§ 6.5  Module 6 — Pumps

NPSH, Cavitation, and Margin

NPSHa belongs to the system. NPSHr belongs to the manufacturer. Margin belongs to the design.

By the end of this lesson

  • Compute NPSHa for flooded suction and for suction lift
  • Quantify what temperature and altitude cost in available margin
  • Set a defensible NPSH margin and say what governs it

6.5.1Pressure must stay above vapour pressure

At the impeller eye, acceleration and curvature lower static pressure. If local absolute pressure falls to the liquid’s vapour pressure, vapour cavities form. They collapse after moving into higher pressure, producing noise, vibration, loss of head, and eventually characteristic pitting. Cavitation is a phase-change and flow problem, not air leaking in.

NPSH available is the suction-side total head above vapour-pressure head, at the pump datum. For an open source, NPSHa = (patm−pv)/(ρg) + zs − hfs. The static term is positive for flooded suction and negative when the impeller is above the source. Every suction loss is subtracted.

barometric head above vapour = 10.167 m
source surface above impeller = +2.000 m
suction loss = 0.800 m
NPSHa = 10.167 + 2.000 − 0.800 = 11.367 m

Flooded suction at sea level, 15 °C

6.5.2Temperature and altitude spend the same budget

Warmer water has higher vapour pressure; a high site has lower atmospheric pressure. Holding the same 2.0 m flooded level and 0.8 m suction loss, moving the example to 1,500 m elevation and 35 °C reduces NPSHa from 11.367 m to 9.297 m—a loss of 2.071 m. The pump did not change. The atmosphere and liquid changed the system’s available head.

For suction lift, change the sign rather than inventing a second equation. If the source surface is 3.0 m below the impeller, zs = −3.0 m. Small suction pipe, a fouled screen, long lift, or entrained gas then consumes what little budget remains.

Interactive 3D instrument

Spend the suction budget

A 3D instrument you drive yourself, one variable at a time. It needs JavaScript and WebGL, so it is not shown in this static copy of the page.

NPSH available is an absolute-pressure calculation, and vapour pressure is not lineartrap

Two errors travel together in NPSH work. The first is using gauge pressure: NPSHa is built from absolute pressure, so atmospheric pressure is a term, and it falls with elevation — about 0.9 m of water head lost per 1,000 m of altitude. A station at 1,500 m has roughly 1.3 m less NPSHa than the same station at sea level, for free.

The second is treating vapour pressure as a constant. It rises steeply and non-linearly with temperature: about 0.17 m of head at 15 °C, 0.32 m at 25 °C, 0.76 m at 40 °C. Warm sewage in a summer afternoon, or a recirculation line that has been running against a closed valve, moves this term by more than the margin many designs carry.

What to do

Compute NPSHa at the worst combination: highest liquid temperature, lowest wet well level, site altitude, and the suction losses at maximum flow — not at design flow. Then require a margin over NPSHr, typically 0.6 m or 1.3 × NPSHr, whichever is greater. The bare curve crossing is not a design.

6.5.3Required does not mean damage-free

NPSHr is supplied by the pump manufacturer at each flow, speed, and trim. Under the 2024 Hydraulic Institute convention, the supplied NPSHr must be at least the tested NPSH3—the NPSH at a 3% head drop. Cavitation can exist before that measured drop. Therefore NPSHa = NPSHr is not a design target.

The Hydraulic Institute’s ANSI/HI 9.6.1 guidance treats margin according to pump type, application, operating point, and suction energy. Apply the governing edition and manufacturer requirement. Do not universalize a remembered “0.6 m (2 ft)” or ratio across every machine.

A test threshold became a design trap

NPSH testing made suction performance comparable by tying “required” to an observable head-drop criterion. The trap is linguistic: required sounds like a cliff between safe and unsafe. It is a repeatable test threshold, while damage mechanisms and reliability vary continuously.

6.5.4Build the minimum case honestly

  1. Use the minimum credible suction water level, not the normal level.
  2. Use maximum liquid temperature and site barometric elevation; evaluate a closed source with its minimum absolute pressure.
  3. Price entrance, pipe, fitting, screen, and approach losses at the actual operating flow.
  4. Read NPSHr at every governing point, including runout and variable-speed extremes.
  5. Apply the named margin criterion and test uncertainty; then check intake vortices and entrained gas separately.

Check your understanding

Protect the eye

1 auto-graded question with an explanation for every wrong answer. Requires JavaScript. (m6-l5-q1)

Lab 6.5

Implement an open-source NPSH budget

Write npsha(barometricAboveVapourM, sourceAboveImpellerM, suctionLossM) . The second argument is negative for suction lift. Graded in the browser against 3 assertions; the editor and harness require JavaScript.

Examination score

The scorecard totals this page’s graded work as you go. It requires JavaScript.

Head, Loss and Lift · Module 6, Lesson 5