--- title: "Cavitation in pumps and NPSH calculation" subject: "Fluid Machines" chapter: 4 key: "semester_5/fluid_machines:cavitation-npsh-numerical" source: https://www.bctengineer.com/q/semester_5/fluid_machines/cavitation-npsh-numerical site: BCT Engineering --- # Cavitation in pumps and NPSH calculation Fluid Machines · Chapter 4: Pump - Asked in: year not given, 3+5 marks - Video: [pump cavitation and net positive suction head](https://www.youtube.com/watch?v=L_kAv0wpST8) — technoworks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/fluid_machines/cavitation-npsh-numerical - Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-4 ## Question (a) What is cavitation in a centrifugal pump? Explain its causes, effects and the measures to prevent it. (b) A centrifugal pump lifts water at 30 degrees C (vapour pressure 4.25 kPa, density 996 kg/m³) from a sump open to the atmosphere at 101.3 kPa. The pump is 4.0 m above the sump water level and the head loss in the suction pipe is 0.8 m. The pump manufacturer gives a required NPSH of 4.2 m. Find the available NPSH and check whether the pump will cavitate; find the maximum permitted suction lift. What happens if the same installation is moved to a site where the atmospheric pressure is 79.5 kPa? ## Short answer Cavitation is vapour bubble formation when local pressure drops to vapour pressure, causing noise, erosion, and performance loss. Prevention: reduce suction lift, use larger suction pipe. (b) Available NPSH = 5.13 m > required 4.2 m, so no cavitation. Maximum suction lift = 4.93 m. At 79.5 kPa, NPSHa = 2.90 m, cavitation occurs. ## Answer #### (a) Cavitation **Cavitation** is the formation of vapour bubbles in a flowing liquid where the local absolute pressure falls to the vapour pressure, and the sudden collapse of these bubbles when they move into a higher-pressure region. **Causes:** high suction lift, low atmospheric pressure (high altitude), high liquid temperature (high vapour pressure), high pump speed, large losses in the suction pipe and an insufficient NPSH. The pressure is lowest at the impeller eye, on the suction side of the vane. **Effects:** - Noise and vibration. - Pitting and erosion of the impeller and casing by the shock of collapsing bubbles. - Fall in head, discharge and efficiency. **Prevention:** lower the pump (reduce suction lift) or use a flooded suction, use a larger suction pipe with fewer bends and valves, reduce the speed, cool the liquid, use an inducer or a pump with a lower required NPSH, use cavitation-resistant materials (stainless steel, bronze). #### (b) Numerical **Available NPSH** (net head above vapour pressure at the pump suction): $$ \text{NPSH}_a=\frac{p_a-p_v}{\rho g}-H_s-h_{fs} $$ $$ \frac{p_a-p_v}{\rho g}=\frac{(101.3-4.25)\times10^3}{996\times9.81}=9.933\ \text{m} $$ $$ \text{NPSH}_a=9.933-4.0-0.8=5.13\ \text{m} $$ Since $\text{NPSH}_a=5.13\ \text{m}>\text{NPSH}_r=4.2$ m, there is a margin of $5.13-4.2=0.93$ m and the pump **will not cavitate**. **Maximum suction lift** (taking $\text{NPSH}_a=\text{NPSH}_r$): $$ H_{s,max}=9.933-0.8-4.2=4.93\ \text{m} $$ (A safety margin of at least 0.5 m is kept in practice.) **At 79.5 kPa:** $$ \frac{p_a-p_v}{\rho g}=\frac{(79.5-4.25)\times10^3}{996\times9.81}=7.702\ \text{m} $$ $$ \text{NPSH}_a=7.702-4.0-0.8=2.90\ \text{m}<4.2\ \text{m} $$ The available NPSH is now short by about 1.30 m, so the pump **will cavitate**. The suction lift must be reduced (or a flooded suction used) to restore the margin. **Answer:** $\text{NPSH}_a=5.13$ m (no cavitation); $H_{s,max}=4.93$ m; at 79.5 kPa $\text{NPSH}_a=2.90$ m, so cavitation occurs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is cavitation? A. Leakage of liquid through the clearance B. Formation of vapour bubbles where local pressure falls to vapour pressure, and their sudden collapse in a higher-pressure region C. Entry of air through the gland D. Boiling of liquid in the delivery pipe due to friction Answer: B. Bubbles form at low pressure (impeller eye) and collapse violently when carried to higher pressure. (Key point: Definition of cavitation) 2. Which change helps prevent cavitation in a pump? A. Increase the suction lift B. Raise the liquid temperature C. Use a flooded suction or lower the pump D. Use a smaller suction pipe with more bends Answer: C. Reducing suction lift raises the available NPSH; high lift, high temperature and suction losses all promote cavitation. (Key point: Causes and prevention) 3. What is the formula used for available NPSH here? A. (pa - pv)/(rho g) + Hs + hfs B. (pa - pv)/(rho g) - Hs - hfs C. pa/(rho g) - Hs + hfs D. (pa + pv)/(rho g) - Hs - hfs Answer: B. NPSHa = 9.933 - 4.0 - 0.8 = 5.13 m, which is above the required 4.2 m so no cavitation. (Key point: NPSH available) 4. What happens when the same installation is moved to a site with atmospheric pressure 79.5 kPa? A. NPSHa rises, so margin improves B. NPSHa falls to about 2.90 m, below the required 4.2 m, so the pump cavitates C. NPSHa is unchanged D. NPSHr falls to match Answer: B. Lower atmospheric pressure reduces (pa - pv)/(rho g) to 7.70 m, giving NPSHa of 2.90 m. 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