Centrifugal process pumps often work in hostile and stressful operating conditions. Consequently, they can fail prematurely – resulting in lost productivity from unplanned downtime. Chris Davies, MD of Brammer Ireland discusses optimising centrifugal process pump performance and efficiency
Improved reliability, reduced maintenance and lower energy consumption are all achievable by ensuring pumps operate at optimum speed and efficiency.
Common problems in pumps
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, or even pipe collapse.
Vibration from component wear or misaligned and poorly balanced shafts can increase energy usage and maintenance costs, and even cause failure.
Efficiency through speed control
Variable speed drives (VSDs) can reduce energy usage and help optimise reliability. A VSD which reduces by 10 per cent the speed of a 22kW pump operating 24 hours a day for 300 days annually can pay for itself through reduced energy usage in eight months.
Energy is key. A small speed increase to up flow rate can potentially increase power demand significantly, meaning contingencies are successively built into the design process. A VSD delivering a four per cent speed decrease typically pays for itself in around two years.
However 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 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.
The latest synchronous carbon belts offer far greater power-carrying capacity than previously and good flex fatigue resistance, meaning they bend more easily around pulleys, and deliver a typical five per cent energy efficiency advantage over v-belt drives.
VSD or soft start?
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 a 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 to feed back any reduced flow requirement to the VSD which will cause the motor to slow accordingly.
‘Ragging’ – fouling of impellers – can significantly affect pump performance. It can cause partial or complete blockage with resulting problems of downtime, cleaning costs, pressure on back-up systems and, in the worst cases, effluent leakage.
A chopper pump – a centrifugal pump with blades attached to the impeller shaft which chop the rags into small pieces before they enter the volute – is normally specified. This is often complemented by a macerator or grinder, which macerates the rags into small pieces. If a solid of sufficient size stops the macerating head from rotating, the unit reverses its rotation to clear the solid and then reverses again to try again to macerate the solid. If this is repeated three times and the matter is still not cleared the macerator stops and an alarm is triggered. The system will need to be stopped at the next opportunity and deragged manually.
An intelligent pump control system which monitors the amperage drawn by the pump and stops it if this exceeds a certain preset range can also be used.
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. Modern software tools even suggest likely causes, and prescribed remedial actions.
Key maintenance points
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. Remember, replacing parts without detailed investigation into the problem’s root cause will not re-establish optimum performance.
A monthly check should be made of: 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 checks and proactive maintenance, coupled with appropriate control methods, will contribute towards optimising efficiency.
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