Austenitic Stainless Steel Materials in Section I boilers 3

[curtis2004]

What is the main reason that austenitic SS pipes and tubes prohibited to use in boiler pressure parts which are water wetted in normal conditions and in the same time they are acceptable in steam-touched service such as superheaters?
Anyone knows?

 

[boilerone]

The reason is stress corrosion cracking of austenitic stainless from halogen element (such as chlorine) content in a water solution. This phenomenon is minimum in either stress-free wet or dry (steam) with stress environments.

 

[curtis2004]

I come across with the situation in our company and really do not know what I should do. Our company supplied hot condensate heater which is installed after feed-water pumps. These heaters are presumably designed to ASME BPVC Section I and made of finned 1"OD x 0.083 tubes (material SA-249 TP304L, manifolds made of 4" Nom pipe (material SA-312 TP304, 600# x 4" RFWN flanges (material SA-182 F304), weldcaps and half couplings (material SA-182 F304) fillet welded into manifols (does this permitted by Section I?). On the outside there is hot TO exhaust (natural gas+bio gas) at ~ 600F. Hot condensate is at 200 psi pressure and heated up from 220 F to 310 temperature and not expected to form steam.
If ferritic steel is not acceptable in water servise in Section I, how come this passed through?
What is the best course of action in this situation? Do I need to rise a concern? What is the concequences of inaction? I face this kind of ethical dilemma...

 

[metengr]

curtis2004;
I doubt these are "stamped" as ASME Section I. Go back and see if you can locate an ASME stamping or data report. I would guess these are ASME Section VIII, Div 1 components, if stamped, or they were exempt from Section I stamping.

 

[boilerone]

This is a similar situation to that of lower furnace tubes of chemical recovery boiler paper mills. These type of boilers use specially-ordered 'composite tube' which has carbon steel tube inside and stainless cladding outside. The exact spec of stainless for the outside depends on your application and lowest grade is 304. I can find a list of suppliers if you need.

 

[curtis2004]

metengr:
I'm sure that these are "S" stamped units. They are designed by technician, who doesn't have formal engineering education, nor pressure vessel training, and stamped by P.Eng. drawings and code calculations are reviewed by TSSA inspector, (two) units are build by ASME Authorised pressure vessel fab shop and shipped to Minnesota.
From my prospective these are 100% Section VIII, Div 1 units. They are separate from boiler, do not have even near access to open fire, although, they installed between boiler and feed waters. Now, what is done it's done.
What would you do in these circumstances? Do I need to rise a concern with the mangement of our company, client, or National Board?

 

[curtis2004]

What is main jurisdictional borderline between Section I and Section VIII? I've taken a Section VIII course where was told that Section VIII vessels can be both fired and unfired.
I've asked technician who designed units why did he designed to Section I? He said: "All feed water economizers should be designed to Section I". I asked him where did he get this info to which his answer was: I worked for other company and they always designed feed water economizers to Section I.
I have a lot of doubts...

 

[metengr]

curtis2004;
It sounds like these components are like feedwater heaters. Obviously, if these components are designed and stamped to Section I, versus Section VIII, Div 1, you have a nonconformance related to original selection of tube material. This does not sound like a clad or composite construction as mentioned in the previous post.

What I would do is contact the local Jurisdiction and explain to them your finding. The Jurisdiction can probably take one of two approaches- either issue a waiver to allow this component to remain in service based on past performance assuming a feedwater heater type component (despite the note prohibiting austenitic alloys in water-wetted service) or require the existing material (if not clad or composite construction) to be changed out. I would bet on the latter because of Section I code requirements.

One can try to have these heaters evaluated using Section VIII, Div 1 requirements and see if you can place these under a state special (administratively).

 

[curtis2004]

Thank You metengr,
What is difference between feedwater economizer and feedwater heater?

 

[metengr]

Here is my take on this;

Actually both perform the same function where heat is absorbed to raise the temperature of feedwater (no steaming). A boiler economizer is part of the boiler proper (fired component) where flue gas heats feedwater to improve cycle efficiency. Some economizers can be isolated from the boiler, but are still part of the boiler proper by design.

A feedwater heater is a Section VIII, Div 1 unfired pressure vessel, separate from the boiler, where cycle steam is used to heat feedwater before entering the boiler. Similar function but not part of Section I design.

 

[curtis2004]

In other words if coils are integrated to boiler and there is no valves between "heat exchanger" and boiler - feedwater economizer.
Consequently if "heat exchanger" is separate (physically) from boiler, and there is check valve between them - feedwater heater.
Both of them can use (reuse) boiler flue in order to heat feed water though. Am I correct?
Thank You a lot it was very helpful.

 

[curtis2004]

boilerone:
It seams somewhat strange that austenitic SS is prohibited from using in boilers water wetted parts, but in the same time you can use them in unfired steam boilers. Is there any restrictions regarding this in Section VIII, Div I?
Thank You,

 

[metengr]

curtis2004;
ASME Section VII does not prohibit the use of austenitic steels in an unfired vessel. The approach by BPV I is that in fired service you can have contaminants that can plate out on waterside tube surfaces. Chlorides as a common contaminant in feedwater can plate out and concentrate resulting in stress corrosion cracking.

For unfired service, you don't have the conditions to concentrate contaminants on the wetted tube surface.

 

[curtis2004]

metengr:
Is it true that stress corrosion cracking occurs near welded areas where there are resudual stresses in Heat Affected Zone from welding?
Thank you for help.
curtis2004

 

[metengr]

Yes, it can occur. You need three factors working together for stress corrosion cracking (SCC);

tensile stress (residual or service applied)
susceptible material(base or heat affected zone)
environment (source of contaminants)

If you eliminate any one of the above factors, the mechanism stops.

 

[eliebl]

Also take a look at Part PFH in Section I. This permits a Section VIII-1 feedwater heater as part of the boiler proper. This may be the case here but you need to confirm that it meets the requirements of Part PFH. I know a few years back there was a HX made but the pressure conditions of Part PFH were not considered. It caused a lot of headaches.

 

[boilerone]

Just want to show what ASME Section I has on austenitic stainless plus reason explained in footnote:

PG-5.5 The use of austenitic alloy steel is permitted for boiler pressure parts that are steam touched in normal operation. Except as specifically provided in PG-9.1.1, PG-12, and PEB-5.3, the use of such austenitic alloys for boiler pressure parts that are water wetted in normal service is prohibited.(footnote 1)

(footnote) 1. Austenitic alloys are susceptible to intergranular corrosion and stress corrosion cracking when used in boiler applications in water wetted service. Factors that affect the sensitivity to these metallurgical phenomena are applied or residual stress and water chemistry. Susceptibility to attack is usually enhanced by using the material in a stressed condition with a concentration of corrosive agents (e.g., chlorides, caustic, or reduced sulfer species). For successful operation in water environments, residual and applied stresses must be minimized and careful attention must be paid to continuous control of water chemistry.

 

[curtis2004]

eliebl,
What pressure conditions of Part PFH were not considered? Is there additional conditions there?
As far as I understand if Part PFH in Section I is applied, feedwater heater can be built to Section VIII, Division I. MAWT should be chosen based on steam saturation temperature @ MAWP and could be far less then 700 F (minimum required by Section I). Correct me if I am wrong.
Thank you

 

[eliebl]

The point that was missed that I am aware of is the reference to MAWP shall not be less than the requirements of B31.1 paragraph 122.1.3.

If memory serves this was a case of an EPC company designing a boiler and subcontracting all of the parts to different fabricators. If each component was looked at in isolation there were no obvious shortcomings. It was only when the entire assembly was looked at as a boiler that there were other provisions, like the one noted above, that had to be considered.