Design of Centrifugal Pumps with Non-Overloading Power Curve: A Review of the State of the Art

Abstract

Non-overloading (NOL) power characteristics of a centrifugal pump is an important performance parameter that ensures safety and reliability in diverse industrial applications, particularly where the pump should continue to run without overloading or tripping the driver over the entire operating range. For most applications, pump designers try to optimize pump efficiency for a given set of rated duty conditions-flow, head and available Net Positive Suction Head. However, there are many applications where a centrifugal pump is required to operate over a wide range of flow and head. This operating requirement is often systemic, such as in municipal water distribution systems, where flow is regulated by turning on or off a number of pumps operating in parallel. There are other situations, such as in fire pump applications, where governing standards (Underwriters Laboratory, Factory Mutual Global) mandate operation of the pump over a wide range of flows. For applications such as these, the pump designer needs to try and optimize not merely efficiency at a single rated duty point, but also peak power consumption over the entire range of operating flows. However, attaining NOL power characteristics poses a significant challenge to the designers. Though the science of fluid movement is quite advanced and the recent years have witnessed a phenomenal improvement in CFD, design of a pump for a specific characteristic from the first principle of fluid mechanics is still a formidable task. Till date experts (pump manufacturers) follow a design strategy, where scientific principle is intervened by empiricism obtained through carefully conducted experiments. It need not be exaggerated that the design of pumps with NOL characteristics from the basic theory of fluid mechanics is a formidable task, if not impossible. In this paper, the importance of NOL power, parameters affecting attainment of the NOL power, different design approaches and modifications in the existing design to achieve NOL power have been discussed in detail. It has been observed that with the exception of Anderson's area ratio method, very few of the design approaches deal with the NOL power characteristics. Considering the usefulness of Anderson's area ratio method, a real-life example has been cited describing this approach for the benefit of the readers. Two dominant schools across the Atlantic have been using empirical design factors on the one hand and a concept known as "area ratio" on the other hand. Both approaches have solid theoretical foundations and they have produced successful pump designs in the second half of twentieth century. The review paper discusses these design approaches and their impact on the shape of the power characteristics of centrifugal pumps. This review paper has been prepared to document the accumulated study, design work and experimentation to achieve NOL power characteristics in centrifugal pumps. Finally, it can be concluded that systematic research endeavors, combining experiments and computation, are needed to achieve further improvement in NOL characteristics of centrifugal pumps.