Electricity + Control October 2018

CONTROL SYSTEMS + AUTOMATION

Optimising water transfer system with VSDs

Christo du Plessis, Zest WEG Group

System optimising opportunity AngloGold Ashanti has for years been at the fore- front of pursuing energy reduction initiatives and has worked closely with universities and energy efficiency institutions in the past. Driven by this culture, it was imperative for the company to im- prove a water transfer system at its Vaal River op- erations in the North West province. The system had been running trouble-free for many years but was probably not as efficient as it could have been and was identified as one of the operations’ big consumers of electricity. The engineering supervisor for surface opera- tions requested a meeting with Zest WEG Group’s Automation Division to discuss possible improve- ments. The first meeting, held early in 2014, included a site visit to the Pump Station to gather information on the installed base of equipment.There was also a discussion regarding the Affinity Law and what impact it could have when incorporating variable speed drive (VSD) technology on the existing cen- trifugal pumps. In summary, the optimisation of the pump station to reduce energy consumption and improve efficiency was the objective. The original system operation The installed motors were 185 kW, 4-pole, 525 V Standard Efficiency IE1 units driving four pumps. The pumps were all connected in parallel to the suction side, with the delivery side in turn paral- leled into a pipe column. The delivery side – feed- ing all areas of the surface operations – had a pres- sure-controlled return valve to the reservoir tank, which opened up if the pressure in the delivery line reached 700 kPa; this level was reached every time the demand for water was reduced. The excess pressure was therefore controlled by the return of water through the return valve passing back water to the reservoir and suction side of the pumps. In addition to this water return system, a

Rising energy costs The fast-increasing price of electricity from South African local utility Eskom has become a major cost factor for South African mining groups. At the same time, pressure has been placed on Eskom, as the leading power utility on the African conti- nent, to supply rapidly growing mining, agricultural and industrial sectors. Historically, its coal-fired power stations have predominately serviced local industry, with some assistance to neighbouring countries. This has changed dramatically, with an ever-increasing de- mand throughout the whole of the southern Afri- can region – putting its aging infrastructure under constraint. The growing demand has led to the construction of two new coal-fired power plants, currently being completed, while new and environ- mentally friendlier generating plants are also being considered. With the expansion of its power station fleet, combined with desperately needed upgrades to its existing plants, the utility had no other option than to raise the rate of the traditionally very con- servative annual increases that it had applied to new and existing customers over previous dec- ades. Over the past five years, the average annual increase in electricity prices for the mining sector was 12,06%.

Take Note!

1. The delivery side had a pressure-controlled re- turn valve to the reservoir tank. 2. The valve opened up if the pressure in the delivery line reached 700 kPa. 3. This level was reached every time the demand for water was reduced.

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Eskom’s Average Price Adjustment for the mining sector.

20 Electricity + Control

OCTOBER 2018