Portland Bulk Terminals, a Canpotex facility for the export of Potash is managed by Kinder Morgan Bulk Terminals Inc. Located in Portland Oregon, the terminal is the site of a recent Belt Scale Permanence test. The test, the final part of the NTEP trade approval process, has established High Precision Belt Scales manufactured by Australian Company, Control Systems Technology Pty Ltd (CST) as the preferred method of weight determination for the export sales of Potash.
Export sales of bulk materials have long been conducted on the basis of ships draft survey. It has become well known that precision of accuracy is quite variable depending upon wave action, the age of the ship and hence the precision of the original volumetric survey and the amount of time allowed for the survey process. The repeatability of ships draft survey can vary between +-0.5% and +-2.0%, while offset errors due to the quality of the ship’s original survey are of unknown size. If the accuracy of the whole ship survey is questionable, the accuracy of partial loads is even more in question. This is why Canpotex has chosen to base all of its export sales upon weighing by High Precision Belt Scales by CST. Canpotex is the world’s largest potash exporter, and is an international marketing and distribution company wholly owned by the Saskatchewan potash products: Agrium Inc., IMC Canada Ltd., and Potash Corporation of Saskatchewan, Inc.
CST has worked closely with their North American agent, TD Micronic, Inc. of Vancouver, BC in having their belt scales approved for trade use in the USA and in Canada. CST had completed NTEP laboratory testing and also the initial round of field testing at Portland Bulk Terminals and then on 12th August 2002 the final Permanence test was conducted.
The National Type Evaluation Program (NTEP) in the USA involves laboratory and field test. The field tests require that 3 consecutive test loads be passed over the belt scale at a flow rate between 35% and 98% of capacity with an error not exceeding 0.25%. The belt scale at PBT is rated at 5000 t/h and it was tested with 600 t loads at a flow rate of 3500 t/h. The reference scale upon which the 600 t loads were pre-weighed was a Toledo track scale with an accuracy equivalent to 0.02%. The normal permanence test period is six months, however the period since the initial tests had stretched out to 13 months. The belt scales is required to perform to an accuracy of +-0.25% at the time of initial tests and again in six months, or in this case 13 months later.
The performance of the belt scale was very convincing at the initial tests, returning errors of -0.04%, -0.03% and -0.01%, while a test rejected by NTEP officials as not sufficiently within the 35% to 98% flow range showed an error of -0.08%. The later tests 13 months later, without any adjustment or maintenance, yielded a very repeatable group of errors or 0.12%, 0.12% and 0.01%. The results were the subject of much favourable comment by the testing officials who were ready to simply restart the tests if the 13 month period had proved to be a problem, as indeed, it is well beyond the required 6 month period. The error of 0.12% high represents a calibration drift in the order of less than 0.01% per month, a drift which is likely to be due to a reduction in the diameter of the tail pulley used for tachometry. On the 1000mm tail pulley which is affected by rust scale, which is in turn related to the corrosive nature of potash, a reduction in diameter of only 0.5mm would account for this level of calibration drift.
While the capital outlay on one of these highly stable belt scales is more than many other units on the market, the low cost of maintenance and the efficiency of certification tests will yield cost savings which will repay the capital over and over during the life of the equipment. More importantly, the high precision of the belt scales is also likely to bring in the right revenue when compared to the relatively variable results available from ships survey.
Bulk Solids Handling – Vol 22 (2002) No. 6