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Using NPSHR to Avoid Cavitation
in Centrifugal Pumps

By Joe Honeywell

 

Introduction

Centrifugal pumps are commonly used in facilities that deal with the transfer of liquids from one energy state to another. They add energy to a liquid stream in an efficient manner and are easily controlled. Maintenance cost is generally lower than other types of pumps and if properly designed and installed, centrifugal pumps will last a long time. To reach these goals the designer must consider many design factors, not the least of which is the Net Positive Suction Head Available of NPSHA.

All facilities with pumping systems have specific characteristics that are related to their suction design. These characteristics may be separated into three areas; hydraulic, operating and physical. This is true regardless of the category of pump, reciprocating or centrifugal. For example, the suction pipe diameter and piping configuration are two important suction design parameters of the hydraulic characteristics. Another hydraulic characteristic is the suction equipment elevation. The pump system will also have operating characteristics. These include such things as the operating pressures, flow rate, temperatures and liquid levels. Finally, the pump system characteristics are represented by the physical properties of the fluid. Viscosity, density and vapor pressure are three of the important properties that affect the pump. Together, the hydraulic, operating and physical parameters form the pump’s suction system characteristic called NPSHA. It is solely a characteristic of the system and not necessarily a characteristic of the pump itself.

Another important term related to this discussion is called Net Positive Suction Head Required or NPSHR. It is a characteristic of the pump and is determined by the manufacturer through shop testing or design. NPSHR is important to this discussion because of its relationship to NPSHA. If NPSHR is greater than NPSHA, pumps will cavitate to some degree. To avoid this harmful condition, system designers need to fully understand NPSHA and how it affects the design of centrifugal pump installations. For facility operators and maintenance staff, understanding how NPHSA affects the operation of pumps will help troubleshoot problematic pumps, especially those with cavitation issues.

This paper will focus on the topic of NPSHA. The subject of cavitation will be discussed and how to avoid the harmful effects it has on centrifugal pumps. A clear understanding of NPSHA is given along with how system characteristics determine NPSHA. Each of the parameters that contribute to NPSHA will be examined and how variations should be considered in the system design. Examples of how to calculate NPSHA are given. In addition, case problems are presented for three of the most common types of pump suction designs. Finally, recommendations and troubleshooting tips are given that will help improve pump longevity, system designs, and avoidance of ongoing cavitation problems.

 

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