Drawing No. EH–FM–002 // Fluid Mechanics & Piping

Cavitation & Pump NPSH Simulator

Reviewed August 2026

Cavitation is a pressure problem, not a power problem. Move the suction level, warm the liquid, increase the flow or add suction friction, and watch the NPSH margin shrink. The 2.5D pump cutaway then shows vapor cavities forming near the impeller eye and collapsing as they move into higher-pressure regions.

Educational model. Steady, incompressible flow of water with one-dimensional suction-line friction and an illustrative parabolic NPSH_r curve. The 3D bubble animation is schematic, not CFD. Real pump suction behaviour depends on impeller geometry, inlet recirculation, liquid properties, dissolved gas and manufacturer test data, so use this to build intuition and preliminary screening — not to accept a design.

2.5D pump cutaway and impeller eye

NPSH margin drives the cavity animation

NPSH_a − NPSH_r

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Parameters

Live readout

Model / visual alignment. The 2.5D cutaway is taken from the visual simulator, but its animation is now driven by this page’s calculated NPSH_a, NPSH_r, margin ratio, vapor pressure, suction losses and pipe velocity. The bubble shapes and collapse trail remain schematic; manufacturer NPSH curves are required for real equipment selection.

What to watch for

Four behaviours the simulation makes visible.

The critical region is inside the pump

The suction system determines NPSH_a, but the lowest local pressure occurs as flow accelerates into the impeller eye and vane passages. The pump-specific NPSH_r curve encapsulates that internal geometry; a suction-flange pressure alone cannot establish the cavitation margin.

Higher flow can squeeze the margin from both sides

As flow increases, suction-system friction rises roughly with v², so NPSH_a falls. For this teaching pump, the illustrative NPSH_r curve also rises with flow. Real NPSH_r-versus-flow shape is pump-specific and must come from manufacturer data.

Temperature can rapidly consume suction margin

As water warms, vapour pressure rises steeply and usually dominates the NPSH_a change. Density and viscosity also change, but their effects are secondary and can partly offset the pressure-head loss. There is no universal temperature at which suction lift becomes impossible; the limit depends on atmospheric pressure, lift, friction and the required pump margin.

NPSH_r is not a zero-cavitation boundary

NPSH₃ is the tested NPSH associated with a 3 % head reduction. Current Hydraulic Institute guidance uses manufacturer-supplied NPSH_r for margin determination and requires it to be at least the tested NPSH₃. The required margin is application-specific; this page uses 1.30× only as an illustrative teaching marker.

Collapse is the erosive part

Vapour cavities can already disturb head, efficiency, noise and vibration while they exist. The strongest erosion mechanism occurs when cavities collapse in a higher-pressure region, producing intense local pressure pulses and micro-jets near solid surfaces.

The cavitation number generalises it

Switch to the last tab: σ = (p − p_v)/(½ρv²) describes any geometry — valve, orifice, propeller, hydrofoil. Cavitation begins when σ falls below the incipient value for that shape, and NPSH is simply the pump industry's version of the same statement.

Fixing a cavitating installation

In the order that usually costs least.

Change	Effect on the margin	Cost and caveats
Raise the source level	Adds head one-for-one; the single most effective lever on a lift installation	Often civil work; sometimes free if a tank can simply be kept fuller
Shorten or fatten the suction pipe	Friction falls with L and approximately with D⁵ at fixed flow (with some dependence through the friction factor), so one pipe size up can be worth far more than it looks	Cheap on a new design, disruptive as a retrofit
Remove suction fittings	Every elbow, strainer and partly shut valve is a K term subtracted from the margin	Suction throttling is not a remedy for insufficient NPSH; added suction loss directly reduces NPSH_a
Cool the liquid	Vapour pressure falls steeply, so a few degrees can be decisive when the available margin is already small	Rarely available in practice
Reduce flow, or trim the duty point	Cuts both friction and NPSH_r	Only if the process tolerates it
Choose a lower-NPSH pump or add an inducer	Moves NPSH_r down rather than NPSH_a up	New hardware; inducers have their own stable operating window
Run two pumps in parallel	Each handles less flow, so both terms improve	Capital and control complexity

Do not treat suction throttling as an NPSH fix. Adding restriction on the suction side increases h_f and therefore reduces NPSH_a. Flow control is normally handled without deliberately consuming an already-limited suction margin.

Sources and technical basis

Vapour pressure and density follow published water correlations; NPSH definitions follow standard pump practice.

Hydraulic Institute — ANSI/HI 9.6.1 NPSH MarginCurrent application-specific NPSH margin framework and the relationship between manufacturer NPSHR and tested NPSH3.IAPWS — saturation properties of ordinary waterReference basis for water saturation pressure over the simulator temperature range.KSB Pump Lexicon — NPSHNPSHA/NPSHR definitions, impeller-inlet pressure behavior and NPSH3/incipient-cavitation criteria.OpenStax — University Physics Volume 1Fluid statics and dynamics: pressure with depth, continuity and Bernoulli’s equation.NASA Glenn Research Center — Beginner’s Guide to AeronauticsDynamic pressure and the non-dimensional pressure coefficient behind the cavitation number.