Performance and efficiency
Jeremy Salisbury of Brammer UK looks examines technologies and techniques for optimising centrifugal pump performance.
Centrifugal process pumps often work in hostile and stressful operating conditions. Consequently, they can fail prematurely - resulting in lost productivity from unplanned downtime. Improved reliability, reduced maintenance and lower energy consumption are all achievable by ensuring pumps operate at optimum speed and efficiency.
Cavitation is a significant cause of wear, especially in pumps required to start quickly. When a liquid undergoes rapid pressure changes, cavities form in its lower pressure regions. When entering high-pressure areas, these bubbles collapse, causing cyclic stressing and, ultimately, surface fatigue to the impeller, pump housing, or both.
Rapid pump stoppage - or failure - can also create significant problems. A sudden flow reduction can mean a valve closes rapidly, resulting in water hammer - a pressure surge or wave which can cause noise, vibration, blown valves, leaks, and even pipe collapse. Meanwhile, vibration from component wear or misaligned and poorly balanced shafts can increase energy usage and maintenance costs, and even cause product failure.
Variable speed drives (VSDs) can reduce energy usage and help optimise reliability while closing the loop with regard to PID (proportional-integral-derivative) functionality. A VSD which reduces by 10% the speed of a 22kW pump operating 24 hours a day for 300 days annually can pay for itself through reduced energy usage in just eight months, depending on electricity costs.
Energy is key given that many pumps are specified larger than needed for the application. A small speed increase to up flow rate can potentially increase power demand significantly, meaning contingencies are successively built into the design process. However, a VSD delivering a 4% speed decrease typically pays for itself in around two years through reduced energy costs.
The only rider is that VSDs cannot be fitted to pumps operating under high head pressures. This is because pressure varies in proportion to speed as per the pump affinity laws. In these cases, speed variation using any method is not advisable without due caution. Other options include v-belts, where potential issues like slippage have been largely circumvented by the latest synchronous belts, which do not require regular retensioning.
Meanwhile, the latest synchronous carbon belts offer far greater power-carrying capacity than was previously achievable and exceptional flex fatigue resistance, meaning they bend more easily around pulleys, and deliver a typical 5% energy efficiency advantage over v-belt drives.
For smaller pumps, a VSD is generally preferable, while soft start typically becomes economical for 22kW pumps or larger. However, there is a crossover area where either method is suitable depending on application requirements. For example, an application starting once per month and running at constant speed will require soft start, however a VSD will be more appropriate if demand from the pump varies.
Overall performance can also be increased by fitting pressure sensors in applications with variable throughput. The sensor will feed back any reduced flow requirement to the VSD which will cause the motor to slow accordingly, delivering energy savings with no risk to the pump.
Ragging - fouling of impellers - can significantly affect pump performance. It can cause partial or complete blockage, with problems of downtime extending over several days, as well as cleaning costs, with back-up systems placed under additional pressure and, in the worst cases, effluent leakage. This can be negated by an intelligent pump control system which monitors overall performance, triggering an automated cleansing cycle if performance crosses pre-programmed 'out of profile' boundaries.
Identifying problems before they occur is integral to optimising efficiency and minimising downtime. Modern condition monitoring systems such as sensors and accelerometers monitor vibration levels and temperature changes with information downloadable to a hand-held system or, for remote or hard-to-access locations, fed into a central system which can provide warnings of unusual or undesirable readings, allowing planned maintenance or a complete if required. Modern software tools even suggest likely causes, and prescribed remedial actions, based on data provided.
Pumps should always operate on a stable baseplate with shafts optimally aligned and lubricated according to manufacturer instructions, otherwise poor performance and ultimately failure are almost inevitable. As with any other component, replacing parts without detailed investigation into the problem's root cause will not re-establish optimum performance.
The following points should be checked monthly: priming speed, capacity, noise in the pump casing, gaskets and O-rings, shaft seal leakage of air and water; hose, hose washers and suction strainer. Six-monthly checks should be made of impeller wear; clearance between the impeller face and the volute; shaft seal wear; and shaft sleeve wear; while the casing and volute passages should also be cleaned.
This combination of regular checks and pro-active maintenance, coupled with appropriate control methods, will contribute significantly towards optimising efficiency, minimising downtime and reducing whole life costs.
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