Cyclic loadings in pipings

[engeserv03]

Hello,

I would like to know experiences about elasto-plastic effects and fadigue under cyclic loadings, in carbon steel pipings.
I have red about "Ratchetting" effect but still do not have clearly the conditions to avoid this or practice cases where already this.
It is not difficult to notice direct applications of B31.1 or B31.3 without to verify suitable the real pulsations loads. What would be in the design practice the boundary to analyse as static or dynamic / fadigue an industrial piping.

I would thank for opinions and experiences !

 

[davefitz]

In my experience, ratcheting has only been associated with stainless steel in very high temperature service. I know of no cases of ratcheting with carbon steel, and only one type of ratcheting with ferritic alloys.

SS has a relatively low yield stress, and a relatively low thermal diffusivity, and a relatively high coeficient of thermal expansion . These factors imply that if the SS part is constrained from growth ( due to stiff endpoint boundary conditions, generally due to being welded to a stiff ferritic steel structure)and then the SS part is overheated, it will yield in compression, and leave a residual tensile stress upon cooling . After the temperature drops to room temperature, the residual tensile stress may lead to cracking if the material is sensitized, etc.

The only generic case of a ferritic alloy ratcheting, in my experience, is a damage mechanism called "alligator cracking" of the furnace waterwall tubes in a supercritical steam generator. Kind of a rare situation.

Usually a carbon steel component will have a stress raiser that concentrates the strain and the result is a fatigue crack, which limits the progress of further thermal stress damage.

 

[C2it]

Surely the self-springing that occurs in carbon steel pipe systems at high temperature is an example of ratchetting ? I have found plenty of cs piping systems that were installed stress free, but when broken out for maintenance have a permament set.

This is accepted in the 'stress range' concept of B31.3 for instance, and its cyclic fatigue effect is recognised by the 'f' factor to limit the stress allowed by a specific number of (static) cycles.

Pulsation is a very different problem which would need dynamic analysis (time history or spectrum).

 

[JohnBreen]

Hello all,

I feel a little lazy today after my "birthday weekend" so I will just post this reference:

http://www.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=9332

Regards, John.

 

[davefitz]

I guess I need to yield to JohnBreen and C2it.

One case I had seen of a ratchet on a SA335P22 main steam line was at Bull Run station- it was reported they had inadvertantly left the piping's spring hanger pins in place after a turbine overhaul, and the blockage of spring movement led to a 9-12" growth of the main steam line; the extreme stress upon heat-up to 1005 F led to creep releif and ratcheting of the pipe. After removal of the pins, the pipeline dispacement was enough to allow it to hit fixed bumpers, with resulting creep releif , and ratcheting. So, ratcheting can occur on ferritic piping if it hits a fixed bumper and remains in the high stress state at creep temperatures- it creep relieves.