Design of Centrifugal Pumps with Computer Aid

Abstract

The design of centrifugal pumps involves a large number of interdependent variables, so that there are several possible alternative designs for the same duty. One of the most difficult design problems is to predict the impeller ‘head slip’, i.e. the difference between the theoretical head for an infinite number of impeller vanes H th ∞ as derived by Euler, and the theoretical head deduced from the net horsepower given to the fluid passing through the impeller after deducting external losses H thn. In a previous experimental paper (1)† the author showed that the head slip was related to the mean blade loading and this has been applied in the present paper. He also suggested that the mean blade loading was an important factor in cavitation problems, having an advantage over the Thoma cavitation constant, as it allows for variations in the number of impeller vanes. For preliminary design of the main dimensions the author has plotted a number of dimensionless parameters to a base of specific speed and derived mathematical equations for them which can be fed into a computer for rapid solution. The suggested computer programmes are in Fortran IV language and in four sections: (1) Internal hydraulic losses, related to Reynolds number and relative roughness of the flow passages, external losses, efficiencies, and horsepower. (2) Impeller outlet dimensions, with allowance for head slip based on the mean blade loading. (3) Impeller inlet, eye diameter, inlet angles, etc. (4) Estimation of the direction and magnitude of the absolute velocity leaving the impeller, and determination of the throat areas of volute casing or diffuser vanes. Also estimation of required net positive suction head (n.p.s.h.), again based on the mean blade loading. These programmes have been check tested on the International Computers and Tabulators (I.C.T.) 1905 computer at Loughborough University of Technology and give results which agree with the test results within 1 or 2 per cent over a wide range, except for very small pumps. To illustrate the method worked examples of impeller and diffuser are given in the Appendices, and corresponding results of a number of other alternative designs for the same duty are shown in a table. The main advantage of the computer aid is that it permits a wide range of design constants to be investigated in a very short time, so that the optimum design can be selected more readily. Consequently the skill and experience of the designer are still required, as the optimum design is a compromise between high efficiency with low running power costs, and low production costs to secure an order against severe international competition.