Drying Challenges in Pipeline Pre-Commissioning: Seeking Solutions

[Finchh]

Hi everyone,

I'm seeking advice on drying a 56-inch diameter, 20 km long carbon steel sales gas pipeline for pre-commissioning. The required dryness level is exceptionally low, with a water content target of less than 0.1 ppmV.

We typically use nitrogen purging for drying, achieving a dew point of around -10°C. However, this method might not reach the ultra-low water content necessary in this case.

My questions are:


[li]Drying Techniques: What drying techniques and methodologies would be most suitable for achieving a water content below 0.1 ppmV?[/li]

[li]Dew Point Correlation: Considering the target water content, what dew point temperature would be necessary?[/li]

[li]Nitrogen Purging Optimization: If nitrogen purging remains a viable option,[/li]
What amount of nitrogen would be required for purging the pipeline?
How long would the purge cycle need to last?
Are there specific equations for calculating these parameters?
What pressure would be recommended for the nitrogen purge?


Any insights or experience with similar ultra-low water content drying projects in pipelines would be greatly appreciated.

Thanks in advance,

 

[LittleInch]

This would have been better in the pipelines forum.... but its here now.

What does that water content mean exactly? Normally you define it in dewpoint. Are you talking amount of water in the vapour space in the pipeline or what exactly?

Why so low for a sales gas pipeline? You need to challenge this as it sounds to me like something someone has just made up. The sales gas spec won't be anywhere close to that tight unless its re-gassified LNG. Even then you need to define it in terms of dewpoint as 1ppmV is difficult to measure.

I've seen a lot of dryness specs, but never seen it in ppmV. Volume of what? Air I can only assume.

If air this little site

https://amsystems.co.uk/moisture-dewpoint-calculator/

is quoting -76C for 1 ppmV. That is NUTS for a sales gas pipeline. Are you sure they didn't mean 1000ppmV - which is -20C dewpoint which is totally normal.

For dewpoint minus 20 or more you often see vacuum drying work effectively. However it takes a loooong time. And longer if you don't first get every single drop of free water out of the line first.

Volume of Nitrogen?

Dunno, but my best guess is probably 10 times the volume of the pipeline at say average pressure of 1 barg to be able to flow that much gas through it and get to your dryness spec. aim for a veleocity of about 3m/sec and then work out the pressure drop you need to flow that much gas through your huge pipeline.

Note you will probably need cryogenic Nitrogen regassified, not a nitrogen generator to get the moisture content down.

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

 

[EdStainless]

Use hot nitrogen. Pull from cryo source and heat to at least 85C, hotter if the pipeline will take it.
And yes LI is correct.
The spec is nuts.
And the 3m/s velocity is correct.
A minimum of 7 volumes is needed, with 10-12 more likely for this diameter.
How did you remove all liquid water first?
Flush with alcohol?

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

 

[LittleInch]

Be careful above about 60C otherwise you'll get upheaval buckling or thermal over stress / movement.

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

 

[Finchh]

Hi all,

Thank you so much for your insightful responses!
As Mr. LI mentioned, the spec does seem extreme, but the client is insisting on it. The maximum dew point we’ve achieved so far is -10°C

Dewatering after the hydrotest will be done with a bi-di pig, followed by swabbing using light density foam pig.

The recommendation to use hot nitrogen at 85°C makes sense, and I appreciate the insight on the nitrogen volumes and flow rate.

Quick questions:

How was the 3m/s velocity determined? Is there a specific guideline or empirical data behind it?

Is there a way to determine the maximum achievable dew point during nitrogen purging to qualify the drying process?

Thanks again!

 

[LittleInch]

As said, do a stress check and upheaval buckling check at 85 C with no pressure. No point in breaking your pipeline trying to achieve a bonkers dryness spec

I would seriously consider walking away from that spec.

3m/ sec is a ROT aimed at achieving turbulent flow to not leave a damp layer next to the pipe and stimulate active drying without excessive pressure drop or waste of gas.

You can prob get within 10C of the nitrogen dew point.

I would go back and say we can do -20. Anything more is not guaranteed and every day trying will be charged at $20,000 per day. Or walk.

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

 

[shvet]

@Finchh @LittleInch
Gentelmen, am I correct that you are discussing 3 m/s of gaseous N2 as a turbulent flow? Not a laminar or stratified?
And where is a velocity reference point? At the start, end, or middle of pipeline?

 

[LittleInch]

Yes,

Anywhere.

At that velocity and distance the pressure drop won't be high

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

 

[georgeverghese]

Before the hot N2 purge step, do a few pure methanol displacement pig runs. This is what I remember from Operating Company pipeline engg guidelines for very low water dewpoint specs.
Agreed, why would you ask for 0.1ppmv water content in a sales gas line out in the Middle East ? This sounds like dry gas / dry air from a 13X or 5A mole sieves drier unit, prior to feed into an air separations unit cold box?

 

[LittleInch]

Methanol is rather old school as it's horrible to deal with.

MEG is double the quantity but far lower risk.

The spec is still nuts.

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

 

[georgeverghese]

Alternatively, ask the Company to provide you with mole sieves dry air from their plant, which usually has a water dewpoint of at least -100degC, which you can then heat up for drying out this line, Mole sieves dry NG may do just as well, but it will have to be flared off. If you use dry NG, air in the line will first have to be replaced with N2.

 

[LittleInch]

Many gas pipelines just do a "direct purge" with a gas /air interface going up the stack. So long as you keep the interface going at 3 to 5 m/ sec you get very little mixing.

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