Control the cost of pumping
Ian Weybourne, water industry specialist with Dowding & Mills, considers the major factors which need to be taken into account to ensure cost-efficient operation of pumping systems
In this context failure to meet performance and efficiency requirements is just as important as a mechanical or electrical failure. It is not possible to tell if a pump is operating inefficiently just by looking at it. The pump may deliver the flow or pressure required and, unless it is in a very poor state, it will not be making a noise or be overheating. Unfortunately, the instrumentation that can provide the information required to evaluate performance and efficiency is often not fitted, not working or out of calibration.
The efficiency of pumps varies significantly depending on their design and the operating conditions. The peak efficiency of new centrifugal pumps can vary between 60% to 90% and their efficiency characteristics are strongly flow dependent. A pump may be operating inefficiently, even though it is in good condition, simply because it is being asked to work outside its normal performance envelope.
Processes change and whilst the original pump design and selection may have been perfectly acceptable it may no longer be suitable for the current requirements. Under these circumstances it may be more cost effective to replace the pump with a more appropriate unit than persevere with consistently poor reliability and efficiency.
Given the time an experienced engineer can reduce or eliminate most in-service pump problems. The real challenge is to be able to identify and quantify the scale of the problem. On a small plant it may be obvious where resources should be allocated. On a more complex plant it becomes increasingly important to have some basic technical and financial information available on the current performance and efficiency of the pumping assets.
To improve the life of pumping equipment and reduce maintenance expenditure the plant engineer must have access to accurate data on operating conditions and running hours, the cause of failures and the frequency and total cost of any repairs. This information can be incorporated into most commercially available computerised maintenance management systems and with relatively simple analysis it can be used to:
- identify the most expensive pumps - the illusive 20% of pumps that generate 80% of the maintenance costs,
- reduce maintenance expenditure by designing-out problems, replacing unsatisfactory equipment or improving maintenance and operating regimes,
- monitor the effectiveness of any modifications to the pump or system and improve future purchase specifications,
- accurately predict operating expenditure at the estimating stage of new projects and more effectively assess products at the time of purchase.
For pump repairs data can be collected on the reported symptoms the observed defects and, in some cases, the probable cause of the failure. The level of detail available depends on the experience and skills of those recording the data and the information required.
Symptoms generally relate to the pump performance (eg no flow, excessive power consumption) or its condition (eg overheating, noisy, leaking). Defects are normally identified on strip-down and relate to components, bearing failure or mechanical seal failure for instance. The probable cause can be harder to determine. For example, a bearing may fail repeatedly due to overload resulting from operating the pump at too low a flow rate. Unless a thorough investigation is carried out, and a connection made between the mechanical effect and the hydraulic operating conditions, the root cause of the problem will not be identified.
With access to this data, companies can benchmark their own performance and develop a detailed understanding of the links between pump and system design features and reliability in different operating environments.
The function of most pumping systems is to transfer liquid from one location to another at minimum cost. The input to this process is electrical energy that is converted into mechanical energy by the motor and ultimately hydraulic energy by the pump. For a well designed system the hydraulic losses within the pump and the system pipework account for about 30% of the total energy consumed and in many cases the system losses can be much higher.
Where a large number of pumps are involved a process needs to be put in place to compare the performance and efficiency of different pumping systems to identify those that should be considered in detail. The effort and costs involved in this process need to be balanced against the potential returns that will be generated by improved pumping system efficiency. A step-wise approach is required to progressively focus down onto the areas that will produce the greatest savings.
Initial screening is most often based on energy consumption. Typically, 80% of the energy costs will be associated with 20% of the plant. For each of the pumping systems being considered data is collected on the motor rating and the estimated annual operating hours. The product of the motor rating and the operating hours then gives an estimate of the annual energy consumption (kW/h). This is not a very accurate estimate as the motor rating refers to the shaft output power not the electrical input power and it also assumes that the motor is operating at its rated load condition.
Prioritising pumping systems on the basis of energy consumption alone makes the assumption that the greatest potential savings are always associated with the high-energy use plant, which may not be the case. If possible some measure of the overall efficiency of the different pumping systems should be introduced. This is usually done on the basis of specific power consumption (ie the power consumed per unit volume pumped, kW/h/Ml). Effort can then be focused on systems that have a highenergy bill and high specific energy consumption.
Where it is possible to obtain data on the actual flow and head required by the system this can be used to further refine the screening process. Pumping systems can be identified where there is an appreciable imbalance between the current system requirements and the pump output and where pumps are being operated significantly away from their specified design condition. The degree of imbalance can be assessed by comparing the product of the actual flow and head generated by the pump with that required on-site.
Pump reliability and efficiency are closely linked and should form part of a company's overall asset management strategy. The traditional approach is to treat reliability and efficiency completely separately but this may mean the duplication of effort and a missed opportunity.
The ultimate objective is to provide sufficient information, at minimum cost, to be able to identify poor performance and implement the work required to reduce operating costs. The type of information required will depend on the range of pumping applications being considered. One thing is certain: if you can not measure it then you can not control it