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RE: Stack Strakes

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I have tried almost anything to avoid helical strakes myself.  All of MY 
previous efforts to avoid bar stock helical strakes failed.

If your client is serious, you will have to run 3 ropes in a helical 
pattern. I have never tried wire rope, but you will have to anchor it and 
support it at intervals to keep it from banging into the stack.  The 
potential problem is in the aspect ratio of the wire rope.  To get the 
profile required to mitigate vortex shedding will take a pretty big wire 
rope.   The equivalent weight of the wire rope will be significant.  I am 
not sure if you would want to run a CFD model, but I have a contact (Mike 
Porter) that can do that for you for a reasonable fee.  THE expert in 
vortex shedding is Bob Belvins.  But I use Mike to do CFD modeling for me. 
You might run this concept by Dr. Blevins first.

It took me a significant and expensive learning curve.  I have a source 
that became an expert on bending bar stock into helical strakes.  I used a 
guy in Chicago for making the strakes.  After some hiccups, he built a 
section of stack and check fit each strake before it shipped.  We welded 
clips on the stacks and attached the strakes to the clips to avoid a 
problem with differential thermal growth.  For my application, the 
atmosphere could go as low as -60F, the design maximum design stack 
temperature was +800F.  We straked the top 2/3 of the stack after a CFD 
analysis.

If you don't have the temperature extremes that I had, you might try using 
a tuned damper.  They are very efficient and much less expensive.  I could 
not use them because of my thermal issues.  If you can use them, a tuned 
mass damper might be the ticket.  Check out Meca Enterprises. 
www.mecaenterprises.com

The last flare stack that I did was guyed.  That was fun.

PS The API has a method of using vertical strakes.  They do not work and 
that is why ASME never picked them up.  Been there, tried that too, and 
failed.

Regards, Harold Sprague

> From: h.d.richardson(--nospam--at)shaw.ca
> Date: Fri, 1 Feb 2013 12:01:20 -0700
> Subject: [SEAINT-SEAOSC] Stack Strakes
> To: seaint-seaosc(--nospam--at)mail-list.com
> 
> Fellow engineers,
> 
> I have a client who manufactures flare systems for the oil and gas
> industry. A major component of these systems is the flare stack itself.
> These stacks vary from 40' to 80' tall with the top 6' to 10' varying 
> from 4" to 8" stainless steel pipe with a burner tip at the top.
> 
> Occasionally, vortex shedding may be a concern; my client is
> willing to go to almost any length to avoid the use of traditional 
> strakes. 
> My question "What do you think of the idea of creating strakes by
> wrapping the top third of the stack with wire rope and securing the rope 
> to the stack with clips that are tack welded to the stack?" The cost of 
> wire rope strakes would seem to be only a small fraction of the cost of
> traditional rolled plate strakes.

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