Cavitation in pipeline by pass

[dkf]

We have severe cavitation in by-pass loop that has the same configuration of an expansion loop.
Main pipelines are 22" and 38", (2)by-passes are 6" dia each.
Product is oil (API < 20)and we start by-passing at a 11Kgf/cm² pressure difference. Flow speed peaks 16 meters/second in the 6" sections.
Result is severe, cavitation, noise and vibration at the beginning of operation, that fades once pressure differece decreases.
There is a proposal to eliminate cavitation by replacing 6" dia pipe for 12" dia, section between valves. Once the proposal is to keep 6" existing conections and valves, there is concern that cavitation could be even worse.
The alternative to change entire 6" section by a pipe of 12" is alleged to be too expensive, once you will have to hot tape pipelines.
Maybe someone already tried this shortcut. Wonder about results. Thank you for commenting.

 

[BigInch]

I think that might not be the solution. If noise and cavitation is being initiated with the 6" valves, it may not stop cavitation in a new 12" pipe, but a 12" pipe will most likely reduce the vibration problem.

A bigger valve would be what I'd try doing next time the pipeline is off. Put in a brace or two to try to reduce vibs until then.

If I had oil moving at 16 mps through even a 2", I THINK I might be able to spring for a hot tap or two.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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[MortenA]

Maybe you should ask yourself: Why is there a by-pass and why is it smaller (why was it put there originally)? In many cases its because in case of a max dP over the main block valve - then you cant open e.g. a ball valve (assuming that its a block valves around a control valve setup)! The bypass is then for equalizing pressure. So replacing the bypass with a larger valve may have unwanted consequences.

Best regards

Morten

 

[dkf]

BigInch, thank you for helping.
MortenA, by-passes ( there are two of them for each pipeline)are used to allow scraper operation without stopping oil production. Installation is at least 20 years old, when production was much lower than present.
I am asking for opinions because experts want to implement 12"section between 6" valves, without further appreciation.
Thank you for caring

 

[MortenA]

OK - it makes sence then - but its a quite common arrangement around large control valves with block valves for isolation - because you cant open the block valves when the dP is too high.
Best regards

Morten

 

[casflo]

The cavitation probably takes place in the valves. If you change to 12" valves, it will be less, but you need to study before if the new valve size is cavitation free to manage the flow and pressure drop required by your system.
Best regards

 

[rgvrider]

Try and do a quick calculation.

If you have a CV curve of the valves, work out what the maximum pressure drop is. This will give you an idea of whether it's the cause.

My limited experience is that for such small run of pipe, cavitation would only be occurring at a fitting. Straight pipe would unlikely to have a large influence. But this is with water, I'm not sure about oil.