Total Dynamic Head determines whether a pump can push water to its delivery point. Net Positive Suction Head (NPSH) determines whether it can draw water in without cavitating. These are separate calculations. Conflating them — or running only TDH and skipping the NPSH check — is the most common surface pump sizing error on dewatering sites. NPSH failure causes impeller pitting, vibration, and flow loss, and it develops quietly: the damage accumulates before the pump shows obvious symptoms.
Two values to know: NPSHa and NPSHr
NPSHa (available) is a site condition. It measures the pressure energy the fluid carries to the pump inlet, expressed relative to the fluid’s own vapour pressure at operating temperature. NPSHr (required) is a pump characteristic — the minimum inlet pressure the impeller needs at a given flow point to avoid vaporisation. The design criterion is NPSHa ≥ NPSHr + 0.5 m.
The NPSHa formula
NPSHa = (Patm/ρg) + Hs − hf − (Pvap/ρg)
| Variable | Meaning | Typical value |
|---|---|---|
| Patm/ρg | Atmospheric pressure head at site | 10.33 m at sea level; reduces ~0.12 m per 100 m elevation |
| Hs | Static suction head — positive if source is above pump inlet, negative for suction lift | Site-dependent |
| hf | Suction pipe friction loss (m) | Calculate from pipe length, diameter, flow velocity |
| Pvap/ρg | Vapour pressure head of pumped fluid | 0.24 m at 20°C; 0.32 m at 25°C; 0.74 m at 40°C |
Worked example: CAP auto-prime pump, 3 m suction lift
A CAP auto-prime pump installed at ground level draws from a construction sump 3 m below grade. Suction pipe is 20 m of 100 mm diameter PVC at 2 m/s flow velocity. Ambient temperature 25°C. Site at sea level.
- Patm/ρg = 10.33 m
- Hs = −3 m (suction lift)
- hf = 1.2 m (20 m run, 100 mm pipe, 2 m/s velocity)
- Pvap/ρg = 0.32 m
NPSHa = 10.33 − 3 − 1.2 − 0.32 = 5.81 m
If the pump curve specifies NPSHr = 4.5 m, the margin is 5.81 − 4.5 = 1.31 m — above the 0.5 m minimum; design is safe. If the suction pipe run were 40 m instead of 20 m, hf would approximately double (2.4 m), and NPSHa would drop to 4.41 m — below the NPSHr threshold, requiring a larger suction pipe or reduced suction lift.
CAP auto-prime pumps operate through the air-clearance cycle on starting without water in the suction line. This dry-run tolerance is a separate design parameter: once pumping starts and flow is established, NPSHa must still exceed NPSHr + 0.5 m.
Submersible vs surface: NPSH comparison
| Factor | CDW submersible | CAP auto-prime surface |
|---|---|---|
| Suction condition | Fully immersed — Hs is positive | Suction lift up to 9.8 m — Hs is negative |
| NPSHa calculation required? | No — flooded suction ensures adequate NPSHa in normal operation | Yes — required for any installation with suction lift |
| Cavitation risk | Low; check only at near-empty sump conditions (<0.3 m above strainer) | Present; verify NPSHa ≥ NPSHr + 0.5 m before final pump specification |
| Key design check | Total Dynamic Head (head-flow curve only) | TDH plus NPSHa verification |
For CDW submersibles, the pump sits below water level — Hs is positive and NPSHa is typically 10+ m above vapour pressure. The NPSH check is necessary only when the sump is nearly depleted and the pump runs at minimum submergence depth, or when pumping hot fluids (elevated Pvap/ρg).
For CAP surface pumps, every installation with suction lift requires the calculation. The 9.8 m suction lift rating is the physical maximum under ideal conditions — sea level, cold clear water, negligible pipe friction. Real-site NPSHa is always lower.
The TDH calculation guide at /total-dynamic-head-calculation/ covers discharge head, velocity head, and static lift for the delivery side. The pump cavitation guide at /pump-cavitation-prevention-guide/ covers cavitation symptoms and prevention. This guide covers the NPSHa calculation — use all three together for a complete surface pump specification.
Frequently asked questions
What is NPSH in a dewatering pump?
NPSH (Net Positive Suction Head) is the pressure margin at the pump inlet above the fluid’s vapour pressure. NPSHa is the available margin at the site. NPSHr is the minimum the pump requires. When NPSHa drops below NPSHr, the fluid vaporises at the impeller eye, causing cavitation: noise, vibration, flow loss, and impeller erosion.
How do you calculate NPSHa for a submersible pump?
For a fully submerged CDW pump, NPSHa is not a constraint — positive submergence means the fluid arrives at the impeller with adequate pressure. Run the calculation only if the pump operates at very low submergence (less than 0.3 m of water above the strainer), or when pumping fluids above 40°C.
What happens when NPSHa is less than NPSHr?
Cavitation. Bubbles form at the impeller eye and collapse against impeller surfaces, causing pitting, vibration, and flow reduction. The fix is to raise NPSHa: increase suction pipe diameter to reduce hf, shorten the suction run, lower the pump closer to the water source, or select a pump model with a lower NPSHr.
What is the NPSH for Cosmos CAP auto-prime pumps?
CAP pump NPSHr values are specific to each model and operating point — individual pump curves state them. As a guide: installations with suction lifts above 5 m and suction pipe runs longer than 15 m should always run the full NPSHa calculation. At the maximum 9.8 m suction lift, NPSHa margin against typical NPSHr values is tight.