The pump curve looked fine on paper. At the required flow rate and the available air pressure, the pump should have delivered comfortably. Then it was installed, and flow was 30% lower than expected. This is one of the most common commissioning disappointments with AODD pumps — and it is almost always the result of misreading the pump curve, not a problem with the pump itself.
AODD pump curves behave differently from centrifugal pump curves in ways that are not obvious until you understand the underlying operating principle.
What an AODD Pump Curve Actually Shows
An AODD pump performance curve plots flow rate on the vertical axis against differential pressure on the horizontal axis, at a specific air supply pressure. Unlike a centrifugal pump, where the curve is fixed by impeller diameter and speed, an AODD pump has a family of curves — one for each air supply pressure. The pump can operate on any of these curves depending on what air pressure is supplied.
At zero differential pressure (no system resistance), the pump delivers its maximum flow for that air supply pressure — all the diaphragm displacement goes into moving fluid, and nothing is lost to overcoming back-pressure. As differential pressure increases, flow decreases. At a differential pressure equal to the air supply pressure, flow reaches zero — the stall point. The pump cannot generate more differential pressure than the air supply provides.
This relationship is approximately linear but not exactly linear. The curve typically shows a gradual flow reduction at low to moderate pressures, then a steeper drop as differential pressure approaches the stall point. The “knee” of this curve — where the rate of flow loss accelerates — usually occurs at around 70–80% of the stall pressure.
The Mistake That Causes Underperformance at Commissioning
The most common sizing mistake is reading the pump’s maximum flow rating and the stall pressure from the datasheet, then specifying a system that requires the pump to operate near the stall point.
Consider a pump rated at 80 L/min maximum flow at 6 bar air supply, with a stall pressure of 6 bar differential. If the system requires 40 L/min at 4 bar differential, you might assume the pump has ample capacity — it is delivering only half its rated flow against a pressure well below stall. But 4 bar differential is already 67% of the 6 bar stall pressure. On the actual pump curve, this may correspond to a flow rate of 35–40 L/min — close to what is required, but with almost no margin left. Any increase in system resistance (a slightly clogged filter, a more viscous fluid at cold startup, an extra fitting added to the line) pushes the operating point further down the curve and flow drops noticeably.
The correct approach is to identify your duty point — required flow at actual system differential pressure — and confirm it sits on the flat portion of the pump curve, well clear of the knee. Aim for an operating point at no more than 50–60% of the stall differential pressure at your intended air supply.
Air Supply Pressure and the Curve Family
Every air supply pressure produces a different curve. A pump running at 4 bar air supply delivers less maximum flow and has a lower stall pressure than the same pump at 6 bar. The curves at different air pressures are not parallel — they scale with the air supply in a way that means the usable range of the curve shifts as pressure changes.
This matters in practice because air supply pressure is often set informally. An operator who “turns up the air” to get more flow from an underperforming pump is effectively moving to a higher curve on the family — which does increase maximum flow, but also increases the stall pressure. Anything downstream that was engineered for 6 bar stall is now seeing 7 or 8 bar stall pressure when the pump deadheads.
Always confirm the air supply pressure you intend to use, specify the pump against the curve at that pressure, and lock the regulator at that setting once commissioned. Changes to the air supply pressure change the operating point on the curve and the stall pressure simultaneously — both consequences need to be understood before making the adjustment.
Viscosity and Its Effect on the Curve
Published AODD pump curves are produced with water. Water has a viscosity of approximately 1 cPs at 20°C. Most chemical process fluids have higher viscosity — some significantly higher.
Viscosity affects AODD pump performance in two ways. First, it increases the pressure losses in the suction and discharge lines, which increases the effective differential pressure the pump sees relative to the same installation with water. The operating point shifts toward higher differential pressure and lower flow — the same effect as operating closer to the stall point.
Second, at high viscosity, the check valves do not open and close as crisply. Ball re-seating after the discharge stroke is slower with viscous fluid, and some backflow occurs on each stroke. This reduces the effective displacement per stroke and lowers the actual flow rate below what the curve at water conditions would predict.
For fluids above approximately 200 cPs, apply a viscosity correction to the pump curve before finalising the selection. Most pump manufacturers publish viscosity correction factors — apply them or contact the supplier to review your duty point with viscosity accounted for.
Reading the Curve for Your Specific Pump Size
Within the Fluimac Phoenix range, pump sizes from P4 (4 mm connection, sub-litre flow rates) through P1000 (50 mm connection, several hundred litres per minute) each have distinct curve families. The shape of the curve — the ratio of maximum flow to stall differential pressure — is similar across sizes, but the absolute numbers scale with the pump displacement volume per stroke.
When selecting between adjacent sizes, choose the size whose duty point sits at 50–65% of the maximum rated air supply pressure at the required flow. This is where the pump operates most efficiently, with the best air consumption per litre pumped and the most margin against system variation. A pump operating at the top of its curve — close to stall — has no reserve capacity and will show significant flow variation with small changes in system conditions.
If you are reviewing pump curve data for a specific application and want a second check on whether your duty point is correctly specified, contact Autoflo at info@autoflotechnology.com. We can walk through the curve family with you and confirm the right model and air supply setting for your conditions.