Hi Sajit
speaking out of school, several issues stand out:
The picture has cut short my response in order to avoid hurting feelings.
No, numerical methods will not solve your problem(s) because they need a source of excitation. Perhaps some of the better programs may predict some self induced effects, but I think you will move faster by changing the natural frequency.
Scary!
Cheers
Steve McKenzie
Hello Steve,
I am glad you have taken up my problem.
We did some Caesar Dynamic runs to find out the natural frequency of the system.
The Caesar results give several modes of vibration starting from 1, 2.887, 4.351, 5.23, 5.809, 7.055, 9.047 Hz etc.
2 cases were modeled.
However during the dynamic run it was noticed that the very same supports that were actually ineffective at site, had lifted off in the analysis. The operating case was selected as the static case in the dynamic run.
The natural frequency results are the same, as the number of effective supports were the same in both the cases.
For a 24" gas line the sustained support span can be as high as 25 mts. The support span that I have is only 4 m.
The vibration must have been significant from the damage. Please see the attached 2 jpgs. The 100_0063 shows the vent line going to the HP flare. This connection had snapped of the 3/4" cross section. 100_0012 shows the pipe after the repair was done, waiting to be put back. The snap was fortunately after a block valve.
With the temporary arrangement that I have, the pipe when it tries to get back to its original length, when temperature falls, will be restricted, this will put additional loads at the end where it tees into the 48". This may not be any problem though.
I can put in hold down supports such as the ones like, spring loaded clamps with a rubber or a synthetic soft material, to suppress the vibration. A suppressed vibration, will it not lead to an eventual fatigue failure? Considering the criticality of the service. Moreover operations have plans to increase the flow rates from the current levels to significantly higher figures.
There is also an on-going job to de-bottle neck the 24" route to the 48" from the system HIPPS valve location with an additional 32" bypass. This line will join the 48" close to where the 24" joins by a hot-tap. The vibration study will have to cover this as well.
Considering all of the above, I think the study cannot be ruled out.
Do you mean to say that all those numerical methods will not be able to identify the source of the vibration?
Sajit
-----Original Message-----
From: Steve McKenzie [mailto:mechproj@xtra.co.nz]
Sent: Monday, December 29, 2003 12:19 PM
To: PipingDesign@yahoogroups.com
Subject: [Man.Distr]RE: FW: [Man.Distr]RE: [PipingDesign] Impressed
vibration
Hi Sajit
if the temporary supports are doing the job of reducing movement, then you
are moving in the right direction.
I saw a more extreme case in Indonesia some years ago. A 36" overland
geothermal steam line ripped about six pipe supports out of the ground when
it was livened. The quick solution was to box in the suspended supports and
concrete them in their "natural" location. Subsequent stress analyses
recommended minor changes, but were insignificant in terms of load
adjustment, and expensive to do, so things were left as they were.
If the small angle braces that you have added have stopped the
vibration/strain problem, then it would appear that the problem may be more
one of appearance than one of unsafe loads.
When preparing your brief for the stress analysists, be sure to instruct
them to analyse your temporary fix. I have a feeling they will say it is
fine as a permanent fix if you add a stiffener/wear saddle at the contact
points.
As for the cause of the excitation, you could spend a lot of investigation
money here without achieving anything useful. I have to deal with a lot of
poorly supported lines, and the only thing I know for sure is that if the
spans are longer than "normal", a higher percentage will flap around. Maybe
the minor volume change as they straighten and bend is sufficient to set
them off; in your case, maybe condensate pooling; who knows? Yes, I have
done the driving frequency thing; vane passing frequencies, Von Karman
shedding etc etc. You do all this, then clamp the pipe which is what a
pipefitter would have done anyway.
Chris will probably rip my head off over this.
My recommendation is to check that yout temporary fix is not causing unsafe stress levels, and to request how the fix can best be made permanent.
If you have some spare money, have a study done on the excitation source. For this type of report, I normally work on $US35,000/kg on the scales, but with packaging, unnecessary simulation printouts, superfluous appendices and covers removed. I would be interested to hear what $/kg you wind up with.
Cheers
Steve McKenzie
-----Original Message-----
From: Sajit Viswan [mailto:sviswan@tebodin.co.om]
Sent: Monday, December 29, 2003 4:39 PM
To: Christopher Wright
Cc: PipingDesign. com (E-mail)
Subject: RE: FW: [Man.Distr]RE: [PipingDesign] Impressed vibration
Those are make shift arrangements of supports. They were put in because after the vibration incident it was noticed that some of the supports were ineffective (not taking the load). This must have been either due to lifting off by thermal (solar) expansion or by settlement. I have established what support has sunk by taking spot levels. The original supporting arrangement had 20 mm rods welded on top of sleeper top plates. The rods had to be removed to put in the plates.
That launcher route is the only export pipeline which cannot be disrupted. The plant is operating at threshold (at which there is no vibration (by feel and sound))flow rates with the makeshift arrangement of making all the supports effective by the shim plates.
For the long term solution we are currently seeking the help of the specialists to study the vibration source. I am currently investigating agencies who can do this reliably. Did send the problem to a few including LMNOeng, swri, DPS.
Sajit
-----Original Message-----
From: Christopher Wright [mailto:chrisw@skypoint.com]
Sent: Sunday, December 28, 2003 10:00 PM
To: Sajit Viswan
Subject: RE: FW: [Man.Distr]RE: [PipingDesign] Impressed vibration
>There are no rubber pads, what you are seeing in the photo are 6 mm steel
>shim plates between 2 angle sections.
I couldn't tell from the picture. It still looks a bit makeshift. And
it's still not clear that you've actually fixed the problem or simply
made one of the symptoms harder to spot.
Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen. ___________________________| John Sedgwick, Spotsylvania 1864)http://www.skypoint.com/~chrisw
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