|
In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation: # The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to boiling, and so to cavitation. # The Required NPSH (NPSHR): the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating. NPSH is particularly relevant inside centrifugal pumps and turbines, 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. == 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 NPSHA at this point is defined as: 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 fluid density is constant: Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSHA: 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 (NPSHR). For simple impeller systems, NPSHR can be derived theoretically,〔Paresh Girdhar, Octo Moniz, ''Practical Centrifugal Pumps'', p. 68〕 but very often it is determined empirically.〔Frank M. White ''Fluid Mechanics'', 7th Ed., p . 771〕Note NPSHA and NPSHR are in absolute units and usually expressed in "ft abs" not "psia". Experimentally, NPSHR 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.〔http://www.pumps.org/content_detail.aspx?id=1770〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「NPSH」の詳細全文を読む スポンサード リンク
|