# Specific speed

﻿
Specific speed

Pump

Specific Speed is an index that, in mechanical terms, represents the impeller speed of a pump necessary to generate 1 gallon per minute at 1 foot of head.

Ns = (n*sqrt(Q)) / (gH^(3/4))

Ns = specific speed (unitless)n = pump rotational speed in rpmQ = flowrate (GPM)H = total head per stage (feet)g = acceleration due to gravity

The specific speed is not a dimensionless quantity.

Standard impellers can have values from 500 to 10,000, with the highest efficiency at values between 2,000 and 3,000. It is effectively an index used to predict certain pump characteristics.

Turbine

The specific speed of a turbine can also be defined as the speed of an ideal, geometrically similar turbine, which yields one unit of power for one unit of head.

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 and flows.

:$n_s=nsqrt\left\{P\right\}/H^\left\{5/4\right\}$ (dimensioned parameter), $n$ = rpm

$N_s=frac\left\{Omegasqrt\left\{P/ ho\left\{gH^\left\{5/4$ (dimensionless parameter),

:$Omega$ = angular velocity (radians/second)

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.

The specific speed is also 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.

Application

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.

Imperial Units

The "specific speed" is defined as ns = n√(P)/h5/4
* where 'n' is the wheel speed in RPM
* P is the power in HP
* h is the water head in feet

Metric Units

The "specific speed" is ns = 0.2626 n√(P)/h5/4
* where 'n' is the wheel speed in RPM
* P is the power in kW
* h is the water head in meters

Well-designed efficient machines typically use the following values: Impulse turbines have the lowest ns values, typically ranging from 1 to 10, a Pelton wheel is typically around 4, Francis turbines fall in the range of 10 to 100, while Kaplan turbines are at least 100 or more [http://mysite.du.edu/~jcalvert/tech/fluids/turbine.htm#Impu Technical derivation of basic impulse turbine physics, by J.Calvert] ] .

Notes

* Water turbine

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Specific Speed — The net suction specific speed is a dimensionless number defined by centrifugal and axial pumps inherent physical characteristics and operating pointcite web| url = http://www.gouldspumps.com/cpf 0008.html| title = NPSH and Suction Specific Speed …   Wikipedia

• specific speed — An index which provides information on the general profile or shape of the impeller of a pump. The specific speed is the speed in revolutions per minute at which an impeller would run if reduced in size to deliver one gallon per minute against a… …   Dictionary of automotive terms

• Speed index — (sometimes Speed rating) is a system of rating the performance of Quarter Horse racehorses. The American Quarter Horse Association (or the AQHA) has used two systems over the history of Quarter Horse racing to evaluate racing performances. The… …   Wikipedia

• speed —  The average rate at which an object is travelling. Measured as a relationship of distance per time e.g., meters per second (m/s), kilometers per hour (kph or km/h), feet per second (ft/sec.), miles per hour (mph), or revolutions per minute… …   Dictionary of automotive terms

• Specific impulse — (usually abbreviated I sp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount …   Wikipedia

• Specific fuel consumption (thrust) — Specific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly… …   Wikipedia

• Specific thrust — is a term used in Gas Turbine Engineering to show the relative bulk of a jet engine (e.g. turbojet, turbofan, etc) and is defined as the ratio: net thrust/total intake airflow. Low specific thrust enginesA modern civil turbofan has a low specific …   Wikipedia

• Speed limits in the United States — are set by each state or territory. Speed limits in the United States vary according to the type of road and land use. These speed limits do not exceed eighty miles per hour. Increments of five miles per hour are used. Additionally, these limits… …   Wikipedia

• specific finding — A decision on a fact made by a jury in its verdict at the judge s request. Often the judge gives a jury a list of decisions to be made on specific findings of fact to help the jurors focus on the issues. For example, a judge may ask the jury to… …   Law dictionary

• Speed Tribes — Speed Tribes: Days and Nights with Japan s Next Generation is a 1995 book by Karl Taro Greenfeld. It is a collection of non fiction short stories, each focusing on a specific Japanese youth in the early 1990s, a turbulent time in Japan following… …   Wikipedia

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.