Vortex Shedding on Tall Process Towers 5

[lmacallndong]

I am designing a 300 ft tall x 26 ft diameter tower. The tower would need a thick skirt and thicker bottom shell sections due to vortex shedding...around 1 5/8" thick. Without considering vortex shedding, the skirt and bottom sections would just require 1 1/4" thk sections.

Alhtough the Client spec states that vortex shedding calcs are required only when the tower L/D ratio is greater than 15, I still prefer to check for vortex shedding especially for a very tall tower.

My checker does not want me to perform the vortex shedding calcs. He is also saying that the presence of ladders and platforms will ensure that vortex shedding will not occur. I don't believe what he is saying. In my opinion, the L&P will just minimize vortex shedding but will not completely eliminate it.

Is it true that towers with ladders and platforms will not experience vortex shedding?

 

[DSB123]

Think you are doing the correct thing. So your checker knows that there is no direction for the wind to hit the tower where there is not a ladder. You must have ladders all the way around the column then?

 

[fegenbush]

While it is somewhat of a black art, you could install helical strakes on the tower to eliminate vortex shedding. That would eliminate the scenario DSB123 describes where there is some angle at which the wind could contact where there is not a ladder or platform present to break the vortex.

 

[lmacallndong]

DSB123,
The ladders or platforms does not go all around. All L&P's and pipes are located within a 180 degree orientation. The rest of the vessel orientation has no attachments. The reason I am wary about not considering vortex shedding is that the trays (120 total) will be installed after erection. During empty or operating conditions, the trays help in damping wind-induced vibrations so there are no issues when the internals are installed. The trays are approx. 2/3 of the vessel fabricated weight.

I will challenge my checker. I am responsible for the vessel design. I don;t want my vessel to vibrate like the 90 meter chimney in this video.

http://youtu.be/hvg0QQt8Ifk

fegenbush,
I have not yet seen helical strakes used on towers. I normally see them on stack or chimneys. I never had vortex shedding issues before because most of the towers I designed were erected with internals installed. With the diameter of the tower I am currently working on, the internals vendor wanted the trays installed after erection.

 

[DSB123]

If you are responsible for the Tower Design then ignore the checker and do what you beleive to be correct ans safe!!!!

 

[fegenbush]

lmacallndong,

You are correct that helical strakes are uncommon for this low L/D ratio. The link you posted recommended using a damper system at the top of the structure. You could use a false tray to contain this damper system and the false tray damper will be removed when the trays are installed.

 

[racookpe1978]

You have already "paid" for the difference n cost between a 1-1/4 tower skirt plate and a 1-5/18 tower skirt plate in the engineering time spent going back and forth.... It's only a few hundred bucks in steel weight.

Use the heavier plate.

 

[jte]

Well, the added weight is roughly 1,200# per foot of height of shell which is increased in thickness. That's not trivial.

This added weight could mean the difference between rigging / erecting choices (quantity and size of crane, tailing beam / baseplate design), it could impact foundation design, it could impact shipping issues and routes (e.g. it may push the vessel over a weight limit of a bridge), etc, etc. Further, it may be the difference between PWHT for code reasons (if not required for process environment / metallurgy). This, naturally relates back to future repair issues as well: A field PWHT of such a vessel would not be something to be taken on lightly.

So I'd say it is well worth a bit more time spent on enginering.

One resource that hasn't been mentioned in this thread is ASME STS-1 which includes a section on dynamics and strakes to combat vortex shedding issues.

 

[JStephen]

Note that if you marginally have a vortex shedding issue, you can add metal to adjust the frequency where it's not an issue, or design for fatigue, with whatever that entails.

I believe in Bednar's book, he mentions the L/D = 15 limit, specifically, that vortex shedding has not been observed at lower L/D. It might be worthwhile looking into the source of that in more detail. There are multiple ways to approach the design, and all of them are going to be dependent on the design methods formulated by others. Using Method A to fix a potential problem at great expense while Method B clearly shows you don't have a problem is not necessarily the ideal approach.

If this is only an issue during erection of the tower, you might be able to use a reduced sustained wind speed to evaluate the design.

 

[lmacallndong]

JStephen,

Bednar also mentioned about the Zorrila methodology, i.e. performing dynamic analysis when W/Ld^2 is less than 20. And so does other authors like Dennis Moss and Kanti Mahajan. Moss even mentioned that just using L/D > 15 as the criteria for performing dynamic analysis is unconservative. I don't know why he mentioned it. Can anybody shed light?

If you are familiar wind vortex shedding, it will occur at lower wind speeds.

My checker wants me to place my faith in him thgat vortex shedding will NOT occur because of the L&P's and because of the low L/d. He is probably right, but as an engineer, I want to exercise due diligence by assuring myself through calculations that even if vortex shedding occurs, my vessel will not be damaged or even cause panic in the construction site when they see the vessel oscillating.

 

[SNORGY]

To me it's really simple. The issue appears to be whether or not to do the calculation.

If you do the calculation and it proves that there is no problem, then there is no problem.

If you do the calculation and it proves that there is a problem, you can correct the problem.

If you don't do the calculation and it turns out that there was no problem, you will have been lucky in that you have succeeded in designing something that you didn't really know enough about but you got away with it.

If you don't do the calculation and it turns out there is a problem...well, that could be a problem.

Me...

I'd do the calculation. It makes no sense to me not to. I can't see myself caving to the arm-waving ill-founded dogma of some checker telling me that towers with ladders and platforms render them immune to vortex shedding effects. I am glad you see it the same way.

 

[TomBarsh]

Snorgy puts it very well.

The question is a bit like the situation encountered in the design of the Citicorp building in New York City in the 1970's or so. A highly prominent structural engineer handled the analysis of the building's unique design that cantilevered out over adjacent property. Sometime later an architectural student studying the building discovered that there was no recognition given to the effect of diagonal wind on the building. She questioned the engineering company and was given a pat answer by a representative. Somehow, the question made its way to the top guy, the famous LeMessurier, who at first thought it was unfounded but then realized the correctness of the student's issue. This resulted in a large project of retrofitting of additional structural components after the building was complete and occupied. There are a number of articles available on the web about this, here is one

http://www.theaiatrust.com/whitepapers/ethics/study.php

The point I make is that you (the OP) may be the one pointing out that the emperor has no clothes.

The science of vortex shedding is a bit of a black art, lots of unknowns and black box equations. But it's certainly a real phenomenon. Crunch the numbers and present them to the vessel owner. The critical wind speed of the completed vessel might be sufficiently high that vortex shedding is unlikely (but does the owner want to take the risk of that "unlikely"? how "unlikely" is it?). But during field erection of the bare vessel the critical wind speed might be low enough that dynamic wind effects may be a realistic, serious issue. Oftentimes in engineering we can say that something is "OK by inspection", generally based on some logical evaluation. But for those situations where there may not be such a logical evaluation or comparison to other load conditions we have to crunch the numbers; with that information then we can evaluate the risk and proceed as desired. It may be that a wind tunnel test of the vessel is desirable, for both erection and operating conditions. Such testing is actually more affordable than I would have thought.

Technically, I think it is correct that the presence of ladders, platforms, etc, will mitigate vortex shedding to some extent. They will serve to increase the critical wind speed. If the critical wind speed is greater than the design wind speed, then 'no problem'. But if it is less than the design wind speed then some judgment needs to be applied. I believe it's stated in the Bednar book that vortex shedding need not be investigated if the critical wind speed is greater than 60 mph; this is based on the idea that winds of this speed are unlikely to be sustained for a long-enough period to induce the dynamic effects on the vessel of vortex shedding. That's a judgment itself (60 mph). But by knowing what the vessel critical wind speed is you can make the judgment as to whether it is a possibility that may govern the design of vessel.