Pump affinity laws calculator

The affinity laws describe how a rotodynamic pump responds when its speed or impeller diameter changes. Flow follows the direct ratio, head follows the square, and power follows the cube. Halve the speed and you get half the flow, a quarter of the head, and one-eighth of the power. That cube is the reason a small speed reduction produces a disproportionately large energy saving — and the reason overspeeding a pump can burn out its motor.

The laws assume efficiency stays constant, which is close to true within a moderate range. Use the calculator above for speed changes, frequency conversion, or impeller trimming on centrifugal and mixed-flow pumps.

A variable frequency drive changes speed by changing frequency: at 50 Hz a four-pole motor runs 2900 rpm, at 40 Hz it runs 2320 rpm. On that same duty, dropping from 2900 to 2320 rpm reduces flow to 80 % of the base, head to 64 %, and power to 51 % of the base — the classic VFD energy story.

Two caveats. First, the saving assumes the pump was throttled before: a pump that already runs at its system duty point saves little when slowed. Second, the affinity idealisation applies to friction-dominated systems; where static head dominates, the operating point does not follow the cube law, and the system-curve tools should be used instead of trusting the headline saving figure.

Indian plants standardise on 50 Hz, but imported equipment and export-bound lines may run 60 Hz. Moving a pump from 50 to 60 Hz raises speed 20 %, flow 20 %, head 44 % and power 73 %. The motor current rises roughly in proportion to power, so a standard 50 Hz motor will almost always need a larger frame (IS 1231) and a check on NPSHr — suction capability rises with speed and can exceed what the tank and piping can supply.

The calculator applies the 1.2 ratio and surfaces both warnings so the decision is made with the motor and suction system in mind.

Trimming the impeller is the cheapest way to match a pump to a lower duty without a VFD. The same affinity ratios apply: 10 % trim (90 % remaining diameter) gives 90 % flow, 81 % head, and 73 % power. A closed impeller can usually be trimmed 20–25 % before efficiency collapses; open and semi-open impellers allow less because tip clearance losses grow quickly.

Beyond the practical limit, machining further costs more in lost efficiency than a new impeller costs to buy. Teflow’s workshop trims, rebalances and re-cuts impellers for chemical pumps, and can advise whether a trim, a new impeller, or a VFD is the right retrofit for your duty.

The laws assume constant efficiency and a fixed system curve, which fails in four common situations. Viscous liquids: affinity still scales the water-duty point, but efficiency drops with viscosity and the cubic power rule becomes unreliable — the Hydraulic Institute (HI 9.6.1) publishes correction factors for viscous service. Very low speed: below about 50 % of rated speed, head may fall below the static head the system needs, and the pump stops delivering. Large trims: beyond 25 % the efficiency collapse makes predictions optimistic. Positive-displacement pumps: flow is set by displacement, not head, so affinity laws do not apply at all.

Once you have the new duty point, the business case follows: multiply the kW saved by running hours and your kWh tariff to get the annual rupees saved. That number decides whether a VFD, a pulley change, or an impeller trim pays for itself — and how quickly. Teflow’s engineers can confirm the retrofit against your actual system curve, trim and rebalance impellers, and supply or fit drives for chemical, textile, dairy and water-treatment plants across Gujarat. Call +91 98251 62709 or send a WhatsApp message with your duty point for a recommendation.