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Introduction to Centrifugal Pump Control Systems

By Joe Honeywell

 

Introduction

Irreversible processes that transport a liquid require energy to complete the path. Assuming the process is not a chemical reaction, the amount of energy needed will vary for many reasons but is a function of the liquid flow rate. Simply stated, an irreversible process requires varying amounts of energy depending on the fluid flow rate. From a hydraulics standpoint the graphic representation of the required energy at various flow rates is referred to as a system performance curve or just system curve. A centrifugal pump is a device commonly used to add the needed hydraulic energy to the system. The pump transfers energy to the liquid velocity as it passes through the impeller and diffuser. The graphic representation of the amount energy produced by a pump at various flow rates is referred to as a pump performance curve or pump curve. Both curves represent, at equilibrium, the energy required by the system and the energy produced by the pump. The equilibrium point is also referred to as the operating point.

The operating point of most processes will change for a wide variety of reasons. For example, if a boiler produces less steam, then the feed pump’s flow rate is reduced. In this case less steam energy is related to a reduced water flow rate. A similar example is a pipeline that transports an increased amount of product. The higher flow rate requires the booster pump to produce more energy to overcome the increased pipeline friction. In both of these examples the changing operating points are not only influenced by the amount of energy but by the fluid flow rate.

The flow rate of a process is commonly altered for a wide variety of reasons. It may be due to changing product demand, equipment failure, cost, environmental constraints, operating conditions and many other reasons. Pump controls are frequently used to accommodate the change in a process’ flow rate. However, there are many methods of controlling a pump. Each method has its own advantages and disadvantages, and the selection will depend on the particular application and the reasons for controlling the process.

This article will introduce the user to centrifugal pump control systems. It will discuss the two categories of pump controls and the various types of controls that are part of each category. The advantages and disadvantages of each control method are reviewed, along with selection criteria that will aid the user in determining the method best suited for a given pumping system. An example problem is given that demonstrates how the selection criteria can be applied to a particular application. Next, the important topic of constructing a system performance curve is reviewed. The theory of energy equilibrium is described along with an explanation of the mathematical equations needed to prepare a system curve. This is followed by a description of the three types of system curves and how they apply to the various types of applications. Two examples that demonstrate how to construct a system curve are presented. Finally, several references are given for the interested reader that discusses the various types of control methods in more detail.

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