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| '''Specific speed''' ''N''<sub>''s''</sub>, is a [[dimensionless quantity]] used to characterize [[turbomachinery]] speed.<ref name="Shepard">
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| {{cite book|author=Shepard, Dennis G.
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| |title=Principles of Turbomachinery
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| |year=1956
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| |publisher=McMillan
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| |isbn=0 - 471 - 85546 - 4
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| |lccn = 56002849}}
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| </ref> Common commercial and industrial practice uses dimensioned versions which are equally useful. One of the most valued uses of specific speed is its application to pumps to define the suction specific speed -- a quasi [[Dimensionless quantity|non-dimensional number]] used to classify [[pump]] [[impeller]]s as to their type and proportions. In [[Imperial unit]]s it is defined as the speed in [[revolutions per minute]] at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot of [[hydraulic head]]. In metric units flow may be in l/s or m³/s and head in m, and care must be taken to state the units used.
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| Performance is defined as the ratio of the pump or turbine against a reference pump or turbine, which divides the actual performance figure to provide a unitless [[figure of merit]]. The resulting figure would more descriptively be called the "ideal-reference-device-specific performance." This resulting unitless ratio may loosely be expressed as a "speed," only because the performance of the reference ideal pump is linearly dependent on its speed, so that the ratio of [device-performance to reference-device-performance] is ''also'' the increased speed the reference device would need to turn, in order to produce the performance, instead of its reference speed of "1 unit."
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| Specific speed is used in engineering design where it is thought of as an index used to predict desired pump or turbine characteristics; e.g., the general shape of a pump's [[impeller]]. Often it is used to predict the type of pump or turbine required for a design flow rate and head. Once the desired specific speed is known, basic dimensions of the unit's components can be easily calculated.
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| Several mathematical definitions of specific speed (all of them actually ideal-device-specific) have been created for different devices and applications.
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| == Pump specific speed ==
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| Low-specific speed radial flow impellers develop [[hydraulic head]] principally through [[centrifugal force]]. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. An axial flow or propeller pump with a specific speed of 10,000 or greater generates its head exclusively through axial forces. Radial impellers are generally low flow/high head designs whereas axial flow impellers are high flow/low head designs.
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| [[Centrifugal pump]] impellers have specific speed values ranging from 500 to 10,000 (English units), with radial flow pumps at 500-4000, mixed flow at 2000-8000 and [[axial flow pump]]s at 7000-20,000. Values of specific speed less than 500 are associated with [[positive displacement pump]]s.
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| As the specific speed increases, the ratio of the impeller outlet diameter to the inlet or eye diameter decreases. This ratio becomes 1.0 for a true axial flow impeller.
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| <math>N_s = \frac { n \sqrt Q } { (gH)^{ 3/4 } } </math>
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| where:
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| :<math>N_s</math> is specific speed (unitless)
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| :<math>n</math> is pump rotational speed (radians per second)
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| :<math>Q</math> is flowrate (m³/s) at the point of best efficiency
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| :<math>H</math> is total head (m) per stage at the point of best efficiency
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| :<math>g</math> is acceleration due to gravity (m/s²)
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| Note that the units used affect the specific speed value and consistent units should be used for comparisons. Pump specific speed can be calculated using British gallons or using Metric units (m<sup>3</sup>/s or L/s and metres head), changing the values listed above.
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| == Net suction specific speed ==
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| The net suction specific speed is mainly used to see if there will be problems with cavitation during the pump's operation on the suction side.<ref name=McNallyInstitute>{{cite web| url = http://www.mcnallyinstitute.com/09-html/09-12.html| title = Specific speed | accessdate = 2007-07-13| author = | publisher = McNally Institute}}</ref> It is defined by centrifugal and axial pumps' inherent physical characteristics and operating point.<ref name=GouldsPumps>{{cite web| url = http://www.gouldspumps.com/cpf_0008.html| title = NPSH and Suction Specific Speed - Goulds Pumps - ITT Corporation | accessdate = 2007-07-13| author = | publisher = ITT Corporation}}</ref> The net suction specific speed of a pump will define the range of operation in which a pump will experience stable operation
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| .<ref name=PumpingMachinery>{{cite web| url = http://www.pumpingmachinery.com/pump_magazine/pump_articles/article_03/article_03.htm| title = Article #3: Suction Specific Speed (NSS) | accessdate = 2007-07-13| author = | publisher = Pumping Machinery}}</ref> The higher the net suction specific speed, then the smaller the range of stable operation, up to the point of cavitation at 8500 (unitless). The envelope of stable operation is defined in terms of the best efficiency point of the pump.
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| The net suction specific speed is defined as:<ref name=EngineeringToolbox>{{cite web| url = http://www.engineeringtoolbox.com/suction-speed-pumps-d_638.html| title = Specific Suction Speed for Pumps | accessdate = 2007-07-13| author = | publisher = Engineering Toolbox}}</ref>
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| <math>
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| N_{ss} = \frac{N\sqrt{Q}} {{NPSH}_R^{0.75}}
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| </math>
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| where:
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| :<math>N_{ss} = </math>net suction specific speed
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| :<math>N = </math>rotational speed of pump in rpm
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| :<math>Q = </math>flow of pump in US gallons per minute
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| :<math>{NPSH}_R = </math> [[Net positive suction head]] (NPSH) required in feet at pump's best efficiency point
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| == Turbine specific speed ==
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| The specific speed value (radian/second) for a turbine is the speed of a geometrically similar turbine which would produce one unit of the specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. This allows accurate calculations to be made of the turbine's performance for a range of heads.
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| Well-designed efficient machines typically use the following values: Impulse turbines have the lowest ''n''<sub>''s''</sub> values, typically ranging from 1 to 10, a [[Pelton wheel]] is typically around 4, [[Francis turbine]]s fall in the range of 10 to 100, while [[Kaplan turbine]]s are at least 100 or more, all in imperial units.<ref name=jcalvert>{{cite web|url=http://mysite.du.edu/~jcalvert/tech/fluids/turbine.htm#Impu |title=Technical derivation of basic impulse turbine physics, by J.Calvert |publisher=Mysite.du.edu |date= |accessdate=2012-07-08}}</ref>
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| <!-- Commented out because image was deleted: [[File:Water_Turbine_Specific_Speed_Comparison.png|thumb|left|250px|Image adapted from European Community's 'Layman's Guidebook (on how to develop a small hydro site)']] -->
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| <!-- Water_Turbine_Specific_Speed_Comparison.png: see image description page at http://en.wikipedia.org/wiki/Image:Water_Turbine_Specific_Speed_Comparison.png for rationale -->
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| <!-- Flashpöhler Timo, desing kaplan turbine runner -->
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| <math> n_s=n\sqrt{P}/H^{5/4} </math> (dimensioned parameter), <math> n</math> = rpm <!-- !!formula missing g ?? --><ref>{{cite book|last=Sayers|first=A. T.|title=Hydraulic and Compressible Flow Turbomachines|year=1990|publisher=Mcgraw Hill Book Co Ltd|isbn=978-0-07-707219-3}}</ref>
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| where:
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| :<math>\Omega</math> = angular velocity (radians per second)
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| :<math>H_n</math> = Net head after turbine and waterway loss (m)
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| :<math>Q</math> = water flow (m³/s)
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| * <math>N</math> = Wheel speed (rpm)
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| * <math>P</math> = Power (kW)
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| * <math>H</math> = Water head (m)
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| ===English units===
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| Expressed in [[English unit]]s, the "specific speed" is defined as ''n''<sub>''s''</sub> = ''n''√(''P'')/''h''<sup>5/4</sup>
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| * where ''n'' is the wheel speed in [[rpm]]
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| * ''P'' is the power in [[horsepower]]
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| * ''h'' is the water head in feet
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| ===Metric units===
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| Expressed in [[metric unit]]s, the "specific speed" is ''n''<sub>''s''</sub> = 0.2626 ''n''√(''P'')/''h''<sup>5/4</sup>
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| * where ''n'' is the wheel speed in [[rpm]]
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| * ''P'' is the power in [[kilowatt]]s
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| * ''h'' is the water head in meters
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| The factor 0.2626 is only required when the specific speed is to be adjusted to English units. In countries which use the metric system, the factor is omitted, and quoted specific speeds are correspondingly larger.{{Citation needed|date=August 2010}} <!-- are the formula's reversed? The metric section HAS the 0.2626 factor thought the text seems to indicate it should be in the English units section... ? -->
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| ===Example===
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| Given a flow and head for a specific hydro site, and the RPM requirement of the generator, calculate the specific speed. The result is the main criteria for turbine selection or the starting point for analytical design of a new turbine. Once the desired specific speed is known, basic dimensions of the turbine parts can be easily calculated.
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| <!-- This was called "Kaplan turbine calculations" without explaining what that meant -->
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| Turbine calculations:
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| :<math> N_s=\frac{2.294}{H_n^{0.486}} </math>
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| :<math> D_e=84.5(0.79+1.602 N_s) \frac{\sqrt{H_n}}{60 * \Omega} </math>
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| :<math> D_e</math> = Runner diameter (m)
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| <!-- What "following formula" ?
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| === Applications ===
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| For a given turbine application, if one knows the water head, desired wheel speed and output power, then the following formula can indicate the appropriate type of turbine.
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| -->
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| ==See also==
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| * [[Pump]]
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| * [[NPSH|Net positive suction head]]
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| * [[Water turbine]]
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| == References ==
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| <References/>
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| {{DEFAULTSORT:Specific Speed}}
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| [[Category:Hydraulics]]
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| [[Category:Fluid dynamics]]
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| [[Category:Pumps]]
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The name of the author is Figures but it's not the most masucline title out there. What I love performing is to gather badges but I've been taking on new issues recently. For years he's been operating as a meter reader and it's some thing he really appreciate. South Dakota is where me and my husband reside and my family enjoys it.
Look into my blog :: buzzbit.net