Acid Gas Material Selection

[trukerox]

Hi

I have a reciprocating compressor application needed for well a head injection disposal. The suction pressure is 12 psig and discharge pressure of 975 psig in 5 stages. Are there any reference, code or standards in terms of material selection that take into consideration H2S and CO2 in the same gas stream? I would typically go with NACE material compliment for H2S alone, but I would like to do some research first for this application.

My gas analysis is 73% H2S, 21% CO2, rest hydrocarbons and saturated water. Please, do not discuss about dehydration process since this is not part of the scope of the unit (maybe done by others).

Best regards,

 

[strider6]

I think you're talking about NACE MR 0175/ISO 15156 on H2S. That standard does not cover corrosion by CO2.

You've to look at DeWaard & Milliams Model for CO2 corrosion and all the improvement of the basic formula.
The basic formula consider only CO2, but the last, see for example L. Smith on NACE website, include also the effect of H2S on corrosion rate.

hope this help

Strider

http://www.corrosionist.com

 

[dcasto]

Nace MR0175 is the standard. You must take into consideration the status of dehydration, why so touchy about the subject?

I've done several AGI's and here's what shakes out. All water wet streams are 304 SS. The gas compressor will be mild carbon steel except valves ans rod's which will be 304 SS. The compressor discharge piping from the cylinder to the cooler can be CS, but it's not really worth the savings, so keep them SS. The coolers will be SS as well as the suction scrubbers/bottles.

The final discharge line is troublesome, if the gas is dehydrated, then CS is OK. At 900 psig you can still have freewater from the compression at around 1000 to 1200, the mixture is "hydrscopic" and it is undersaturated and therfore non corrosive and carbon steel works. I perfeer SS on my discharge line because I put the compressor at the wellhead and the run is short so the added security is worth the cost even if the gas is dehydrated. Not to be offensive, but I've never dehydrated the acid gas because I beleive the risk in that step is more than just designing without dehydration. Look at the radius of exposure calculations for H2S and you'll see what I mean. It's a tough one to convince most people about the risk.

Typically I've installed 4 stage units for up to 1100 psig and if you need to go over 1200 (criticle pressure) I'd seriously look at a pump for over that pressure. Remember to look the heat of condensation in the cooler design!

 

[EdStainless]

For places than need stainless you might be better off looking at 2205. It will be a lot stronger, so you will use less metal. And you will have some CSCC resistance, hey you never know what may happen down the road.

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[trukerox]

Thanks for the help strider6 and dcasto.

Yes, you are right strider6, I am talking about NACE MR0175-2002.

About dcasto reply, I am trying to get as far as possible from the gas mixture’s dew point line. I am also aware that the discharge line is trouble, since at that high pressure and low temp (150F) is hard to avoid some condensations. I am still considering what way to go in terms of cost effectiveness, CS for discharge lines and SST 316L for the after cooler discharge or everything SST.

However, I was more concern about any material selection mandatory regulations for acid gas in the United Sates. In other words, piping, vessels, and all instrumentation in contact with the process stream (liquid level controls, dump valves for scrubber drains, relief valves, etc.) shall meet the full requirement of NACE MR0175? Or I have to be aware of any extra regulation that includes H2S and CO2, since as strider6 said (and I agreed with him); NACE-MR0175 does not covered corrosion of CO2 in conjunction with H2S.

Thanks

 

[trukerox]

I forgot to add this.

Strider6, could you provide the link for the DeWaard & Milliams Model for CO2 corrosion, L. Smith example?

Thanks

 

[strider6]

trukerox

I don't know the mandatory regulations in the United Sates, but in my opinion there are some points to clarify:

-NACE 0175 now is NACE 0175/ISO 15156 and has some important changes respect the old version of 2002 in terms of materials requirements, for example SS 316L has a 60°C limit of temperature

-NACE/ISO give you requiremnts to select materials that resist SSC (sulphide stress cracking) and other corrosion mechanisms related to the presence of H2S, chloride... but it does not cover CO2 corrosion and other form of corrosion

-There are no, for what i know, standard for CO2 corrosion but only corrosion model that allow you to calculate the corrosion rate in presence of CO2 and H2S.


I can't find the paper of smith, but you can use this, you can purchase it from NACE website:

NACE PAPERS 02235 IMPROVEMENTS ON DE WAARD-MILLIAMS...
by Bert.F.M. Pots, Randy C. John, Ian J.Rippon, Maarten J.J.Simon Thomas, Sergio D. Kapusta, Magdy M. G

http://portal.nace.org/custom/Custo.../nacestore/search.asp?txtSearchCriteria=02235

Also on this website you can find a lot of technical publication on CO2 / H2S corrosion:

http://cheserver.ent.ohiou.edu/CorrosionCenter/publications.htm

hope this help

S.

http://www.corrosionist.com

 

[dcasto]

This topic has been up before on CO2 partial pressures and whats in NACE 0175. There have been many papers published by the GPA in the last 10 years. The most recent paper by Duke Energy showed that the latest version of NACE 0175 would allow the use of CS on thier final wet discharge line and all hot compressor discharge lines. If you think you need SS or 2205 on the abovegeound pipe, just think of the mess going downhole!!! Everyone in the industry sticks with 304, that I know of, and these are on services of up to 10 gpm of injection fluids (thats about 1.5 mmscfd).

In carbon steel piping where CO2 is present, H2S improves the resistance to corrosion by laying down a protective layer of FeS that inhibits Carbonic acid attach. At one facility they added a few ppm H2S to the Amine still overhead to protect the piping.