1. 24. Pumps 2: Cavitation
CH EN 374: Fluid Mechanics

2. Cavitation Remember “cavitation” from your homework?
Cavitation: pressure drops below liquid vapor pressure
Bubbles form, collapse, damage equipment

3. Pump Cavitation Where in a pump should pressure be lowest? Bonus:
OUT
Bonus:
Where in a turbine do you expect the lowest pressure?

4. Net Positive Suction Head
NPSH defined to help us avoid cavitation
𝑁𝑃𝑆𝐻= 𝑃 𝜌𝑔 + 𝑣 2 2𝑔 𝑝𝑢𝑚𝑝 𝑖𝑛𝑙𝑒𝑡 − 𝑃 𝑣 𝜌𝑔
Stagnation Pressure

5. Pump Find NPSH in terms of 𝑃 1 , 𝑣 1 , 𝑧 1 , 𝑧 2 , ℎ 𝐿 𝑃 1 , 𝑣 1 , 𝑧 1
𝑃 2 , 𝑣 2 , 𝑧 2 1 2 3
𝑁𝑃𝑆𝐻= 𝑃 𝜌𝑔 + 𝑣 2 2𝑔 𝑝𝑢𝑚𝑝 𝑖𝑛𝑙𝑒𝑡 − 𝑃 𝑣 𝜌𝑔
Find NPSH in terms of 𝑃 1 , 𝑣 1 , 𝑧 1 , 𝑧 2 , ℎ 𝐿

6. Pump Find NPSH in terms of 𝑃 1 , 𝑣 1 , 𝑧 1 , 𝑧 2 , ℎ 𝐿 𝑃 1 , 𝑣 1 , 𝑧 1
𝑃 2 , 𝑣 2 , 𝑧 2 1 2 3
𝑁𝑃𝑆𝐻= 𝑃 𝜌𝑔 + 𝑣 2 2𝑔 𝑝𝑢𝑚𝑝 𝑖𝑛𝑙𝑒𝑡 − 𝑃 𝑣 𝜌𝑔
Find NPSH in terms of 𝑃 1 , 𝑣 1 , 𝑧 1 , 𝑧 2 , ℎ 𝐿
𝑁𝑃𝑆𝐻= 𝑃 2 𝜌𝑔 + 𝑣 𝑔 − 𝑃 𝑣 𝜌𝑔
= 𝑃 1 𝜌𝑔 + 𝑣 𝑔 + 𝑧 1 − 𝑧 2 − ℎ 𝐿 − 𝑃 𝑣 𝜌𝑔

7. 𝑁𝑃𝑆𝐻= 𝑃 1 𝜌𝑔 + 𝑣 1 2 2𝑔 + 𝑧 1 − 𝑧 2 − ℎ 𝐿 − 𝑃 𝑣 𝜌𝑔
Pump
𝑃 1 , 𝑣 1 , 𝑧 1
ℎ 𝐿
𝑃 2 , 𝑣 2 , 𝑧 2 1 2 3
𝑁𝑃𝑆𝐻= 𝑃 1 𝜌𝑔 + 𝑣 𝑔 + 𝑧 1 − 𝑧 2 − ℎ 𝐿 − 𝑃 𝑣 𝜌𝑔

8. Problem
In the chemical plant where you work, waste is dumped into a waste treatment pond. After being treated there, it is pumped into another pond for a second treatment. After years with no problems, technicians report loud popping noises—the pump between the ponds is experiencing cavitation. What are some things you can check to determine the problem?

9. 𝑯𝒆𝒂𝒅 𝑽 Curve will be different for different liquids—why?
Curve will be different for different temperatures—why?

10. NPSH Required
How much NPSH is needed to avoid cavitation in pump?
Depends on flowrate
Determined experimentally for each pump
𝑯𝒆𝒂𝒅 𝑽

13. 𝑯𝒆𝒂𝒅 𝑽 NPSH NPSHrequired
What does the intersection point tell us, physically?

14. 𝑯𝒆𝒂𝒅 𝑽 NPSH NPSHrequired
Cavitation Happens
NPSHrequired
What does the intersection point tell us, physically?