Some time ago, pump our engineering team introduced the concept of "NPSH" (Net Positive Suction Head) to you. However, we received feedback indicating that the previous explanation lacked sufficient detail and was difficult to fully grasp, so we are revisiting the topic today to provide a more comprehensive explanation.
When a pump operates, the liquid at the impeller inlet generates vapor due to the vacuum pressure; the resulting vapor bubbles, driven by the impact of liquid particles, cause erosion on metal surfaces (such as the impeller), thereby damaging the metal components. The vacuum pressure at which this vaporization occurs is known as the vaporization pressure. NPSH refers to the excess energy possessed by a unit weight of liquid at the pump suction inlet—beyond the vaporization pressure—and is measured in meters. Suction lift corresponds to the required NPSH (denoted as Δh); it represents the permissible vacuum level for liquid suction (or the allowable installation height of the pump), measured in meters. Suction Lift = Standard Atmospheric Pressure (10.33 meters) - NPSH - Safety Margin (0.5 meters). Standard atmospheric pressure can support a vacuum column height of 10.33 meters in the pipeline.
NPSH is defined as the difference between the total head of the liquid at the pump inlet and the pressure head at the point of vaporization; it is measured in meters (of water column) and denoted as NPSH. It is categorized as follows:
NPSHa — Installation NPSH (also known as available NPSH); the higher this value, the less likely cavitation is to occur.
NPSHr — Pump NPSH (also known as required NPSH or pump inlet dynamic pressure drop); the lower this value, the better the cavitation resistance.
NPSHc — Critical NPSH; the NPSH value corresponding to a specific drop in pump performance.
[NPSH] — Permissible NPSH; the NPSH value used to determine pump operating conditions, typically calculated as [NPSH] = (1.1–1.5) × NPSHc.
Definition of NPSH for centrifugal pumps.
During operation, the liquid pressure drops as the fluid moves from the pump inlet to the impeller inlet; the pressure reaches its minimum value ($p_K$) at point K, near the leading edge of the vanes. Subsequently, as the impeller performs work on the liquid, the pressure rises rapidly. When the pressure $p_K$ near the vane inlet falls below the saturated vapor pressure ($p_v$) at the operating temperature, the liquid vaporizes. Simultaneously, dissolved gases are released from the liquid, forming numerous vapor bubbles. As these bubbles are carried by the flow to regions of higher pressure within the impeller channels, the external liquid pressure exceeds the internal vapor pressure; consequently, the bubbles condense and collapse, creating cavities. Surrounding liquid rushes into these cavities at extremely high speeds, causing liquid-to-liquid impacts and sudden, localized pressure spikes (which can reach hundreds of atmospheres). This not only impedes normal flow but—more critically—if the bubbles collapse near the impeller walls, the liquid acts like countless tiny projectiles continuously striking the metal surface. The high frequency of these impacts (up to 2000–3000 Hz) causes the metal surface to flake off due to impact fatigue. If the bubbles contain reactive gases (such as oxygen), the heat released during condensation (local temperatures can reach 200–300°C) can facilitate thermoelectric effects and electrolysis, leading to electrochemical corrosion that further accelerates the rate of metal erosion and damage. This combined phenomenon—involving liquid vaporization, condensation, impact, and the generation of high-pressure, high-temperature, high-frequency impact loads that result in mechanical flaking and electrochemical corrosion—is known as cavitation.
Where is cavitation most likely to occur?
a. At the front shroud where curvature is greatest, near the low-pressure side of the vane inlet edge;
b. In the discharge chamber, near the low-pressure side of the volute tongue or the inlet edge of the guide vanes;
c. In the sealing clearance between the outer diameter of the vane tips and the casing, and on the low-pressure side of the vane tips (specifically in high specific-speed impellers without a front shroud);
d. The first-stage impeller of a multistage pump.
What is Net Positive Suction Head (NPSH)? What is suction lift? What are their respective units of measurement and symbols? Answer: When a pump is operating, the liquid at the impeller inlet generates vapor due to the vacuum pressure; the resulting vapor bubbles collapse and impact metal surfaces (such as the impeller), causing material erosion and damage. The pressure at which this vaporization occurs is known as the vaporization pressure. Net Positive Suction Head (NPSH)—specifically the required NPSH, denoted as (NPSH)r—refers to the excess energy possessed by a unit weight of liquid at the pump suction inlet above the vaporization pressure. It is measured in meters of liquid column.
Suction lift (Δh) represents the permissible vacuum level for liquid suction—or the allowable geometric installation height of the pump—measured in meters. It is calculated as: Suction Lift = Standard Atmospheric Pressure (10.33 m) – NPSH – Pipeline Losses – Safety Margin (0.5 m). (Standard atmospheric pressure supports a vacuum lift of 10.33 meters in the pipeline.) Example: If a pump has an NPSH of 4.0 meters, calculate the suction lift (Δh).
Solution: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters.
Cavitation occurs when the pressure of a liquid at a given temperature drops to its vaporization pressure, causing vapor bubbles to form. This phenomenon of bubble formation is known as cavitation. When these bubbles flow into a high-pressure zone, they shrink and eventually collapse; this process of bubbles disappearing into the liquid due to rising pressure is termed cavitation collapse.
During pump operation, if the absolute pressure of the pumped liquid in a localized area of the flow passage (typically just downstream of the impeller blade inlet) drops to the liquid's vaporization pressure—due to specific operating conditions—the liquid begins to vaporize, generating a large volume of vapor bubbles. As the bubble-laden liquid moves into the high-pressure zone within the impeller, the surrounding high-pressure liquid causes the bubbles to contract rapidly and collapse. As the bubbles condense and collapse, liquid particles rush to fill the resulting voids at high velocity, generating intense water-hammer effects. These impacts strike the metal surface at frequencies reaching tens of thousands of times per second, with impact stresses ranging from hundreds to thousands of atmospheres; in severe cases, this can puncture the wall material.
The process involving bubble formation and collapse, which damages the pump's flow-passage components, constitutes cavitation in the pump. Beyond damaging these components, cavitation generates noise and vibration and degrades pump performance; in extreme cases, it can interrupt liquid flow, rendering the pump inoperable.
Measures to Improve Cavitation Resistance
a. Measures to enhance the inherent cavitation resistance of centrifugal pumps
(1) Improve the structural design of the area between the pump suction inlet and the impeller. Increase the flow passage area; enlarge the radius of curvature at the impeller shroud inlet to minimize rapid flow acceleration and pressure drops; reduce the blade inlet thickness and round off the leading edge to create a streamlined profile, thereby reducing acceleration and pressure drops as fluid flows around the blade tip; improve the surface finish of the impeller and blade inlet areas to minimize resistance losses; and extend the blade inlet edge toward the pump suction inlet so that the fluid begins to receive work input earlier, thereby raising the pressure. (2) Employ an inducer to initiate work on the fluid flow upstream, thereby increasing the fluid pressure.
(3) Utilize a double-suction impeller, allowing fluid to enter from both sides simultaneously; this doubles the inlet cross-sectional area and halves the inlet flow velocity.
(4) Adopt a slightly larger positive angle of attack at the design operating point to increase the blade inlet angle; this reduces blade inlet curvature and blockage while increasing the inlet area, thereby improving operating conditions at high flow rates and minimizing flow losses. However, the positive angle of attack should not be excessive, as this would adversely affect efficiency.
(5) Use cavitation-resistant materials. Practical experience shows that higher material strength, hardness, and toughness, combined with superior chemical stability, result in greater cavitation resistance.
b. Measures to increase the available net positive suction head (NPSHa) of the suction system
(1) Increase the pressure of the liquid surface in the suction-side reservoir to raise the NPSHa.
(2) Reduce the installation height of the pump in a suction-lift configuration.
(3) Convert the suction-lift configuration to a flooded-suction (gravity-feed) configuration.
(4) Minimize flow losses in the suction piping. Examples include shortening the pipeline as much as possible within required limits, reducing flow velocity, minimizing the number of elbows and valves, and maximizing valve openings.
(5) Lower the temperature of the pumped medium at the pump inlet (when the medium is close to its saturation temperature).
The measures above should be applied appropriately based on a comprehensive analysis of factors such as pump selection, material choice, and on-site operating conditions., Ltd.