These pumps are based on bladed impellors which rotate within the fluid to impart a tangential acceleration to the fluid and a consequent increase in the energy of the fluid. The purpose of the pump is to convert this energy into pressure energy of the fluid to be used in the associated piping system.
The specific speed of a rotodynamic pump is a numerical value used to classify and compare various pump types and for the analysis and graphical representation of design parameters..
Note: Various sources (See links below) provide different values of Q (m 3/h, m 3/s, m 3/min) please refer to all reliable sources before completing any detailed design evaluations..
The majority of rotodynamic pumps have specific speeds of 900 - 16000. The following factors are important related to the specific speed.
International standard ISO 3548 provides a dimensionless form of the specific speed K
defined as below ;
K = 2 . p . n. Q 0.5 / (g.H) 3/4
There are three main classifications of rotodynamic pumps
Rotodynamic pumps include a number of parts in their construction..
In radial flow and Mixed flow pumps may enter the impeller from one side in the single entry pump or from both directions in the double entry pumps. The range of impeller designs include for closed impellers, open impellers and semi-open impellers. e.g. in the closed impeller design the fluid is directed through the impeller through a number of enclose channels fabricated in the impeller body.
The prime purpose of the casing is to energy from the fluid leaving the impeller into useful pressure energy. The design of the casing is of equal importance to that of the impeller. There are two types of casing design
The sketch below related to the centrifugal pump is included to illustrate the difference.
The shafts bearings and seals are provided to support, and allow controlled rotation of the impeller
and to ensure that the fluid is contained within the system.
These are important items and are generally the limiting factors relating to the the pump reliability and convenience of maintenance.
Rotodynamic pumps can be mounted horizontally or vertically. The trend for sealed pumps is to use mechanical seals . Notes on mechanical seals are provided on this website Mechanical Seals
The radial pump is the most common type of pump used in industry.
These pumps have low specific speed (Ns = 900 to 4500) . This means that low shaft velocities
are required to deliver unit flow at unit head. Single stage centrifugal pumps
with one or two inlets genally have volute casing. Multi-Stage pumps generally have diffuser casing
with return guide-vanes..
The radial centrifugal pump is the most commonly used pump . It has the advantage of providing a smooth flow and is capable of handling fluids with solids contents. The Centrifugal pump is a very simple low cost design and is reliable for continuous operations over extended time periods.
A typical radial flow centrifugal pump is shown below;-
Mixed Flow Pumps
These have specific speed ranges Ns = 3500 to 7000.
They are recognised by having screw like
impellers. Mixed flow pumps are generally single inlet.
These pumps have a high specific speed range Ns = 9000 to 16500. These pumps are always
single entry and are generally single stage. The pump cases are concentric with inline inlet and outlet connectons.
The casing is normally fitted with guide vanes.
The relationship of the pump developed head with the pump discharge flow at constant
speed in generally called the pump characteristic.
The complete definition of the pump performance also includes the efficiency and the NPSH and the power requirements over
the flow range.
The following relationships are useful in predicting the performance of centrifugal pumps.
Generally a pump head developed (measured in m of the fluid being transferred) is the
same for any fluid. The head will be the same but the actual developed
pressure measured in bar will be related to the fluid density.
The power therefore will also vary with the density of the fluid. On a normal pump characteristic curve provided
by a pump supplier the curve will be based on water as the fluid. Therefore
to obtain the power required from a characteristic curve based on water it is necessary
to multiply the power curve by the s.g. of the fluid being pumped.