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| {{about|Net positive suction head|the fire hose coupling thread|Glossary of firefighting equipment#N}}
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| {{Incomplete|date=May 2009}}
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| In a [[hydraulic]] circuit, '''net positive suction head''' ('''NPSH''') may refer to one of two quantities in the analysis of [[cavitation]]:
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| # The Available NPSH (NPSH<sub>A</sub>): a measure of how close the fluid at a given point is to [[boiling]], and so to [[cavitation]].
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| # The Required NPSH (NPSH<sub>R</sub>): the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.
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| NPSH is particularly relevant inside [[centrifugal pump]]s and [[turbine]]s, which are parts of a hydraulic system that are most vulnerable to cavitation. If cavitation occurs, the [[drag coefficient]] of the [[impeller]] vanes will increase drastically - possibly stopping flow altogether - and prolonged exposure will damage the impeller.
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| == NPSH in a Pump ==
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| [[Image:NPSH.PNG|thumb|A [[hydraulic circuit]]]]
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| In a pump, cavitation will first occur at the inlet of the impeller.<ref>Frank M. White ''Fluid Mechanics'', 7th Ed., p. 771</ref> Denoting the inlet by ''i'', the NPSH<sub>A</sub> at this point is defined as:
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| <math> NPSH_A = \left( \frac{p_i}{\rho g} + \frac{V_i^2}{2 g} \right) - \frac{p_{v}}{\rho g}</math>
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| Applying [[Bernoulli's principle]] from the suction free surface ''0'' to the pump inlet ''i'', under the assumption that the kinetic energy at ''0'' is negligible, that the fluid is inviscid, and that the fluidy density is constant:
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| <math> \frac{p_{0}}{\rho g} + z_{0} = \frac{p_i}{\rho g} + \frac{V_i^2}{2 g} + z_i + h_f</math>
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| Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSH<sub>A</sub>:
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| <math> NPSH_A = \frac{p_{0}}{\rho g} - \frac{p_{v}}{\rho g} - ( z_i - z_{0} ) - h_f</math>
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| This is the standard expression for the Available NPSH at point. Cavitation will occur at the point ''i'' when the Available NPSH is less than the NPSH required to prevent cavitation (NPSH<sub>R</sub>). For simple impeller systems, NPSH<sub>R</sub> can be derived theoretically,<ref>Paresh Girdhar, Octo Moniz, ''Practical Centrifugal Pumps'', p. 68</ref> but very often it is determined empirically.<ref>Frank M. White ''Fluid Mechanics'', 7th Ed., p . 771</ref>Note NPSH<sub>A </sub>and NPSH<sub>R </sub> are in absolute units and usually expressed in "ft abs" not "psia".
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| == NPSH in a Turbine==
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| The calculation of NPSH in a [[reaction turbine]] 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]].<ref>''Cavitation in reaction turbines'', http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/machine/ui/Course_home-lec31.htm</ref> Denoting the entrance of the draft tube by ''e'', the NPSH<sub>A</sub> is defined in the same way as for pumps:
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| <math> NPSH_A = \left( \frac{p_e}{\rho g} + \frac{V_e^2}{2 g} \right) - \frac{p_{v}}{\rho g}</math><ref>Frank M. White ''Fluid Mechanics'', 7th Ed., p. 771</ref>
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| Applying [[Bernoulli's principle]] 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:
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| <math> \frac{p_e}{\rho g} + \frac{V_e^2}{2 g} + z_e = \frac{p_{0}}{\rho g} + z_{0} + h_f</math>
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| Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSH<sub>A</sub>:
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| <math> NPSH_A = \frac{p_{0}}{\rho g} - \frac{p_{v}}{\rho g} - ( z_e - z_{0} ) + h_f</math>
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| Note that, in turbines minor losses (<math>h_f</math>) alleviate the effect of cavitation - opposite to what happens in pumps.
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| == NPSH design considerations ==
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| [[Vapour pressure]] is strongly dependent on temperature, and thus so will both NPSH<sub>R</sub> and NPSH<sub>A</sub>. [[Centrifugal pump]]s are particularly vulnerable especially when pumping heated solution near the vapor pressure, whereas [[Pump#Positive displacement pump|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.
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| 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.
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| == Relationship to other cavitation parameters ==
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| The NPSH appears in a number of other cavitation-relevant parameters. The suction head coefficient is a [[dimensionless]] measure of NPSH:
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| <math> C_{NPSH} = \frac{g NPSH}{n^2 D^2} </math>
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| Where <math>n</math> is the angular velocity (in ''rad/s'') of the turbomachine shaft, and <math>D</math> is the turbomachine impeller diameter. The [[Thoma cavitation number]] is defined as:
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| <math> \sigma = \frac{NPSH}{H} </math>
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| Where <math>H</math> is the head across the turbomachine.
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| == Some general NPSH Examples ==
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| (based on sea level).
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| '''Example 1:''' A tank with a liquid level 2 metres above the pump intake, plus the [[atmospheric pressure]] of 10 metres, minus a 2 metre [[friction loss]] into the pump (say for pipe & valve loss), minus the NPSHR curve (say 2.5 metres) of the pre-designed pump (see the manufacturers curve) = an NPSHA (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.
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| Remember that (+ or -) flow duty will change the reading on the pump manufacture NPSHR curve. The lower the flow, the lower the NPSHR, and vice versa.
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| 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!).
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| '''Example 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 NPSHR curve (say 2.4 metres) of the pre-designed pump = an NPSHA (available) of (negative) -9.4 metres. NOW we add the atmospheric pressure of 10 metres. We have a positive NPSHA of 0.6 metres. '''(minimum requirement is 0.6 metres above NPSHR),''' so the pump should lift from the well.
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| Now we will try the situation from example 2 above, but will pump 70 degrees Celsius (158F) water from a hot spring, creating negative NPSH.
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| '''Example 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 NPSHR curve (say 2.4 metres) of the pre-designed pump, minus a temperature loss of 3 metres/10 feet = an NPSHA (available) of (negative) -12.4 metres. NOW we add the atmospheric pressure of 10 metres and we have a negative NPSHA of -2.4 metres remaining.
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| Remembering that the minimum requirement is 600 mm above the NPSHR 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).
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| '''A minimum of 600 mm (0.06 bar) and a recommended''' 1.5 metre (0.15 [[bar (unit)|bar]]) [[head pressure]] “higher” than the NPSHR pressure value required by the manufacturer is required to allow the pump to operate properly.
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| Serious damage may occur if a large pump has been sited incorrectly with an incorrect NPSHR value and this may result in a very expensive pump or installation repair.
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| NPSH problems may be able to be solved by changing the NPSHR or by re-siting the pump.
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| If an NPSHA 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.
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| Example: A pump with a max. pressure head of 8 bar (80 metres) will actually run at 18 bar if the NPSHA is 10 bar.
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| i.e.: 8 bar (pump curve) plus 10 bar NPSHA = 18 bar.
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| This [[phenomenon]] is what manufacturers use when they design [[multistage]] pumps, (Pumps with more than one impeller). Each multi stacked impeller boosts the previous 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.
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| == References ==
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| {{reflist}}
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| [[Category:Hydraulics]]
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| [[Category:Fluid mechanics]]
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| [[de:Pumpe#NPSH-Wert]]
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