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Oct 24, 2017

Stress Analysis on turbines after Mexico City Earthquake

4 comments

Edited: Oct 25, 2017

After Mexico City Earthquake, the PEMEX (Petroleos Mexicanos) Refiney located in Salina Cruz, Oaxaca, Mexico required a shut down to inspect some vapor lines connected to turbines, steel structures and foundation. This system has been operating for 40 years without any problems.

 

After a visual inspection on the lines, it was found out that several spring supports are not working or are not set properly. In order to design and size the springs, a Piping Stress analysis was performed. The calculation shows several elements overstressed and the loads on nozzles exceding the allowable loads.

 

For safety requirements, the operator has decided to replace all the spring supports based on the stress analysis results.

 

As stress engineers, how can we justify that this system is failing according to Autopipe but it has worked for over 40 years without any issues?

arturo n,
Oct 25, 2017

I feel that several situations has been ocurring in these zones, the ground or basis were moving and in the same way the supports, thus the piping systems also had movements relative to the others trying to be in a fix or adjust by weight and thermal expansions as appendages in others levels. I think that it was necessary a field survey of the positions and dimensions as built.

regards to J.Arana.

Antonio Domingo
Apr 15, 2018Edited: Apr 16, 2018

 

These kinds of situation are frequently presented in piping stress analysis and in modifications of old installations. Piping installations of 40 and 50 years old without apparent problems, but when we use modern piping stress analysis software they predict failures that never occur.

 

The right approach to these kinds of installation begins with the paragraph: “319.4.1 Formal Analysis Not Required. No formal analysis of adequate flexibility is required for a piping system that: (a) duplicates, or replaces without significant change, a system operating with a successful service record (b) can readily be judged adequate by comparison with previously analysed systems” ASME-ANSI B31.3, 2014

 

The “successful service record” are the key words here.

 

RECOMMENDED APPROACH:

 

As in the case exposed above, is a good practice check first with the owner (or client), with their operational and maintenance personal (and their reports) that there are not operational or mechanical problems in the old piping system, prior to the piping stress analysis, and before we make any changes. This to prevent any future blaming from them, if there is a problem not properly reported previously.

 

Also, is recommended to analyse and predict the “remaining live” of the old system to be studied and to be changed. Maybe, we can suggest further changes than improve it, and guarantee that it will operate longer enough without problems after the new piping, strings or supports will be installed.

 

From my experience, we need two types of piping stress reports for these kinds of installation: 1) Stress report(s) of the existing piping system with the actual stresses and loadings -even if they show red numbers- and 2) Another stress report(s) for the piping installation after modifications.

 

In agreement with the owner or client, the purpose of these reports is to ensure that the stresses and the loadings of the old parts of the modified system, are least equal, or better, below than of the old system before the modifications.

 

The question about, why old systems works fine, even in the eventuality of piping stress software apparently not approve their design, if very difficult to answer and only we can guess. I will go to this point later. (Also, look my other and final comment)

 

First, the piping stress software (like Caesar II or Autopipe) is addressed mainly to analysis new installations. They are widely and technically (and even legally) recognized tools to verify that our design is in the “safe zone” of operation. The existing old installations are based in more rudimentary approach, sometimes based in manual methods, or worse by the “trial and error method". Some errors were paid by piping failures or even by explosions.

 

In the above case, like in some others, one explanation is that the real stresses are between the standard allowable and the ultimate stress value of failure. In equipment, the explanation can be as simple like their nozzles can with more loads than designed or showed it in their old drawings and documents; sometimes the old correspondence within clients, engineers and manufactures were missing, with the approve of the old loads.

 

At the end, some mystery remains, a piping stress software approximates reality, using computer models to approach the reality of the round piping using some beam theory method. But, they are our most reliable tools to design fast and with safety. The evidence is they are worldwide and extensively used everywhere with near no catastrophic event to report. And in the case to report one, usually is because a bad o partial use of the tool, or by an error to interpret the model or their reported results.

 

Tony Martínez, Piping Stress Technical Advisor

Antonio Domingo
Apr 15, 2018Edited: Apr 15, 2018

 

Remember always that the piping stress software are tools that use the standards to help to approve the design. The standard is what approves the design.

 

Tony Martínez

Piping Stress Technical Advisor

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4 days ago

 

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