Specific speed
   HOME

TheInfoList



OR:

Specific speed ''N''''s'', is used to characterize
turbomachinery Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from ...
speed. Common commercial and industrial practices use dimensioned versions which are of equal utility. Specific speed is most commonly used in pump applications to define the suction specific speed

a quasi Dimensionless quantity, non-dimensional number that categorizes
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
impeller An impeller or impellor is a 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. In pumps An impeller is a rotating component ...
s as to their type and proportions. In
Imperial unit The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thro ...
s it is defined as the speed in
revolutions per minute Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
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 Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
. 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. 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 A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. Examples *Clock rate of a CPU * Calories per serving *Contrast ratio of an LCD *Frequency response of a speaker * ...
. 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 evice-performance to reference-device-performanceis ''also'' the increased speed at which the reference device would need to operate, in order to produce the performance, instead of its reference speed of "1 unit." Specific speed is an index used to predict desired pump or turbine performance. i.e. it predicts the general shape of a pump's
impeller An impeller or impellor is a 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. In pumps An impeller is a rotating component ...
. It is this impeller's "shape" that predicts its flow and head characteristics so that the designer can then select a pump or turbine most appropriate for a particular application. Once the desired specific speed is known, basic dimensions of the unit's components can be easily calculated. Several mathematical definitions of specific speed (all of them actually ideal-device-specific) have been created for different devices and applications.


Pump specific speed

Low-specific speed radial flow impellers develop
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
principally through
centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
. 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. In theory, the discharge of a "purely" centrifugal machine (pump, turbine, fan, etc.)is tangential to the rotation of the impeller whereas a "purely" axial-flow machine's discharge will be parallel to the axis of rotation. There are also machines that exhibit a combination of both properties and are specifically referred to as "mixed-flow" machines.
Centrifugal pump Centrifugal pumps are used to transport fluids by the 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 a sub-class of dynamic ...
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 An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted ...
s at 7000-20,000. Values of specific speed less than 500 are associated with
positive displacement pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
s. 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. The following equation gives a dimensionless specific speed: N_s = \frac where: :N_s is specific speed (dimensionless) :n is pump rotational speed (rpm) :Q is flowrate (l/s) at the point of best efficiency :H is total head (m) per stage at the point of best efficiency Note that the units used affect the specific speed value in the above equation and consistent units should be used for comparisons. Pump specific speed can be calculated using British gallons or using Metric units (m3/s or L/s and metres head), changing the values listed above.


Net suction specific speed

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. It is defined by centrifugal and axial pumps' inherent physical characteristics and operating point. The net suction specific speed of a pump will define the range of operation in which a pump will experience stable operation . 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. The net suction specific speed is defined as: N_ = \frac where: :N_ = net suction specific speed :n = rotational speed of pump in rpm :Q = flow of pump in US gallons per minute :_R =
Net positive suction head In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation: # The Available NPSH (NPSH''A''): a measure of how close the fluid at a given point is to flashing, and so to cavitation. ...
(NPSH) required in feet at pump's best efficiency point


Turbine specific speed

The specific speed value for a turbine is the speed of a geometrically similar turbine which would produce unit power (one kilowatt) under unit head (one meter). 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. Well-designed efficient machines typically use the following values: Impulse turbines have the lowest ''n''''s'' values, typically ranging from 1 to 10, a
Pelton wheel The Pelton wheel or Pelton Turbine is an impulse-type water turbine invented by American inventor Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the impulse of moving water, as opposed to water's dead weight like the trad ...
is typically around 4,
Francis turbine The Francis turbine is a type of water turbine. It is an inward-flow reaction turbine that combines radial and axial flow concepts. Francis turbines are the most common water turbine in use today, and can achieve over 95% efficiency. The proces ...
s fall in the range of 10 to 100, while
Kaplan turbine The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by Austrian professor Viktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to ach ...
s are at least 100 or more, all in imperial units.


Deriving the Turbine Specific Speed

To derive the Turbine specific speed equation we first start with the Power formula for water then using proportionalities with η,ρ, and g being constant they can be removed. The power of the turbine is therefore only dependent on the head H and flow Q. : P=\eta \rho gQH :so P \propto QH let: :D = Diameter of the turbine runner :B = Width of the turbine runner :N = Speed of the turbine (rpm) :u = Tangential velocity of the turbine blade (m/s) :N_s = Specific Speed of the Turbine :V = Velocity of water at turbine (m/s) Now utilising the constant speed ratio at the turbine tip, the following proportionality can be made that the tangential velocity of the turbine blade is proportional to the square root of the head. : V = \sqrt :Speed ratio = \frac = \frac :so u \propto \sqrt But from rotational speed in RPM to linear speed in m/s the following equation and proportionality can be made. : u = \frac :so D \propto \frac The flow through a turbine is the product of flow velocity and area so the flow through a turbine can be quantified. : Q = \pi DBV_ :with B \propto D and as shown previously: : V_ \propto V \propto \sqrt \propto \sqrt So using the above 2 , the following is obtained :Q \propto D^2\sqrt By combining the equation for diameter and tangential speed, with tangential speed and head a relationship between flow and head can be reached. :Q \propto \left ( \frac \right )^2 \sqrt \therefore Q \propto \frac Substituting this back into the power equation gives: : P \propto \frac H \therefore P \propto \frac To convert this proportionality into an equation a factor of proportionality, say K, must be introduced which gives: : P = K \frac Now assuming our original proposition of producing 1 kilowatt at 1m head our speed N becomes our specific speed N_s. So substituting these values into our equation gives: : 1 = K \frac \therefore K = ^2 Now we know K we have a complete formula for specific speed,N_s: : P = ^2 \frac So rearranging for Specific Speed give the final following result: :N_s=\frac where: * N = Wheel speed (rpm) * P = Power (kW) * H = Water head (m)


English units

Expressed in
English unit English units are the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at d ...
s, the "specific speed" is defined as ''n''''s'' = ''n'' /''h''5/4 * where ''n'' is the wheel speed in
rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
* ''P'' is the power in
horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the ...
* ''h'' is the water head in feet


Metric units

Expressed in
metric unit The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Internati ...
s, the "specific speed" is ''n''''s'' = 0.2626 ''n'' /''h''5/4 * where ''n'' is the wheel speed in
rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
* ''P'' is the power in
kilowatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
s * ''h'' is the water head in meters 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.


Example

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. Turbine calculations: : N_s=\frac : D_e=84.5(0.79+1.602 N_s) \frac : D_e = Runner diameter (m)


See also

*
Pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
*
Net positive suction head In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation: # The Available NPSH (NPSH''A''): a measure of how close the fluid at a given point is to flashing, and so to cavitation. ...
*
Water turbine A water turbine is a rotary machine that converts kinetic energy and potential energy of water into mechanical work. Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now, t ...


References

{{DEFAULTSORT:Specific Speed Hydraulics Fluid dynamics Pumps