High temperature S&Tmaterial

[vessexst]

Hi all,

We are designing a high temperature S&T exchanger having the following design conditions

Shell side
Design P: 2.5 Bar
Design T: 160 Deg C

Tube side
Design P: 2.5 Bar
Design T: 480 Deg C

We used the following materials for tube side owing to high temperature
Channel shell: SA 387 Gr.P91
Tubes: SA 335 Gr. P91

This equipment is in severe sour service and NACE MR0175 compliance is mandatory.

However, client has rejected this material owing to its high hardness value. To this, we gave a response that if we order this material with NACE and HIC compliance, the material should be having a hardness of HRC22 (max. Limit specified by NACE MR0175). As per ASME SECII, Part A, the hardness levels of SA 335 P91 Pipe vary upto HRC 25. So, if we specify NACE + HIC for this material, it should be supplied as HRC 22 max hardness.

To this, client has stated that NACE MR0175 is applicable for low alloy steels whose definition according to NACE MR0175 is 5% max. Since, our proposed material has 9% chormium, it does not fall under this category. However, as per ASME Sec II, Part A, this material falls in low and intermediate alloy elements. I am confused now.
Should I give any explanation to client or is the client right in this.

 

[MJCronin]

I believe that you need to engage the services of an experienced metallurgist ....

MJCronin
Sr. Process Engineer

 

[r6155]

It is the equipment user's responsibility to select materials suitable for the intended service.

Regards

 

[vessexst]

I think this material is not covered by NACE. It has an alloy designation of K90901 and P No. 15 E

Can anyone confirm this understanding?

 

[EdStainless]

There really is a 5% alloy limit?
I know that there is a 1% Ni limit for steels.
I used to by P91 to the HRC 22 max all of the time. It was Q&T with high temp temper.
I believe that HRC 22 is also the limit for martensitic SS, so it works for me.
But 6155 if correct, unless you were hired specifically to select material then it is the end users job.
And if they hired you to do it what they doing second guessing you.
Is this a u-bend HX?

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[vessexst]

@edstainless steel

Yes. NACE MR0175 defines low alloy steel as having 5% alloy element.

But,I think this material is not covered under NACE MR0175. I have checked the specific alloy designation and P No.in NACE standard and I could not locate one.

However, you are saying that used to buy P91 having HRC 22 hardness. Can you please elaborate?

Yes. It is a U tube exchanger

 

[EdStainless]

We had written our own spec that referenced the ASTM/ASME and then also had additional requirements.
One was that it had to be Q&T (N&T or SR not permitted) and another was the HRC22 max (though our spec actually had strength limits in it that I don't remember).
We did allow re-tempering so that if material was available Q&T at a higher strength they could just give it a higher temp temper and meet the spec.
We were using this in oil wells though we didn't have to comply with MR0175, We used it for high Cl + CO2 applications.
I asked about Ubends because you will need to also pay attention to how they are HT after bending, typically it would be re-tempered.

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[vessexst]

My question now is whether this P91 grade is suitable for sour service or not. Since, I did not find it listed in NACE MR0175 standard

 

[EdStainless]

What category of service?
Any CO2? Any water? Any Cl? What is the in-situ pH?
It will be better than 1% Cr, but is it good enough?

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[waliq]

Moreover, can anyone tell if SS 321 and SS 347 grades are covered under NACE MR0175. I could not find the exact grades and hardness, temperature limits. I intend to use them for an equipment having design temperature of 400 Deg C. However, as per Annex A of NACE MR0175, the Austenitic stainless steels have a temperature limit of 60 Deg C. Can anyone confirm? Am I misinterpreting something. Please consider the attachment

 

[EdStainless]

You don't need 321/347 you can do this with 304L.
These alloys fall under that second row, in low Cl they might be usable.
The limits of them are not the HS2, it is other factors.
You do have to design using the lower allowed stresses and you will loose heat transfer.
But you can use higher flow velocities.

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This kind of application is out of my wheelhouse, but it seems to me there are less exotic alloys than P91 that would work for your design conditions.
p.s., I believe that if we knew then what we know now, P91 might never have made its way into the Code, at least not for very high temperature creep situations.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[vessexst]

@edstainless steel: Regarding what @waliq asked about 321/347, is there a limit of temperatures in NACE??
If so, even 304L might not work since the deisgn temperature is 400 Deg C in his case and 480 Deg C in my case

And can Austenitic steels only be used upto 160 Deg F as per the attachment of NACE MR0175??

 

[weldstan]

With the stated pressure and temp, why choose grade 91 when grade 22 would be a more appropriate and I believe less costly and certainly easier to meet NACE hardness requirements with PWHT? Grade 11 has also frequently been used at the 480C temp.

 

[EdStainless]

Yes Stan, 22 would have been my first thought, unless the service category is too severe.
But then we haven't been told the pH2S, CO2, pH, or anything else.
As to the SS read the remarks in row 2 of the table, " These materials have been used without restrictions ..."


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[r6155]

Avoid cold forming of tubes. Change design to floating head.

Regards

 

[weldstan]

Ed,
One would assume a chemical process engineer and corrosion metallurgist would be involved in material selection but it appears not to be the case. Without knowing what the tube side and shell side fluids are, it's anyone's guess.

 

[EdStainless]

r6155, I would rather see u-bends with proper post-bend HT.
With this kind of temp swing it reminds me of a power plant feedwater heater, though this is much lower pressre.

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[vessexst]

@weldstan: You are right. However, we had preferred P91 then due to its higher allowable stress.
The H2S in the stream is about 2gram mole%.
Can you specifically tell me wherecare the hardness requirements for 304L , 321 and 347 mentioned in NACE. I could not locate them

 

[weldstan]

vessexst,
I think Ed answered your question re hardness of SS, "used without restrictions..."
Unfortunately, I do not have access to MR0175.
With the low pressure given, you need not be so concerned with allowable stress values re costs.