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Discharge Control of Centrifugal Pumps

By Joe Honeywell

Introduction

A process system and pumping system must work together to balance the energy equilibrium point. The First Law of Thermodynamics demands it. Without some type of control the process and pump systems are left to find their own equilibrium. Process conditions frequently change and so will the corresponding system energy requirements. The change may be induced by the operators, such as changing the flow rate and pressure. Unexpected upsets may occur caused by equipment shutdowns. In any event, the result is an energy imbalance, one that forces the pump to react to the new operating condition. When this happens the energy produced by a pump must be controlled to match the system energy requirements.

Whenever a process system changes to a new operating condition, this must occur in a stable and controllable manner. First, it should react to gradual changes that normally occur in any process. Second, it must react to quick unexpected changes, before the process must be shut down. Here again the pump’s control system plays a part in balancing the energy of the process and pump systems, whether the change is gradual or fast acting, whether the change occurs over a wide range of conditions or is merely for safety purposes. The pump controls must accommodate the energy needs of the process system in a controllable manner.

Arguably, the most common method of controlling a centrifugal pump utilizes a modulating valve on the discharge side of the pump. It is commonly called discharge control or throttle control. Despite other methods that have resulted from advances in technology, the discharge control method has remained relatively unchanged and preferred for many applications. Why then do many users prefer this control method? The most likely answer is probably related to the user’s person preference. For most, it is a proven method that does not require a high level of understanding to implement and maintain. However, there are many technical and commercial reasons that favor discharge control.

Control valves (CV) are frequently used whenever liquid flow must be regulated over a range of process conditions. Common applications include regulating flow into or out of tanks, pressure vessels, boilers, exchangers and many other types of equipment. They may also be used to control flow in systems such as pipelines, power plants, mills and chemical facilities. CV’s are also used in many types of services ranging from simple residential applications to complex process plants. In general, control valves are used in all types of applications, systems and services, including the control of centrifugal pumps.

This article will examine the advantages and disadvantages of utilizing the discharge control method for centrifugal pumps. It will explain the theory behind a discharge control valve and how it influences the system curve. Topics such as valve selection, design versatility, stability, cost effectiveness, system response, and series and parallel pump configurations are discussed. It will also discuss different applications and how discharge control can be used. Three case examples related to common applications are reviewed. Finally, recommendations are given that list where a discharge control method should be considered.

 

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