Carbon Steel Never Fails in Steam Service

I've not heard it said outright, but my perception is many "seasoned" contractors and engineers think that carbon steel does not fail in steam service due to fatigue. The perception seems to be that carbon steel is so forgiving with such an ideal elastic stiffness, the number of cycles is generally so low, that the only reason pretzels are not made of it is because of the bitter taste of flaking pipe lacquer. And yes, I'm talking about the materials and conditions that probably bore most of the people active in this forum.A106 and A234 carbon steel in steam service at temperatures below creep.

When I think back on some 25 years engaged in steam systems and recall what failures I've seen and have been charged to remedy: bellows failure due to stress corrosion cracking, plastically deformed pipe due to frozen slip joints, valve failure due to ball joint loads, plastically deformed pipe due to improper bellows replacement, fractured supports due to piping overloads, plastically deformed supports due to pipe loads and improper spring hanger sizing. I've not seen failures clearly due to fatigue of piping or fittings in steam service.

Markl's work demonstrated the bounds of fatigue, particularly at relatively high cycling piping. For steam systems with a relatively small number of full temperature cycles, as John Luf has pointed out the Markl data is scattered.

For the present system under review, I've performed a cumulative usage analysis estimating cycles and associated expansion stress levels of 300 to 500%. The results, based on what I identified as the most likely operating scenarios and cycle history, indicate to me that the system is in jeopardy with respect to fatigue. A dry magnetic particle inspection at 700 Deg. F on the fittings was performed with no cracks larger that 1/16" x 0.007 deep found. The owner and other attending engineer breathed a sigh of relief.

I recommend this piping be replaced because of the results of the fatigue analysis and because no cracks found does not mean no cracks present or no cracks in the next cycle. I'm bracing for the response that I'm crying wolf, and would like to find examples of fatigue problems in steam systems (not from nuclear plants or other electric generating stations where the piping material is more sophisticated and the application is more critical). But I realize when fatigue failures do occur in process or central steam systems they are not problems that are written up and put before the engineering public. So, I've reviewed what current literature I thought might present some example cases (Thielsch, Wintle, some ASME PV&P Conf. pubs). Most of what I've read through these sources deal with more exotic and more critical systems.

So, my questions are these:

1. What first-hand (or second hand) experiences have forum members had with carbon steel under low-cycle fatigue?
2. Any recommendations as to where such a narrow field of failure examples might be written up or discussed?

Ken A. Nisly-Nagele, P.E.

Project Engineer, Mechanical

Applied Engineering Services, Inc.

7999 Knue Road

Indianapolis, IN 46250

317-585-8920

[Non-text portions of this message have been removed] Received on Sun May 28 11:05:00 2006