# NPSH - Net Positive Suction Head

## A definition and an introduction to Net Positive Suction Head - NPSH

Low pressure at the suction side of a pump can encounter the fluid to start boiling with

- reduced efficiency
- cavitation
- damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.

### Suction Head

Based on the Energy Equation - the **suction head** in the fluid close to the impeller can be expressed as the sum of the **static** and the **velocity head**:

h_{s}= p_{s}/ γ + v_{s}^{2}/ 2 g(1)

where

h_{s}= suction head close to the impeller

p_{s}= static pressure in the fluid close to the impeller

γ=specific weight of the fluid

v_{s}= velocity of fluid

g= acceleration of gravity

### Liquids Vapor Head

The **liquids vapor head** at the actual temperature can be expressed as:

h_{v}= p_{v}/ γ(2)

where

h_{v}= vapor head

p_{v}= vapor pressure

**Note!** The vapor pressure in fluids depends on temperature. Water, our most common fluid, starts boiling at *20 ^{o}C* if the absolute pressure in the fluid is

*2.3 kN/m*. For an absolute pressure of

^{2}*47.5 kN/m*, the water starts boiling at

^{2}*80*. At an absolute pressure of

^{o}C*101.3 kN/m*(normal atmosphere), the boiling starts at

^{2}*100*.

^{o}C### Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like

NPSH=h_{s}-h_{v}(3)

or, by combining (1) and (2)

NPSH=p_{s}/ γ + v_{s}^{2}/ 2 g - p_{v}/ γ (3b)

### Available NPSH - NPSH_{a }or NPSHA_{ }

The Net Positive Suction Head made available the suction system for the pump is often named NPSH_{a}. The NPSH_{a} can be determined during design and construction, or determined experimentally from the actual physical system.

The available NPSH_{a} can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h_{0}= h_{s}+ h_{l}(4)

where

h_{0}= head at surface

h_{s}=headbefore the impeller

h_{l}=headloss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h_{0}=p_{0}/ γ =p_{atm}/ γ(4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

h_{s}=p_{s}/ γ + v_{s}^{2}/ 2 g + h_{e}(4c)

where

h_{e}= elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

p_{atm}/ γ = p_{s}/ γ + v_{s}^{2}/ 2 g + h_{e}+ h_{l}(4d)

The head available before the impeller can be expressed as:

p_{s}/ γ + v_{s}^{2}/ 2 g = p_{atm}/ γ - h_{e}- h_{l}(4e)

or as the available NPSH_{a}:

NPSH_{a}= p_{atm}/ γ - h_{e}- h_{l}- p_{v}/ γ(4f)

#### Available NPSH_{a} - the Pump is above the Tank

If the pump is positioned above the tank, the elevation *- h _{e} -* is positive and the NPSH

_{a}decreases when the elevation of the pump increases.

At some level the NPSH_{a} will be reduced to zero and the fluid starts to evaporate.

#### Available NPSH_{a} - the Pump is below the Tank

If the pump is positioned below the tank, the elevation *- h _{e} -* is negative and the NPSH

_{a}increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSH_{a} by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.

### Required NPSH - NPSH_{r} or NPSHR_{ }

The NPSH_{r}, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSH_{r} for a particular pump is in general determined experimentally by the **pump manufacturer** and a part of the documentation of the pump.

The available NPSH_{a }of the system should always exceeded the required NPSH_{r} of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSH_{a} should in general be significant higher than the required NPSH_{r} to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.

Note that the required NPSH_{r} increases with the square capacity.

Pumps with double-suction impellers has lower NPSH_{r }than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

### Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.

At the maximum elevation *NPSH _{a}* is zero. The maximum elevation can therefore be expressed by (4f):

NPSH_{a}= p_{atm}/ γ - h_{e}- h_{l}- p_{v}/ γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:

h_{e}= p_{atm}/ γ - p_{v}/ γ(5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature* 20 ^{o}C*

*is*

*2.3 kN/m*. The maximum theoretical elevation height is therefore:

^{2}

h_{e}=(101.33 kN/m^{2}) / (9.80 kN/m^{3}) - (2.3 kN/m^{2}) / (9.80 kN/m^{3})

= 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

### Suction Head as Affected by Temperature

Temperature | Vapor Pressure | Max. elevation | ||

(^{o}C) | (^{o}F) | (kN/m^{2}) | (m) | (ft) |

0 | 32 | 0.6 | 10.3 | 33.8 |

5 | 41 | 0.9 | 10.2 | 33.5 |

10 | 50 | 1.2 | 10.2 | 33.5 |

15 | 59 | 1.7 | 10.2 | 33.5 |

20 | 68 | 2.3 | 10.1 | 33.1 |

25 | 77 | 3.2 | 10.0 | 32.8 |

30 | 86 | 4.3 | 9.9 | 32.5 |

35 | 95 | 5.6 | 9.8 | 32.2 |

40 | 104 | 7.7 | 9.5 | 31.2 |

45 | 113 | 9.6 | 9.4 | 30.8 |

50 | 122 | 12.5 | 9.1 | 29.9 |

55 | 131 | 15.7 | 8.7 | 28.5 |

60 | 140 | 20 | 8.3 | 27.2 |

65 | 149 | 25 | 7.8 | 25.6 |

70 | 158 | 32.1 | 7.1 | 23.3 |

75 | 167 | 38.6 | 6.4 | 21 |

80 | 176 | 47.5 | 5.5 | 18 |

85 | 185 | 57.8 | 4.4 | 14.4 |

90 | 194 | 70 | 3.2 | 10.5 |

95 | 203 | 84.5 | 1.7 | 5.6 |

100 | 212 | 101.33 | 0.0 | 0 |

### Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use with **cold water**. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.

Fluid | Temperature (^{o}C) | Vapor Pressure (kPa abs) |

Ethanol | 20 | 5.9 |

65 | 58.2 | |

Methyl Acetate | 20 | 22.8 |

55 | 93.9 |

Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.

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