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Centrifugal Pump NPSHR Testing

By Joe Honeywell

Introduction

A topic that is frequently mentioned when discussing pumps is “cavitation”. This phenomenon applies to all categories of pumps including rotary, reciprocating and centrifugal types. Cavitation is fluid flow instability. This flow instability can have a far reaching impact on the pump’s hydraulic performance or it can be so minor that it goes unnoticed for years. The consequence on the metallurgy and mechanical components can range from severe impairment to a small annoyance. In essence, pump cavitation can affect any pump in many different ways.

Pump designers and users have found that cavitation can be controlled by many different methods. For example, not all pumps cavitate in the same manner and severity. By careful pump selection and design, the cavitation characteristic of a pump can be identified and made to suit the applications. But pump design and selection is not the only factor that influences cavitation. Knowledgeable users can adjust operating parameters such as temperature, pressure, flow rate and speed, to prevent pump cavitation from occurring. The physical behavior of the fluid is another factor that plays an important part. One good example is the vapor-liquid phase relationship. If fluid vapor does not form inside the pump, then cavitation will not occur. Industry designers and users have found many methods of controlling cavitation, and understanding how each plays a part is important to this discussion.

Pump cavitation is a complex phenomenon and has been categorized by its characteristics. Many technical articles have been published that explain this topic in great detail. One of the most common types and arguably the most severe is the focus of this article. It is sometimes referred to as high flow cavitation or just cavitation for short. Without going into great detail, this specific type of cavitation will be briefly reviewed and compared to the various other types of impairment.

The topic of cavitation and how it occurs in centrifugal pumps is especially useful to engineers, mechanics and operators who are involved in pump performance. For experienced engineers, it will help in the design of facilities and preparations of pump specifications. Maintenance and operating staff will gain a better understanding of problematic equipment or how to maintain a healthy machine. For young engineers witnessing a pump test for the first time, it will help in understanding the science of pump testing and expected outcome. Briefly stated, much of the information presented in this article is meant to provide an understanding of pump users’ and manufacturers’ proven approach to cavitation issues.

This article will focus on explaining one of the controlling factors that contribute to centrifugal pump cavitation. Pump manufacturers typically conduct a specialized test called NPSHR test. It identifies the characteristics that predict when cavitation will occur. The purpose of this test and how it can be used to determine when cavitation occurs is a primary topic of this paper. Recommendations are given for when pump users should request this test. The testing requirement of the various pump codes and standards is another topic of importance and will be reviewed. Finally, this paper will examine shop testing procedures and give an example of typical test results.

 

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