Cavitation describes the change in state of liquid to vapor and its transport to a higher-pressure region where the vapor bubble collapses or implodes back to the liquid phase (Image 1). In rotodynamic pumps, cavitation is typically associated with suction pressure at the pump inlet being lower than it should be. As the liquid accelerates into the impeller eye, there is a region of low pressure where local pressure is less than the vapor pressure, resulting in cavitation. Cavitation can also be the result of operating a pump incorrectly. There may be too low of a flow rate, where suction and discharge recirculation result in low-pressure regions, or there may be too high of a flow rate, where the pump is not designed to operate and the system is not capable of supplying adequate suction pressure. Potentially negative consequences of cavitation include reduced performance, noise, vibration and erosion damage to the impeller. Cavitation occurs in almost all pumps, but it is often not at a level that will cause performance reduction or reduced reliability.

A method to quantify cavitation in rotodynamic pumps is to conduct laboratory net positive suction head (NPSH) testing per American National Standards Institute (ANSI)/Hydraulic Institute (HI) 14.6 Rotodynamic Pumps for Hydraulic Performance Acceptance Tests. This standard defines four types of NPSH tests that can be conducted, while the following focuses on an NPSH test to quantify the level of performance reduction due to cavitation.

Within ANSI/HI 14.6, NPSH3 is the NPSH required (NPSHr) benchmark used and is the value of NPSHr at which the first-stage total head drops by 3% due to cavitation. As the definition implies, NPSH3 testing quantifies a level of cavitation where the total head of the first-stage impeller is reduced by 3% due to cavitation while maintaining a constant flow rate and rotational speed and measuring pump total head while the available NPSH is reduced (Image 2). An implication of NPSH3 is that cavitation starts before a 3% reduction in head occurs. NPSH3 has been adopted by industry standards because it is the smallest total head reduction due to cavitation that can be practically measured and is reproducible.

Quantifying when cavitation is initiated or when it first begins to cause a performance reduction is complex and often not repeatable. For most pumps, with some exceptions, cavitation that does not result in performance reduction is not damaging and does not reduce the life of the equipment.

However, if you are dealing with pumps that operate with high head per stage, high power per stage, high rotational speed or are designed for low relative NPSHr for a given flow rate and rotational speed, additional available NPSH may be required to limit erosion damage. For these pumps, it is recommended to discuss the NPSH margin requirements with the pump manufacturer.

For additional information on cavitation and NPSH testing, refer to ANSI/HI 9.6.1 Rotodynamic Pumps – Guideline for NPSH Margin and ANSI/HI 14.6 at pumps.org/standards.

There are many design, installation, operation and testing standards that can and should be used for municipal water pumps. The specific standards depend on the pump type and application. The American Water Works Association (AWWA) has published three specifications for the procurement of submersible vertical turbine pumps (E102), horizontal and vertical line shaft pumps (E103) and solids-handling water pumps (E110) for municipal water applications. Refer to these standards at awwa.org for the specific scope and minimum requirements. These AWWA standards leverage Hydraulic Institute standards for definitions of the pump types, some design and application information and instructions for installation and operation.

• ANSI/HI 9.6.1 Rotodynamic Pumps – Guideline for NPSH Margin
• ANSI/HI 9.6.3 Rotodynamic Pumps – Guideline for Operating Regions
• ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurements and Allowable Values
• ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping
• ANSI/HI 9.6.8 Rotodynamic Pumps – Guidelines for Dynamics of Pumping Machinery
• ANSI/HI 9.8 Rotodynamic Pumps for Pump Intake Design
• ANSI/HI 14.1-14.2 Rotodynamic Pumps for Nomenclature and Definitions
• ANSI/HI 14.3 Rotodynamic Pumps for Design and Application
• ANSI/HI 14.4 Rotodynamic Pumps for Manuals Describing Installation, Operation and Maintenance
• ANSI/HI 14.6 Rotodynamic Pumps for Hydraulic Performance Acceptance Tests