2 Pump selection fundamentals

Pump selection fundamentals

This chapter is primarily concerned with the basic principles of pump selection. Experienced pump system designers and users are well aware of the importance of choosing the right pump and the problems that follows not doing it.

The content of this chapter intend to give a brief overview of how to overcome the obstacles connected with pump selection. In addition to explain the basic pump selection fundamentas there’s information on common pitfalls and how to avoid them.

Good pump selection involve many factors to consider, including “open books” type of comunication with the pump supplier. A simple duty point enquiry will most often result in a less suitable pump selection.

The pump selection process often starts with the duty point as reference but there are many things to consider as e.g. how the pump is supplosed to be controlled, how many hours per day or days per year it´s supposed to run. What media (pumpage) is to be pumped, what is the temperature, denisity, viscosity of that media? Is it consistent or does it change?

Same question should be asked regarding flow, is it consistent or does it change? For consistent flow a fixed speed pump with as less control (throttling) as possible is to prefer in order to pump as economically efficient as possible.

Head (or pressure) will also change with different flows. The frictional resistance (between the fluid and the pipe) is also called dynamic head as it change dynamically with the flow. If flow are increased or decreased, head will increase or decrease proportional to the square of the flow. In other chapters we describe how to calculate the pump work load and how to control the pump properly, what materials to use and what seals are favourable with regards to fretting, temperature and pressure. These are examples of what a proper pump selection should include, however, if you want efficient, troble free operations of your pumps, there are more considererations and factors to be accounted for.

In order for people with less experience to make a general pump selection we have made a table, with a numbered column for every common type of pump matching pump application to use as a guide in discussions with pump suppliers. The chart is not intended to overrule the principles of pump selection but mainly to guide the inexperienced pump user.

Pump types and categories

What categorize a pump is described in detail in Chapter 3 but there are some fundamentals which needs to be understood for someone interested in pump selection. There are two major groups of pumps used for pumping fluid, namely Rotodynamic pumps and Positive Displacement pumps.

A rotodynamic pump is a kinetic machine in which energy is continuously imparted to the pumped fluid by means of a rotor often named impeller or propeller depending on the design of the rotor. The exact designation or group a specific rotodynamic pump belongs to is determined by the pump specific speed. The concept of specific speed is explained thoroughly in Chapter 3.

Why rotodynamic pumps equals centrifugal pumps

In our daily conversation about pumps, the word rotodynamic pump is seldom used. Insted we use the word centrifugal pump since the rotating impeller is pushing the fluid from its center to its periphery using centrifugal force. One may argue that the use of centrifugal pump insted of rotodynamic pump is not correct from a technological perspective, but for the ease of reading this book and to understand the concepts around pump selection we use the word centrifugal pump for any rotodynamic pump if not specifically mentioned.

Positive displacement pumps

Positive displacement pumps are divided into two major groups, rotating and reciprocating pumps. Common rotary positive displacement pump are e.g. gear pumps, screw pumps, vane pumps or sliding vane pumps, circumferential piston pumps, flexible impeller pumps, peristaltic pumps, processing cavity pumps followed by many more.

Reciprocating positive displacement pumps are e.g. piston pumps, plunger pumps or diaphragm pumps.