Introduction

When designing a pump a number of design variables need to be determined:

• impeller rotational speed

• impeller inlet or suction dimensions

• impeller outlet diameter

• impeller blade number

• impeller blade passage geometry, including inlet and outlet blade angles

• impeller position relative to the casing

• collector leading dimensions (volute throat area or diffuser geometry)

• pump construction and materials.

There are a number of approaches to design, chief among which are: small changes from existing designs to give a slight change in head or flow range; design using empirical information, tabular and graphical; and computer based approaches which are in some instances based on empirical data and more recently use finite element or finite difference approaches. The use of these techniques will be discussed later. The sections which now follow survey some of the empirical information available. Typical pump cross-sections of single-stage end suction, and double suction designs and of a multi-stage machine are shown in Figures 3.1, 3.2 and 3.3.

Choice of rotational speed

As will be clear from a reading of the later chapters on design, the choice of rotational speed is interlocked with other parameters, but there are empirical speed limits as given, for example, by the American Hydraulic Institute Standards (1983) reproduced in many handbooks. Clearly the rotational speed is limited to a range of synchronous speeds when using electric motor on a 50 or 60 HZ supply frequency. For large pumps, turbine or diesel drive is used, and the eventual rotational speed is a compromise between hydraulic design and driver considerations.