What is npsh?

Low pressure at the suction side of a pump may cause the fluid to lớn start boiling with

reduced efficiency cavitation damage

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

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To characterize the potential for boiling & cavitation the difference between

the total head on the suction side of the pump - cđại bại to lớn the impeller, và the liquid vapor pressure at the actual temperature

can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid cthua kém to lớn the impeller*) can be expressed as the sum of the static and velođô thị head:

hs = ps / γliquid + vs2 / 2 g (1)

where

hs = suction head cđại bại to the impeller (m, in)

ps = static pressure in the fluid cthất bại to lớn the impeller (Pa (N/m2), pyêu thích (lb/in2))

γliquid = specific weight of the liquid (N/m3, lb/ft3)

vs = velođô thị of fluid (m/s, in/s)

g = acceleration of gravity (9.81 m/s2,386.1 in/s2)

*) We can not measure the suction head "cđại bại to the impeller". In practice we can measure the head at the pump suction flange. Be aware that - depending of the thiết kế of the pump - the contribution khổng lồ the NPSH value from the suction flange to the impeller can be substantial.

Liquids Vapor Head

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

hv = pv / γvapor (2)

where

hv = vapor head (m, in)

pv = vapor pressure (m, in)

γvapor = specific weight of the vapor (N/m3, lb/ft3)

Note! The vapor pressure in a fluid depends on the temperature. Water, our most common fluid, starts boiling at trăng tròn oC if the absolute pressure is 2.3 kN/mét vuông. For an absolute pressure of 47.5 kN/m2 the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere) the boiling starts at 100 oC.

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be defined as

the difference between the Suction Head, & the Liquids Vapor Head

& can be expressed as

NPSH = hs - hv (3)

or, by combining (1) và (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ (3b)

where

NPSH = Net Positive Suction Head (m, in)

Available NPSH - NPSHa or NPSHA

The Net Positive sầu Suction Head available from the application to lớn the suction side of a pump is often named NPSHa. The NPSHa can be estimated during the design và the construction of the system, or determined experimentally by testing the actual physical system.

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The available NPSHa can be estimated with the Energy Equation.

For a comtháng application - where the pump lifts a fluid from an open tank at one level to lớn an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller và can be expressed as:

h0 = hs + hl (4)

where

h0 = head at surface (m, in)

hs =headbefore the impeller (m, in)

hl =headloss from the surface khổng lồ impeller - major and minor loss in the suction pipe (m, in)

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

h0 = p0 / γ = patm / 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:

hs = ps / γ + vs2 / 2 g + he (4c)

where

he = elevation from surface khổng lồ pump - positive sầu if pump is above the tank, negative sầu if the pump is below the tank (m, in)

Transforming (4) with (4b) và (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ (4f)

where

NPSHa = Available Net Positive Suction Head (m, in)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above sầu the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases (lifting the pump).

At some cấp độ the NPSHa will be reduced khổng lồ zero và the fluid will start lớn evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative & the NPSHa increases when the elevation of the pump decreases (lowering the pump).

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It"s always possible lớn increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don"t increase it more). Note! It is important - and comtháng - to lower a pump when pumping a fluid cthất bại khổng lồ evaporation temperature.

Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prsự kiện cavitation for safe & reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.

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

chú ý that required NPSHr increases with the square of capathành phố.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible khổng lồ an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When elevating a pump located above a tank (lifting the pump) - the fluid starts lớn evaporate at the suction side of the pump at what is the maximum elevation for the actual temperature of the pumping fluid.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by modifying (4f) to:

NPSHa = patm / γ - he - hl - pv / γ

= 0

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

he = patm / γ - pv / 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 oC is 2.3 kN/mét vuông. The maximum theoretical elevation of a pump when pumping water at trăng tròn oC is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

= 10.1 m

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

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Maximum theoretical elevation of a pump above sầu an open tank at different water temperatures are indicated below.

Suction Head & Reduction in Suction Lift for Water & Temperature

Suction head for water - or max. elevation of a pump above a water surface - is affected by the temperature of the water - as indicated below:


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