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Hello everyone*
For stack* designed as per STS-1* i am in a situation that critical velocity is lower than design velocity.
So far i know by introducing helical strakes or other attachments* critical velocity can be increased.
But my question is how to evaluate critical velocity after introducing strakes.
Please provide your opinion/ guidance on same.
Izzy
Further to above:
In Dennis Moss* Chapter 6: special designs* it says that Vortex shedding can be eliminated by introducing helical strakes upto top 1/3 portion of stack.
As per above* does it mean that by introducing strakes there is no need to check for vortex shedding.
If so than how to validate it on paper.
After increasing thickness of components when i checked output for STS in pvelite* it is observed that still critical velocity for vortex shedding is less than the mean critical velocity.
to investigate further for the cause of failure* i checked ovalling frequency.
But i found that ovalling frequency is more than natural frequency of vessel even with less thickness of shell components. See below image for reference
Still i am not able to conclude the reason for failure.
I request you to explain the same if possible.
thanks.[ATTACH]10336[/ATTACH][ATTACH]10337[/ATTACH]
israrkhan*
Can you upload the input file or the input echo. Perhaps then we can understand better.
Regards*
Linus*
See attached Pvelite file.
Dear Lins
Not able to open this files.
regards,
sml
[QUOTE=israrkhan;348295]Further to above:
In Dennis Moss* Chapter 6: special designs* it says that Vortex shedding can be eliminated by introducing helical strakes upto top 1/3 portion of stack.
As per above* does it mean that by introducing strakes there is no need to check for vortex shedding.
If so than how to validate it on paper.[/QUOTE]
israrkhan
I've been Designing Stack , in my Experience if the Strakes Guidelines are Followed it is assumed that the Helical
Strakes is effective. Below is the Guidelines in the Stack Design Software we use.
Helical Strakes have been specified, and so no further checks will be performed for
vortex shedding, because the design codes provide no guidance on how to quantify
the effectiveness of helical strakes. The additional wind drag due to the addition
of the strakes is considered, using the shape factor entered by the user; however,
determining the effectiveness of the strake to suppress vibration, is up to the Designer.
The design codes assume that if properly designed, the helical strakes will suppress
the wind induced vibration to an acceptable level. Generally the following
guidelines are suggested for helical strake design/application:
1) 3 Continuous Strakes, 120 Degrees apart, over the top 1/3 of the stack height
2) Pitch of Strakes is 5 times the Outer Diameter of Stack
3) Strake should project one tenth of Outer Diameter, perpendicular to Shell
4) Breaks allowed at stiffening rings and attachments, but should be minimized
5) Maximum gap between stack shell and strake should be 0.1*Strake Width
6) Segments of flat vertical straks at helical locations are NOT acceptable.
7) Strakes to be applied to the top 1/3 of the stack.
8) Not to be used when interference from another tall structure is less than 15 x OD