Pumps

A pump adds energy to water, expressed as head, so that it can overcome elevation and friction and move around the loop. In a recirculating system pumps carry the recirculation duty, and often also sludge transfer, backwashing, makeup water and dosing.

• Recirculation pumping runs continuously, so it is generally the largest single electrical load in the plant.
• Loss of pumping is loss of life support, because gas transfer and filtration both depend on flow.
• Efficiency at the actual duty point, not the rated point, determines the energy cost.
• A pump run far from its best efficiency point wears faster, vibrates more and fails sooner.
• Pump behaviour is the most sensitive available indicator of change elsewhere in the system.

• Select at the duty point, not the rated point, and confirm that the intersection sits near the best efficiency point.
• Calculate the system curve from the real pipework, including fittings, filters and their fouled condition, not the clean condition alone.
• Check suction conditions. Available net positive suction head must exceed the pump's requirement with margin, or the pump cavitates.
• Prefer variable speed where the duty varies, since speed reduction reduces power steeply, whereas throttling wastes it as heat.
• Anticipate that head rises as filters and pipework foul, so the duty point migrates over the maintenance interval.
• Design redundancy around the consequence of stopping, not around the cost of the pump.

• Establish the system curve before selecting any pump, and include a fouling allowance.
• Provide standby capacity for life-support duty, with automatic changeover and an alarm on the changeover event.
• Consider several smaller pumps rather than one large one, since this gives partial capacity on failure and better turndown.
• Design suction pipework generously and short, with the pump flooded wherever possible.
• Provide isolation valves and removable spool pieces so a pump can be lifted without cutting pipe or draining the loop.
• Specify materials for the water chemistry, including salinity, ozone and any dosing chemicals.
• Provide a flow measurement and a power measurement, since the relationship between them is the diagnostic.
• Place pumps where noise and vibration do not reach the culture tanks, and mount them on suitable bases.

• Trend power against flow. Rising power at falling flow is the earliest reliable indication of fouling in the loop.
• Run standby pumps regularly rather than leaving them idle, since an unrun standby is frequently unavailable when needed.
• Test automatic changeover functionally, including the alarm, rather than assuming it works.
• Listen to pumps. Cavitation, bearing wear and impeller damage are audible before they are terminal.
• Record vibration and temperature where the plant warrants condition monitoring.
• Never throttle the suction side to control flow.

• Inspect impellers and wear rings, since clearance loss reduces efficiency long before it stops the pump.
• Service seals and bearings on a schedule based on running hours.
• Check and record alignment on directly coupled units after any disturbance.
• Exercise standby pumps and log the run.
• Verify variable speed drives are modulating rather than pinned at maximum.
• Keep spares for the pumps installed, including seals, bearings and at least one complete critical unit where the consequence of failure warrants it.

• Selecting on rated flow rather than on the duty point where the curves actually intersect.
• Calculating system head from the clean condition and ignoring fouling.
• Oversizing and then throttling, which wastes energy continuously and runs the pump away from its efficient point.
• Ignoring net positive suction head and then replacing cavitation-damaged impellers repeatedly.
• Providing no standby on a life-support duty.
• Providing a standby but never running or testing it.
• Omitting flow or power measurement, leaving no basis for diagnosis.
• Throttling the suction side to control flow.

Parallel and series operation behave less intuitively than expected. Two identical pumps in parallel do not deliver twice the flow, because adding flow raises the system head and moves the combined duty point up the system curve; the gain depends on how steep that curve is. On a steep, friction-dominated system the second pump adds relatively little. Series operation adds head rather than flow and is appropriate where the head requirement rather than the flow requirement is the constraint.

The affinity relationships explain why variable speed is so effective: flow varies with speed, head with the square of speed, and power with the cube. A modest speed reduction therefore produces a disproportionate power saving, provided the system curve is friction-dominated. Where static lift dominates, the saving is much smaller, and the choice between variable speed and staged fixed-speed units should be made against the actual system curve.

Pumps deliver flow at the intersection of their curve and the system's, so selection begins with the system curve and a fouling allowance. Sit the duty point near best efficiency, protect the suction to avoid cavitation, prefer variable speed on friction-dominated systems, provide and actually exercise standby capacity on life-support duty, and trend power against flow as the plant's earliest fouling signal.