On a construction site or during pipe line (well point) dewatering, engineers often face the choice between electric-driven or diesel-driven pumps. Because of the often temporary character of dewatering, calculating the cost of a dewatering job has not been that much of an issue. However due to the worldwide price increase of fuel, this attitude is changing. While an electric (submersible) pump appears easier and more cost effective than a diesel driven pump, there are other factors to consider.
As dewatering is usually the first activity on a construction site, often there is no power supply immediately available. If there is power, it is usually from a temporary source of dubious reliability. The consequences of having a failure in the dewatering system can be huge and damage to new foundations not easy to repair and are extremely costly.
During pipe line dewatering, the time frame of having a pump in one place is often very short. A flexible solution is needed that does not require transporting cumbersome extension cords, with the added risk of them becoming damaged by heavy equipment. In these situations, there is an argument that diesel-driven pumps could be the most effective choice.
Central generator
If a dewatering project is long-term, electric-driven pumps are often installed with a central generator as a power source. Filling one fuel tank instead of several one at a time makes logistics easier. However, efficiency losses in the generator and the electric motor may result in the total fuel consumption being more rather than less than the diesel alternative. Furthermore, diesel-driven dewatering pumps with noise levels as little as 46 db at 10 m is another point in favour of their adoption in preference to electric driven pumps.
Maintenance on diesel engines is needed far more frequently than the electric-driven variety, but new techniques have extended service intervals on engines by factor six. Nowadays, most standard diesel engines need oil and filter change every 250-300 hours. Running them without maintenance up to 1500 hours and still having factory warranty is possible. The choice of the type of dewatering pump is essential from a cost perspective. The difference in purchase price between the systems becomes less important when calculating the cost of ownership during the pump's life time. Differences in energy efficiency mean that break-even point can be reached within months.
Commonly used diesel-driven (well point) pump solutions are vacuum-assisted centrifugal pumps. They are vacuum-assisted because they create a dry self-priming pump unit with good air handling capabilities as needed in well point situations. The centrifugal pump ends can be of a self-priming type, having a limit of maximum pump efficiency of as low as 45%, or are normal end-suction models. These can be divided into two types: ‘normal’ efficient dewatering pump ends with a Best Efficiency Point (BEP) maximum 50% and ‘high’ efficient pump ends, with a BEP of up to 85% or more.
Example of possible savings revealed
Even today, the majority of dedicated dewatering pumps available to the market belong to the ‘normal’ efficiency category. Comparing a commonly used ‘normal’ centrifugal pump with a ‘high’ efficient type in a small flow rate situation, major savings can be revealed.
For example, a small to medium sized dewatering project is set on a flow rate of 120 m3/h at 15 m of head. This duty point is selected in a curve of a normal efficient pump with a BEP of 44% versus a curve of a high efficient pump with a BEP of 75% resulting in efficiency rates at duty point of 41% and 68%.
The energy saving is calculated with the formula kW = ((Q×H×Sp.Gr)/367) × (1/eff1-1/eff2) ((120×15×1)/367) × (1/0.61-1/0.41) = 4.75 kW.
Comparing the results it leads to a difference of 4.75 kW/h. This with a specific fuel consumption of 0.252 gr/kwh for smaller diesel engines leads to a saving of diesel per hr of 4.75 × 0.252/0.85 = 1.41 l/h. Based on running hours per years of a well point pump at, say, 5000 hours a different selection of pump is calculated to save 7050 l/yr.
In a dewateringjob with larger flow and head, of 500 m3 at 80 m for example, a difference in efficiency in duty point of only 7,5%, would lead to a saving of 16.15 kW/h. Running 3000 hours per year in a mining application, with a specific fuel consumption of 0.215 gr/kwh for bigger diesel engines, should save 12240 l of fuel per year.
Iceberg comparison
As submersible pumps are (partly) under water, the comparison with an iceberg can be made. Submersible pumps do have advantages compared to dry installed pump systems. They are less sensitive to frost and do not run the risk of having failures on a suction line, or have suction lift limitations. Also, the compact design assures good logistic handling.
In terms of energy usage, it can be argued that the majority of the common used dewatering submersible pumps (including bore hole pumps) do not perform that well. This is related to their limited pump efficiency. Additionally to this, when comparing to a diesel driven pump, is that you have to add the efficiency loss of the electric motor of the submersible pump; this is often indicated as Wire-to-Water calculation.
So in open pit dewatering (eg open mines), the choice of a dry installed pump can be shown to be more cost effective in a relatively short time frame. If the power source has to be a generator rather than a permanent supply, the case for a diesel-driven pump is an even stronger one.
In deep well dewatering applications, large wells are required because of the diameter of the submersible pump. The advantages here can be the need for less piping around the pit itself, but usually the total amount of water needed to be pumped away to dry the pit is larger than using well points in combination with a dry installed pump system. A larger sized discharge pipe system can be necessary or as seen is often the case, the size stays the same, but bigger booster pumps are required.
Due to their construction, positive displacement pump systems like a piston pump or so called ‘PT’ pump have limitations in applications. Having limited solids handling capabilities, these pumps are not suitable for open sump pumping. In filter well point systems or horizontal sock drainage systems however, they show the best pump performance possible.
When comparing the maximum head and flow with a centrifugal pump, piston pumps may seem to be small, but do not forget that centrifugal pumps show a curved flow/head performance as positive displacement pump show a constant line. A centrifugal pump with a maximum flow of, say, 180 m3/h can or will give less volume of water at a basic head of 15-20 m than a positive displacement pump with a stated maximum flow of 90 m3/hour. This in combination with a pump efficiency of up to 90% can easily cut fuel consumption with more than 60%.
Depending on the variety of dewatering jobs you meet, selecting the proper pump can save you money.
