What price efficiency?

In the wake of recent steep increases in energy tariffs, Andy Cruse of ERIKS explains how pump users can improve efficiency levels of critical machinery such as centrifugal pumps.

Right now, close to where you are, there's probably a pump working quietly in the background. Large and small pumps are used throughout industry and to every production and maintenance engineer will be almost as familiar as their own family (perhaps more so in some cases!). What many engineers may not be aware of, however, is the true cost of operating pumps.

Consider these findings from the many installations managed by ERIKS' engineers: the purchase price of a typical centrifugal pump represents a mere 5% of its total life cost. Maintenance costs normally add only a further 8% to total life costs. That leaves a staggering 87% of total cost attributable solely to energy consumption.

There is therefore huge potential for companies to make major savings in running costs, both through efficient pump operation and effective maintenance programmes; and of course, the more, or larger the pumps you operate, the greater the potential savings.

Controlling the cost of ownership
Pump operating costs begin from the first time you press the start button and some of these costs are unnecessary if the pump has been incorrectly installed or specified; it is not, for example, uncommon to find pumps that are greatly over specified for a particular application running continuously 24 hours a day.

Similarly, once in service many pumps exhibit wear characteristics that can significantly increase energy consumption, while older units may by modern standards be so energy inefficient that it is a false economy to keep them running; if this is the case then replacement and consideration of using a a variable speed device, where appropriate, will provide a fast return on investment.

Ideally, pump operating costs should be controlled through careful monitoring and selection based on tried and tested calculations. This should form the basis of a pump action plan that delivers significant gains in performance and the lowest possible running costs.

Calculating pump performance
There are a number of factors to consider when determining optimum pump efficiency. Firstly, it is necessary to accurately determine the flow rate and the total pump head of the system. The intended design head and flow the pump operates at is called the duty point. Variations in actual pump can impact on the efficiency of a pump by anything up to 60% of its efficiency.

Once the calculation of the pump duty has been determined, comparisons can be made against manufacturer's specifications, shown by plotting the measurements on a graph (see Graph 1). Critically, pump performance is achieved where the flow/head curve intersects the hydraulic resistance curve.

This resistance curve represents the actual head and flow potential of a given pumping system, which depends on the frictional characteristics of the pipework, the velocity, distance and height to be pumped. The curve line on the graph can steepen or lessen when, for example, when valves are closed or open, although where this line intersects the head/flow curve, this represents actual performance of the pump on-site.

Graph 1 Pump dynamics - relationship of pressure and flow rate

Pump dynamics
The relationship between high head and low flows compared with high flow and low head also needs to be understood in order to appreciate the performance of a working pump in scenarios such as these (see Graph 2).

For example, operating to the far left of the curve can reduce bearing and seal life, while high flow rates operating at the right run the risk of motor overload. However, striking the right balance between head and flow will provide the best efficiency for pumps.

Graph 2 Variation in pump duties

As well as ensuring that pumps run efficiently, operators also need to assess whether wear and tear is hindering pump performance. Pump components such as neck rings may wear, resulting in greater clearance; this can cause leakage and recirculation to occur from high pressure areas of the pump back to the suction side of the impeller, leading to reduced head and flow.

Similarly, erosion/corrosion from abrasive particles in the pumping medium, along with scale build-up and cavitation, can impair the impeller, not only affecting performance but causing mechanical imbalance and thus reducing the life of bearings and seals.

Making a plan for sustainable pumps
By having a clear understanding of how pumps are performing, such as measuring the overall energy consumed and pinpointing areas of high energy use or problem spots, much can be done by the retrofitting of energy saving features. This can be part of an all-encompassing action plan for pumps that identifies which steps can be taken to achieve maximum effect.

An on-going assessment of pump condition will boost operating efficiency and this is where condition monitoring techniques play a valuable role in ensuring the benefits of reduced pump wear, reduced energy consumption and increased productivity. Monitoring pumps in this way can identify any leaks or potential improvements in flow control rather than the use of throttling or by-pass valves. It can also identify whether any process interruptions are occurring during operation; if so, pumping could in future be minimised by the use of simple power on/off control devices.

As well as providing continuous monitoring, the action plan enables the implementation of a fully structured maintenance programme to establish a repair and replace schedule. The replacement of leaking seals, the application of efficiency/wear resistant coatings and the investigation and rectifying of faults such as cavitation can extend pump life considerably and avoid costly and unnecessary pump replacement. Pump maintenance information can then be recorded to help identify future problems and justify the need for the replacement of faulty pumps when they do not achieve the duty level specified by the manufacturer.

Finally, the plan will help operators to develop a better understanding of pump operation, provide them with a data log of estimated savings, costs and payback potential, and also inform them of appropriate energy efficiency initiatives.

Pumping for savings
Today's leading specialists in plant and machinery maintenance such as ERIKS use calculation and action plan methodology as part of a condition monitoring regime for a large number of companies throughout industry. This approach to pump management is proven to offer significant savings in terms of running costs, extended pump life and, through a planned maintenance programme, the virtual elimination of unexpected breakdowns that can prove costly to productivity.

With rising energy costs, the need for companies to adopt a more sustainable approach is greater than ever. The depletion of non-renewable energy sources and increasing legislation to curb carbon dioxide emissions is also shifting perceptions of manufacturers towards this goal. Armed with all the necessary information about your pumps and an action plan for sustainability, it's now time to turn on the pumps and start saving money.