ENGINEERING TOOL
NPSH available
NPSH available (NPSHa) is calculated as NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf: absolute pressure at the liquid surface minus vapor pressure, in meters of liquid, plus the static suction head and minus the friction loss. This page gives the formula, water vapor pressure from 0 to 100 °C and a worked calculation.
At a glance
- NPSH available (NPSHa): NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf, in meters of liquid. Surface pressure and static head add; vapor pressure and suction friction loss subtract.
- Water per NIST: vapor pressure rises from 2.34 kPa at 20 °C to 47.41 kPa at 80 °C. As head, the Pv/(ρ·g) term grows from 0.24 m to 4.98 m of NPSHa.
- The margin between available and required NPSH must be defined for the fluid, pump and operating range. Request manufacturer criteria and installation data; a fixed allowance cannot replace that assessment.
- For an FBCN, request the curve for the intended model, speed and impeller diameter. Read required NPSH at duty flow; a series-wide range cannot replace that point.
Updated
Answers to guide your selection
What is the NPSH available formula?
NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf, in meters of pumped liquid. Pa is the absolute pressure at the liquid surface, Pv the vapor pressure at pumping temperature, Hz the static head and Hf the suction friction loss. It is the Hydraulic Institute definition, NPSHA = h_atm + h_s − h_vp, written from the liquid surface.
How do I calculate NPSH step by step?
Divide the absolute surface pressure and the vapor pressure by ρ·g, at the liquid temperature, to express both in meters. Subtract one from the other, add the static head with its sign (negative when the pump sits above the level) and subtract the suction friction loss at duty flow. The worked example below does the math for water at 40 °C.
Does the calculation work for any liquid?
Use actual fluid properties at operating temperature. Pure-water data should not automatically represent mixtures, solutions or hydrocarbons. Attach the fluid data sheet and identify composition and startup conditions.
How do I request the NPSH calculation?
Send the fluid, temperature, flow, reservoir levels, pressure and suction layout. If a pump is already specified, attach its curve and identify the model and speed. These data support an engineering review of margin and service conditions.
Data for assessing Available NPSH
Use the information below to start the assessment with the technical team.
Discuss your duty point with engineering
Send the data below so FB engineering can check the installation NPSH available against the NPSH required on the model curve at duty flow.
Selection depends on process conditions and confirmed data. The assessment and supply scope will be defined with the technical team.
Information for the assessment
- Pumped fluid and pumping temperature
- Site altitude
- Suction type (flooded or lift) and static head in meters
- Suction line layout (diameter, length, fittings) — or the friction loss, if already known
- Tank pressure, if closed or pressurized
- Pump NPSHr, if a pump is already defined
The NPSH available formula
NPSH available (NPSHa) is the pressure energy left at the pump suction after subtracting the liquid vapor pressure. It is computed by adding the surface head to the static head and subtracting the friction loss and the vapor pressure — all terms in meters of liquid column:
NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf| Term | What it represents | Effect on NPSHa |
|---|---|---|
Pa / (ρ·g) | Pressure head at the liquid surface (atmospheric + tank pressure), divided by ρ·g, the specific weight | Adds (+) |
± Hz | Static suction head: positive if the pump is flooded (liquid above), negative if it lifts from below | Adds or subtracts (±) |
Hf | Total friction loss in the suction line (piping, valves, strainer, bends) | Subtracts (−) |
Pv / (ρ·g) | Vapor pressure head of the liquid at pumping temperature (rises with heat) | Subtracts (−) |
The Hydraulic Institute defines NPSHA = h_atm + h_s − h_vp: total absolute suction head minus absolute vapor pressure, both as head of liquid (Pump Principles, Eq. 1.D.6). The formula above is that definition written from the liquid surface. The concept, NPSHr from the curves and the margin by application are covered in the article NPSH in centrifugal pumps: NPSHa, NPSHr and margin.
Water vapor pressure from 0 to 100 °C
Saturated water per the NIST Chemistry WebBook. The Pv/(ρ·g) column shows how much vapor pressure takes from NPSHa. The last column already gives (Pa − Pv)/(ρ·g) for an open tank at sea level, with the standard atmosphere of 101.325 kPa: in that case, just add Hz and subtract Hf.
| Temperature | Pv (kPa) | ρ (kg/m³) | Pv/(ρ·g) (m) | (Pa − Pv)/(ρ·g) at sea level (m) |
|---|---|---|---|---|
| 0.01 °C | 0.61165 | 999.79 | 0.06 | 10.27 |
| 10 °C | 1.2282 | 999.65 | 0.13 | 10.21 |
| 20 °C | 2.3393 | 998.16 | 0.24 | 10.11 |
| 30 °C | 4.2470 | 995.61 | 0.43 | 9.94 |
| 40 °C | 7.3849 | 992.18 | 0.76 | 9.65 |
| 50 °C | 12.352 | 988.00 | 1.27 | 9.18 |
| 60 °C | 19.946 | 983.16 | 2.07 | 8.44 |
| 70 °C | 31.201 | 977.73 | 3.25 | 7.31 |
| 80 °C | 47.414 | 971.77 | 4.98 | 5.66 |
| 90 °C | 70.182 | 965.30 | 7.41 | 3.29 |
| 100 °C | 101.42 | 958.35 | 10.79 | −0.01 |
Worked NPSH calculation
Water at 40 °C in an open tank at 1,000 m altitude, with the pump 3.0 m above the lowest level (suction lift) and a friction loss of 1.2 m in the suction line at duty flow. Atmospheric pressure at 1,000 m is 89.88 kPa per the U.S. Standard Atmosphere, 1976; ρ = 992.18 kg/m³ and Pv = 7.38 kPa at 40 °C come from the table above.
| Step | Math | Result |
|---|---|---|
| ρ·g at 40 °C | 992.18 × 9.80665 ÷ 1000 | 9.73 kN/m³ |
| Pa ÷ (ρ·g), at 1,000 m | 89.88 ÷ 9.73 | 9.24 m |
| Pv ÷ (ρ·g), at 40 °C | 7.38 ÷ 9.73 | 0.76 m |
| Hz, suction lift | pump 3.0 m above the lowest level | −3.0 m |
| Hf in the suction line | loss at duty flow | −1.2 m |
| NPSHa | 9.24 − 0.76 − 3.0 − 1.2 | 4.28 m |
In the same installation with water at 80 °C, the Pv/(ρ·g) term rises to 4.98 m, and NPSHa drops to about 0.26 m. Hot liquid with suction lift is the most critical case: comparing with the NPSHr on the curve, at duty flow, tells whether the installation must change (flood the suction, cut the loss or cool the liquid).
Fluid properties
Include the liquid data sheet in the suction assessment. Record the unit, temperature and source of each property.
| Data | What to provide | Where to check |
|---|---|---|
| Temperature | Startup, operation and expected variations | Process conditions |
| Vapor pressure | Value and unit at liquid temperature | Fluid data sheet or identified reference |
| Density | Value, unit and corresponding temperature | Data sheet or measurement of the actual fluid |
How to prevent cavitation
The responsible team should assess installation and model conditions. These are investigation points; priority depends on the case.
- 1Raise the reservoir level or lower the pump: every meter of flooding goes straight into NPSHa.
- 2Reduce suction friction loss: larger pipe diameter, shorter run, fewer bends, and a clean, generous strainer.
- 3Check process temperature range and liquid properties during startup and operation.
- 4Provide actual reservoir pressure and its variations; any change must be part of the system design.
- 5Compare required-NPSH curves for the proposed flow, speed and configuration. Do not extrapolate a curve to another speed without manufacturer confirmation.
- 6Check the manufacturer operating region, including the distance from the best efficiency point.
Hf comes from the suction-line head loss; NPSHr, from the model curve at duty flow.
Frequently Asked Questions
What is NPSH in a pump?
NPSH is total suction head above the liquid vapor pressure, expressed in meters of that liquid. During selection, compare the installation available value with the model requirement and a duty-appropriate margin. Assessment considers flow, temperature and operating conditions.
What is the difference between NPSH available (NPSHa) and required (NPSHr)?
NPSHa describes available installation conditions. The required value belongs to the pump and must be checked in the curve and declared test criterion. NPSH3 corresponds to a 3% head drop during testing; it does not mean zero cavitation. Assess the margin for the liquid, model and duty.
How is NPSH available calculated?
NPSH available is calculated as NPSHa = (Pa − Pv)/(ρ·g) ± Hz − Hf, where Pa is the absolute pressure at the liquid surface, Pv the vapor pressure at pumping temperature, ρ·g the fluid specific weight (density × gravity), Hz the static suction head (positive if flooded, negative if the pump sits above the liquid) and Hf the friction loss in the suction line. The result is given in meters of liquid column. It is the Hydraulic Institute definition, NPSHA = h_atm + h_s − h_vp, written from the liquid surface.
What safety margin is recommended between NPSHa and NPSHr?
The margin between available and required NPSH must be defined for the fluid, pump and operating range. Request manufacturer criteria and installation data; a fixed allowance cannot replace that assessment.
What causes cavitation in a centrifugal pump?
Cavitation involves vapor formation when local pressure falls and bubble collapse in higher-pressure regions. It can affect performance and components. Check suction, fluid, temperature, flow and operating range. Do not use NPSHa = NPSH3 as the boundary for cavitation onset or absence.
How do temperature and altitude affect NPSH available?
Atmospheric pressure varies with altitude and local conditions; vapor pressure depends on liquid and temperature. Use fluid properties at operating temperature and system pressure data. Also assess tank levels, suction losses and changes during operation.
What is the NPSH required of FB Bombas centrifugal pumps?
Consult the FBCN model curve at the intended flow, speed and impeller configuration. The document should identify its NPSH criterion. Send the curve and suction data to engineering; do not use a series-wide range as the requirement at your duty point.
Are NPSHd and NPSHa the same thing?
Yes. NPSHd (from the Portuguese and Spanish "disponível" and "disponible") and NPSHa (available) name the same value: the NPSH the installation offers at the suction, calculated with the formula on this page. NPSHr (required) belongs to the pump and comes from the model curve.
How do I convert pressure to meters of head in the NPSH calculation?
Divide the pressure by ρ·g: h [m] = p [kPa] ÷ (ρ·g) [kN/m³]. With water at 20 °C (ρ·g = 9.789 kN/m³), 1 bar equals 10.22 m, 1 kgf/cm² equals 10.02 m and 1 kPa, 0.102 m. In a pressurized tank, Pa is the absolute pressure: local atmospheric plus the tank gauge pressure. Convert Pa and Pv with the density at the same temperature.
Hydraulic Institute: interpreting pump curves
Hydraulic Institute: Pump Principles, NPSH available
NIST Chemistry WebBook: thermophysical properties of water
U.S. Standard Atmosphere, 1976 (NASA NTRS)
Assessment depends on installation data, the curve and requested scope. Confirm conditions in the technical proposal. For fire systems, documentation must meet the project and standards and requirements applicable to the location.
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