When to do a dynamic stress analysis? 2

[garfio]

I've seen many recommendations about when a formal stress analysis should be done (based on line size, temperature, etc), however, I still haven't seen any practical recommendations about when a dynamic analysis should be included.

Could any of the many experienced engineers in this forum give some guidelines, examples, criteria regarding when (or when not) to include a dynamic stress analysis, and probably elaborate a little more about the types of analyzes done for specific cases?

Thanks

 

[C2it]

Dynamic stress analysis comes in three levels of complexity ...

Modal analysis, just derives natural frequencies and shapes, without any magnitudes for loads displacements or stresses.

Response Spectrum, gives loads and stresses etc., based on dynamic input in fairly simple, worst case form.

Time History, gives accurate results from time based load or displacement application at one or many locations in a pipe geometry.

Whether any dynamic analysis is required will depend on what the world (or a connected thing) does to the pipe, and what the pipe contents do to the pipe. For instance a compressor may cause vibration, earthquake may impose dynamic loads. Slug flow may cause flow induced vibration.

It is often recommended to check out the natural frequencies of pipe systems particularly offshore and some limits are recommended in codes. Those limits cannot generally be imposed on larger refinery pipework. Response spectra analysis for earthquake is now a requirement for most nuclear pipework in the UK. Time history takes alot of effort to generate the input data but gives good results.
Time history might be used to look at for example, steam pipe system response to a rapid closing turbine stop valve.

It is critically important to get masses reasonably accurate and get boundary conditions exactly correct for a decent dynamic result. Remember to include things like a valve actuator mass on a long stem for instance.

 

[LSThill]

PLEASE READ AND REVIEW

DNV-RP-D101: Structural Analysis of Piping Systems Recommended Practice DNV-RP-D101, October 2008. Page 10 design codes more or less limited to linear- static and dynamic analysis. ...
exchange.dnv.com/OGPI/OffshorePubs/Members/rp-d101.pdf


L S THILL

 

[garfio]

Thanks, that is a good document. Any other reference to the case of steam lines in power plants?

 

[LSThill]

garfio (Mechanical)

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