Let’s talk efficiency
Marc Redit, group operations director of ABS Pumps service spares and rental, explores the issues with life-cycle costing of sewage pumps
Life-cycle costing (LCC) evaluation is a useful tool that can reduce the costs of operating pumping equipment.
But it is not a quick comparison that can be used as part of a framework negotiation or tender process.
If not used correctly, traditional LCC modelling can penalise many design features that have been proven to significantly reduce operator support costs and efficiency losses as a result of wear. For many years, LCC calculations have been used to evaluate pumps to determine which model is most cost-effective. We have seen some development recently in the type of calculation used, but most users still use the most basic form of LCC (initial cost, as new energy cost and maintenance cost).
When used for sewage applications, this drives specifiers towards selecting equipment that does not give the best value. To benefit from life-cycle modelling, users must focus on all major costs including operator support, dealing with blockages and the increased energy costs that result from efficiency fall-off.
LCC must be carefully calculated, based on the size and type of equipment and the application it is being used for. One must measure the actual LCC, and not just create a convenient benchmarking figure that does not reflect the true cost of operating equipment. It should be based on all costs – not just those that are easy to get.
Looking at typical municipal pumping stations, there are huge differences in operator support costs between the best and the worst. While the best stations spend about e300 a year on support, the worst stations spend e1,100 per year on support. Over the 20-year design life of a pumping station, the contrasts are even more stark, with the best stations spending e6,000, while the worst spend e22,000 on support.
LCC must be linked to clear maintenance and operating procedures. Equipment that has a high as-new efficiency can have significant support costs to deal with blockages and very high efficiency losses as a result of wear. Designs that have good solids handling capabilities and lower on-site support costs, are penalised when compared to high-efficiency designs that block on a regular basis.
In many cases, improved efficiency is a direct result of increasing the number of impeller vanes – this, however, greatly reduces the solids handling capability. Designs that have adjustment features to allow as new efficiencies to be restored without replacing parts and therefore giving lower energy costs over the life of the unit are penalised against low-cost simple designs without this feature.
The LCC calculation was originally for large pumps with high utilisation running in clean, non-aggressive applications.
If we try to use this same calculation for sewage applications, we see that many costs of running this type of equipment are not included in the calculation. The benefits of improved designs for solids handling or wear resistance therefore cannot be evaluated since their cost saving benefits are not seen in the traditional LCC calculation.
Support costs have not been traditionally included in LCC calculations, because most users believe them very difficult to accurately define. But just because a cost is difficult to measure, does not mean it should be ignored. Support costs can be very significant and, if not considered, will very quickly offset any benefit achieved from installing high-efficiency pumps.
Fifty five percent of breakdowns are pump-related with blockage the most common cause of pump breakdown. By analysing breakdown data, we can conclude that equipment size is a major consideration as, in general, the smaller the pump the more inaccurate the basic LCC calculation becomes.
This is a major consideration since many pumps installed on sewage applications are below 30kW. On-site support costs of smaller equipment is the most critical factor. The reason for this is that the cost of attending site remains very similar for all pumps up to approximately 50kW. This means these costs become much more significant for smaller pumps.
The factors causing pump breakdown are listed below:
Operator support costs are higher for small pumps, as they suffer more blockages.
Maintenance has a major influence on operator support costs. If pumps are not maintained, certain design features such as those for handling rags become less efficient and blockages increase. Performance reduces, increasing energy costs. The cost of maintenance depends on the type of activity and its frequency. It is important to look for pumps that can retain as-new efficiency by adjustment rather than parts replacement, to keep maintenance costs to a minimum. This makes it possible to reduce additional energy costs and maintenance frequency. ABS sees a trend towards smaller pumps being run to destruction and replaced. This only works if pumps are selected with impellers that retain their efficiency without requiring intervention (Vortex) and do not need maintenance to retain rag-handling capability.
Where a run-to-destruction strategy is adopted, there is a significant increase in the number of calls before the pump actually fails. If considering this strategy, additional energy costs should be taken into account. To minimise additional energy costs associated with wear, it is necessary to maintain equipment and return it back to its as-new efficiency. If as-new efficiency is not restored, additional energy costs will be significant for high-utilisation equipment that is fitted with high-wear impellers. All impellers wear, but the amount of efficiency loss depends very much on the impeller design. If we look at a vortex impeller, we can see that the efficiency loss as a result of wear is very low, compared with some high-efficiency designs that rely on close-running clearances to achieve the initial high efficiency.
The cost of this efficiency loss when looking at the LCC depends on three key elements: the motor size, the number of running hours (utilisation) and how often the pump is maintained to restore the efficiency. When considering LCC calculations, time should also be taken to agree maintenance standards. For most commonly used LCC calculations, the cost of maintenance is the only concern – whereas, in reality, the quality of maintenance is of far greater importance.
3. Equipment age
Equipment age is another significant factor in breakdowns and is closely linked to the type and quality of maintenance.
4. Rag & Grit content
Rag and grit content varies considerably between pumping stations. The amount increases the number of blockages on smaller pumps with closed impellers. A further factor is the accelerated wear caused by this, which again reduces both the effectiveness of the rag handling and the efficiency of the pump, if it is not maintained on a regular basis.
5. Impeller type
It is now possible to purchase a large number of impeller designs for sewage applications. Breakdown statistics suggest that incorrect impeller selection significantly increases breakdowns.
Blockage testing of different impeller types again confirms a large range of capabilities when it comes to handling solids. It is important to ensure the impeller selected matches the maintenance strategy used. Do not select high-efficiency impellers that require regular maintenance if your strategy is to run to destruction.
When calculating LCC, understand the impact of pump blockage when looking for improved efficiencies. Efficiency should not be promoted at the expense of increased support costs. Users should develop clear strategies based on pump size and application as to which impeller type should be used and the consequences of that choice. Pump size should be a major factor in deciding this strategy, along with the need for more regular adjustment to maintain high-efficiency pumps.
To calculate LCC accurately and evaluate pump designs correctly, it is essential that additional energy costs are considered. The amount of efficiency loss can vary considerably depending on the application, the type of impeller used and the maintenance strategy. As a result of wear and reduced efficiency, many calculated energy costs for pumps over their design life are not accurate. Time should also be taken to review a pump’s construction to assess how easy it will be to restore the as-new efficiency.
It is possible to achieve a balance between maintenance and additional energy costs resulting in the optimum LCC for the pumping equipment. This will be achieved when additional energy costs effectively pay for regular maintenance.
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