Electricity + Control October 2018

CONTROL SYSTEMS + AUTOMATION

It was imperative to understand that, at any time, the running hours could be affected by the required demand, and it was agreed that the report would not look at a payback period but purely at the esti- mated electrical demand reduction; this was con- servatively calculated to be 15%. The saving was due to the combination of the Affinity Law gains when reducing the speed of the pump’s impeller and decreasing of the amount of water circulated through the return valve, requiring the same water to be pumped because of excessive pressure in in- stances where the required delivery rate would fall.

Communication was established between the VSD and PLC via Modbus TCP to incorporate all the field signals with immediate flow rates and pressures. This allowed the system in turn to ad- just required speed references to the two VSDs running pump #1 and pump #2 at the same speed. Pump #3 would still run as a ‘direct on-line’ con- trolled starter and would be controlled via the PLC to switch on only in high demand times where the flow rate required exceeded 1 100 m 3 /h. Once the flow rate required dropped below 950 m 3 /h for 10 minutes, it would again switch off. Pending the flow rate requirements in the water transfer system, pump #1 and pump #2 would con- tinuously speed up and down, optimising the system requirements based on a set point which was estab- lished as being 520 kPa. Pump #4 would remain a standby pump as with the previous control system. In the PLC programme, pump #2 was made the ’base’ or ‘master’ pump and pump #1 the ’slave’ pump – where the slave would simply fol- low the master’s speed reference. Provision was also made in the programme to change the master pump to pump #1, if need be. The pressure-con- trolled return valve remained in the system as a precautionary measure.

Mechanical power required in a centrifugal pump is directly proportional to the cube of the flow rate ((PᾳQ3).

Valuable feedback was received from AngloGold Ashanti regarding the report, with suggestions on an improved control philosophy which would fur- ther optimise the water transfer system. Project scope and implementation AngloGold Ashanti placed the order on Zest WEG Group in July 2014 and delivery was scheduled for the end of September 2014 for two 185 kW CFW11WEGVSDs complete in floor standing pan- els manufactured by Shaw Controls, a division of the Zest WEG Group. After some communication regarding the con- trol wiring requirements and the AngloGold Ashan- ti standards, the wiring diagram for construction was approved. Delivery to site was made in ac- cordance with the project timelines. Installation of the two panels was completed during the first week of October 2014. The new up- graded system now also had a new added feature: a PLC was incorporated to improve the control side.

Two 185 kWWEG VSD panels supplied by Shaw Controls.

22 Electricity + Control

OCTOBER 2018