Pipe spans designed to ensure that the natural period of the piping system less than 0.1s

[KevinNZ]

We have a client's specification state:
"Pipe spans for larger piping shall be designed to ensure that the natural period of the piping system is less than 0.1 seconds."
Have others seen this requirement and know what is the reason behind it?

 

[RVAmeche]

The logic is to prevent/reduce the risk of vibration/resonance from occurring that will cause damage to the piping or equipment.

I don't have it on hand but I believe that comes from Kellogg's Piping Design, or it has a very similar statement.

 

[MJCronin]

This is a lot easier said than done .... A piping system is not entirely "spans" ..

Piping systems contain valves, tees, small branches and other significant items that strongly affect the fundamental frequency of systems.

Vibration can occur in any of three orthogonal directions, but the piping is only supported in one.

I suggest that you ask the client if an operational walkdown/ vibration evaluation would be a reasonable substitute for this requirement. If piping is found to vibrate excessively, supports may be adjusted/added ...

Years ago, the nuclear power plant piping were qualified by a signed/certified vibration walkdown during initial operations.... I thought it was an effective



MJCronin
Sr. Process Engineer

 

[KevinNZ]

The client did clarify it was only the period in the vertical direction between supports they wanted design checked. They seemed happy with a sample calculation of a long piping run with even spaced supports. We then used the calculated max span in the actual design.

Not an issue with other clients. In the rare case where vibration is an issue we fix that point with more support. These problems have never been a straight run of pipe. Usually at expansion loops.

 

[1503-44]

It's far more common near reciprocating compressors, where unequally spaced pipe spans may help.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

I had to get a vibration study done when using recip pumps in parallel and it was as complex if not more so than a stress analysis.

Never see it in writing before, but a good check.

At that sort of frequency (>10 htz) you avoid all sorts of wind induced vibration and more common vibrations from machinery.

I did once spot a fairly long span small pipe vibrating in the wind one time and it's frequency was about 5 Htz.

I stopped the vibration, then let it go again and the wind just started it again by vortex shedding.

Quite interesting to work out what wind speed you need to generate a 10 htz frequency on a pipe.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saplanti]

KevinNZ,

If the client does not give you information about the process there not much that you can do. The possibilities are too many as you may guess. If I were you I would ask the reason of this analysis, clarification at least on the process and details (If you are lucky enough) in case you are going to share the liability. If their focus on the period only I believe you would not have any problem with it.

Kind regard.

 

[saplanti]

I additionally suspect that the vertical branch high pressure piping has two phase flow which could be piping to blowdown vessel. This is just a guess only for boiler applications.