Acceptable Limit of CO2 in Compressible Natural Gas 3

[DERAILH]

I am working on a natural gas compressor application where we will be compressing natural gas (1050 BTU rating) from 0-20 psig suction pressure to 150 psig discharge pressure. I am concerned about 10% CO2 content in the natural gas along with low percentage of water vapor in the natural gas. What are guidelines for acceptable concentrations of CO2 when potential of carbonic gas can be created when natural gas is compressed with presence of water vapor. Any help would be appreciated.

DERAILH

 

[dcasto]

you don't have a problem. here is a link about 1 recommendation.

http://octane.nmt.edu/WaterQuality/corrosion/CO2.htm

I've always set the cut off at a partial pressure of 25 psia before going to a softer metalurgy or stainless.

If you are concerned, just make the 2nd cooler tubes, piping and scrubber of 304 stainless. Or huse heay wall materials and monitor the thickness.

 

[SJones]

You will have to look at the phase envelope of the system to see what the water dew point will be. There will only be corrosion if there is liquid water available. Generally, for unsaturated gas, this would only occur upon shut down and cooling of the compressor. So, all you have to do is work out a predicted metal loss under the predicted periods of aqueous condensation. You can do this with various CO2 corrosion prediction models but I would recommend that you leave it to process engineers and corrosion engineers to sort out between them.

Steve Jones
Materials & Corrosion Engineer

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

 

[BigInch]

Given the CO2 percent and pressure, I guess its probably a gathering system, so I would think that liquid water and perhaps other condensates will be around somewhere. Knock those out before entering the compressors. Then be sure you use a corrosion allowance in the gathering lines and for any water condensing after the compressor, add a corrosion allowance to the discharge piping too. You might also need some drip legs in the dischrge system as well. Continue as/if necessary until you reach a dehy plant.

**********************
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[SJones]

BI,

The gas will usually exit the compressor at a temperature far in excess of the water dew point which will allow carbon steel piping with some nominal corrosion allowance to cater for any shut in condensation scenarios (the same as for the compressor). The CO2 corrosion problem tends to rear its ugly head again at the discharge cooler where the temperature is brought down near the water dew point. It needs a proper process and corrosion engineering assessment to ensure technical integrity.

Steve Jones
Materials & Corrosion Engineer

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