The pump curve is the manufacturer’s statement of what a pump will do, and the operating point is what it is doing. The distance between the two is where energy, bearings and impellers go. Most districts have the curve in a binder and the operating point nowhere; putting them on the same chart is the single most useful thing a well record can do.
The three curves
- Head–capacity — total head developed at each flow, falling from shut-off head at zero flow to run-out at the right. For a vertical turbine this is per stage; multiply by the stage count and subtract column and shaft losses to get the pump’s curve as installed.
- Efficiency — the bowl or pump efficiency at each flow, peaking at the best efficiency point (BEP). Wire-to-water efficiency will be lower by the motor, drive and column losses.
- Brake horsepower — the power the pump demands at each flow; for a turbine it rises with flow, which is why a well that produces more than expected can overload its motor.
Where the pump should run
Hydraulic Institute guidance puts the preferred operating region (POR) at roughly 70–120% of BEP flow and lets the manufacturer define a wider allowable operating region (AOR) beyond which vibration, recirculation and radial loads shorten life. A pump whose operating point sits left of the POR — high head, low flow — is usually fighting a lowered water level or a throttled valve; one to the right is a well giving more than the pump was picked for. Both cost efficiency, and both tell you something about the well.
Speed: percent, hertz and rpm
On a variable-frequency drive, speed is quoted three ways and they mean the same thing: a percentage of full speed, the drive frequency in hertz, and the shaft speed in rpm. At 60 Hz a four-pole induction motor turns about 1,770 rpm under load (1,800 synchronous); a two-pole motor about 3,550. So 80% speed is 48 Hz is roughly 1,415 rpm on a four-pole machine. A pump curve is drawn for one speed; every other speed has its own curve, and the affinity laws generate it:
- Flow scales with speed: Q₂ = Q₁ × (N₂/N₁)
- Head scales with speed squared: H₂ = H₁ × (N₂/N₁)²
- Power scales with speed cubed: P₂ = P₁ × (N₂/N₁)³
The cube law is the reason VFDs pay for themselves on pumps that spend hours at part load — and the reason they do not on a well pump that must always lift the same static head; below a certain speed the pump simply cannot reach the water surface and produces nothing. That minimum useful speed belongs on the chart.
Critical speeds and skip bands
Long lineshaft pumps have natural frequencies; running at one shakes the column and eats bearings. Drive manufacturers provide skip frequencies to jump over them. A well record should hold the skip band so that nobody sets a VFD minimum inside it, and so the curve shows the operating range that is actually usable — normally from start-up to 60 Hz with the skip band marked.
Putting the test on the curve
Each efficiency test gives a flow and a head. Plot that point on the curve at the test speed and the story is immediate: on the curve near BEP, all is well; below the curve, the impellers are worn or the stages are plugged; far left of BEP, the water level has fallen and it is time to reconsider the setting or the pump. Waterwell IQ draws the curve from the design data entered with the well — BEP, design head, shut-off head, stage count, speed — scales it to any speed with the affinity laws, and places every test on it, from a 200 gpm domestic-scale pump to a 10,000 gpm plant.
See your pump curves drawn from your own design data, with the operating point from every test. See the platform →
Sources
- Hydraulic Institute. ANSI/HI 9.6.3, Rotodynamic Pumps – Guideline for Operating Regions. 2024 edition. The governing standard for preferred and allowable operating regions. It is a paid standard, cited here by designation; it sets the regions by pump type, so the 70-120% and 50-130% bands this site draws are the values in general use, not a quotation from the standard.
- Hydraulic Institute. Pump FAQs – affinity rules. current. Source for flow proportional to speed, head to speed squared, power to speed cubed.
- NC State Extension. Pumping Plant Performance. rev. 2024. Source for water horsepower = gpm x TDH / 3,960.
Every link above was retrieved and read on 2026-09-09. Paid standards are named by designation so you can check them against your own copy.