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# Maximum allowable H2S concentration while using SS and also while using CS? 3

## Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Hi everyone,

I have a question regarding the pipe/tubing material to use for Sour gas:

- What is the maximum allowable concentration of H2S while using SS 316?
- What is the maximum allowable concentration of H2S while using A106?
- Incase of over 200ppm of H2S concentration, what materials are recommended to be used in this case?

Temperature 20C to 60C max

Any hint or general answer or point me to where I can find the answer will be much appreciated!

Regards,
Zee

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
I appreciate it luis! :)

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

If you use the NACE spec (or similar EN) you would have to figure out which service class that partial pressure and temperature put you in.
Then you can go back to the material tables and see what is acceptable for use in those conditions.

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P.E. Metallurgy, consulting work welcomed

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

If you are referring to an upstream application, then your base documents will be ISO 15156-1/2/3. If it is a downstream application it will be ISO 17945. ISO 15156 works with partial pressure of H2S rather than concentration, thus, you will work in reverse: calculating the mole fraction that, for the system pressure, does not give rise to an H2S partial pressure that exceeds the defined limits. Bear in mind that these documents pertain to cracking only; there will be other degradation mechanisms to consider.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Thank you so much guys for replying.

@SJones reading through the proposed H2S limit, it shows that for 316, temp 60C the partial pressure H2S max is 0.1 bar

"Calculating the mole fraction that, for the system pressure, does not give rise to an H2S partial pressure that exceeds the defined limits"
I didn't get how to take that partial pressure of 0.1 bar and use it to calculate the concentration of H2S in ppm. Can you please try to explain it to me using the above mentioned H2S partial pressure max or using an example?

Again, I appreciate it so much!!

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

ZZ, what is your max system pressure?

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P.E. Metallurgy, consulting work welcomed

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

Partial pressure = system pressure x mol fraction

Thus, when you have the system pressure, then 0.1 bar a/system pressure = mole fraction.

Remember: ppm is a fraction based on a whole of a million units.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Okay maybe it is better to explain the process.

Natural gas extracted in different locations varies in hydrogen sulphide concentrations. Those natural gas suppliers say "this is our natural gas and it contains 50ppm H2S"
other suppliers say "this is our natural gas and it contains 30ppm H2S" and so on. Hence what I am trying to figure out is what material (pipes, fittings,
valves etc) can we use to transfer this sour natural gas to be treated? If I can use 316ss at those H2S concentrations, would it still be viable if for example for another supplier that says their natural gas contains 200ppm H2S?

Our company take natural gas from suppliers, compress it to 250bar, transfer it by trailers, then decompress it using prms (pressure reduction and metering station) down to 2-4 bar depending on our clients need. Flow can range depending on client need ranging from 150 scmh up to 1500 scmh. Temperature min 5C and max 60C.

I am just learning as an intern..Thanks!

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

You need to know the maximum system pressure (absolute) at the battery limit of the receiving equipment. There will then be an argument between those that think it's maximum operating pressure, and those that think it should be design pressure - let's park that one for another thread

50 ppm = mole fraction 0.00005

pH2S = selected maximum pressure (absolute) x 0.00005

200 ppm = 0.0002

and so on.

Check your calculated pH2S values against the limits set in ISO 15156-3 for stainless steels. For carbon steel, effectively, if the necessary manufacturing, hardness, and chemical composition controls have been instituted for H2S service, there is no real limit to the amount of H2S as there is with the stainless steel.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

What other equipment do you have apart from piping? What are they made of?

Inconel has been used as material for knock out drums in flare systems where H2S of different concentrations are received.

DHURJATI SEN
Kolkata, India

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Thank you once again for explaining it to me;

The maximum pressure that we attain in our system is 250bar, so I will use this pressure as the maximum system pressure, and lets say we have a 100ppm H2S sour gas, what I need to do is:

Calculate pH2S = (250bar) x (100ppm H2S)
pH2S = 250bar x 0.0001
pH2S = 0.025bar
Now I take this partial pressure (0.375psi) and check ISO 15156-3 Table A.2 which has material 316SS. It indicates for max Temp 60C (which is the max temperature of our system), max partial pressure H2S = 15psi, regardless of the chloride concentration and pH value. Therefore 100ppm H2S is way under the limit for my system. In fact, if I want to get what is the maximum H2S concentration for my system given the ISO 15156-3 H2S partial pressure limits
H2S conc = pH2S/ system pressure
H2S conc = 1 bar / 250 bar = 0.004 = 4000ppm
Is my calculation correct? feels like the H2S concentration is so high

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

Looks OK to me. Now you can get into the vagaries of ISO 15156 when looking at Table A.2 where you can compare the limits for 316 versus those shown for 316L (S31603) - vastly different. Also, bear in mind that it's a brave person who actually believes there isn't a chloride and pH limit for "austenitic stainless steel." Also, carefully, hidden from view, is that the table does not apply to welds unless they have been post weld solution annealed, and comply with the chemical composition requirements.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

The pH and Cl limits for 316L are rather low, especial at 60C
At neutral pH the pitting limit is 100-200ppm Cl.
At a pH of 5 (easy to see if there is any CO2) the pitting limit is more like 10-20ppm.
Crevice corrosion under deposits wills start more easily and welds will also corrode more easily.
Then there is the issue of chloride stress corrosion cracking.
While most references will tell you that to also need oxygen I can assure you that while this may work for downhole hardware any fluid at the surface has oxygen in it. No system is that tight. And S aggravates CSSC. And 60C is plenty hot for it happen.
The risk with steels and H2S is related to hydrogen embrittlement, and austenite will not hydrogen embrittle except under extreme conditions.
Choose your steel alloy (and HT and welding) to resist the effects of hydrogen.
This is the specific thing that 15156/1-175 are designed for.
Choose your SS alloy (fab and welding) to resist corrosion.
You will need other guides for this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Amazing thank you so much,

Reading through online, I came across API 5L X65 pipe recommended for sour gas pipeline transportation (that is the application I am looking to use the pipe for - transportation) however I believe its a carbon steel. Is this a possible solution for me?

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

Carbon steel should always be the first solution to be examined, and is on the table until it is ruled out. If the gas is corrosive, it will be subject to metal loss/pitting, and that will probably become your next question. So, to preempt that, check this: https://www.eng-tips.com/search.cfm?q=Corrosion+al...

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
I have checked out the corrosion allowance threads, thanks. That made me go ahead and search all the sour gas threads on the forum.

Reading through, seems like all API 5L can be used for sour gas. However, when do you choose API 5L x52 over API 5L x65 for example? If you can shed some light on what I should be looking for?

For me, when I look at both API 5L x52 over API 5L x65, the difference that I can see is the mechanical properties of both are a bit different with API 5L X52 having lower Yield and Tensile strength. What am I missing if any?

Thank you so much!!

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

Carbon steel pipe grade selection is another question that should be asked in the Pipelines, Piping & Fluid Mechanics forum.

Not all API pipe - it has to have Annex H compliance.

The key is in the numbers: 52 = 52 ksi specified minimum yield strength; 65 = 65 ksi SMYS

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Maximum allowable H2S concentration while using SS and also while using CS?

(OP)
Oh I see, that makes sense.
Thank you so much SJones, I really appreciate your help, it means a lot !

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