Bending Stress of a vessel in lifting/rigging

[s1111]

Hi all,

Could someone tell me how to hand calculate bending stress of a pressure vessel during lifting/rigging analysis.
I am using the following, please let me know if this is correct approach:

1. Solving forces and moments for Equilibrium.
2. Calculating Bending Moments at each force
3. Bending stress at each point
sigmab= M * y / I

where M= bending Moment
Y= Inner Diameter/2
I= Area Moment of Inertia = pi/4 (Ro^4 - Ri^4)
where,
Ro= Outer Radius
Ri= Inner radius

I get the correct point of highest bending stresses, however my stress values are highly overestimated (so obviously incorrect).

Please advise with a corrected approach if applicable.

Thanks in advance for your comments.

 

[desertfox]

Hi S111

It may help if you gave some dimensions of the vessel in question, for instance if your vessel is large in diameter
compared to length the beam theory might not be applicable

regards

desertfox

 

[s1111]

dear desertfox,

The vessel is approximately height of 127 feet with 8 cylinders and skirt of diameter 188 inch and 5 cylinders of 102 inch and a transition section.
In line with your comment, my query is exactly as your comment.

Thank you in advance for your help.

Regards,
s1111

 

[vesselguy]

s1111,

Two solutions to your problem:
1. Buy a copy of Compress software.
2. Buy a copy of the pressure vessel handbook by Dennis Moss. The method is essentially the methods used in Fluor for many years and proven in thousands of lifts. You can this book from Amazon.com

 

[DSB123]

s1111,
If you are performing an analysis wereby the vessel is being lifted from the horizontal to the vertical then you must determine where the reactions are the greatest which may not be with the vessel either vertical or horizontal. The worst case for the design will probably be somewhere between which has to be determined.

 

[TomBarsh]

Currently, COMPRESS does not perform a calculation for bending stress in the vessel between the lifting lugs and tailing lugs. At one time, a utility was provided that determined the distribution of shear stress, bending moment, and bending stress along the vessel in the horizontal position as picked at two points. This utility was removed when the options to add lifting/turning lugs and tailing lugs were added. The previous utility was reasonably accurate but could be "fooled" by certain configurations of vessels. We see the need for such an analysis and plan to add this back to COMPRESS at some point in time in a more robust version.

Regarding the original question as to how this should be done, your approach is correct (although I think you should take the outside surface of the vessel for the section modulus calculation, not the inside surface).

Of course, "the devil is in the details". Determining the bending moment distribution for a complicated vessel (especially if the support skirt is not attached to the bottom head) can be very tedious. But if you are doing this for a specific case, all you really need to check are certain critical cross-sections that you can likely determine by inspection.

Tom Barsh
Codeware Technical Support

 

[s1111]

Dear vesselguy,

Thank you for the information.
I will buy the PV handbook as recommended by you.

Thank you and regards,

s1111

 

[JStephen]

Why is y the inside radius? (not that it should make a big difference on typical vessels)

How do you know that your stresses are calculating too high?

What are you taking as allowable stresses?

Also, you don't mention units, but check that they are consistent throughout. (Maybe you're calculating PSF instead of PSI).

 

[s1111]

Thanks everyone for your valuable input, Tombarsh, DSB123, JStephen.

I'm comparing hand calculation results with those reported by PVelite.
I realized later that I should be using outer radius not inner and have already fixed that in my hand calcs.

I agree with TomBarsh, Compress does not perform Rigging analysis. (that's why our company bought PVelite).

My units are psi. The Allowable compressive stress range for different parts of the vessel is fairly 14600 psi as predicted by PVelite. but my question is not with allowable stress but rather with calculated bending stress.

At this point I'm not sure how this package performs the rigging analysis and I am trying to validate results with hand calcs.

Thank you in advance for your help.

Regards,
s1111

 

[LSThill]

s1111 (Mechanical)

Did your company buy the PVelite Verify/Validation manual?

L S THILL

 

[s1111]

I don't think they bought PVelite Verify/Validation manual...

 

[desertfox]

Hi s111

Your length to diameter ratio is less then 20 to 1 which is usually the ratio reccomended for use of beam theory.
Maybe bending of thick curved beams as found on this link
might help:-

http://www.mech.uwa.edu.au/DANotes/MST/thick/thick.html

regards

desertfox

 

[LSThill]

s1111 (Mechanical)

Technical Note:
April 2006 I sent TomBarsh (Structural) am excel spread sheet perform a calculation for bending stress in the vessel between the lifting lugs and tailing lugs /rigging analysis.








L S THILL

 

[LSThill]

s1111 (Mechanical)

SEE WEB PROGRAM SAMPLE: TOWER LIFTING LUG CALCULATION

http://www.pes21.com/skin/bobos21/plantengineering_e.asp

VESSEL WEIGHT

LEG SUPPORT

ASME DIV.2 FATIGUE

WIND-SEISMIC LOAD

H/EXCHANGER COST

P/VESSEL COST

TOWER LIFTING LUG

ASME DIV.1 NOZZLE

ASME DIV.2 NOZZLE

ASME SEC.1 NOZZLE

ASME IMPACT TEST

WRC107-STRESS

ASME DIV1 HEAD

VESSEL DRAFTING

The above may help



L S THILL