NPSH
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hydraulic Hydraulics () is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counterpart of pneumatics, which concer ...
circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of
cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...
: # The Available NPSH (NPSH''A''): a measure of how close the fluid at a given point is to flashing, and so to cavitation. Technically it is the absolute pressure head minus the vapour pressure of the liquid. # The Required NPSH (NPSH''R''): the head value at the suction side (e.g. the inlet of a pump) required to keep the fluid away from cavitating (provided by the manufacturer). NPSH is particularly relevant inside
centrifugal pump Centrifugal pumps are used to transport fluids by the Energy transformation, conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are ...
s and
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...
s, which are parts of a hydraulic system that are most vulnerable to cavitation. If cavitation occurs, the drag coefficient of the
impeller An impeller, or impellor, is a driven rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid. Strictly speaking, propellers are a sub-clas ...
vanes will increase drastically—possibly stopping flow altogether—and prolonged exposure will damage the impeller.


NPSH in a pump

In a pump, cavitation will first occur at the inlet of the impeller.Frank M. White ''Fluid Mechanics'', 7th Ed., p. 771 Denoting the inlet by ''i'', the NPSH''A'' at this point is defined as: \text_A = \left( \frac + \frac \right) - \frac where p_i is the
absolute pressure Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in unit of measurement, units of force per unit of surface area. Many techniques have been developed for the me ...
at the inlet, V_i is the average velocity at the inlet, \rho is the fluid density, g is the acceleration of gravity and p_v is the
vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
of the fluid. Note that NPSH is equivalent to the sum of both the static and dynamic heads – that is, the stagnation head – minus the equilibrium vapor pressure head, hence "net positive suction head". Applying the
Bernoulli's equation Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...
for the control volume enclosing the suction free surface ''0'' and the pump inlet ''i'', under the assumption that the kinetic energy at ''0'' is negligible, that the fluid is
inviscid Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...
, and that the fluid density is constant: \frac + z_ = \frac + \frac + z_i + h_f Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSH''A'': \text_A = \frac - \frac - ( z_i - z_ ) - h_f This is the standard expression for the available NPSH at a point. Cavitation will occur at the point ''i'' when the available NPSH is less than the NPSH required to prevent cavitation (NPSH''R''). For simple impeller systems, NPSH''R'' can be derived theoretically, but very often it is determined empirically. Note NPSH''A''and NPSH''R'' are in absolute units and usually expressed in "m" or "ft," not "psia". Experimentally, NPSH''R'' is often defined as the NPSH3, the point at which the head output of the pump decreases by 3 % at a given flow due to reduced hydraulic performance. On multi-stage pumps this is limited to a 3 % drop in the first stage head.


NPSH in a turbine

The calculation of NPSH in a
reaction turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical p ...
is different to the calculation of NPSH in a pump, because the point at which cavitation will first occur is in a different place. In a reaction turbine, cavitation will first occur at the outlet of the impeller, at the entrance of the
draft tube A draft tube is a diverging tube fitted at the exit of a turbine's runner and used to utilize the kinetic energy available with water at the exit of the runner. This draft tube at the end of the turbine increases the pressure of the exiting flui ...
. Denoting the entrance of the draft tube by ''e'', the NPSH''A'' is defined in the same way as for pumps: \text_A = \left( \frac + \frac \right) - \frac Applying
Bernoulli's principle Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...
from the draft tube entrance ''e'' to the lower free surface ''0'', under the assumption that the kinetic energy at ''0'' is negligible, that the fluid is inviscid, and that the fluid density is constant: \frac + \frac + z_e = \frac + z_ + h_f Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSH''A'': \text_A = \frac - \frac - ( z_e - z_ ) + h_f Note that, in turbines minor friction losses (h_f) alleviate the effect of cavitation - opposite to what happens in pumps.


NPSH design considerations

Vapour pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
is strongly dependent on temperature, and thus so will both NPSH''R'' and NPSH''A''.
Centrifugal pump Centrifugal pumps are used to transport fluids by the Energy transformation, conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are ...
s are particularly vulnerable especially when pumping heated solution near the vapor pressure, whereas positive displacement pumps are less affected by cavitation, as they are better able to pump two-phase flow (the mixture of gas and liquid), however, the resultant flow rate of the pump will be diminished because of the gas volumetrically displacing a disproportion of liquid. Careful design is required to pump high temperature liquids with a centrifugal pump when the liquid is near its boiling point. The violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as "pumping gravel". Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.


Relationship to other cavitation parameters

The NPSH appears in a number of other cavitation-relevant parameters. The suction head coefficient is a
dimensionless Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that align with another sy ...
measure of NPSH: C_\text = \frac Where n is the angular velocity (in rad/s) of the turbo-machine shaft, and D is the turbo-machine impeller diameter. Thoma's cavitation number is defined as: \sigma = \frac Where H is the head across the turbo-machine.


Some general NPSH examples

(based on sea level). Example Number 1: A tank with a liquid level 2 metres above the pump intake, plus the
atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...
of 10 metres, minus a 2 metre
friction loss Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
into the pump (say for pipe & valve loss), minus the NPSH''R'' curve (say 2.5 metres) of the pre-designed pump (see the manufacturers curve) = an NPSH''A'' (available) of 7.5 metres. (not forgetting the flow duty). This equates to 3 times the NPSH required. This pump will operate well so long as all other parameters are correct. Remember that positive or negative flow duty will change the reading on the pump manufacture NPSH''R'' curve. The lower the flow, the lower the NPSH''R'', and vice versa. Lifting out of a well will also create negative NPSH; however remember that atmospheric pressure at sea level is 10 metres! This helps us, as it gives us a bonus boost or “push” into the pump intake. (Remember that you only have 10 metres of atmospheric pressure as a bonus and nothing more!). Example Number 2: A well or bore with an operating level of 5 metres below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSH''R'' curve (say 2.4 metres) of the pre-designed pump = an NPSH''A'' (available) of (negative) -9.4 metres. Adding the atmospheric pressure of 10 metres gives a positive NPSH''A'' of 0.6 metres. The minimum requirement is 0.6 metres above NPSH''R''), so the pump should lift from the well. Using the situation from example 2 above, but pumping 70 degrees Celsius (158F) water from a hot spring, creating negative NPSH, yields the following: Example Number 3: A well or bore running at 70 degrees Celsius (158F) with an operating level of 5 metres below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSH''R'' curve (say 2.4 metres) of the pre-designed pump, minus a temperature loss of 3 metres/10 feet = an NPSH''A'' (available) of (negative) -12.4 metres. Adding the atmospheric pressure of 10 metres and gives a negative NPSH''A'' of -2.4 metres remaining. Remembering that the minimum requirement is 600 mm above the NPSH''R'' therefore this pump will not be able to pump the 70 degree Celsius liquid and will cavitate and lose performance and cause damage. To work efficiently, the pump must be buried in the ground at a depth of 2.4 metres plus the required 600 mm minimum, totalling a total depth of 3 metres into the pit. (3.5 metres to be completely safe). A minimum of 600 mm (0.06 bar) and a recommended 1.5 metre (0.15
bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar ** Chocolate bar * Protein bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a laye ...
)
head pressure Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually measur ...
“higher” than the NPSH''R'' pressure value required by the manufacturer is required to allow the pump to operate properly. Serious damage may occur if a large pump has been sited incorrectly with an incorrect NPSH''R'' value and this may result in a very expensive pump or installation repair. NPSH problems may be able to be solved by changing the NPSH''R'' or by re-siting the pump. If an NPSH''A'' is say 10 bar then the pump you are using will deliver exactly 10 bar more over the entire operational curve of a pump than its listed operational curve. Example: A pump with a max. pressure head of 8 bar (80 metres) will actually run at 18 bar if the NPSH''A'' is 10 bar. i.e.: 8 bar (pump curve) plus 10 bar NPSH''A'' = 18 bar. This
phenomenon A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...
is what manufacturers use when they design multistage pumps, (Pumps with more than one impeller). Each multi stacked impeller boosts the succeeding impeller to raise the pressure head. Some pumps can have up to 150 stages or more, in order to boost heads up to hundreds of metres.


References

{{reflist Hydraulics Fluid mechanics de:Pumpe#NPSH-Wert