A Basic Thermal Analysis question

[scadds289]

We are doing a solid finite element liner static study of a steel pressure vessel which is subjected to dead loads, internal pressure and a temperature load of 700°F. While the allowable stress for the material under ambient temperature is 20,000 psi, it is 18,100 psi at 700°F. While the actual stress under dead loads and internal pressure (without the thermal load) is 16,600 psi, the stress at combined dead+pressure+thermal loads is 29,300 psi. Under this scenario, I would conclude that the vessel is not structurally safe since the actual stress of 29,300 psi exceeds the allowable stress of 18,100 psi under operational conditions. However, others in my team are of the opinion that since the allowable stress at 700°F is the criterion for comparison, the effect of temperature is already taken care of and therefore, the thermal load should not be applied on the finite element model. Only dead+pressure loads should be applied and the maximum stress under this condition is to be compared with the allowable stress at 700°F to decide on structural safety. I feel this opinion opposes the very basics of Statics. Please let me know if my decision is right or wrong.

I thank you all for your precious time and valued opinion.

NK
Structural Engineer

 

[TGS4]

Are performing an evaluation of an ASME pressure? What is the Code of Construction?

 

[scadds289]

Yes, it is a pressure vessel designed per ASME Sec. VIII, Div. 1. Allowable stresses are taken from Section II, Part D, Table 1A.

 

[TGS4]

Read U-2(g).

Now, general "good practice" in the field of pressure vessel engineering to to follow the rules in ASME Section VIII, Division 2, when it comes to using FEA to evaluate pressure vessels.

Judging from your post, this is the first time that you have done this sort of thing. Take this piece of advise: hire a competent consultant to help you out. This is not the type of thing that you can learn overnight, or through an online forum. (It takes about 1 year for me to train a young engineer on the ins and outs of FEA for pressure vessels - even longer if they have no experience regarding pressure vessels.)

The long and short answer to your original post, however, is to refer you to Division 2. The answer depends on the cause of the stress, as well as whether or not you are talking about a membrane stress or a membrane-plus-bending stress, or even a peak stress. And have you given any thought to transient thermal stresses, or for that matter, fatigue?

 

[rb1957]

i think the fact that the allowable is reduced at temperature is completely unrelated to the stresses reacted at temperature. the structure is clearly constraining the thermal strains and so the structure is creating internal stresses.

therefore i think your assessment (that 29.3ksi > 18.1ksi) is correct and the structure is unsafe.

mind you, isn't 20ksi a pretty low allowable for steel ?

 

[corus]

The stresses have to be classified under primary (dead loads etc) and secondary (thermal) stresses. The limit for primary plus secondary is 3f or twice yield, whereas the limit for primary membrane is f (or Sm as it is in ASME I think). I'd recheck your allowable stresses for the two cases, and of course use the value of f at the design temperature.

corus

 

[desertfox]

Hi scadds289

I agree with you, I believe the vessel is over stressed as well, but check the code to make absolutely sure.

desertfox

 

[Pawel27]

Thermal load should not be applied. Result (stress) should be compared to allowed stress at temperature 700. I do not know your structure but thermal expansion is rather not blocked at pressure vessel and can be classified as a self - limiting. If thermal expansion would be blocked, then there can arise very large stresses and coupled analysis
is necessary (mechanical - thermal).

 

[prex]

Both the judgments you express are wrong.
Of course no temperature stress is accounted for in the allowable stress at temperature! But also your vessel is not necessarily unsafe.
As already stated above, you must classify the stresses into the primary and secondary categories, and in almost all cases (but by no means all!) a thermal stress is secondary (or peak). Now, under the coverage of ASME VIII Div.1, you are not necessarily required to consider, or even calculate, secondary stresses. An example that comes to mind is the thermal stress at the skirt to head junction of a vertical vessel at temperature (if fatigue is not an issue), where high thermal stresses are present even at relatively low temperatures.
Not knowing the origin of your thermal stresses, it is impossible to say more. Come back with more information if you want further advice.

prex

http://www.xcalcs.com

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http://www.megamag.it

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[scadds289]

I give below the main design specs for the vessel:

1. Internal Design Pressure = 50 psi
2. NO external pressure
3. Other loads = Dead Loads due to internal refractory and self-weight
4. Operating Shell temperature = 700°F
5. Shell material = Carbon Steel (SA 516 Gr. 70)
6. Design Code = ASME
7. It is a direct fired vessel, horizontally supported on ground.

8. In addition to two saddle supports, the vessel has another equipment connected at one end. This equipment has its own supports and thus, induces a third support constraint on the connected end. This third constraint does not allow the vessel to expand freely in that direction.

9. Method of Analysis = 3D Finite Element study using Solidworks Simulation Professional (formerly known as Cosmosworks)

Please note that I am seeking expert opinions on the basic design philosophy that must be considered in this problem while arriving at a conclusion, after performing additional FEA studies, if necessary.

I individually thank everyone of you for your advice/comments made so far, that have helped me have a revised outlook on the whole study.

NK
Structural Engineer

 

[corus]

The addition of the saddle supports will also induce some thermal stress due to differential expansion, but the limit will be twice yield as the stress is classed as secondary. You should really do a thermal analysis first and add these thermal loads to your mechanical loads.

You also need to consider the thermal stresses due to the expansion of the internal refractory. This can be the most significant stress when assessing furnaces and such like. These peak during the initial heat-up of the furnace when the refractory is hot on the inner face but still relatively stiff, and the shell is still cold.

corus

 

[prex]

You are exactly in one of those situations where a thermal load induces primary stresses!
This is because the thermal load coming from outside the vessel must be considered as if it was a mechanical external load, because you cannot asses how much of the thermal stress is self limiting (a basic characteristic of a secondary stress).
However, if you could make, as suggested by corus, a thermo-structural model of the assembly (vessel+external equipment), you could assess how much of the thermal deformation is taken by your vessel, the balance being taken by the equipment. This could dramatically lower your thermal stresses.
Another point to be clarified is whether your 29,300 psi is 100% membrane or it is partly bending: you have different allowables for membrane and membrane+bending stresses.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[jte]

Good reference for the argument that thermally induced loads can cause primary stresses here:

http://store.asme.org/product.asp?c...apers&category_name=&product_id=PVP2005-71535

If you look stirctly at the numbers given, even if the primary membrane is below the 18.1 ksi S, the 29.3 ksi exceeds the allowable for combined membrane plus bending. However, I strongly suspect that the 29.3 ksi is not really a primary membrane plus bending but more likely a peak stress, which would get an altogether different evaluation.

Finally, lets not forget that the top of the horizontal vessel shell is in compression and should be compared against the allowable compressive stress. Be sure to include a hot shutdown condition (no pressure, but hot) when checking the compressive stress. At 700°F the allowable compressive stress will be somewhat low.

Finally, regardless of what the FEA says, you still have to meet the requirements of VIII-1.

Like TGS4 stated, this vessel is not the one you want to learn about vessel design with. There are plenty of consultants around (no, I don't accept outside work, so no bias) who are competent with both FEA and Section VIII. But be careful in screening them. There are also plenty of consultants who state that they are experienced and competent just to land the job, then they go to internet forums to figure out how to do the work...

jt