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Fig. 2. A 53 – 73 micron filter calibration standard.
By accurately measuring the particle size distribution of the filter standards both by microscopy and a precision electroformed sieving method, a calibration graph is constructed where the percentage of the beads passing an unknown mesh can then be used to calculate the filter cut point (Fig. 3). As the results are traceable to international standards such as the National Institute of Standards and Technology (NIST), the derived filter cut points are also NIST traceable. Having a precision range of traceable microspheres is only the first stage in measuring filter cut points. Tere must also be an accurate and repeatable way of presenting or challenging the filter medium with the standards. Te sonic filter tester (Fig. 4) is a unique system that uses intense sonic energy to produce an oscillating column of air, which flows through the body of the mesh. Tis process energises the individual microspheres at rates of 3600 cycles per minute rather than mechanically shaking the mesh as in conventional sieve shakers. It therefore has the potential of sifting down to five microns. An on-board computer programmes the entire test sequence thus eliminating any operator bias. Woven sand screens can be measured in a few minutes using the sonic sifting device. A known weight of the calibrating microspheres is placed on the sand screen under test and sifted for one minute. From the per centage of the standard passing the filter, the calibration graph is used to determine the cut point of the mesh. Because the particle size distribution of the standard is so narrow, a five per cent error in weighing only results in a two micron difference in cut point. Once the accuracy, repeatability and confidence levels have been
Fig. 3. A 127 - 175μm filter calibration graph.
established it is then possible to use the method as a quality control instrument in the weaving process. In one production facility, 25 random samples
were taken from several rolls of sand screen meshes having target cut points 270µm, 230µm and 150µm. Te results showed excellent consistency over a prolonged period and illustrated the precision that can be achieved in the weaving process. Te sonic filter tester has had a dramatic effect
on the measurement of pore sizes in sand screens. Tere is now an accurate and repeatable method of certifying the meshes to international standards.
Fig. 4. The sonic filter tester.
Having an independent laboratory issuing the test certificates gives users, from sand screen fabricators to oil companies, confidence in the results. It is hoped that there will never again be a US$150m disaster from wrongly specified sand screens. l
Dr Graham Rideal is Managing Director, Whitehouse Scientific Ltd, Chester, UK
www.whitehousescientific.com 10
www.engineerlive.com
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