Cavitation in a centrifugal pump often begins on the suction side of the system. When pressure at the pump inlet drops too close to the liquid’s vapor pressure, vapor bubbles can form and then collapse as they move into higher-pressure areas of the pump.
This process can damage the impeller and other pump components. Understanding net positive suction head (NPSH) helps engineers and operators determine whether the pump has adequate suction conditions for reliable operation.
What Is NPSH?
Net positive suction head describes the pressure available at the pump inlet relative to the vapor pressure of the liquid being pumped. It is typically expressed in feet or meters of head.
Two NPSH values are important when evaluating a centrifugal pump system:
NPSH required (NPSHr): The minimum suction conditions required by the pump at a particular operating point.
NPSH available (NPSHa): The suction conditions provided by the actual piping and process system.
NPSHr is established by the pump manufacturer and is typically shown on the pump performance curve. NPSHa must be determined from the conditions of the installed system.
How Low NPSH Causes Pump Cavitation
Cavitation occurs when pressure in the pump inlet drops low enough for the liquid to begin forming vapor bubbles.
As those bubbles move through the impeller into areas of higher pressure, they collapse. Repeated bubble formation and collapse can damage the impeller and other internal components and interfere with stable pump operation.
To reduce the risk of cavitation, the NPSHa at the pump inlet must be greater than the pump’s NPSHr at the intended operating condition.
What Affects NPSH Available?
NPSHa is determined by the suction-side conditions of the system rather than by the pump alone. Several factors can influence the pressure available at the pump inlet, including:
The elevation of the pump
The liquid level in the suction tank or sump
Friction losses through the suction piping
The vapor pressure of the liquid being pumped
Changes to any of these conditions can affect the suction pressure reaching the pump and therefore the margin between NPSHa and NPSHr.
Pump Elevation
The pump’s position relative to the liquid source affects suction conditions. Changes in elevation can increase or decrease the pressure available at the pump inlet.
Liquid Level
The height of liquid above the pump suction provides additional pressure at the inlet. If the level in a tank or sump drops, NPSHa can also decrease.
Suction-Piping Losses
Resistance in the suction piping reduces the pressure that ultimately reaches the pump. Friction losses through the piping therefore need to be considered when determining available NPSH.
Liquid Vapor Pressure
Every liquid has a vapor pressure. If local pressure within the pump falls below that vapor pressure, part of the liquid can vaporize and create the bubbles associated with cavitation.
NPSHa vs. NPSHr
The relationship between NPSHa and NPSHr is one of the key considerations when evaluating pump suction conditions.
NPSHr comes from the pump manufacturer. It represents the suction conditions the pump requires at a particular flow rate and can be found on the manufacturer’s pump curve.
NPSHa comes from the system. It is influenced by the installation, liquid level, elevation, suction piping losses, and properties of the pumped liquid.
If the available NPSH is insufficient relative to the pump’s requirement, the likelihood of cavitation increases. This is why evaluating the pump curve without considering the surrounding system can result in an incomplete pump selection.
Why NPSH Should Be Considered During Pump Selection
Selecting a centrifugal pump based only on required flow and discharge pressure does not account for whether the pump can reliably draw liquid from the suction system.
NPSH requirements should be evaluated alongside the system’s actual suction conditions. This helps determine whether the selected pump can operate at the intended duty point without encountering cavitation caused by inadequate inlet pressure.
If a pump is already experiencing cavitation, evaluating suction conditions can also help identify whether pump elevation, liquid level, suction-piping losses, or another system condition is contributing to the problem.
Protecting Centrifugal Pumps From Cavitation
Pump cavitation is not simply a pump problem. It reflects the interaction between the pump, the liquid, and the suction-side system.
Comparing NPSHa with the manufacturer’s NPSHr is an important step in both pump selection and troubleshooting. Maintaining adequate suction pressure helps prevent vapor formation at the pump inlet and supports more reliable centrifugal pump operation.
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Cavitation in a centrifugal pump often begins on the suction side of the system. When pressure at the pump inlet drops too close to the liquid’s vapor pressure, vapor bubbles can form and then collapse as they move into higher-pressure areas of the pump.
This process can damage the impeller and other pump components. Understanding net positive suction head (NPSH) helps engineers and operators determine whether the pump has adequate suction conditions for reliable operation.
What Is NPSH?
Net positive suction head describes the pressure available at the pump inlet relative to the vapor pressure of the liquid being pumped. It is typically expressed in feet or meters of head.
Two NPSH values are important when evaluating a centrifugal pump system:
NPSHr is established by the pump manufacturer and is typically shown on the pump performance curve. NPSHa must be determined from the conditions of the installed system.
How Low NPSH Causes Pump Cavitation
Cavitation occurs when pressure in the pump inlet drops low enough for the liquid to begin forming vapor bubbles.
As those bubbles move through the impeller into areas of higher pressure, they collapse. Repeated bubble formation and collapse can damage the impeller and other internal components and interfere with stable pump operation.
To reduce the risk of cavitation, the NPSHa at the pump inlet must be greater than the pump’s NPSHr at the intended operating condition.
What Affects NPSH Available?
NPSHa is determined by the suction-side conditions of the system rather than by the pump alone. Several factors can influence the pressure available at the pump inlet, including:
Changes to any of these conditions can affect the suction pressure reaching the pump and therefore the margin between NPSHa and NPSHr.
Pump Elevation
The pump’s position relative to the liquid source affects suction conditions. Changes in elevation can increase or decrease the pressure available at the pump inlet.
Liquid Level
The height of liquid above the pump suction provides additional pressure at the inlet. If the level in a tank or sump drops, NPSHa can also decrease.
Suction-Piping Losses
Resistance in the suction piping reduces the pressure that ultimately reaches the pump. Friction losses through the piping therefore need to be considered when determining available NPSH.
Liquid Vapor Pressure
Every liquid has a vapor pressure. If local pressure within the pump falls below that vapor pressure, part of the liquid can vaporize and create the bubbles associated with cavitation.
NPSHa vs. NPSHr
The relationship between NPSHa and NPSHr is one of the key considerations when evaluating pump suction conditions.
NPSHr comes from the pump manufacturer. It represents the suction conditions the pump requires at a particular flow rate and can be found on the manufacturer’s pump curve.
NPSHa comes from the system. It is influenced by the installation, liquid level, elevation, suction piping losses, and properties of the pumped liquid.
If the available NPSH is insufficient relative to the pump’s requirement, the likelihood of cavitation increases. This is why evaluating the pump curve without considering the surrounding system can result in an incomplete pump selection.
Why NPSH Should Be Considered During Pump Selection
Selecting a centrifugal pump based only on required flow and discharge pressure does not account for whether the pump can reliably draw liquid from the suction system.
NPSH requirements should be evaluated alongside the system’s actual suction conditions. This helps determine whether the selected pump can operate at the intended duty point without encountering cavitation caused by inadequate inlet pressure.
If a pump is already experiencing cavitation, evaluating suction conditions can also help identify whether pump elevation, liquid level, suction-piping losses, or another system condition is contributing to the problem.
Protecting Centrifugal Pumps From Cavitation
Pump cavitation is not simply a pump problem. It reflects the interaction between the pump, the liquid, and the suction-side system.
Comparing NPSHa with the manufacturer’s NPSHr is an important step in both pump selection and troubleshooting. Maintaining adequate suction pressure helps prevent vapor formation at the pump inlet and supports more reliable centrifugal pump operation.
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