I'm going to be performing pipe stress analysis on the blow lines for a batch digester piping system. The piping moves significantly when digesters blow (not a harmonic movement). Has anybody done stress analysis on a similar system? If so, what type of dynamic analysis did you perform and what guidance can you give me?

Dave B,

The first evaluation of the piping dynamics would be a modal analysis. Any low natural frequencies found would then indicate where to add restraints to the piping to eliminate those low frequency mode shapes. Some designs use a cutoff frequency as little as 2 or 3 Hz, while I have seen other systems with more restraints able to eliminate mode shapes below 20 Hz on systems having a process similar to steam injection to heat water. The vibration is from pressure pulses of random low frequencies and amplitudes as steam bubbles collapse. If you need some magnitude for restraint loads, then add a static seismic load case to get some restraint load results. The general rule would be to locate at least one line stop plus one or two guides in each run of pipe, except where another restraint is already located near the elbow. The line stop / guides would have gaps set for the thermal expansion of piping.

The Poyry-Montreal office did some work for us back in 2008. You might try to contact Adrian Telishewsky, as he did some vibration analysis on an item similar to a digester system.