Two curves, one operating point
A pump curve sheet answers a question people keep trying to answer with a single number: how much water will this pump move? The honest answer is that the pump alone does not decide — the piping votes too. The manufacturer's curve records what head the pump delivers across its flow range, your system curve records what head the piping demands at each flow, and the installation operates where the two agree: the duty point. Everything else on the sheet exists to qualify that point, and the most safety-critical qualifier is the NPSH-required line, which decides whether the pump cavitates.
This guide builds both curves for one running example — a transfer pump moving 20 °C water at 79 gpm (5.0 L/s) up a net 33 ft (10 m) of static rise through 165 ft (50 m) of 4-inch (100 mm) pipe — then finds its duty point and runs the suction check. By the end, every line on the sheet has a number attached to it.
The system curve is yours: static head plus friction
The system curve belongs to the piping, not the pump, and it is built from the total dynamic head: TDH = static head + friction head + velocity head. Static head is the elevation the fluid must gain, and it does not care about flow — pump nothing or pump everything, the water still rises 10 m. Friction is the flow-dependent part: each pipe segment loses h_f = f·(L/D)·v²/(2g) by Darcy-Weisbach, with the friction factor from the flow regime and minor losses added from the fitting K-values, and a small velocity head v²/(2g) rides along.
Because velocity is proportional to flow, the friction terms grow with the square of the flow rate — which gives the system curve its shape. Plot TDH against flow and it starts at the static head on the zero-flow axis and bends upward, gently for a fat, short pipe and steeply for a long, skinny one. For the running example at 5.0 L/s: velocity 0.637 m/s in the 100 mm bore, velocity head 0.021 m, friction about 0.31 m over the 50 m run with its five fittings, so TDH = 10 + 0.31 + 0.02 ≈ 10.33 m (about 33.9 ft). Repeat that arithmetic at a few more flows and the curve is drawn.
The pump curve is the manufacturer's
The other curve arrives printed. A pump's head-versus-flow characteristic is measured on a test stand under the Hydraulic Institute and EN ISO 9906 test procedures, and it is a property of the machine — the same curve applies whether the pump ends up filling a tower or feeding an irrigation line. This division of labor is the deep logic of the sheet: the manufacturer publishes what the pump can do, you compute what the system demands, and neither party can compute the other's half.
The same division applies to the suction data. The curve sheet's NPSH-required line is also the manufacturer's measurement — a property of the pump's inlet geometry, published as a function of flow — while the NPSH available is a property of your installation: its surface pressure, water temperature, elevation, and suction-line friction. Reading a pump sheet well mostly means never confusing which numbers are yours to change and which are cast in the iron.
Where they cross: the duty point
Lay your system curve over the manufacturer's head curve and the intersection is the duty point — the one flow and head at which what the pump delivers equals what the piping demands, and therefore the condition the installation will actually run at. Selection is the art of making that intersection land where you want it: for the running example, a pump whose curve passes through roughly 5 L/s at 10.3 m of head hits the design intent on the nose.
The duty point also prices the electricity. The shaft power is P = ρ·g·Q·H divided by the pump and motor efficiencies, and the motor is rounded up to the next standard rating so there is margin above the hydraulic duty. Note what the duty point is not: it is not a promise the pump will stay there. Valves throttle, filters load up, and levels change — which is exactly why the suction check that follows is run at the worst credible flow rather than the nominal one.
The other line on the sheet: NPSHr
Below the head curve sits a quieter line labeled NPSHr, and it is the one that breaks pumps. Net positive suction head required is measured by the manufacturer under the HI/ISO 9906 criterion: at a given flow, the suction head at which the pump's total head has already dropped 3% from cavitation. Read that definition twice — the published NPSHr is not the edge of trouble, it is a point where cavitation is already well developed. By the time your available suction head has fallen to NPSHr, vapor bubbles are collapsing against the impeller and eating it.
Your side of the comparison is NPSHa = (P_atm − P_vap)/(ρ·g) + z_static − h_friction: the absolute pressure on the supply surface minus the liquid's vapor pressure, converted to head, plus the surface's elevation above the pump centerline (negative for a suction lift), minus suction-line friction at operating flow. Temperature hides inside the vapor-pressure term and dominates it — water at 20 °C pushes back with only 2.34 kPa, but at 80 °C it is 47.4 kPa, which is why hot service is the classic cavitation victim even when the plumbing never changed.
Margin: how much NPSHa over NPSHr is enough
NPSHa exceeding NPSHr is the entry requirement, not the target. Industry practice wants daylight between them: the commonly cited floors are 0.5–1 m of absolute margin or a 1.1× ratio, and the Hydraulic Institute's ANSI 9.6.1 margin guidance runs higher — in ratio terms up to 1.35–2× — for high-energy, hot, or variable-flow services where cavitation damage accumulates quickly. A margin that clears the floor on paper can still be thin in practice once the impeller wears or the flow creeps upward.
Both terms of the margin move against you as flow rises: NPSHr climbs steeply with flow on the manufacturer's curve, and your suction friction grows with the square of flow. The consequence is a rule worth tattooing on the calculation: verify the margin at the maximum credible flow, not just the duty point. A check that passes at nominal and fails at runout has not passed.
A 33-ft lift at 79 gpm, plotted and checked
Run the example end to end. System side: 79 gpm (5.0 L/s) of 20 °C water, net static rise 10 m — the supply surface sits 2 m above the pump centerline and the discharge 12 m above it — through 50 m of 100 mm pipe with ΣK = 5 in fittings. TDH at design flow: 10 + 0.31 + 0.02 ≈ 10.33 m, and the system curve rises from 10 m at shutoff along the Q² friction growth. Duty point: a pump whose tested curve passes through ~5 L/s at ~10.3 m. Suction side: sea-level open tank, so (101,325 − 2,339)/(998.2 × 9.80665) = 10.11 m of pressure head, plus the 2 m flooded suction, minus 1 m of suction friction — NPSHa = 11.11 m.
Against a pump curve demanding NPSHr = 8 m at duty flow, the margin is 3.11 m and the ratio 1.39 — clear of both the 0.5 m and 1.1× floors with room for wear and flow creep. One stress test before signing off: the same installation moved to 80 °C water sees its NPSHa collapse to 6.66 m, below the required 8 — temperature alone flips the verdict. The pump sizing calculator builds this entire sheet-side workflow — Darcy-Weisbach friction from your actual segments, the generated system curve, duty-point head, motor power, and the NPSH comparison — so the by-hand arithmetic above becomes a cross-check instead of a chore.
The curve sheet in one pass
Recap the transfer pump: system curve from TDH = 10 m static + friction growing with Q², landing at 10.33 m at 5 L/s; duty point where the manufacturer's tested head curve crosses it; motor sized from ρgQH over the efficiencies and rounded up a standard rating. Suction: NPSHa = 10.11 + 2 − 1 = 11.11 m against NPSHr = 8 m — margin 3.11 m, ratio 1.39, adequate at 20 °C and doomed at 80 °C. That is the whole grammar of the sheet: one curve is yours, one is the manufacturer's, the crossing is the operating point, and the NPSH lines are the safety check that must be read at the hottest temperature and highest flow the installation will ever see.