Is there a general rule for allowable vibration transmission into structural steel? Is there an ISO spec for it?

This is a vibrating fluid bed sorter which was originally mounted in the structural steel support tower but has been relocated to the ground and mounted on a fairly substantial concrete base. There is still some degree of vibration transmitting into the tower despite isolation with flexible inlet, discharge and de-dusting connections. I haven't been to the site yet and nothing has been measured yet as far as frequency or amplitude. I work in another plant that has a vibrating fluid bed dryer and it shakes the entire tower. That one is mounted in the steel, though. The people at the new site are telling me that they were elated by the difference they achieved by relocating it to the ground floor, but the big boss is concerned about some special control piping requirements.

I developed a "let's deal with vibration" commissioning program for Nuc power plant about 30 years ago. We included structural vibration. There was a German (VDI) standard that dealt with structural vibration, and we referred to it. I can't remember "the numbers", but will try to dig it out (basement - we just moved) in a day or two.
You might find something in Harris (Harris and Crede?) Shock and vibration handbook if you have access to it
If you do any survey, use velocity units - that will help you assess severity.
For the big boss's concern re critical control piping, pay attention to layout. One of the worst layouts for piping per se is if you have a section of piping where a mass, eg a valve, is vibrating on a non-flexible section of piping. Common sense, eyes, ears works here.
For a very rough preliminary guide, I'd suggest that piping per se is probably OK at 1 ips, but sensitive control components could be in distress.

Check the below link. It may help.

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...281099291#4281099291

Further to my initial posting above:
Our work was done 25 year ago.
The Standard we referred to was the German Code DIN 4150. I no longer have a copy.
Our internal documents (developed by an in-house team, and not necessarily just adopting the standard) came up with 6 mm/sec 0-to-peak as an acceptance limit above which further investigation / correction required, and 10 mm/sec, 0-to-peak as an "immediate concern" level requiring prompt attention.
In the context you described, it would seem advisable to focus on the resulting vibration of the systems / piping/ components of concern, rather than on the structure that's driving it.

Ron,

Thanks for looking and for the input. It is helpful as amplitudes in general are on the edge of the limits you cite. In some cases they are much higher. The highest I measured on my initial walk-through was on a 2" diameter nitrogen pipe that is located four floors up. The items of most concern are the connections on the inlet, discharge and de-dusting connections which are 26" and 31" diameter 10 ga duct with rolled angle bolt flanges. These operate under 30" wg of negative pressure and must be kept charged with nitrogen. This makes air ingress a problem, so they are worried about fatigue in the connections. The half-strap hangers that they used to hang the duct work are made of 3/8" tick by 3" wide flat bar rolled into a smile with ears and bolt holes. They are hung from the support steel by all-thread rod-a fairly typical method. The first problem I see is with the location of the hanger rods. They are twisted significantly in many places to work around control piping, conduit, etc. and this is probably creating lots of stress. It may or may not be the cause, but it needs to be addressed if I am to tune any natural frequencies.

One thing I need to correct in my original post. The sorter was not relocated to the ground floor . It was isolated from the support tower and new structural steel supports were built on a concrete slab of 24" thickness. This seems to be adequate as there is very little vibration in that structure. The only transmission paths are through the flexible connectors to the duct works. On all the big ducts there is around .15 ips transmission through the connectors. On the small ones there is about .45 ips. (All at operating speed of the sorter).

As far as whether to focus on the cause or the result, the result is so widespread that it might be better to find the cause. I'm waiting on word (read: purchase order) as to how to proceed from here. I think I have a pretty good grip on whats happening here and it is a situation where we can have complete use of the tower and sorter since the process is still months from starting up. I'll collect some data, do some bump tests, and maybe some other stuff if need be then we'll decide how to approach it. I hope to go and collect initial data, then have them correct the duct hangers and collect again. I'll let you know how it turns out.

Thanks again, and thanks Ali M. Shurfura for the help.

Danny, ductwork is always a problem because of the relative lack of stiffness and because you have so many different lengths... in congested areas every section of duct may be a different length. Along with handrails, lighting masts, conduit runs, etc, this can make even a little vibration seem really bad.... something, somewhere will be resonant.

I've not seen many problems with round duct like you have, but then I've never seen anything this big under a vacuum. Flat duct panels are the worst and I've seen lots of fatigue failures, often associated with machinery that is running well below allowable limits.

Have you been able to isolate the forcing frequencies? Coherence (if your meter is 2-channel) helps a lot with that. If the duct is simply sitting on top of the cradles, it should be easy enough to insert some soft rubber under it to see it that helps (you might be surprised). You'll laugh, but there are a number of sleeping bag "pads" that have very good isolating properties. They are very cheap and readily available (Wal-Mart) and work good for experimenting.

Good luck.....

Rusty,

Rubber pads or spring isolators are possibilities. First I need them to relieve the stress so I can know what the stiffness really is.

Thanks