A pump adds energy to a liquid, expressed as head (metres of fluid). The rate of useful work it does is P = ρ·g·H·Q — pressure rise times flow.
Pumps split into two great families: dynamic (centrifugal — they trade velocity for pressure) and positive displacement (they trap and squeeze a fixed volume). The difference shows up entirely in their curves.
A pump's head curve meets the system curve at the operating point. NPSH tells you whether the liquid will stay liquid at the impeller eye; lose that margin and you get cavitation — boiling, then imploding, then eroded metal.
1 · What a pump actually is
A pump is a machine that adds mechanical energy to a liquid. That is the entire definition. It does not "create pressure" out of nothing and it does not "suck" — both are useful shorthand, but both will mislead you later. A pump takes shaft power from a motor or engine and hands it to the fluid as an increase in the fluid's energy.
That energy can show up in three forms, and Bernoulli's principle is just the bookkeeping that says they are interchangeable:
- Pressure energy — the static pressure of the fluid (what a gauge reads).
- Kinetic energy — the fluid's velocity.
- Potential energy — its elevation.
Pumps move liquids. A machine that adds energy to a gas is a fan, blower, or compressor — the physics rhymes, but liquids are essentially incompressible and that changes everything about how the machine behaves.
Head: the one concept everything hangs on
Engineers describe a pump's output not in pressure but in head — the height of a column of the pumped liquid that the pump's energy could support. Head is energy per unit weight of fluid, and its unit is simply metres.
Why bother with head instead of pressure? Because a centrifugal pump develops the same head regardless of the liquid's density. Spin the impeller at a given speed and it throws the fluid to a fixed velocity, which converts to a fixed head. The pressure that head represents depends on density, but the head does not. Curves are published in head precisely so one curve works for water, oil, or brine.
2 · The physics: energy, work, and power
This section is the spine of the whole guide. Get it right and pump curves, throttling, efficiency, and the closed-valve start rule all follow without memorisation.
Work, in mechanics, is force acting through a distance: W = F · d. For a fluid being pushed through a pipe, the "force" is pressure acting over the pipe's cross-section, and the "distance" is how far the fluid moves. Multiply it out and the messy geometry cancels, leaving something clean:
Look hard at that equation, because it contains a result that surprises most people the first time:
If flow is zero, the useful work is zero — no matter how high the pressure. Q = 0 makes Phyd = ρgH×0 = 0. A pump running against a fully closed valve builds maximum pressure and does no useful work on the fluid whatsoever. Nothing moves, so nothing is transported, so by the definition of work, none is done.
Where does the shaft power go, then, if the motor is clearly still turning? Into heat. The impeller churns the trapped liquid, friction and recirculation convert that shaft energy into a temperature rise. This is exactly why a centrifugal pump can be started against a closed valve safely (the load is light) but must never be run that way for long (the trapped liquid cooks). We unpack that fully in Part 2 of this series — and the interactive below lets you see the work appear and disappear in real time.
Useful work versus wasted work: what a throttle valve really does
Here is the subtlety that the “closed valve = no work” idea sets up. Open the valve a little and flow starts. Now Q > 0, so the pump is doing work — but not all of that work is useful. Throttling a valve does not reduce the work; it redirects it.
At any operating point, the head the pump produces is spent on two things:
So a partially open valve sees a great deal of work: there is real flow being moved (the useful part) and the pump is fighting the artificial resistance the valve adds (the wasted part). A throttle valve controls flow by deliberately destroying energy. The interactive model makes this visible — it shades the useful work green and the wasted work orange, and you can watch the orange band balloon as you pinch the valve.
Interactive 1 — The work a pump does
Live modelThe shaded rectangle is hydraulic power, ρgHQ — its area is the work rate. Drag the valve from open to closed and watch it collapse to nothing. The split shows useful lift (green) versus energy thrown away across friction and the throttle (orange).
Head vs Flow — the rectangle is the work
H = 50 − 0.0009·Q² on cold water at fixed speed. Hydraulic power ρgHQ; useful = ρg·Hstatic·Q; wasted = the rest. Brake power (what the motor draws) is higher still because of pump inefficiency — covered in the curves section.3 · The two great families of pump
Every pump ever built belongs to one of two classes, distinguished by how they add energy to the fluid. This single split predicts almost everything about how a pump behaves.
How a centrifugal (dynamic) pump works
A spinning impeller flings liquid outward by centrifugal action. The liquid leaves the impeller tip at high velocity — high kinetic energy, but still low pressure. It then enters the volute (the spiral casing) or a diffuser, which is shaped to slow the flow down. As the velocity drops, Bernoulli demands the pressure rise: kinetic energy is converted to pressure energy. A centrifugal pump is fundamentally a velocity machine.
Because it works by velocity rather than by trapping volume, a centrifugal pump's flow depends entirely on what the downstream system will accept. Raise the resistance and flow falls; the pump simply rides up its own curve to a higher head. This is why it can be throttled, and why it can sit happily at zero flow for a moment.
How a positive displacement pump works
A PD pump captures a discrete pocket of liquid — between gear teeth, inside a cylinder behind a piston, in the cavity of a screw — and physically carries or squeezes it from inlet to outlet. Each revolution or stroke delivers essentially the same volume regardless of pressure. Flow is set by speed, not by the system.
That makes the PD curve almost vertical: tell it to deliver 10 m³/h and it will deliver 10 m³/h whether the discharge pressure is 2 bar or 200 bar — building whatever pressure it takes to push that volume through. Which leads to the defining safety fact of PD pumps:
Never start or run a positive displacement pump against a closed valve. With nowhere for its fixed volume to go, the pressure rises without limit until something fails — a relief valve must lift, or the casing, pipe, or coupling breaks. This is the exact opposite of the centrifugal start rule, and the reason is right here in the curve shape. A PD pump always needs a relief valve in its discharge.
| Property | Centrifugal (dynamic) | Positive displacement |
|---|---|---|
| Flow vs pressure | Flow falls as pressure rises (drooping curve) | Flow nearly constant with pressure (vertical curve) |
| Best for | High flow, moderate head, thin liquids | High pressure, low flow, viscous liquids, metering |
| Viscosity | Performance drops sharply with viscosity | Handles high viscosity well |
| Closed discharge | Safe momentarily (start position) | Dangerous — over-pressure; needs relief valve |
| Flow control | Throttle valve or variable speed | Variable speed or stroke; not a throttle |
| Flow smoothness | Smooth, continuous | Often pulsating (especially reciprocating) |
The rest of this guide concentrates on centrifugal pumps — they are roughly 70–80% of the pumps you will meet, and their curve behaviour is where curves, NPSH, and cavitation all come alive.
4 · Reading pump curves
The characteristic curve is the pump's fingerprint. A manufacturer's data sheet overlays four curves on one flow (Q) axis:
- Head curve, H–Q — head produced at each flow. For a radial pump it droops from a maximum at zero flow (the shutoff head) down to zero at maximum flow (run-out).
- Efficiency curve, η–Q — peaks at one flow, the Best Efficiency Point (BEP). This is where you want to run: lowest vibration, lowest wear, longest life.
- Power curve, P–Q — brake power the shaft needs. For radial pumps it rises with flow (which is the whole basis of Part 2).
- NPSH-required curve, NPSHr–Q — how much suction margin the pump needs, rising steeply with flow. More on this below.
The system curve and the operating point
A pump never operates in isolation — it operates into a system. The system curve plots the head the piping demands at each flow:
The pump runs where the two curves cross: the only flow at which the head the pump produces equals the head the system demands. Everything you do operationally — throttling, opening branches, fouling filters, changing speed — works by moving one curve and letting the operating point find its new home. The interactive in Part 2 lets you drag all of these.
The affinity laws
When you change a centrifugal pump's speed (or, with care, trim its impeller), the whole curve scales by simple ratios. These are the affinity laws:
5 · NPSH — keeping the liquid liquid
Everything so far has been about the discharge side. NPSH is about the suction side, and it is where most real-world pump trouble lives.
As liquid accelerates into the impeller eye, its velocity rises and — by Bernoulli — its local static pressure falls. If that pressure drops to the liquid's vapour pressure, the liquid boils, right there inside the pump, at ambient temperature. NPSH (Net Positive Suction Head) is the bookkeeping that tells you whether you have enough pressure margin to stop that happening.
There are two NPSH numbers and the whole game is keeping one above the other:
- NPSHa (available) — what your system delivers to the pump suction. You calculate it from the installation.
- NPSHr (required) — what the pump needs to avoid cavitating. The manufacturer measures it; it rises with flow.
Read that equation and the field problems explain themselves. Hot liquid raises Pvapour and eats your margin — pumping near-boiling condensate is the classic NPSH killer. A long or clogged suction line raises friction. A suction lift (pump above the source) makes the static term negative. Altitude lowers Patm. The calculator below lets you feel each one.
Interactive 2 — NPSH & cavitation margin
Live calculatorSet up a suction system and watch the available margin. When the blue bar (NPSH available) falls to the red line (NPSH required), the pump cavitates. Try dragging the temperature up — it is the most common cause in the field.
NPSHa = (Patm−Pv)/ρg + Hs − Hf at sea-level atmospheric pressure; NPSHr ≈ 1.5 + 0.00007·Q² as a representative pump. Always use the manufacturer's measured NPSHr curve for real designs.6 · Cavitation — what happens when you lose the margin
Cavitation is the consequence of letting suction pressure fall below vapour pressure. It happens in three stages, and the third one is what destroys pumps:
- Formation. At the low-pressure impeller eye, the liquid flashes to vapour — thousands of tiny bubbles form.
- Transport. The impeller carries those bubbles outward into the higher-pressure region of the passages.
- Implosion. Back above vapour pressure, the bubbles collapse violently. Each collapse fires a microscopic jet of liquid at the metal at velocities that produce local pressures measured in gigapascals.
The damage looks like the metal has been hammered with a tiny chisel or eaten away — engineers call it "cavitation pitting," and a cavitating pump famously sounds like it is pumping gravel. Beyond the noise and erosion, cavitation also causes:
- Head breakdown — the vapour blocks the passages and the pump loses head. By convention, NPSHr is defined as the suction head at which head has already dropped 3%, so a pump "meeting NPSHr" is already lightly cavitating.
- Vibration and bearing/seal damage — the implosions are violent and random.
- Loss of flow — in severe cases the pump vapour-locks and stops delivering entirely.
Cavitation is a suction problem, not a pump fault. The cure is almost always on the suction side: lower the liquid temperature, raise the suction tank level, shorten or de-restrict the suction line, reduce flow, or — if the installation can't deliver the margin — select a pump with a lower NPSHr. Throwing a "tougher" pump at it without fixing the suction just gives the bubbles harder metal to chew.
7 · Putting it together
You now have the full chain of reasoning a rotating-equipment engineer carries:
- A pump adds energy as head; the rate of useful work is ρgHQ, which is zero at no flow and is split between useful lift and throttle/friction losses whenever there is flow.
- Dynamic vs positive-displacement is the master distinction, and it lives entirely in the shape of the curve — drooping versus vertical — which in turn dictates how each is controlled and how each must be protected.
- The operating point is the pump curve meeting the system curve; you move it by changing the system or the speed (affinity laws).
- NPSH decides whether the liquid survives the trip into the impeller; lose the margin and cavitation erodes the pump from the suction side.
With that, the closed-valve start rule from Part 2 is no longer a piece of folklore — it is a direct reading of the ρgHQ equation and the radial power curve. That is the test of whether this guide worked: the rules should now feel inevitable, not memorised.