# NPSH Available Calculator for Oil and Gas Pumps

> Source: https://netray.co/tools/npsh-available-pump-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Compute NPSH available in meters and compare it with the pump NPSH required to see the cavitation margin.

## Category

industry-templates

## Industry

Oil and Gas Equipment

## Audience

- Process engineer
- Pump engineer
- Sales engineer
- Maintenance engineer

## Tool

**Kind:** calculator

**Inputs:**

#### Absolute pressure at suction surface

**Key:** ps

**Type:** number

**Default:** 101.3

**Min:** 0

**Step:** 1

**Unit:** kPa abs

**Help:** Atmospheric is about 101.3 kPa abs. For a closed vessel use the vessel absolute pressure.

#### Vapor pressure at pumping temperature

**Key:** pv

**Type:** number

**Default:** 12

**Min:** 0

**Step:** 0.5

**Unit:** kPa abs

**Help:** Use the fluid vapor pressure at the actual temperature.

#### Liquid density

**Key:** rho

**Type:** number

**Default:** 850

**Min:** 100

**Step:** 5

**Unit:** kg/m3

**Help:** Density at pumping conditions.

#### Static head (positive if liquid level above pump)

**Key:** hs

**Type:** number

**Default:** 2.5

**Step:** 0.1

**Unit:** m

**Help:** Use a negative value for a suction lift.

#### Suction line friction and fitting loss

**Key:** hf

**Type:** number

**Default:** 0.8

**Min:** 0

**Step:** 0.1

**Unit:** m

#### Pump NPSH required (NPSHr)

**Key:** npshr

**Type:** number

**Default:** 3

**Min:** 0

**Step:** 0.1

**Unit:** m

**Help:** From the pump curve at the operating flow.

**Outputs:**

#### Pressure head term

**Key:** ph

**Expr:** (ps - pv) * 1000 / (rho * 9.80665)

**Format:** decimal

**Description:** Head from the pressure difference between surface and vapor pressure.

#### NPSH available

**Key:** npsha

**Expr:** ph + hs - hf

**Format:** decimal

**Primary:** true

**Description:** In meters of liquid.

#### Margin over NPSHr

**Key:** margin

**Expr:** npsha - npshr

**Format:** decimal

**Primary:** true

**Description:** Positive means margin. Many specifications ask for a minimum margin or a ratio.

#### NPSHa / NPSHr ratio

**Key:** ratio

**Expr:** if(gt(npshr, 0), npsha / npshr, 0)

**Format:** decimal

**Chart:**

#### NPSH available vs static head

**Type:** line

**X:** hs

**From:** -5

**To:** 10

**Steps:** 30

**Series:**

- npsha

**Disclaimer:** Calculation uses the standard NPSH available equation. Your project specification may require a minimum margin above NPSHr, correction for dissolved gas and temperature, and flow dependent friction losses. Confirm with the pump manufacturer.

## Content

**Intro:** Net positive suction head available is the pressure margin at the pump suction above the liquid vapor pressure. If it falls below the pump's required value, the liquid flashes inside the pump and cavitation damages the impeller. This calculator works the metric equation and shows the margin.

**Sections:**

#### How to use the calculator

**Body:** Enter the absolute pressure acting on the liquid surface, the vapor pressure at the pumping temperature, the liquid density, the static head and the friction loss in the suction line. Add the NPSH required from the pump curve at the flow you plan to run. The result shows NPSH available, the margin in meters and the ratio. The chart sweeps static head so you can see how raising a tank or sump level changes the margin.

#### The equation

**Body:** NPSHa = (Ps - Pv) / (rho x g) + hs - hf, with pressures in pascals, density in kilograms per cubic meter, g as 9.80665 and heads in meters. Ps is the absolute pressure on the surface of the liquid, Pv the vapor pressure, hs the static height of the liquid above the pump centerline and hf the friction loss in the suction pipe and fittings. A suction lift enters as negative static head.

**Bullets:**

- Always use absolute pressures, not gauge.
- Vapor pressure rises sharply with temperature.
- Friction loss rises with the square of flow.

#### Worked example

**Body:** With 101.3 kPa on the surface, 12 kPa vapor pressure and 850 kg/m3 density, the pressure head term is (101.3 - 12) x 1000 / (850 x 9.80665) = about 10.71 m. Add 2.5 m static head and subtract 0.8 m friction to get NPSHa of about 12.41 m. Against an NPSHr of 3 m, that is a margin of about 9.4 m and a ratio above 4.

#### Common mistakes

**Body:** Frequent errors are using gauge pressure instead of absolute, taking vapor pressure at the wrong temperature, ignoring the extra friction when the strainer fouls, and checking only at design flow when the pump may run at runout. Also note that NPSHr on a pump curve is usually the 3 percent head drop value, so many specifications require extra margin above it.

**Cta:** If pump data sheets and duty points are kept in spreadsheets, we can help connect them to your equipment records and service history in the ERP.

**Faq:**

#### What margin should NPSHa have over NPSHr?

**Answer:** It depends on the specification, the fluid and the pump. Many users ask for a margin of at least 0.5 to 1 m or a ratio above 1.1 to 1.3, and more for hydrocarbons or high energy pumps. Follow the project or API 610 data sheet requirements.

#### Do I use gauge or absolute pressure?

**Answer:** Absolute pressure for both the surface and vapor pressure. If you only know gauge pressure, add atmospheric pressure at the site elevation. Using gauge values will understate NPSHa by about 10 m of water equivalent.

#### How does temperature affect NPSHa?

**Answer:** Higher temperature raises the vapor pressure, which reduces the pressure head term and NPSHa. Hot services such as condensate or hydrocarbon near its boiling point can have very little margin, so check at the maximum pumping temperature.

#### Is NPSHr constant for a pump?

**Answer:** No. NPSHr rises with flow and speed, and it is read from the manufacturer's curve at the operating point. Always compare NPSHa to NPSHr at the highest flow the pump will see in service.

## Search terms

NPSH available calculator, NPSHa formula, pump cavitation margin calculator, net positive suction head calculation, NPSHa vs NPSHr margin, NPSH calculator metric, pump suction head calculation, hydrocarbon pump NPSH

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