NPSH (Net Positive Suction Head) is the net head surplus that keeps water flowing into the pump without vaporising. Where it is inadequate, water boils at the pump inlet, the bubbles collapse at the impeller and the pump loses both its flow and itself.
There are two NPSH values
- NPSHa (available): the net head the system offers the pump. It is calculated from the tank level, atmospheric pressure, suction pipe losses and water temperature. It is under the designer's control.
- NPSHr (required): the net head the pump needs to run without cavitation. It is read from the manufacturer's curve and rises with flow.
The rule is simple: NPSHa must exceed NPSHr. EN 12845 adds a margin on top.
The 1 metre margin
EN 12845 requires the suction pipe, including all valves and fittings, to be designed so that the NPSH available at the pump inlet exceeds the NPSH required by at least 1 m at maximum pump flow. That margin is not arbitrary: strainer fouling, a falling tank level and seasonal water temperature rise consume that metre quickly.
Water temperature: the input most often skipped
EN 12845 requires the calculation at the maximum anticipated water temperature. In an above-ground steel tank, 30 to 35 °C is easily reached in summer. The vapour pressure rises from about 0.032 bar at 25 °C to 0.056 bar at 35 °C, and that alone reduces NPSHa by 0.2 to 0.3 m. A calculation made in winter runs short in summer.
Suction pipe design rules
| Item | Positive head | Suction lift |
|---|---|---|
| Minimum diameter | DN65 | DN80 |
| Maximum velocity | 1.8 m/s | 1.5 m/s |
| Foot valve | Not required | Mandatory |
| Automatic priming | Not required | Mandatory |
The diameter may never be smaller than the pump flange. The suction pipe is laid horizontally or with a slight rise towards the pump; a reverse slope or a dip at mid-point creates an air pocket and the pressure falls when the pump starts.
The eccentric reducer rule: the flat face upwards and the tapered face downwards. Installed that way, no air pocket forms at the top of the pipe. A concentric reducer, or an eccentric one fitted the wrong way round, collects air at the pump inlet and leads to cavitation.
Vortex: the invisible NPSH loss
Where the difference between the tank outlet and the pump centreline is large, water enters the suction pipe at speed and rotation forms at the tank floor. If the distance between the suction pipe and the low water level is inadequate, air begins to be entrained. An anti-vortex plate reduces that distance substantially: a DN300 pipe needs 0.90 m without a plate but only 0.10 m with one — on a tank with a 100 m² footprint, that is about 80 m³ of effective capacity.
Field symptoms of cavitation
- A gravel-like noise comes from the pump casing, as if it were pumping stones.
- The discharge gauge swings unstably.
- The flow stays below expectation and fluctuates.
- Over time, erosion and pitting appear at the impeller vane tips.
- Vibration and bearing temperature rise.
These symptoms show most clearly in the annual full load test, because NPSHr rises with flow and the pump reaches its most demanding point at 150 percent capacity.
Frequently Asked Questions
What is the difference between NPSHa and NPSHr?
NPSHa is the net head the system offers the pump and is under the designer's control. NPSHr is the net head the pump requires to run without cavitation, read from the manufacturer's curve.
What margin is required?
EN 12845 requires NPSHa to exceed NPSHr by at least 1 m at maximum pump flow. That margin covers strainer fouling, falling tank level and temperature rise.
Which water temperature is used in the calculation?
The maximum anticipated water temperature. In an above-ground tank, 30 to 35 °C is easily reached in summer, and that reduces NPSHa by 0.2 to 0.3 m.
How is an eccentric reducer fitted?
Flat face up, tapered face down. Fitted the wrong way round, an air pocket forms at the top of the pipe and cavitation begins at the pump inlet.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 · NFPA 13 (2025) · NFPA 20 (2025) · EN 12259-1. Definitions are for information; the full text of the standard governs in design.