1. What is NPSH and why it matters
Every hydraulic pump has a physical limit defining how far it can suction a liquid before that liquid begins to "boil" inside the pump itself — this limit is expressed by NPSH. When the absolute pressure of the liquid at the impeller inlet (or gear inlet in the case of positive displacement pumps) drops below the fluid's vapor pressure at that temperature, vapor bubbles form spontaneously.
When these bubbles travel to higher-pressure regions inside the pump, they violently collapse into microjets that erode the metal — this phenomenon is cavitation.
Engineers often treat NPSH with less rigor in gear pumps than in centrifugal pumps — and this is a mistake. Although external gear positive displacement pumps (such as the FBE series) are more tolerant of NPSH variations than centrifugals, they also cavitate when poorly sized, especially in applications with hot, volatile or viscous fluids. The symptoms are the same: characteristic noise, vibration, flow drop, erosion on the suction side of the gears, and dramatic reduction in seal and bearing life.
2. Available NPSH (NPSHa): the formula
Available NPSH is a property of the hydraulic installation, not of the pump. It is the project engineer who calculates it, summing and subtracting pressure contributions on the suction line up to the pump inlet flange. The canonical formula is:
NPSHa = (Patm / γ) ± Hs − Hf − (Pv / γ)NPSHa canonical formula
3. The formula variables, explained
Each term of the equation has direct physical meaning and consistent units in meters of liquid column (m.l.c.). Working in m.l.c. is more intuitive than in bar or kgf/cm² because it allows direct comparison between different fluids.
| Symbol | Name | Description |
|---|---|---|
| Patm / γ | Local atmospheric pressure | At sea level: 10.33 m.w.c. (water). For other fluids, divide by specific gravity. Subtract about 1.0 m per 1,000 m of altitude. |
| ± Hs | Geometric suction height | Positive (+) if pump is below liquid level (flooded suction). Negative (−) if pump is above (lift suction). |
| Hf | Suction friction loss | Sum of friction losses in straight pipe, bends, valves, filters and fittings from reservoir to pump flange. Increases with flow squared and with viscosity. |
| Pv / γ | Fluid vapor pressure | Fluid property at operating temperature. Water at 20°C: 0.024 m.w.c. Water at 80°C: 4.8 m.w.c. Thermal oils: tabulated by manufacturer. |
4. The hidden impact of viscosity on NPSHa
The biggest trap in NPSH sizing for gear pumps is underestimating friction loss (Hf) when the fluid is viscous. The Darcy-Weisbach equation shows that friction loss varies linearly with friction factor f, and this factor — in laminar flow — is inversely proportional to Reynolds number: f = 64/Re. Since Reynolds number is Re = (v·D·ρ)/μ, high-dynamic-viscosity fluids (μ) generate low Reynolds numbers, predominantly laminar regime and friction factors much larger than those seen with water.
In practice, this means that SAE 30 oil at 40°C (viscosity ~150 cSt) will have a specific friction loss between 10 and 30 times greater than water in the same piping and flow. Ignoring this factor is the number one cause of NPSHa undersizing in oil, resin, asphalt and thermal fluid pumping projects.
5. Numerical example: lubricating oil at 60°C
Let's size the NPSHa of a real installation: FBE 2" gear pump transferring SAE 30 lubricating oil at 60°C from a reservoir elevated 1.5 meters above the pump, with 8 meters of 2-inch piping, 3 ninety-degree bends and 1 fully open ball valve. Location: Cabreúva-SP, altitude ~600 m.
Fluid properties: SAE 30 oil at 60°C has specific gravity ~0.88 and kinematic viscosity ~60 cSt. Vapor pressure is negligible (Pv/γ ≈ 0 m.l.c.) — lubricating oils do not vaporize in this temperature range.
- Patm / γ = 10.33 / 0.88 × (600 m altitude = −0.7 m) ≈ 11.0 m of oil
- + Hs = +1.5 m (flooded suction)
- − Hf ≈ 2.8 m (calculated via Darcy-Weisbach with actual viscosity)
- − Pv / γ ≈ 0 m
- NPSHa = 11.0 + 1.5 − 2.8 − 0 = 9.7 m of oil
6. Final check: NPSHa vs NPSHr
The FBE 2" operating at 1,750 rpm has NPSHr typically between 2.0 and 3.5 m.l.c. (consult the FBE technical manual for the exact value at your speed and flow). With calculated NPSHa of 9.7 m and NPSHr of 3.5 m, the margin is 6.2 m — well above the recommended minimum of 1.0 to 1.5 m. The installation operates with comfortable margin and there is no cavitation risk under the described conditions.
If the same installation had lift suction (Hs = −3.0 m) instead of flooded, NPSHa would drop to 11.0 − 3.0 − 2.8 − 0 = 5.2 m. Still acceptable, but the margin would be only 1.7 m — within the recommended minimum, but without room for operational variations such as filter partial clogging or viscosity increase at cold start. The lesson: pump position relative to the reservoir has dramatic impact on NPSHa.
7. Cavitation prevention checklist
FB Bombas' engineering experience with gear pump installations in Brazilian and Latin American industries since 1944 has produced the following practical checklist for cavitation prevention in viscous fluids:
- Position the pump as close as possible to the reservoir and preferably below the liquid level (flooded suction).
- Use suction piping with diameter equal to or larger than the pump suction flange — never smaller.
- Minimize bends, reductions, valves and filters on the suction line. Each fitting adds friction loss equivalent to several meters of straight pipe.
- Calculate friction loss using viscosity at minimum operating temperature (cold start), not at nominal temperature.
- In hot fluids, verify vapor pressure at maximum process temperature — fluids near vaporization point require larger margins.
- Install a suction pressure gauge with adequate scale for low pressure (including vacuum) and monitor during commissioning and operation.
- If NPSHa is tight, consider increasing suction pipe diameter or lowering the pump — this is usually cheaper than replacing with a pump of lower NPSHr.
8. Practical conclusion
Sizing NPSH for gear pumps is not complicated math — it is math that must be done, and must be done with the actual fluid viscosity at actual operating temperature. The most common mistake is not getting the formula wrong, it is using an outdated or overly optimistic viscosity.
If your application involves oils, asphalt, resins, biodiesel, chocolate, molasses or viscous chemicals, our engineering team in Cabreúva-SP can review NPSHa sizing per actual installation conditions and recommend the most suitable FBE line — from 1/8" for precise dosing to 6" for large volumes.