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Variable Speed Control of Centrifugal Pumps

By Joe Honeywell

 

Introduction

It has been said that the best control method is the one that is the simplest. This is certainly true for a pump system that moves product from one place to another under operating conditions that never changes. But is the simplest control method the best if the system operates under a wide range of process conditions? The applications may demand a more complex pump control method, one that can accommodate a range of changing system parameters. What if operating cost is a major consideration in selecting a control method? What good is a simple control system if it is inefficient, costly and potentially harmful to the pump? Will the simplest method still be the best? The answers to these questions are not always obvious and will require the facility designer to consider many factors such as operating flexibility, life cycle costs, reliability, controlled response, start-up, shutdown and much more. Today’s pump control systems can affect a facility in many ways and may require a comprehensive evaluation to arrive at the best control method.

A type of pump control that has been used for many years and is gaining in popularity is called variable-speed control. As the name implies, the pump is controlled by varying its speed and thereby changing the head, flow and power of the pump. Various types of drivers are available that not only provide power to the pump but can also vary the speed, thereby meeting varying operating conditions of the process. If the pump is properly designed and installed, changing its speed can be done effectively and still reduce operating and maintenance costs. Technology has also improved the reliability and controlled response of pump drivers to the level where speed control is often preferred to other types of control methods. However, variable-speed control may not be the best selection for all applications. System design factors such as static head, pressure and friction play a part in determining which control method is the best selection. Even though variable-speed control has many advantages and disadvantages, it remains on the most common types of pump control.

This article will discuss the many aspects of centrifugal pump variable-speed control. It will explain the theory behind variable pump speeds and how the energy produced by the pump interacts with changing system requirements. Such topics as operating cost, reliability, system response, series and parallel operation will be discussed. It will also explain the advantages and disadvantages of changing pump speeds and how they can be applied to various types of applications. Three case studies of common applications will be examined; each is representative of how best to use a variable-speed pump control method. Finally, recommendations are given that lists where variable-speed pump control should be considered.

Variable-speed drivers, by themselves, are a complex subject with many topics that require a lengthy discussion for a full understanding. This article will only make reference to the driver and how it affects the performance and controllability of a pump. There are several references for the interested reader that discusses this topic in more detail.

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