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By-Pass Control of Centrifugal Pumps

By Joe Honeywell

 

Introduction

Any control system should consider all aspects of the process operating conditions and the interaction of components that are part of that system. Normal operating conditions may be well known but are the abnormal conditions well-defined? For example, what happens to the process when flow rate or pressure upsets occur? Will the process and related components be harmed if the system shuts down or restarts frequently? When normal system wear and degradation occurs, will the controls be able to handle the changing operating parameters? These questions and many more should be considered when evaluating the type of pump control best suited for the overall system design. Understanding the various types of pump controls and selecting the ones best suited for the application are part of this evaluation.

Centrifugal pumps are intended to add energy to the process system. Merely adding a pump to add energy is not enough; it must be controlled in an efficient and cost effective manner. A control system must also prevent harm to the system and related components. The designer has many control methods from which to choose. One of these methods is called by-pass control. This control method basically splits the liquid flow path at the pump discharge. A portion of the liquid returns to the pump inlet through a by-pass line and the remainder continues to flow through the process piping. The liquid returning into inlet, however, must be throttled to the lower pressure of the pump inlet. By splitting the liquid flow in two directions and throttling pressure in the return line, energy is wasted. Before the split, however, the combined flow rate is much higher inside the pump, which allows it to operate in a more efficient manner.

Utilizing a by-pass control method can influence the process design and operating philosophy in many ways. By-pass control is primarily intended to affect a pump’s operation at low flow conditions. As a consequence, it can extend the normal operating range by controlling a pump’s minimum flow restrictions. By-pass control can also prevent process shutdowns, facilitate equipment startup, and forestall equipment failures caused by high temperature and vibration at low flow conditions. Even with these many advantages, a by-pass control system is not without drawbacks. In essence, the operator and designer should consider all aspects of control systems, including their limitations and influences on the process.

This paper will discuss by-pass control systems and how they are applied to centrifugal pumps. The basic reasons for considering this type of control system, as well as the advantages and disadvantages, will be examined. A survey of pump industry standards and how they influence the use of by-pass controls will be reviewed. Typical by-pass control schematics are presented along with advantages and disadvantages of each arrangement. Finally, an example problem is given which discusses how a by-pass control influences the operation and protection of a centrifugal pump.

 

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