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REPAIR AND REFURBISHMENT OF A CENTRIFUGAL PUMP

ABST

CHAPTER ONE

INTRODUCTION

According to Reti, the Brazilian soldier and historian of science, the first machine that could be characterized as a centrifuge pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini. True centrifugal pumps were not developed until the later 17th century, when Dennis Papin made one with straight vanes. The curved vane was introduced by the British inventor called John Appold in the year 1851.

A centrifugal pump uses a spinning impeller which normally has backward swept blades that directly push water outward. Like most pumps, a centrifugal pump converts mechanical energy from a motor to energy  to energy of a moving fluid, some of the energy goes into kinetic energy of fluid motion, and some into potential energy, represented by a fluid pressure or by lifting the fluid against gravity to a higher level.

The transfer of energy from the mechanical rotation of the impeller to the motion and pressure of the fluids is usually described in terms of centrifugal force (a force which makes things move away from the centre of something) especially in older writtens before the modern concept of centrifugal force as a fictions force in a rotating reference frame was well articulated. The concept of centrifugal force is not actually required to describe the actions of the centrifugal pump. In the modern centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Invariably, some of the energy also pushes the fluid into a circular motion, and this circular motion can also convey some energy and increase the pressure at the outlet. The relationship between these mechanisms was described with the typical mixed conception of centrifugal force known as that time, in an 1859 article on centrifugal pumps thus:

To arrive by a simpler method than that just given at a general idea of the mode of action of the exterior whirl pool in improving the efficiency of the centrifugal pump, it is only necessary to consider that the mass of water revolving in the whirl pool chamber round the circumference of the wheel, must necessary exert a centrifugal force, and that this centrifugal force may readily be supposed to add itself to the outward force generated within the wheel, or, in other words, to go to increase the pumping power of the wheel. The outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force if the vanes of the wheel be straight and radial; but if they be curved, as is more commonly the case, the outward force is partly produced through the medium of centrifugal force and partly applied by the vanes to the water as a radial component of the oblique pressure which in consequence of their wheel for impelling the water outwards becomes purely centrifugal force, and the more nearly will the pump become what the name ordinarily given to it would seem to indicate – a purely centrifugal pump. When, however, a centrifugal pump with vanes curved backwards in such forms as are ordinarily used in well constructed examples of the machine, is driven at a speed considerably above that requisite merely to overcome the pressure of water, and the cause lifting or propulsion to commence, the radial component of the force applied to the water by the vanes will become considerably, and the water leaving the circumference of the wheel will have a velocity less than that of the circumference of the wheel in a degree having some real importance in practice. The statement the mass of water … must necessarily exert a centrifugal force” is interpretable in terms of the reactive centrifugal force – the force is not an outward force on the water, but rather an outward force exerting by the water, on the pump housing (the volute) and on the water in the outlet pipe. The outlet pressure is a reflection of the pressure that applies the centrifugal force that curves the path of the water to move circularly inside the pump (in the space just outside the impeller, the exterior whirl pool as this author calls it).

On the other hand, the statement that the “outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force is best understood in terms of centrifugal as a fictional force in the frame of reference of the rotating impeller the actual forces on the water are inward, or centrifugal, since that is the direction of force needed to make the water move in circles. This force is supplied by a pressure gradient that is set up by the rotation, where the pressure at the outside, at the wall of the volute, can be taken as a reactive centrifugal force. This is typical of 19th and early 20th century writing, to mix these conceptions of centrifugal force in informal descriptions of effects such as that in the centrifugal pump.

Differing conceptions and explanations of how a centrifugal pump works have long engendered controversy and anima diversion. For example, the American Expert commission sent to the Vienna Exposition in 1873 issued a report that included observations that “they are misnamed centrifugal, because they do not operate by centrifugal force at all; they operate by pressure the same as a turbine water wheel; when people understand their method of operating we may expect much improvement”. John Richards, the editor of the San Francesco – based journal industry, in his in-depth essay on centrifugal pump, which also down played the significance of centrifugal force in the working of the pump remarked. This extra-ordinary reports stands printed in a Government publication, signed by men who were or are, eminent in mechanics, and we can only deplore the stupidity, as well as presumption of the commission who thus disposed of a subject that had twenty years before been carefully investigated by such men as sir John Rennie, professor Cowper, Mr Whitelaw, Dr. James Black, professor Rankine, and many others. The most astonishing part is, however, that this report was passed and signed by men who can hardly suppose would fail to perceive its absurdity. Modern sources say things like that the fluid “flows radially under centrifugal force”, or “centrifugal force flinge the liquid outward”. Others counter that :there is no force at all, and a great deal of confused thinking”.

Some are more careful, attributing the outward force to the impeller, not to centrifugal force, “The impeller case. This centrifugal action is what creates the pressure”. Even serious texts that explain the working of the pump as one in which the mechanical energy is converted into pressure energy by means of centrifugal force acting on the fluid.

STATEMENT OF THE PROBLEM

Repair and refurbishment of a centrifugal pump.

AIMS/OBJECTIVES OF THE STUDY

It was aimed that upon completion of this course, the particular pump will not only be put back in good working condition for future practical purposes, a training certificate will also be received indicating the ability to:

-              Identify different types of centrifugal pumps and their parts

-              Understand pump case repair methods,

-              Develop a pump maintenance program,

-              Troubleshoot, install and lubricate bearings,

-              Understand  shaft alignment techniques,

-              Identify different pump drivers and their uses,

-              Trouble shoot wetted end pumps,

-              Understand vibration measurements,

-              Assemble and disassemble an ANSI Pump etc.

SCOPE AND LIMITATION

The scope of the study includes

-              The electric motor

-              The wire connection

-              Painting

However, due to financial difficulties experienced as a result of having spent much money on part of the pump replaced, research and laboratory practical undergone, coupled with limited time had due to other departmental works that needs to be done, the two bearings from the electric motor was merely greased with oil instead of replacing it with  new ones.  

METHOD OF RESEARCH

Most of the information was gotten from the internet from different websites, while others was gotten from the school and National Library, workshop/laboratory and consultations or interviews made.

SIGNIFICANCE OF THE STUDYThe significance of the study is to restore and renovate a centrifugal pump to its normal working condition by adopting a corrective maintenance program.