Vapour Pressure & Pump NPSH

Vapour Pressure 

Traditionally, we can boil the liquid or turn liquid into a vapour by increasing its temperature and crossing the boiling point.

But there is another way of achieving it.

i.e., by reducing the surrounding pressure. Just like the example video that I am sharing it with you. ( Wait until the video ends)

This phenomenon of liquid turning into a vapour at a particular temperature and pressure is called Vapour Pressure. 

What happens when liquid turn into a vapour in a pump ? 

You experience what is called a cavitation.

Cavitation is a process of liquid turning into a vapour and vapour turning into a bubble.

The rumbling sound that you hear in a pump are mostly the cavitation.

Just like the animation that I am sharing with you, the bubble implodes/collapses to induce micro cracks on the surrounding parts.

Effects of Cavitation

Typically, it is impeller that gets affected. Just like what you see in your screen right now.

NPSH

That’s why Net Positive Suction Head is very important for a pump system.

For a system, it is typically a Net Suction Pressure derived from the pressure loss including friction loss of pipes, fittings and equipments minus the vapour pressure of the pumping liquid.

For the pump alone, it is the suction head value when the Differential head reduces by 3% during the pump testing.

Typically, the NPSH margin is to be kept around 0.6m or more to ensure a longer impeller life and in general a longer pump life.

Conclusion

That’s all I have for this session.

Hope you all learnt something today about the pump hydraulics.

Please provide your valuable feedback on this segment. This will personally help me to improve our training segment in the comments section. If you have questions , please pop them in the comments section as well.

Thank you everyone for tuning into this  session. Have a great day and hopefully see you soon in the future sessions

Image Credits:

Collapsing Bubble - Licensed under CC by NC 2.0