Air drying subsea pipeline - Dew point

[rsdorn84]

Hi all.
I need to figure out the resulting dew point when air drying a subsea pipeline from topside.

I figure my inputs are: Topside dew point, Topside temperature, topside pressure, Relative humidity.

By pushing the air subsea, the pressure will increase to hydrostatic pressure. Given that temperature stays the same, relative humidity will increase, but I have not been able to figure out the increase in partial water pressure and therefore the increase to RH.

Not sure if this is the right way to do it. Anyone with experience in this?

Thanks beforehand.

 

[LittleInch]

So are you trying to blow air out the end subsea?

With just compressed air?

If you need a decent setpoint vacuum drying is quite common, but can take a while.

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

 

[1503-44]

Pressure will not equal sea hydrostatic pressure, unless the pipeline is made of flexible material, like a hose, or the pipe has an opening exposed to sea water. You are trying to dry it, so I hope that is the case. If your pipe inlet and outlet is above water, the pipeline will experience only atmospheric pressure plus whatever other pressure you add.

Assume any vapor space is initially occupied by saturated water vapor. Water's partial pressure is a function of temperature. If hydrostatic pressure is higher than that, there will be no vapor space. Otherwise the pipe's pressure will be the sum of partial pressures of all vapors and gases in the mixture inside.

 

[EdStainless]

And it is typical to dry the air that you are using, the dryer the better.
To analyse this you need to use absolute partial pressures, otherwise you can drive yourself crazy.

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P.E. Metallurgy, consulting work welcomed

 

[TiCl4]

RSDorn84,

As others have said, the pressure will only be based on the system losses - depth under the sea will not add any additional pressure unless the outlet of the pipeline is also underwater.

How dry do you need this, and what is the volume of the pipeline? Compressed air can be dried very well, with -40 C to -60 C being more common dew point temperatures. Bottled N2 can be used as well if you can't get a compressor and dryer , and often comes in with a dew point of -70 C. It would, however, be prohibitively expensive if you need large volumes.

For figuring out relative humidity and specific conditions, look up "psychrometric chart" on Google. This will provide you with the needed data for making your evaluations.