No, you can't prevent Cavitation

So truth be told I'm a horribly random consumer of information. Even after the advent of the internet (more on this in a moment), you would often find me in a second hand store leafing through the books section. Many of my most interesting reads over the years have been come upon by this happenstance.

For example in the summer of 1989 I taught myself to speak and read really mangled German from a Werner comic book I came upon while working as an intern in Crailsheim. My work mentors were unimpressed but at least I avoided having to learn noun genders ;)

Of course the advent of the interweb opened up many more avenues for randomness as I am sure readers can attest to being trapped in a YouTube autoplay* or clicking on link after link working deeper into some subject du jour.

(* ok technically this one isn't random since the technology gods are deciding for you, so do yourself a favor and turn it off. Your brain will thank you later...)

Cavitation misinformation delivered crisply

So it was in that spirit of randomness that the previous evening I ended up at the June 2021 edition of Pumps & Systems. As a magazine it has much to recommend it. I've found it valuable in keeping up with news about the industry - who bought who and what new products are being launched etc. However what caught my eye further down the page was a section offering White Papers on various interesting subjects.

One of them titled "What is Pump Cavitation & How Do I Prevent It?" sounded interesting. Readers who have waded through some of my articles will know that I find a strange joy in cavitation and the suggestion that cavitation in centrifugal pumps could be prevented (more on this later) was something I was curious to learn about.

It didn't go well after that...

Misinformation never sits well with me, particularly in an age where so much of it permeates the interweb. It is even more concerning when that misinformation comes in the form of a White Paper from a major supplier of pumps.

I'm not going to inflict the whole document on you - you can do that for yourself if so inclined by going to this link. Instead I'll cover the key error that is repeated several times in the paper. The White Paper claims:

"Suction cavitation occurs when NPSHa (available) is less than NPSHr (required)."

This is completely wrong and a misunderstanding of the physics involved in a centrifugal pump. Here is why:

• Consider that NPSHr is normally determined where the pump head has already degraded by 3%. By that point there is already extensively developed cavitation in the impeller.
• Simply raising NPSHa slightly above NPSHr does not result in that cavitation magically disappearing. You need to go to much higher suction pressures to completely eliminate cavitation.
• For an "average" centrifugal pump, to completely eliminate cavitation, the NPSHa would need to be >500% of the NPSHr (specifically the NPSHi point shown on the example below).

Since no centrifugal pumps operate with that amount of NPSH margin we can say that :

All pumps operate with suction cavitation. The only question is whether it is damaging cavitation

For those of you curious to get further details, you can go here or watch the video below for more information (the most relevant section is at time stamp 26:10 to 30:00).

There are plenty of other technical errors in the White Paper such as talking about the impeller vanes "striking the vapor" and the claim that the cavitation implosion strikes the "leading edges of the impeller". But they are less significant compared to the primary error discussed above.

When your sources fail you, find better sources

So in summary, we can conclude the following:

1. You cannot prevent centrifugal pump cavitation** Instead please consult expert sources for guidance such as the Hydraulic Institute Standard 9.6.1 on the recommended NPSH margin for reliable pump operation.
2. Beware of anonymously authored but authoritative looking White Papers regardless of the publishing source. Real subject matter experts (SMEs) in this field will stand behind authorship of what they write.
3. Simon's joy in exploring obscure and tedious pumping subjects remains undiminished.

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Obsesio positivum

**I am certain someone in the comments will point out that some critical pumps in BFW and water injection service are designed to operate with "zero bubble" i.e. without cavitation inception. This is true. However the chances of an average pump user encountering such a pump are 2 to the power of 276,709 to 1 against ;)