Net Positive Suction Head Available (NPSHA) is the margin of pressure at the pump suction above the point where the liquid would vaporize (boil). If NPSHA drops below what the pump requires, vapor bubbles form in the pump and cause cavitation, lost flow and damage. Roth regenerative turbine pumps are designed for 1 ft NPSH required, so for most applications the question is simply: does my system provide at least 1 ft of NPSH, with some safety margin? Use this calculator to find out.

New to NPSH? Read What Is NPSH and Why Does It Cause Cavitation? for a plain-English explanation with worked examples.

Download the full bulletin (PDF): How to Interpret and Use Performance Curves

How to Use the Calculator

  1. Select your system case. Pick the case that matches how your pump is piped to the supply vessel (see the case descriptions below).
  2. Enter the liquid. For water, enter the temperature and the vapor pressure is looked up for you. For any other liquid, enter its vapor pressure (psia) at the pumping temperature and its specific gravity.
  3. Enter the system data: static suction head (or suction lift for Case II), friction loss in the suction line, and atmospheric pressure. At altitude, enter the elevation and click Apply to calculate atmospheric pressure.
  4. Optional: enter the NPSH required by your pump to see whether your system has an adequate margin.
  5. Click Calculate NPSH. The result is shown in feet of liquid along with the formula and the numbers used.

NPSH Calculator

Vapor pressure is interpolated from steam-table data (32–212 °F).

If the suction line is sized for under 3 ft/sec so that friction loss is under 0.5 ft, it may be disregarded (enter 0).

14.7 psia at sea level.

Calculates atmospheric pressure from elevation.

Available NPSH








Pressure to Feet of Liquid Converter












 




Disclaimer: This calculator is provided for general informational and estimating purposes only. Results are based on the formulas in Roth Pump Company’s technical bulletin and on the values you enter, and are not a guarantee of pump or system performance. Actual NPSH available depends on conditions this tool does not capture, such as fluid properties, temperature changes, suction piping design and fittings, entrained vapor, and altitude. Always have your final pump selection and system design verified by a qualified engineer, and contact Roth Pump Company before relying on these results. Roth Pump Company assumes no liability for the use of this calculator or the results it produces.






The Four System Cases and Their Formulas


All values are absolute pressures converted to feet of the liquid being pumped (feet = psi × 2.31 ÷ specific gravity). For the low-flow range of turbine pumps (up to 250 GPM), velocity head can be ignored.


    

  • Case I – liquid level above the pump (static suction head): NPSH = hs + ha − hvp − hf. This is by far the most common industrial arrangement, such as hot water pumped out of an elevated vessel.
    • 

  • Case IA – closed vessel under vacuum or liquid at its boiling point: the vapor pressure equals the pressure on the liquid, so those terms cancel and NPSH = hs − hf. With large suction piping (friction loss under 0.5 ft), NPSH equals the static suction head.
    • 

  • Case II – liquid level below the pump (suction lift): NPSH = ha − hvp − hs − hf. Atmospheric pressure is the only positive force; vapor pressure and suction lift both reduce NPSH. Liquid at its boiling point can never be lifted.
    • 

  • Case III – liquefied gases and refrigerants: these liquids are nearly always at their boiling point, and heat from the surroundings causes vapor in the suction line, so only 1.0 ft of NPSH should be counted on regardless of static head. A minimum static suction head of 4 ft is recommended.
    • 



Where: hs = static suction head (or suction lift), ha = atmospheric pressure, hvp = vapor pressure of the liquid at pumping temperature, hf = friction loss in the suction line.


Good Suction Piping Practice for Low NPSH Systems


    

  • Size suction pipe and fittings for liquid velocities under 3 ft/sec.
    • 

  • Locate the pump below the supply source and as close to it as possible.
    • 

  • Avoid vertical risers or high points along the top of the suction line, where vapor can collect.
    • 

  • Install a compound gauge in the suction line.
    • 

  • Vent the pump back to the vapor space of the supply vessel, and install an automatic bypass line back to the vessel in the discharge line.
    • 

  • At elevation, available NPSH drops roughly 1 ft for every 1,000 ft above sea level.
    • 



Reference: Available NPSH for Boiler Feed Water at Sea Level


Use this table to check your results for water between 190 °F and 212 °F (friction loss ignored).








Temp °F
Vapor pressure (psia)
Equiv. ft of water
Static suction head (ft) → available NPSH (ft)




0
1
2
3
4
5
6
7
8
9
10




212
14.7
34
0
1
2
3
4
5
6
7
8
9
10




210
14.1
32.6
1.4
2.4
3.4
4.4
5.4
6.4
7.4
8.4
9.4
10.4
11.4




208
13.6
31.4
2.6
3.6
4.6
5.6
6.6
7.6
8.6
9.6
10.6
11.6
12.6




207
13.3
30.7
3.3
4.3
5.3
6.3
7.3
8.3
9.3
10.3
11.3
12.3
13.3




206
13
30
4
5
6
7
8
9
10
11
12
13
14




205
12.7
29.3
4.7
5.7
6.7
7.7
8.7
9.7
10.7
11.7
12.7
13.7
14.7




204
12.5
28.9
5.1
6.1
7.1
8.1
9.1
10.1
11.1
12.1
13.1
14.1
15.1




200
11.5
26.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5




190
9.3
21.5
12.5
13.5
14.5
15.5
16.5
17.5
18.5
19.5
20.5
21.5
22.5