Cavitation Explained
Introduction
Popping a bubble can be a pleasant experience and it is a great
stress buster. But rotating equipment like a pump would totally disagree this though.
Cavitation is a process of
formation of vapour bubble, then its journey towards implosion and
microjet formation. Let’s look at each phase in detail.
Cavitation Phases:
Phase 1 – Formation of vapour
Vapour is formed by two different ways.
First is by heating.
For Eg: Let’s take the case of the water in a container.
At room temperature. The watermolecules in
the water have lower energy and moves slowly within the vessel. Whereas when
the water is heated, its molecules absorb the heat and move faster.Then at 100
deg C , the vapour bubbles form within the water,that is enough to
overcome the surface pressure to let the water molecules escape as vapour. The
temperature that causes this to happen is known as the boiling point of a
liquid .
For liquid mixtures, this gets a bit
complicated since the mixtures generally have multiple components with varied
boiling points. The vapour bubble happens at a point called bubble point which is
the boiling point of one of the component in the mixture.
Second method of vapour bubble formation is by exposing the liquid to a state of vacuum
.
Liquids under vacuum have significantly lower
surface pressure. Hence the liquid molecules needs less energy to form a bubble
and escape as a vapour. That’s why at vacuum conditions, you can boil the water
at room temperature.
Phase 2 is where the Vapour bubble deforms into a random shape while experiencing the pressure from the surrounding areas when it enters into a further high pressure region.
Phase 3 - And the final phase is where the the bubble cannot hold the surrounding pressure any more and implodes to produce a microjet. In pumps, this microjet could generate pressures upto 10 000 MPa for some cases. That is equivalent to 1 million metric tonnes of force / sq meter area.
Recirculation :
While discussing about cavitation, we cannot ignore a phenomenon called recirculation. Recirculation plays a major role inducing or amplying a cavitation.
It is a whirlpool effect within a pump which increases the liquid temperature and hence is a catalyst for vapourisation.
Recirculation can happen due to 3 possible reasons :
1) First is when the pump operates away from its sweet spot which is called the best efficiency point.
This flow point is where the pump yields the maximum performance and minimal recirculation.
Deviating the operating point away from the sweet spot , will first result in a discharge recirculation .
Then, deviating further away will cause the suction recirculation which will impact the Net Positive Suction Head Required and amplifies the cavitation.
2) Now moving on to the Second reason for recirculation which is the Pipe System design.
Shorter pipes leading to or exiting the pumps will create turbulence and thereby induces or amplifies the recirculation. Hence a poor pipe design can really amplify the recirculation.
3) Third possible reason could related to the pump hydraulics:
The pump impeller vane design, impeller tips and casing volute design does influence the recirculation within the pump. Poor design can lead to greater recirculation away from the best efficiency point which limits the pump operating range and thereby creates a lot of headache to the pump operators.
That’s all for the recirculation.
Now let’s look at the tips to control them.
Tips :
Disclaimer
Just a disclaimer on tips :
With these tips, we are trying to minimise the cavitation and not completely eradicate it from the system. Studies have shown that cavitation will always be there in the system . The key is to make sure that it is not a damaging type that leads to catastrophic pump/ system failure.
Tip no 1 :
Try to operate the pump around the sweet spot of the pump. There is a reason why API 610 ( well recognised std for pumps in oil and gas service) recommends the operating point to be within 80- 110% of the sweet spot & preferred operating range to be within the 70-120% of the sweet spot.
Nearer the operating point , the better the performance and lesser the cavitation issues.
Tip no 2 :
Please follow the good engineering practices when it comes to piping design.
One of them is the Minimum suction straight length requirements.
Recommendation is 10 times the suction flange diameter , but atleast keep the straight length to 5 times the diameter to minimise the recirculation.
Have smoother transitions when you have reducers or elbows along the way. Rougher the path for the liquid, higher the frictional losses or turbulence.
Tip no 3:
Focus on the cavitation resistant materials. Studies have shown that some materials are more resistant to cavitation erosion than the other. Invest on right materials, if you feel that cavitation damage is unavoidable in your system.
For Eg : Material A is much better than Material B. ( Source : Simon Brad paper)
Tip no 4:
Talk to pump manufacturers and get their best pump design to meet the lower Net Positive Suction Head needs. Suction performance and operating window used to be inversely proportional to each other. Suction Specific Speed which is an indicator of suction performance provided lower Net Positive Suction Head req design at the best efficiency point or the sweet spot but at the cost of the operating window. Hence NSS = 11000 US units used to be upper limit for the pumps.
But for the past few decades, CFD and other design and analysis improvements have been a game changer and it helped the pump engineers to design a pump with the best possible suction performance without compromising the operating window.
That’s all I have for today. I hope you now have rough idea of what is cavitation, how it happens and some effective tips on how to control them effectively.
If you would like to learn in the video format, below is the link for you :
Regards
Karthik
( Pump Universe)
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