Dear All

I need some clarification reg modal analysis.

* In modal analysis if the natural frequency of piping system is 4HZ and above then we said the system is safe.

* I want to know how they arrive/fix this value ie 4HZ??

-Is it based on deflection of piping/pipe span?

-Is it then how to know the max allowable deflection?

Please explain in detail.............

Thanks & Regards
GS

Search the forum . You will get ample references on the same.

A simple response: There is no theoretical basis for 4Hz ( not to the extent of my knowledge). It is based on a generalization of most systems where a 4Hz lowest natural frequency exhibits a "well supported system".Hence it is experience based rather than theory or experiments.

This is not applicable where exciting frquencies are close to this value and in terms of acoustic induced vibration.

Regards

Here is what I know about natural frequency (fn). If any comment from others welcomed. Every structure (pipe is tubular structure) has natural frequency (fn) with unlimited number of value lie in one interval. One fn has corespondent with one unique shape mode. When dynamic load imposes structure with f = or near to fn of structure then it will deflect with shape mode related to that unique fn.

It is important to set our piping system, especially the lowest value of fn still above the f value of dynamic load. 4 Hz is practical.

Talking about span we only interest w/ shape mode = deflection regarding to sag. We can generate fn for this one. See Chapter X of Sam Kannappan Book. The equation there considers end condition +Y, ANCH, ect. Think which one will generates the highest deflection regarding to end condition (different value of moments at the end).

I have been ever involved in one project that many branch connections have problem at their weld in 55 branches. The plan still 10 years in age. There is pre assumption that may be the vibration caused by fluid in high freq more than 4Hz for example. The vibration measured. Most are in range 2 to 3 Hz. Only a little bit (1,2,..3) has vibration in range 4,1 to 4,3 Hz. So we have taken a conclusion that it was pure caused by poor welding at construction stage. We repaired the branch w/ change stub on to stub in & add pad also or changed using reducing tee.

If anyone has experience the same and has any comment, I'll be very happy to hear.

Dear,
Thanks.....

unlimited number of value lie in one interval, I mean "unlimited number of value spread in one interval" sorry about my English...:)