Minimum pipe wall thickness to avoid collapse due to vacuum 5

[yukon09]

Hi, all,
I am working on a suction gas duct (thin wall big size CS duct) with negative internal pressure. and wondering how to calculate the minimum duct wall thickness to avoid collapse? I searched the ASME B31.3. The code only gives minimum required pipe wall thickness due to positive internal pressure. I aslo ran CAESAR II program but got an error for negative internal pressure.
Any advice will be appreciated greatly!

 

[CNRock]

Hi,
I think you should do buckling analysis to determine it.Because the wall is compressed not tensiled.

Rock

 

[BigInch]

Try adding equal internal and external pressures thereby maintaining the proper pressure differential across the wall, until you get a positive internal pressure.

If that fails there's always this,

http://www-rnc.lbl.gov/~wieman//TopThinPipeAnalysis/sp8007.pdf

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MJCronin]

yukon....

Go back and read B31.3 again.....it refers you to the rules of ASME-VIII foe external pressure evaluation.

It's important that you understand that there are different modes of failures for different geometries.

There are a good discussionz of the theory of external pressure failures in the Harvey and Bednar books on pressure vessels.

http://www.amazon.com/Pressure-Vessel-Design-Handbook-Bednar/dp/089464503X

http://www.amazon.com/Theory-Design-Pressure-Vessels-Harvey/dp/0442232489

Excellent investments......

What is your shell diameter, wall thickness, material type and external pressure ?

Spill........?

-MJC

 

[yukon09]

Thank you guys!
MJC, I read ASME B31.3 again and move forward to ASME -VIII for external pressure evaluation. And Pressure Vessle Design Handbook recommended by BigInch provides excellent reference as well to get the minimum required thickness.
But when I use those reference, I find that all the shell material are metal. I can't find a material chart from Section II for FRP or HDPE to get factor A or B.
The duct I am working on is 78" Dia. 3/8" thick FRP type II.
the internal pressure is about -1 psi. I have to remove the client's concern that the duct is not collapsed at -1 psi.

 

[racookpe1978]

Design point is -1 psid.

But what is the absolute highest pressure differential that the duct will face? Startup, shutdown, purging or even transient ops could easily take the "steady-state" nominal -1 psid down to -3 psid - or up to +2 psid.

Going from 225 psia to 227 psia (or down to 222 psia) is really meaningless in practical terms - the pipe is still in compression at very nearly the same level. But here, even a little swing as gasses and eqpt cool (or heat up) means a swing from vacuum to positive pressure. And a different failure mechanism.

 

[prex]

The simple way is to use the formula for a long thin cylinder (see Roark and many other refs):
critical external pressure p=0.25E/(1-[ν]²)(t³/r³)
Using E=1500 ksi and [ν]=0.3 you get p=0.3 psi , so you are out (and you need a safety coefficient of, say, 3 with respect to the theoretical buckling pressure).
You definitely need stiffeners at intervals, then the formula becomes more involved, but Roark has an approximate one.
BTW FRP vessels are also the object of some standards, they will likely supply a method for checking.

prex

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[eliebl]

For FRP you would need to look at ASME Section X. It has been a while since I opened it. I am not sure if it contains much on external pressure or not.

Regards,
EJL

 

[yukon09]

Hi, Prex. What is the source of the external pressure equation in your post? Is it for buckling analysis?

 

[prex]

It is a very basic formula found in any textbook dealing with the buckling of shells. It gives the critical pressure of thin cylindrical shells with unrestrained ends (or very long). The buckling mode is with the pipe becoming an ellipse.
For real situations, there will be some restraint (heads, miters), and to account for that more complex formulae exist: various standards for pressure vessels use this approach enforcing a safety margin with respect to the critical pressure.
Note however that, if you need to do a definitive assessment of the allowable external pressure, you should use a standard for FRP, as the margin of safety is related to ovalization errors that may be related to the fabrication process. Also how a large flexible pipe is supported may influence the buckling behavior.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[JohnGP]

ISO 14692-3 (design standard for GRP piping) has a similar looking formula for designing against external collapse pressure:
p_c=2(1/F_e).E_h(t_r/D)³ F_e is a safety factor and E_h is hoop modulus

Stresses due to pipe support are also covered, but if you don't have access to that standard, then advice by others above should see you through.

Cheers,
John