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Rotodynamic Pumps

Rotodynamic Pumps

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.  

Specific Speed

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..

specific speed = ns = sqrt( n 2. q / h 3/2 )

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..

  • N = Rotational Speed... Revolutions/Minute
  • Q = Flow Rate ...m 3/Hour
  • H = Total Developed Head ...m

The majority of rotodynamic pumps have specific speeds of 900 - 16000.   The following factors are important related to the specific speed.

  1. The specific speed is normally evaluated for the design, duty or best efficiency point
  2. The specific speed is not a dimensionless unit and different values will result it different units are used
  3. For multi-stage units the specific speed is normally evaluated for one stage only
  4. The specific speed is effectively the speed required to deliver unit flow against unit head.   It is only used for general comparison.  It has no practical use for pump design.

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

  • Q = Flow ..m 3/s
  • n = rotational speed ..Revolutions/s
  • Total head developed For all stages...m

There are three main classifications of rotodynamic pumps

  1. Radial Flow (Centrifugal Pumps
  2. Mixed Flow (Screw )Pumps
  3. Axial Flow (Propeller) 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

  1. Volute
  2. Guide Vane ( Diffuser)

The sketch below related to the centrifugal pump is included to illustrate the difference.

volute diffuser arrangement

Shafts , Bearings and Seals.

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

Radial Flow Pumps

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;-

Centrifugal Pump

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.

Axial-flow Pumps

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.

Pump Characteristics

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.

    Impeller Speed = s

  1. Flow rate is directly proportional to the rotation speed s
  2. Differential head is directly proportional to the s 2
  3. The power required is directly proportional to (Flow x Head) i.e s 3

    Impeller Diameter = d

  1. Flow rate is directy proportional to the rotation speed d
  2. Differential head is directly proportional to the d 2
  3. The power required is directly proportional to (Flow x Head) i.e d 3

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.

Useful Links
  1. Fluid Design .. A Site with information and formulas related to Pump calculation :
  2. The Engineering Page.. A site including various Engineering Calculators - Very good pump calculator
  3. ER_Online.. Useful software downloads - Very good pump calculator
  4. McNally Institute .. An excellent reference site for all matters concerning pumps
  5. Engineers Toolbox.. Provides useful information on this subject
  6. Verder Pumps.. Supplier of a wide range of industrial pumps

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