Hello,everybody.

Greetings from China.

I am doing the stress analysis of a primary reformer furnace in a project of fertilizer factory, using CAESAR II. The operating temperature is 890℃, and the design temperature is 930℃.

I would like to know if ASME B31.3 is the proper codes to check the stresses in this system at such a high temperature. Or are there other validated methods that I can adopt to check the stresses?

Every expert please enlighten me somewhat.

Dear Du Wei,

You have not told anything about your pipe material for 930C i.e. 1706 F. You may have to perform life cycle cost analysis of such piping to justify material selection. In Oct,1,2000 issue of Ch Engg you may get some study material for such temperature application.

regards,

sam

Can you explain a bit on process, what's inside the pipe and how it works wih respect to the furnace? Is it inside/outside furnce?

API 560 and API 530.

Upper pigtai tubes:(the parts with red color in the above picture)
Material: (156)A312 TP304H
T：580℃

Catalyst tubes:(the parts with blue color)
Material: INCOLOY 800H (centrifugally cast)
T：890℃

Lower pigtai tubes:(the parts with green color)
Material: ASME SB407 N08810
T：810 ℃

Now the maximum stress (primary and secondary,99%) occurs on the parts of Lower pigtai tubes, because the allowable stress of the material is very low at such temperature.

Do you have API 530? Have you worked with Larson-Miller parameter curves? Are you aware of rupture design with linearly changing temperature or equivalent tube metal temperature? These and many more topics are addressed in API 530.
Do you have API 560? Lots of applicable stuff in there also.

Du Wei,

I think you may benefit from using a program called Mat/PRO, which is part of FEPipe. Below is the extract from the manual regarding High Temperature Guidelines.

“One of the major omissions of the piping Codes is the consideration of creep-fatigue interaction. High primary stresses reduce the ability of the high temperature material to sustain cyclic stresses. The interaction between primary and fatigue stress categories is not included in the B31 piping codes. ASME III Subsection NH includes these effects in the creep-fatigue interaction diagram. This evaluation is included automatically in the Subsection NH reporting capability in MatPRO.

1 Guidelines for High Temperature Applications

Primary, secondary and fatigue rules for elevated temperature service have similarities to rules used for non-elevated temperature service. The principle differences include:
1) High Temperature primary stress allowables are based additionally on rupture and strain limits as well as yield and tensile stress limits.
2) Secondary stresses that are calculated using linear elastic analyses must conservatively be assumed to have primary qualities because of creep-driven elastic follow-up. These potentially excessive conservatisms can be removed when a nonlinear material analysis is performed to quantify the actual strains.
3) Creep-fatigue interaction occurs. The effect can be considered many ways. The four most common methods are:
a) Reduced cyclic reduction factor based on material and temperature.
b) Reduced cycle life based on material, primary stress state, weld quality and temperature
c) Creep-Fatigue Interaction Envelopes
d) Specific Problem Correlations and rules i.e. API 579 Part 10 rules for bimetallic welds.
High temperature weld embrittlement in heat affected zones has led to a significant number of failures of steam piping components. Modern weld technologies produce welds that tend not become embrittled in high temperature service, but older weld joints, or extreme service conditions can result in welds that become embrittled after years of service and suffer creep-fatigue cracking.
The interaction of creep and fatigue is a very complex phenomena, and is very difficult to accurately quantify. The rules outlined herein take a simplistic approach to these problems following guidance provided in ASME Section III Subsection NH, further simplified by:
1) The assumption that the problem loading can be comprised of a single major pressure and temperature cycle.
2) Any nonlinear strains associated with the single pressure and temperature cycle exist equally for each cycle. The
material may strain harden, but there will be no relaxation of the strain range due to a redistribution of stress due to
strain hardening. (This assumption is considered conservative.)
3) A creep-fatigue interaction diagram gives good general guidance for the interaction of cyclic stresses and sustained loads occurring at the same point in a stressed high temperature geometry.
The methods described below are taken in part from ASME III Subsection NH, ASME III Subsections NB, NC and ND, ASME VIII Division 2, API 579, and API 530, and are intended to provide an improvement over the high temperature rules used in the B31 piping codes and ASME Section VIII Division 1. These guidelines are intended to provide allowables and stress combination methods that apply above the temperature limits given for ASME Section VIII Division 2, Appendices 4 and 5. (ASME Section VIII Division 2 only provides rules for temperatures where creep is not a mechanism of strain or failure.) A summary of the guidelines is given below

1.1 Summary of guidelines
1) Primary (or sustained) stresses must be limited to creep-rupture related stress and displacement limits.
2) High Temperature primary (or sustained) stresses add to the effect of cycling and this effect can be reasonably
considered by use of a creep-fatigue interaction diagram.
3) High Temperature ratcheting rules are similar to low temperature ratcheting rules but are not identical. These rules
are generally not employed, but can be addressed using MatPRO and the ASME NH reporting feature.
4) In high temperature stress states secondary (or thermally induced stresses) can take on qualities of primary stresses
resulting in elastic follow-up and excessive strains. It is typically too conservative to always include these stresses into primary evaluations and so designers must be:
a) Aware of the condition and avoid it, or
b) Run a nonlinear material analysis using plasochronous stress strain curves to determine the inelastic strain and
degree of elastic follow-up present.
MatPRO can help the designer be aware when creep allowables begin governing the design and with FE/Pipe provides a nonlinear approach to evaluating maximum strains to be sure that creep-fatigue and elastic follow-up does not occur.
5) Peak cyclic stresses should be compared to reduced, high temperature allowables.
6) Peak cyclic stresses should be evaluated with primary (sustained) stresses and evaluated using a creep-fatigue
diagram.”

Du Wei,

U are resposible for analysing furnace tubes as well is it?

Thanks for the enthusiastic reply from every expert.

I will refer to the relative codes and standards, and the mentioned software for more information. I do think I will learn a lot from the assets of forum.

My main job is piping stress analysis. In the current project I do the stress analysis of furnace tubes system.

Hi, API 530 declares (par. 1.3.6): "[...] Limits for stresses developed by weight, supports, end connections, and so forth are not discussed in this standard".
How do you have to analize sustained stress?
Is FEA software necessary to verify any internal furnace piping (radiant and convection) or is it enough to make calculation related to "API std 530 calculation sheet" (calculation of minimum thickness)?
Thanks for Your reply.