In a report prepared for RPS Aquaterra, Iain Rea and Daragh Monaghan discuss the significant reductions in energy, greenhouse gas emissions and overall costs that can be achieved for a range of dewatering scenarios.
Pumping water for dewatering and water supply is a major component of electricity consumption at mining operations and hence is a significant contributor of greenhouse gas emissions.
Pump systems at a typical mine site can be broadly split between above ground centrifugal pumps (generally used for water transfer) and submersible borehole pumps (generally for dewatering and/or water supply). In contrast to above ground pumps, submersible borehole pumps can be subjected to varying duty points (total pumping head and flow requirements) as a result of changing bore water levels and yields, particularly in mine dewatering bores. Typically, much higher pumping rates are required at the commencement of dewatering to remove groundwater storage and to intercept sufficient groundwater throughflow to achieve target drawdowns. Once target drawdown levels are reached it often only requires much lower pumping rates to maintain target drawdowns. Pumps sized to remove the large initial volumes of stored groundwater often end up too large for the lower flow rate required for ‘maintenance pumping’. This often results in pumps operating inefficiently, a problem which can be made worse when pumps are throttled to reduce the pump output to better match the reduced bore yield.
Current standard operating practice places little emphasis on the electricity consumed by pumps, with the focus often being on minimising capital expenditure (‘If it ain’t broke don’t fix it’). Submersible pumps can have extended service lives of ten to 20+ years. However as the capital cost of a pump typically represents only five percent of the life cycle cost, the vast majority of the life cycle cost is energy usage.
Through the implementation of a regular review of the operation of borehole pumps, significant reductions in electricity use and associated greenhouse gas emissions can be achieved without impacting the dewatering schedule. In most cases, the capital cost of replacing pumps is more than offset by savings in operating costs. These potential savings become even more significant when considering proposed future carbon tax levies.
Due to the nature of typical dewatering requirements, higher pumping rates are required at the initial stages of dewatering to remove groundwater storage and to intercept sufficient groundwater through flow to achieve target drawdowns. However, once target drawdown levels are reached it often only requires much lower pumping rates to maintain target drawdowns, typically known as maintenance pumping rates.
As saturated aquifer thickness decreases as a result of the dewatering process, it is common practice to continually ‘throttle’ the bore pump in an attempt to reduce the pump output to match the decreasing bore yield. Whilst throttling can be achieved by the use of flow control valves or pressure sustaining valves located on the bore headworks, it is more typical to see it achieved by partially closing isolation valves on the bore headworks.
This approach generally works satisfactorily up to a certain point. However, as a pump is throttled back further, it becomes increasingly difficult to maintain a consistent flow rate due to the lack of control in the partially closed valve (ie with a butterfly valve it can be commonly observed that the majority of flow reduction is achieved only when the valve is 75 per cent or greater closed), and as a result the pumps can start to trip out on low flow. The cause of the low flow cut-out can be either due to a lack of sensitivity in the flow switch on the headworks or as a result of the pumping water level in the bore being pulled down to the pump inlet (bore pumping rate is higher than the bore yield). At this point it is common to see dewatering bores being abandoned because they ‘ran out of water’ or they are pumped only intermittently in response to rising aquifer water levels.
This process may take a number of years depending on the rate of decline of the bore yield, and as a result the required duty point for the dewatering bore pump may move from being efficient to very inefficient – and hence expensive – when throttled back and used as a maintenance water level pump. In the latter stages it also becomes very ineffective as a dewatering bore as it is generally accepted that effective dewatering is better achieved by pumping consistently at lower pumping rates rather than pumping for only one day a week at a higher rate for example.