UNS S17400 Material for Valve Stem 2

[Bilal7590]

Hi all,
At our process plant, H₂S gas at about 17 bar pressure is in use. All the butterfly valves have UNS S17400 stems and SS-316L discs.
I was studying NACE MR 0175 (2009 edition) which states in Table A.27 that "No data submitted to ascertain whether these materials are acceptable for service in the presence of elemental sulfur in the environment."

Now I have 2 questions.

1) Is it OK to use stems of S17400 despite that fact that we recently found two stems broken due to Sulfide Stress Corrosion? To replace these valves with NACE acceptable material will be a uge investment.

2) The maximum H₂S partial pressure given in this Table A.27 is 3.4 kPa. This confuses me because in our system, pure H₂S gas at 17 bar (1700 kPa) is in use. Kindly clarify the meaning of this limit in the table.

Regard,
Bilal

 

[EdStainless]

What heat treat condition (or strength level) is your 17-4PH?
This is not the best choice, especially if it is at the high end of its strength range.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

1) Can you afford to wait until they all break, or even break again after replacement?

2) How is it confusing - someone has either used alternative selection criteria, or they have ignored the ISO 15156-3 limits? The meaning of the limit is: below that H2S partial pressure (and, simultaneously, with a fluid pH of 4.5 and higher) the material is considered to be qualified for H2S service without the need for further qualification testing subject to it meeting the required supply condition. The two failures so far are demonstrating the efficacy of such limits.

Steve Jones
Corrosion Management Consultant

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

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

 

[Bilal7590]

Table A.3 "Environmental and materials limits for austenitic stainless steels
used as valve stems, pins and shafts" proposes a material UNS S20910 for valve stems for any combination of temperature, partial pressure, chloride concentration and pH; but it still doesn't guarantee its resistance to elemental sulfur.
What should be the suitable material for this application along with resistance to sulfur?

 

[EdStainless]

You would typically see Ni alloys used in high H2S + S applications.
I would look for cold finished (for higher strength) material in alloy 22, 686, or 59.

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P.E. Metallurgy, Plymouth Tube

 

[TomEun]

NACE/ISO TG 299 committee (for MR0175/ISO15156) will continuously update the restriction on use of UNS S17400 for the next version because the premature failure of this material has been frequently reported.

You may have to confirm whether the 17 bar (1700 kPa) is for system pressure or H2S partial pressure. If it is for partial pressure, S17400 shall not be used. UNS N07718 (Alloy 718) or others may be considerable under the limitations in NACE MR0175/ISO15156 (latest version).

Some oil producers have more detail application guideline for the use of UNS S17400 in sour service. Here is a sample of the guideline in addition to NACE MR0175/ISO15156 requirements.

1. Maintenance of Existing S17400 Valve Stem : In-kind replacement may be acceptable for the following conditions.
(1) H2S levels of the future process design will be maintained within the original process data
(2) The in-situ pH has not reduced below the original process data so far, and also will not be reduced in the future.
(3) No failure of SSC, SCC, HIC, SOHIC, or other H2S related mechanisms has occurred so far.
(4) No excessive levels of other components, such as chloride, total sulfur, temperature, CO2 are intended. That is the original design limits will not be changed.

2. New construction of Valve Stem (S17400 of existing piping material class)
The use of UNS N07718 instead of S17400 is recommended in accordance with the updated piping materials specification.

Thomas Eun
Corrosion and Materials Selection/Design Specialist/ P.E.

 

[SJones]

"pure H2S" at 17 bar system pressure will give a partial pressure of 17 bar.

The background to the current NACE requirements is given in NACE Corrosion 2014, Paper 2014-3816

Steve Jones
Corrosion Management Consultant

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

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

 

[pradipgoswami]

Hi Bilal,
17-4 PH material is a Cr-Ni low carbon martensitic stainless steel with Cu and Nb precipitation hardening elements. It’s to be solution annealed and aged (or double aged) to acquire different levels of strength.

Strength(UTS) levels are usually higher than 100 KSi YS. 17-4 ph is very cost effective steel for valve stems, springs and fasteners, among others, where high mechanical properties, together with corrosion resistance, are required.

However due it's intrinsitc proprieties, the ductility of these alloys very seldom exceed 10-12%(QUITE LOW). All put together in environments where EAC is always a concern 17-4 Ph steel could always be a suspect.

Many offshore projects put restrictions on the usage of this alloy. However in similar vein such restriction applies to onshore oil and gas and petrochemical plants also.

Here is an Extract from ,API RP-553, Refinery Valves and Accessories for Control and Safety Instrumented Systems

5.6.3.3 Napthenic Acid Corrosion

• Basing the entire trim selection on the minimum molybdenum requirement can be quite challenging.
• One issue is shaft selection. S17400 (17-4 ph Stainless Steel) is not acceptable for resistance.
• S20910 has a range of 1.5 % to 3.0 % Molybdenum, typically around 2.2 %. Many users have no issue with this selection.
• One option is to upgrade to N07718.

A cost effective solution could a switch to Nitronic 50, UNS S20910 , if the process conditions permits. (

http://www.azom.com/article.aspx?ArticleID=9201).

Thanks.



Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299