Tuesday, November 8, 2011

RE: Flare and Degassing Stacks

ASCE 7 will cover the extreme wind event, but will not cover you for vortex shedding.  Vortex shedding is where the devil lives in stack design.  If you are even close on vortex shedding, use helical strakes to mitigate the effect.  Don't bother with the straight strakes, they are not effective.  If vortex shedding is even close, go with a computational fluid dynamics model.  There are some very skilled dynamicists (like Porter McGuffie) can run this effectively and with good fidelity. 
If you need to guy the stacks, you will need to reference Tubular Steel Structures by Troitsky, Chapter 6.  The stacks are prone to vortex shedding.  Guys are prone to galloping, which is similar.  Both are wind induced dynamic responses.  Galloping guys are in the same direction as the wind.  Vortex shedding is perpendicular to the direction of the wind. 

Regards, Harold Sprague

Date: Mon, 7 Nov 2011 18:40:33 -0600
From: bill@polhemus.cc
To: seaint@seaint.org
Subject: Re: Flare and Degassing Stacks

On 11/7/2011 6:31 PM, Harold Sprague wrote:
I am not aware of any significant thrust loads from the startup or operation of a gas flare.  The operational pressures are low.  The thrust from ignition is minimal.  A petrochemical blast from a vapor cloud in the general area could create some unintended loads.  A deflagration load can approach about 4 psi. 
I presume you are designing to STS 1?

Yes I am. The client had their junior engineers draw it up for fabrication. I pointed out that it MIGHT be a good idea if they would check the shell thickness as well. Silence.

"Um, well, they're mechanical engineers."

"Well, mechanical engineers typically design equipment structures."

"Our guys don't even know what that is."


"All right, but you'll have to pay me for it."

"No problem! Thanks!"

I had never even seen STS-1 - I thought that was the name of the first Space Shuttle mission. Anyway, I located a copy online - 2006, not 2010 or 11 or whatever. Close enough.

Wind loads are the only biggie, and I've got that covered by ASCE 7-10. STS-1 has wind load criteria but it looks like ASCE 7-95 or -98 or so.

I'm finished with the degas stack, a 45-footer. Now I hear the flare stack may be between 200 and 300 feet. They claim it's to be free-standing.

At V = 150 mph? I don't think so.