Oxygen content is carbon steel pipe stalks

[Glenhoy]

Can anyone please help me. I am about too start cleaning some 8" x 1500m pipe stalks. This will consist of running some brush pigs and foam swabbing pigs through each of the pipe stalks with dry compressed air to try and remove as much rust, millscale, debris and the like as possible. Once the pipe stalk has been sufficiently cleaned in order to prevent any further oxidisation the plan is to purge and pack each of the pipe stalks with nitrogen to approximately 0.5 - 1.0 barg.

Can anyone please advise what oxygen content (i.e less than 5% oxygen) and dewpoint I should be looking to achieve to prevent any further oxidisation.

thanks for your help and prompt responces on this.

 

[metengr]

The key is to keep the pipe sticks dry (below 20% humidity) and not worry about exposure to air (oxidation). The nitrogen blanket will prevent any corrosion in the event moisture contacts the pipe ID surface.

 

[Glenhoy]

Metengr, thanks for your response.

In order to minimize the chance of any further oxidization/rust occurring what oxygen content and dewpoint should I be looking to achieve?

I am going to have a dewpoint meter for checking the dewpoint of the discharging compressed air upon completion of the pipe stalk cleaning and an oxygen meter for checking the percentage of oxygen following the nitrogen purge.

Thanks for help with this.

 

[metengr]

No need to go through all of this. After cleaning and drying the pipe sticks cap one end, back fill with nitrogen and cap. You are done.

 

[gr2vessels]

A small compliment to above;- filling the pipe with nitrogen, you completely displace the oxygen from the pipe, so you dont need to worry measuring the oxygen content.

 

[SJones]

The question usually relates to the quality of nitrogen. Nitrogen generation units end up with about 95 to 97% N2 and the remainder as oxygen. This triggers the thoughts about corrosion and, more specifically, corrosion product quantities. The stated treatment is already a good pipeline precommissioning standard and the reoccurence of rust even in a 95-5 N2 atmosphere will not get the pigging people in a sweat as it will be a miniscule amount compared to what you will have removed onshore.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Glenhoy]

Thanks Steve

I'm not going to be using nitrogen membrane units for the nitrogen purge nor am I going to displace the oxygen using a pig propelled with nitrogen.

The plan is, once I've finished cleaning the pipe (pigs ran with -40 def C dewpoint compressed air) to then go on an purge the pipe stalk with approximately 2 - 3 line volume of nitrogen. The nitrogen will be introduced into the pipe straight out of a nitrogen (300 bar) quad using a regulator directly into the pipe stalk. I'll have an O2 meter at the discharging end to check the oxygen content.

All of the work will be carried out onshore in a fabrication yard.

Am still trying to establish however what dewpoint I need to see from the discharging dry air along with the oxygen content to be satisfied that no rust will form whilst in storage.

Thanks for your help everyone.