Here is the sentence the whole lesson hangs on: a centrifugal pump does not suck. It lowers the pressure at its eye, and something else — atmosphere, a static level, a pressurised vessel — pushes the liquid in. If the pressure at that eye ever falls to the liquid's own vapour pressure, the liquid boils where it stands, and the bubbles collapse a few millimetres downstream with enough violence to pit stainless steel. That is cavitation, and the margin that prevents it is NPSH.

$NPSH_a = h_{atm} + h_s - h_f - h_{vp}$ — read aloud N-P-S-H available equals h-atm plus h-s minus h-f minus h-v-p, and every term is in metres of the liquid. $h_{atm}$ is the absolute pressure on the liquid's surface written as head — 10.3 m for an open vessel at sea level. $h_s$ is the static suction head, and it is the one signed term on the page: positive when the level is above the pump (a flooded suction) and negative when the pump is above the level (a suction lift). $h_f$ is the friction head lost in the suction piping, always subtracted. $h_{vp}$ is the liquid's vapour pressure written as head, also always subtracted, because the margin is measured ABOVE boiling.

Two of those terms are where the trouble lives. A suction LIFT subtracts, which is why the atmosphere's 10.3 m is a hard ceiling on how far below a pump you may draw — and in practice the ceiling is far lower once friction and vapour pressure take their cut. And vapour pressure climbs fast with temperature: for water it is worth 0.2 m at 20 °C, 2.0 m at 60 °C and 4.8 m at 80 °C. A condensate pump that ran perfectly all winter can cavitate in August for no other reason.

Then the verdict. The pump's own curve prints $NPSH_r$, the required figure — measured on a test rig as the suction head at which that pump has already lost 3 % of its head to bubbles. The rule is $NPSH_a > NPSH_r$, with margin. Positive alone is not the test.

The same margin has a dimensionless form worth meeting: the cavitation number $\sigma = \dfrac{p - p_v}{\tfrac{1}{2}\rho v^{2}}$, where $p$ is the reference absolute pressure in pascals, $p_v$ the vapour pressure in pascals, $\rho$ the density and $v$ the reference velocity in m/s. It counts the margin in velocity heads. There is no universal critical value — the threshold belongs to the SHAPE, roughly 0.2 for a good hydrofoil and several for a sharp-edged throttling valve — which is exactly why valve makers publish it per trim.