Procedure for Vacuum Drying of a Long Pipeline

[SIAVASH54]

Dear Sirs/Madams

We are working on a 26" sub-sea pipeline and its length is 75 Km .
We hydro-tested the line and now we are going to dry it and prepare it to nitrogen purging stage.
The service of pipeline at this stage is sweet natural gas.
We tried injecting dry air and propelling foam pigs to drying of pipeline but this method takes long time , so we decided to use a vacuum pump for this matter.
We are buying a vacuum pump and we are going to be ready to do this but owners asks us to submit a procedure for this work. We now general steps of this job but we do not have any procedure for it.



Pipeline specification are as follows :

size: 26"
Length: 75 Km
Sub-Sea
Temp: 34 C
Thk: 12.7mm
des press: 70 Bar
Vacuum pump capacity: 500 M^3/Hr
Vacuum pump type: Oil sealing

Best Regards
Siavash

 

[Artisi]

Has anyone actually looked at the pumping time to evacuate the pipeline down to a low enough vacuum to actually vapourise the water so it can be exhausted from the pipeline?

This is not my field of expertise but I don't think the solution is as simple as it appears, the following might help.

http://hydraulicspneumatics.com/200/FPE/Pneumatics/Article/True/6460/Pneumatics

 

[quark]

Artisi,

You hit the nail straight on the head.

If I presume that the temperature in the pipe is 30C, corresponding saturation pressure is 0.042 bara. This is about 30 Torr.

With a 500 cu.mtr/hr pump, it takes,

3.142x(0.66^2)x75000xln(760/30)/(4x500) = 165.88 hours. Then the water traces start vaporising.

 

[zdas04]

It is even harder than that. Evaporation rate is a function of both pressure and temperature. The water temp is not 34C (that is proably the wellhead gas temp), it's probably closer to 10C. As pressure comes down, the air can hold significantly more water vapor per mass of air, but the mass of air is also coming down (not as fast as the water-carrying capacity is going up, but pretty fast).

The question I always ask at this point is "how dry do you have to get the line?". I'll assume that the pipeline is carrying field gas that is saturated with water vapor. When it cools off, some will condense and re-wet the line. This can be expected to start within seconds of turning the line on. So how much value did you get from sucking on this pipe for 6 days with a high-cost compressor?

On the other hand, assume that it is dehydrated. You always shoot for some amount of dew-point depression, so any water left behind by the pigs will evaporate within a few days. Either way, no harm, no foul.

Run turbo pigs after the foam pigs. Run them with an air compressor with the after cooler bypassed. Call it good.

David

 

[BigInch]

In 35 years I never had to vacuum a gas line.
Be careful you don't colapse it.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[btrueblood]

The problem gets worse - as the water boils, its latent heat of vaporization is provided by the water remaining on the walls (first), then by conduction from the walls (which is surprisingly slow compared to the heat you need to boil water, when you only have a few degrees of delta-T to drive the boiling with). Typically, some water somewhere can/will eventually freeze, and the frozen droplets (frost) will be carried along the pipe to a low point...leaving you with an ice plug.

 

[pmwinter]

You are correct that the water will cool due to evaporative cooling, and can reduce the temperature down to the freezing point. As the water temperature drops, so does the vapor pressure, so the depth of vacuum you need to achieve to continue drying also drops.
To handle this application it is important to get the right vacuum system design, along with the right size. Evacuation time is certainly one of the criteria you need to consider, but also an estimate of the amount of water remaining in the piping. The temperature of the piping will be a starting point on the vacuum you need to pull. Lastly an estimate on air leakage (all systems under vacuum will leak to some degree, and at higher vacuum levels this is critical). Lastly the vacuum pump design needs to be able to handle the water vapor without becoming contaminated by the water.

Paul Winter
Wintek Corporation

http://www.wintek-corp.com

 

[dcasto]

run 130 air with pigs until it has no free water coming out with the pigs, then run a 5 to 15 Km slug of N2 at 150 psig, then put the gas in.