hygro-expansion orientation needs to be carefully managed, the instrument’s TSO-angle measurement capabilities can help.
“By providing manufacturers timely access to key measurements, papermakers can save on time and raw materials by catching potential issues early in the production cycle,” says Thomas Fürst, Product Manager, ABB/Lorentzen & Wettre.
A typical cross-directional measurement is performed within five minutes, and provides complete information on the full web width, including edges. The TSI and TSO profiles can be determined at every reel change. Early action can be taken against possible deviations in the following reel, keeping the TSI and TSO values, as well as any calculated strength properties, within pre-determined limits.
To use the instrument, a test strip of paper is cut from the reel, using the profile sample cutter from Lorentzen & Wettre. The strip is attached to the instrument’s feeder and the measurement sequence is automatically performed by pressing the start button. The measurement speed is about six seconds per measuring point. Thus, the total time required for a complete profiling sequence depends on the length of the sample and the distance between measuring points.
The L&W TSO Tester consists of a table-top ultrasonic measuring unit with feeder attachment and PC software for logging of test results. The colour touchscreen has intuitive menus, a user-friendly interface, and a protective surface for easy cleaning and durability. TSI- and TSO-values are presented on the touchscreen, either in tabular or graphic form. Results can also be printed on the built-in printer, on a network printer or exported via Ethernet. The instrument acts as an FTP-server and test results can be retrieved by an FTP-client. The complete measuring station can be placed either in the machine control room or in a side laboratory. All results can be stored for further analysis or for transfer to other computers.
Theory of Operation
The speed of an ultrasonic pulse in the plane of a paper depends on the elastic properties of the paper – its tensile stiffness index (TSI). TSI can be compared with Young’s modulus (E) for other materials.
The relationship can be expressed as: TSI = v² × c
TSI = tensile stiffness index (kNm/g or MNm/kg) of the paper measured using the ultrasonic method
v = propagation velocity (km/s) of the ultrasonic pulse
c = a dimensionless constant close to 1, depending on Poisson’s ratio for the paper
Velocity is measured in eight directions. The result is converted by the processor using a Fourier transformation to an ellipse. The ellipse shows the elasticity in all directions of the plane of the paper. This allows the identification of the direction of the greatest stiffness.
This property is known as TSIMAX.
The difference between the machine direction in the sheet and the direction for TSIMAX is referred to as the TSO angle. Other properties reported by L&W TSO Tester are:
TSIMD – tensile stiffness index in the machine direction
TSICD – tensile stiffness index in the cross direction
TSIMD/CD – tensile stiffness index ratio – the anisotropy in the sheet
TSImax – tensile stiffness index, maximum value
TSImin – tensile stiffness index, minimum value
TSIAREA – the tensile stiffness index, ellipse surface area