Electricity + Control

by Christo du Plessis, Zest WEG Group

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 continent, 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 demand throughout the whole of the southern African 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 environmentally friendlier generating plants are also being considered.

Optimising water transfer system with VSDs

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 conservative annual increases that it had applied to new and existing customers over previous decades.

Over the past five years, the average annual increase in electricity prices for the mining sector was 12,06%.

System optimising opportunity

AngloGold Ashanti has for years been at the forefront 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 improve a water transfer system at its Vaal River operations 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 operations requested a meeting with Zest WEG Group’s Automation Division to discuss possible improvements. 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 centrifugal 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 paralleled into a pipe column. The delivery side – feeding all areas of the surface operations – had a pressure- 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 pressure switch controlled the switching on and off of the mostly three-pump operation system, with the fourth pump being on standby. It was also noted that in normal operating conditions, only two pumps would run. The third would switch on and off only at intervals where a sudden increase in demand was reached. This caused challenges both in the flow dynamics of the long piping system, and in terms of mechanical stresses.

Upgrade requirement

The water transfer system needed a smooth flow and pressure control to balance the supply and deman of water. This needed to be achieved within the allowed budget. It was also noted that the allocated budget did not include upgrading to Premium Efficiency IE3 Motors, and that this would be incorporated in the next phase of the project. With AngloGold’s requirements explained – and the information obtained on the existing equipment on site – the Zest WEG Group team developed a solution and returned the following week with an offer on VSDs in panels to run the pumps.

The offer was within the allowed budget, but AngloGold also wanted assistance with calculations on the energy reduction anticipated with the proposed upgrade of the system.

Baseline and opportunities to reduce speed on pump

Calculations like this needed proper measurement of flow rates (work done), as well as the energy (kilowatt hours) used to create the flow rates. A baseline needed to be established, and fortunately there was instrumentation already installed: a flow meter on the delivery pipe and a kilowatt-hour meter measuring the electricity used at the pump station.

Data was obtained with the help of the control and instrumentation department, and this was used to create a baseline showing the flow rate demand over a period, with the electrical power required for the same period.

Zest WEG Group compiled the energy reduction report, considering the pump curves (power and flow rate) provided, and presented the estimated savings to AngloGold Ashanti for discussion.

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 estimated electrical demand reduction; this was conservatively 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 instances where the required delivery rate would fall.

Valuable feedback was received from AngloGold Ashanti regarding the report, with suggestions on an improved control philosophy which would further 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 CFW11 WEG VSDs complete in floor standing panels manufactured by Shaw Controls, a division of the Zest WEG Group.

After some communication regarding the control wiring requirements and the AngloGold Ashanti standards, the wiring diagram for construction was approved. Delivery to site was made in accordance with the project timelines.

Installation of the two panels was completed during the first week of October 2014. The new upgraded system now also had a new added feature: a PLC was incorporated to improve the control side.

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 adjust 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’ controlled starter and would be controlled via the PLC to switch on only in high demand times where the flow rate required exceeded 1 100 m3/h. Once the flow rate required dropped below 950 m3/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 continuously speed up and down, optimising the system requirements based on a set point which was established 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 follow 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-controlled return valve remained in the system as a precautionary measure.

Results and conclusion

Optimising results were done with the system requirements maintained. Variable required flow rates on demand were achieved with the system pressure remaining in the required limits as well.

Over and above these improvements, there was a considerable energy saving achieved, as shown in the graph below, which reflects data from the energy logger at the pump station. The figures indicate an average energy reduction of 20% when compared with energy loggings before the upgrades were made – thus improving substantially on the estimated 15% predicted in Zest WEG Group’s original report.

Measuring the capital expenditure costs of R320 000 against the savings gained since implementation in October 2014, the payback time amounted to just over a year. The savings for the first year amounted to R300 000, based on the tariff at the time of R0,69 per kWh. With the annual average tariff increase of 12,06% compounded, and the 20% energy reduction, it can be said that the water transfer system upgrade was a good investment.

Other benefits

In addition to the energy reduction, the system benefitted from the more accurately controlled process variables through built-in features on the VSD such as longer ramp-up and ramp-down times. This had an impact on pipe friction losses during pipe filling times and a reduction on water hammering during periods where a system stop command would be given. Mechanical stresses were also reduced on the coupling between the motor and the pump, and fewer losses would be seen due to the extended ramp-up and ramp-down times programmed.

Other savings are also realised through man hours reduced during the start-up process – as the automation of controls meant there was now no need to have personnel present. In the event of power failures, the system can now also automatically start and run to its system required demands.

Acknowledgements

Picture credits: AngloGold Ashanti

Further credits: Sydney Higgo, instrument technician at AngloGold Ashanti.

Andre Olivier, chief electrician at AngloGold Ashanti Marchant de Kooker, electrical/cic/systems engineer at AngloGold Ashanti.

Christo Landman, electrical foreman at AngloGold Ashanti

 
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