Carbonic Acid Corrosion in High Pressure Nat Gas Pipeline 3

[mcmidkiff]

I am operating high pressure natural gas pipelines with 3% CO2. The gas is typically dehydrated and the lines are piggable. We have issues with loosing our dehydration for periods of several days at a time. The gas is water saturated upstream of the dehydrator and cools 20 or 30 degrees F as it flows through the pipeline. How big of a concern do I have with carbonic acid corrosion? The area is very hilly. Is gas velocity a major issue? Is there a velocity above which I would not have an issue with this corrosion mechanism?

 

[zdas04]

Some recent work at Gas Unie indicates that gas velocities above 3 m/s (call it 11 ft/s) will keep water mobile. I've found lines in very hilly terrain to still collect some condensation above that velocity at a nominal 50 psig operating pressure. At high pressures, I'd be pretty confident in a line with a superficial gas velocity above 11 ft/s.

As you obviously know, acid attack is only a problem if liquid water absorbs the CO2. If you have very firm rules that pigs will be run within a week of a dehy outage, I would be very suprised if you ever see any CO2 corrosion. The low-pressure system I operated had 22% CO2 in wet-gas gathering service (no dehy's) and with a rigerous pigging schedule we never saw any CO2 corrosion. Non-piggable steel lines were another issue that wasn't nearly as pretty.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[SJones]

Have a play with the corrosion model that can be downloaded at:

http://www.standard.no/imaker.exe?id=1369

Then work out how many upset events it takes to consume the corrosion allowance over the lifetime of the pipeline. This sort of information may guide you in determining the necessary response to upsets.

Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman