Cavitation in canned pump/ sealless pump

[Nick47]

Greetings everyone,
I appreciate your advise on below question:

Can cavitation on sealless pump/ canned pump cause any leakage to the environment?
The pumping liquid has high h2s and we are looking for possible risk of leakage in case of pump failure- cavitation.

Thanks in advance for your useful responses.

 

[TugboatEng]

Sure, there will be leakage every time you open the pump to repair it.

Cavitation can be devastatingly destructive to pumps. If you know it's a problem it's probably going to result in very short life expectancy for the pump. There are many options to mitigate cavitation. Can you tolerate a lower flow rate? A simple orifice plate can solve your problem. In fact, with the right orifice plate you may see your flow output increase due to less vapor in your impeller.

 

[LittleInch]

Nick,

A few more details would be appreciated.

When you say "Can / seal less", what exactly do you mean?

For me a Can pump is simply a vertical turbine pump located within a vertical "can". Usually used where you have a very volatile fluid which exists at its vapour pressure. Therefore any NPSH is generated by the length of the can.

The terminology seems to be "canned" pumps where the whole pump and motor is sealed inside a single pressure envelope.

"seal less" to me sounds like sealed pumps with mag drives, but can be "canned" pumps also.

Any cavitation is not a good idea as you impellor will fairly soon cease to exist. However it is rare for seal less pumps to start leaking, but the internal bearings could fail prematurely.

If you think you going to get cavitation then you have the wrong pump or the wrong design. IMHO.
Note that onset of cavitation is often 1 to 2m higher than the NPSH figure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

It would only directly lead to leakage if your impeller failed and mechanically damaged the can.
The other possible failure mode is the cavitation causing bearing failure, and then the entire rotating assembly damaging the can.
These should both be very rare as you can detect the onset of cavitation by pressure, power, vibration, and acoustics.
If it is a risk you could easily run multiple sensors.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Nick47]

Thank you all for the responses,

@EdStainless
I am in the same page with you, this would be highly unlikely to happened.

@LittleInch
This is API 685 sealless pump, there is a slight change to the feed liquid characteristics due to some process change in the upstream which cause suction screen plug up now and then, so looking for possible risk to the system

 

[LittleInch]

They look mainly like mag drives to me.

Suction screens eh? Urghhhh.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

Maybe someone forgot to remove the commissioning strainer and started calling it a suction screen to cover their keister.

LI, the word "canned" refers to the metal housing that encloses the pump side of the mag drive coupling. The sheet metal ones look a lot like a can.

Nick47, can your downstream system tolerate a reduction in flow to protect the pump?

 

[georgeverghese]

From what I know, mag drive pumps should only be used in clean to very clean service application (no dirt, solids, corrosion debris etc). There is only so much a coarse mesh suction strainer can do. If you use fine mesh, you'll block the feed to the pump.
Also take into account heat exchangers can and will leak at some time or other, and then process fluids get into cooling circuits / heating circuits if cooling water / heating media pressure is lower than process fluid pressure.

 

[TugboatEng]

Mag drive pumps are lubricated by the fluid which is why they require cleanliness. Ceramics help. EdS brought up some interesting points in a previous post about iron contamination in the fluid sticking to the magnets and even abrading the "can".

 

[TheHandModel]

George, there is no such thing as a clean service. There is only dirty and less dirty. There will always be little surprises that don't quite make it into the data sheet.

Every centrifugal pump(sealed, mag-drive, canned motor) has to address 3 issues in solids applications; Clotting the impeller, abrasion, and clogging the lubricating flowpath.

For clogging the impeller, this generally dictates maximum particle size and could force impeller design changes from closed to (semi)open.

Abrasion will impact materials selected and sizing as less abrasion will happen with lower speeds and minimized internal recirculation.

Bearing frames aside, all pumps require some type of lubrication. Seals need fluid to pass across seal faces. It can be a liquid or a gas and the flow can possibly go either way but that lubrication is still needed. The standard off the shelf centrifugal pump is not put into any sort of slurry service. The manufacturer would use some combination of flow modifiers and/or external flushes to keep that lubricating flowpath clean. Sealless pumps can offer similar features and the product group has come a long way since they first were introduced but the solids loading limits of sealless pumps still lags sealed pump technology.