FB Bombas Engineering Team12 min read
NPSH (Net Positive Suction Head) is the suction head above the vapour pressure of the liquid. NPSHa, available, belongs to the installation: NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf, in metres of pumped liquid. NPSHr, required, is the minimum the manufacturer measures in testing and publishes on the curve, and it rises with flow.
The NPSHa − NPSHr margin follows ANSI/HI 9.6.1, by application, at the lowest NPSHa of the operating range.
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With water at sea level, Hz = +2.0 m and Hf = 1.0 m, NPSHa is 11.11 m at 20 °C and 2.77 m at 95 °C.
FB Bombas pump for this application
FBCN Series
Checking the NPSH of a centrifugal pump?
The FBCN is the FB Bombas normalized centrifugal pump for water and low-viscosity liquids, mechanically conforming to ASME B73.1.
Source: manual FBCN MTEC-03/00, p.1
At a glance
- NPSHa is the NPSH available at the installation; NPSHr is the minimum the manufacturer measures in testing and publishes on the curve, rising with flow. The margin is NPSHa − NPSHr, checked at the lowest NPSHa of the operating range.
- In water at 20 °C, standard atmospheric pressure equals 10.35 m at sea level and 9.18 m at 1,000 m; vapour pressure rises from 0.24 m at 20 °C to 8.97 m at 95 °C (NIST).
- With Hz = +2.0 m, Hf = 1.0 m and an NPSHr of 4.0 m, NPSHa falls from 11.11 m at 20 °C to 6.66 m at 80 °C and to 2.77 m at 95 °C, with a negative margin.
- The 2024 ANSI/HI 9.6.1 measures the margin over the published NPSHr, equal to or greater than NPSH3, with recommendations by application; the DOE/HI sourcebook cites the common rule of NPSHa 25% above NPSHr.
- On the FBCN curves for clean water at 20 °C, NPSHr ranges from about 2 m on the 150-500 to more than 20 m on the 300-340; a lower-speed model does not guarantee a lower NPSHr at the same duty point.
Installation NPSHa, NPSHr and NPSH3 from the curve, the formula with atmospheric pressure by altitude and water vapour pressure, a worked example from 20 to 95 °C, application-based margin under ANSI/HI 9.6.1, suction specific speed and the NPSHr curves of the FBCN series.
What are NPSHa and NPSHr?
NPSH stands for Net Positive Suction Head, the suction head above the vapour pressure of the liquid. NPSHa (available) is a system characteristic: the absolute total suction head at the first-stage impeller datum, minus the absolute vapour pressure of the liquid, both expressed as head.
NPSHr (required) is a pump characteristic: the minimum the manufacturer states for the specified performance at a given flow, speed and liquid.
The difference between the two is the NPSH margin, and NPSHa must always exceed NPSHr (Hydraulic Institute). NPSHr is not calculated for the installation: it comes from pump testing, is plotted on the model curve together with head, power and efficiency, and rises with flow.
On the FBCN curves, the 50-160 at 3,500 rpm requires about 1.7 m at 40 m³/h and about 4.7 m at 100 m³/h.
Sheet CC-124.07 for the FBCN 65-315, in the curves at 1,750 rpm (p. 25), specifies clean water at 20 °C. Its required NPSH graph is marked for the 320 mm impeller. Check these identifiers before applying the graph to another configuration. For a selection enquiry, send the curve sheet, duty point and installation data.
| Criterion | NPSHa (available) | NPSHr (required) |
|---|---|---|
| Who determines it | The installation (system designer) | The manufacturer, by pump testing |
| What it represents | Absolute suction head above vapour pressure, at the pump datum | Minimum NPSH for the specified performance (≥ tested NPSH3) |
| How to obtain | NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf | Model curve, at the duty flow and speed |
| Varies with | Level, reservoir pressure, temperature, altitude and suction losses | Flow, speed, impeller and wear |
| Unit | m of pumped liquid column | m of pumped liquid column |
| Selection criterion | NPSHa ≥ NPSHr + margin, at the lowest NPSHa of the range (ANSI/HI 9.6.1) | Insufficient margin: noise, vibration, head drop and erosion |
NPSH concept in centrifugal pumps
In a centrifugal pump, the liquid enters through the impeller eye, and pressure drops at that inlet. If local pressure falls below the vapour pressure at the operating temperature, vapour bubbles form and collapse when they reach a higher-pressure region inside the impeller: this is cavitation.
According to the Hydraulic Institute, the collapse releases enough energy to erode the impeller passages, and the vibration it causes also damages seals and bearings.
NPSH measures the clearance against this phenomenon: NPSHa describes the installation, NPSHr describes the pump, and the margin is the difference between them. A positive margin does not guarantee freedom from cavitation.
HI notes that visible cavitation, increased noise and vibration, the onset of head or efficiency drop and erosion can occur with margin above NPSHr; that is why ANSI/HI 9.6.1 recommends margins by application.
NPSHa calculation — installation formula
NPSHa depends on the suction reservoir, the piping, the liquid temperature and the site altitude. HI writes NPSHa = h_atm + h_s − h_vp: atmospheric pressure, total suction head and vapour pressure, all as head of liquid.
In the form below, each term is evaluated at actual operating conditions, and the check uses the lowest NPSHa across the whole range (ANSI/HI 9.6.1, Appendix A).
NPSHa = (Pa − Pv) / (ρ · g) ± Hz − Hf [m]Available NPSH with a suction reservoir; all terms in metres of pumped liquid
The four NPSHa terms in detail
Pa is the absolute pressure at the liquid surface. In an open tank, it is the local atmospheric pressure: the standard atmosphere is 101.325 kPa at sea level (CGPM), and the U.S. Standard Atmosphere, 1976 gives 89.88 kPa at 1,000 m and 79.50 kPa at 2,000 m.
In water at 20 °C (998.16 kg/m³, NIST), this equals 10.35 m, 9.18 m and 8.12 m of head. In a pressurised tank, Pa is the absolute pressure above the liquid.
Pv is the vapour pressure at the liquid temperature. According to the NIST saturation table, water has 2.34 kPa at 20 °C, 47.41 kPa at 80 °C, 84.61 kPa at 95 °C and 101.42 kPa at 100 °C. Convert Pa and Pv with the density at the same temperature, as in the table below.
At 100 °C, Pv already exceeds the standard atmosphere, and (Pa − Pv) is practically zero for an open tank at sea level.
Hz is the difference between the liquid level and the pump datum: positive with flooded suction (level above) and negative with suction lift (level below). Use the minimum operating level. Hf is the suction line friction loss at actual flow and viscosity, including pipe, bends, valves and strainers.
Per the DOE/HI sourcebook, air pockets or pipe fouling that starve the suction also cause cavitation damage.
Scroll the table to see every column. The first column stays on the left.
| Temperature | Pv (kPa) | ρ (kg/m³) | Pv (m) | Pa (m) | Pa − Pv (m) |
|---|---|---|---|---|---|
| 20 °C | 2.34 | 998.2 | 0.24 | 10.35 | 10.11 |
| 80 °C | 47.41 | 971.8 | 4.98 | 10.63 | 5.66 |
| 95 °C | 84.61 | 961.9 | 8.97 | 10.74 | 1.77 |
| 100 °C | 101.42 | 958.4 | 10.79 | 10.78 | −0.01 |
Safety margin — ANSI/HI 9.6.1
NPSH3 is the NPSH at which first-stage total head drops by 3% due to cavitation; at that point cavitation is already present. The 2024 edition of ANSI/HI 9.6.1, which supersedes the 2017 edition, now measures the margin from the published NPSHr, which must be equal to or greater than the tested NPSH3, because not every manufacturer publishes NPSH3.
Recommended margins are by application, across ten market segments, not a single number.
The guideline assesses the margin through three groups of factors: the liquid (temperature, abrasives, dissolved or entrained gases, solids), the pump (design, speed, wear and ageing, which change NPSHr over time) and the system (operating region, inlet conditions, transients, recirculation lines and parallel pumping).
Since suction pressure is rarely measured continuously, the margin also covers the deviation between estimated and actual NPSHa.
The DOE/HI sourcebook records a common design rule of NPSHa 25% above NPSHr at all expected flows, and notes that near BEP a small margin over NPSH3 is enough, while away from it the margin needed to suppress all cavitation is much larger. The 25% rule is a quick test; for the service, the application-based margin of ANSI/HI 9.6.1 applies.
Engineering note
For an FBCN assessment, provide the liquid, maximum temperature, reservoir pressure or site altitude, minimum level, suction losses and flow range. The margin is defined for these conditions and compared with the model curve. Send the data to the technical team: comercial@fbbombas.com.br.
How to calculate the NPSH margin?
The margin is NPSHa − NPSHr at the operating point. Illustrative example, with a hypothetical model of NPSHr 4.0 m at the operating flow: open tank, flooded suction with Hz = +2.0 m and Hf = 1.0 m, the same in every case.
With water at 20 °C at sea level, NPSHa = 10.35 − 0.24 + 2.0 − 1.0 = 11.11 m, and the margin is 7.11 m.
At 80 °C, vapour pressure reaches 4.98 m and, even with the larger atmospheric head of hot, less dense water, the (Pa − Pv) term falls to 5.66 m. NPSHa is 6.66 m, still above the 5.0 m the 25% rule calls for (1.25 × 4.0 m), with a margin of 2.66 m.
At 95 °C, NPSHa falls to 2.77 m, below the NPSHr of 4.0 m: the margin is negative, and the selection fails. To get back to the 5.0 m of the 25% rule, the minimum level would have to rise by about 2.2 m, or the installation would have to change in another term of the formula.
Redo the calculation when the temperature changes: vapour pressure, density and losses change together, and the check applies to the most demanding condition of the range. The published FBCN curves are valid for clean water at 20 °C, as each curve header states; another liquid or another temperature goes into the engineering selection review.
Scroll the table to see every column. The first column stays on the left.
| Condition | Pa − Pv (m) | NPSHa (m) | Margin (m) | NPSHa/NPSHr |
|---|---|---|---|---|
| 20 °C, sea level | 10.11 | 11.11 | 7.11 | 2.78 |
| 20 °C, 1,000 m | 8.94 | 9.94 | 5.94 | 2.49 |
| 80 °C, sea level | 5.66 | 6.66 | 2.66 | 1.66 |
| 95 °C, sea level | 1.77 | 2.77 | −1.23 | 0.69 |
What happens when NPSHa is lower than NPSHr?
The pump cavitates. The signs appear in cascade: crackling noise, which the DOE/HI sourcebook compares to marbles passing through the pump; vibration; head drop against the curve, which reaches 3% at NPSH3; and, with continued exposure, impeller erosion. Erosion can unbalance the impeller and create alternating bearing loads, and the vibration stresses the foundation and the piping.
The sourcebook also notes that cavitation usually occurs at high flow, with the pump to the right of its curve, where NPSHr rises, and that temperature above design or suction pressure below design can cause it. The correction raises NPSHa (level, losses, temperature or reservoir pressure) or lowers NPSHr with another configuration, checked on the curve.
Where some cavitation is unavoidable, the sourcebook recommends an impeller of high-tensile-strength material, compatible with the fluid.
Suction-specific speed (Nss) and internal recirculation
Suction specific speed (Nss) is an index of pump suction performance, determined at BEP with the maximum impeller diameter. In HI’s US convention, Nss = n × √Q′ / NPSH3^0.75, with n in rpm, Q′ in gpm per impeller eye (half the flow for double suction) and NPSH3 in ft.
Example: 1,750 rpm, 500 gpm and an NPSH3 of 10 ft give Nss ≈ 6,960. The metric index S converts as Nss = 51.6 × S.
HI cautions that calculation methods differ between sources: compare values only on the same basis. Because it is determined at BEP, the index does not describe part-load flow: at low flow, recirculation at the impeller suction or discharge causes cavitation-like damage (DOE/HI sourcebook).
On the FBCN, Table 1 (MTEC-03/00) limits flow from 0.1 Qot or 0.15 Qot, depending on the configuration, up to 1.1 Qot.
NPSH in FBCN DN200+ — critical selection variable
In the 14 large FBCN models, with DN 200, 250 or 300 suction (from the 150-200 to the 300-340), the 1,750 rpm curves show why nominal size does not sum up NPSHr.
On the 150-500 (CC-0202.09), with a 504 mm impeller, NPSHr goes from about 2 m at low flow to about 12 m at the end of the curve; on the 300-340 (CC-0211.09), from about 13 m to more than 20 m.
A lower-speed model does not guarantee a lower NPSHr at the same duty point.
At 60 m³/h, the 50-160 at 3,500 rpm (CC-158.07) requires about 2.4 m; the 50-315 at 1,750 rpm (CC-119.07), with the same 80 mm suction, requires about 2.7 m, already near the end of its curve; and the 65-315 at 1,750 rpm (CC-124.07), with 100 mm suction, requires about 2.0 m.
Compare the curves at the operating point, at the same flow.
With NPSHa low because of a tank close to saturation, altitude or a long suction line, the options are to raise the level or flood the suction, enlarge the suction line to reduce Hf, pressurise the reservoir, install a booster pump or choose another configuration from the curve. Each one requires system analysis.
What data to send to check the NPSH of a centrifugal pump
For FB Bombas to check the NPSH of an FBCN centrifugal pump, send the liquid and its maximum temperature, the minimum level and the pressure on the liquid, the suction layout (diameter, length and fittings), the flow and speed of the duty point and, if you have it, the curve sheet you used.
With that, engineering compares NPSHa with the curve NPSHr and with the application margin.
If the liquid is viscous, such as oil, asphalt or molasses, the calculation changes basis: the inlet requirement becomes the NPIPR and cold start weighs more, the subject of the article on NPSH in gear pumps.