Hello, friends! According to WRC-4

[Sajjad2164]

Hello, friends!
According to WRC-470, when we have a high temperature in the vessel, there are some methods to reduce the stress level occurring in the skirt-to-head junction including hot-box configuration and using slots to make the region more flexible. I am modeling a vessel on which the designer has put several slots to overcome the high level of thermal stresses. I've encountered a high level of stress concentration. Does anyone have any idea what I should do?
Thanks

 

[Sajjad2164]

Typically in the skirt, you will have high secondary bending stress but low primary stress. IMO for ratcheting to be likely, you need to have a reasonable sustained primary stress in combination with your cyclic thermal stresses. Therefore, if you can't ratchet, it will either shakedown to elastic fatigue or result in cyclic plasticity, both of which can be addressed. In the absence of high stresses without thermal, your secondary stress can be reasonably high, if the appropriate analysis is performed. Were you running refractory failure cases that caused the exceedance?
I don't involve refractory in the simulation, actually, the high temperature comes from an assumption that says the refractories are damaged.
The HB can be used in conjunction with slots. Out of interest, how did you model the HB? What certainty do you have of the actual efficiency relative to the analysis performed?
I modeled the HB as exact as it could be possible. I used solid elements for the insulations and considered cavity radiation in the box between the head, skirt, and insulation. It worked because I realized how much thermal stress came down. Now, we decide to involve the slots to see if they're enough to reduce the thermal stresses, but we are facing an overstressed region in the notches.
That is what the slots are intended for. The design isn't actually as simple as just adding slots near the skirt junction. There are lots of factors that influence the design, and there has been significant research in the area. As I mentioned, you should search for some of the publications on this topic. I wasn't saying you need to remove the slots, only presenting another option to consider since you had the ability to redesign the vessel. If it is what the client wants then go with it. However, the 'advantage' of using slots in high cyclic thermal applications is offset by the introduction of fatigue initiation locations. Aside from the design perspective of the slot location relative to the head, which is another discussion, the distance for crack propagation is decrease if you have the slots right at the head junction.

The local failure assessment is actually the trivial assessment for this design. To answer your question, you are allowed to use elastic analysis, what specifically gives you reason to question it? Maybe if you refer to the EP load combination for the local failure criteria in Table 5.5 it might clear this up. Were the stresses in the table you posted actually 'primary' principal stresses?
The stresses I reported in the above tables are typical and mixed primary and secondary stresses.

 

[saplanti]

Slots are used to stop the thermal gradients in that area. But, depending on the temperature difference between the vessel and skirt, it may not be required, most of the time this is the case. I have never seen the used on a pressure equipment skirt until now.

The slots will produce stress concentration in addition to a lot higher stresses under shear, axial load and bending moment on the vessel. Do you use the head material for the skirt? If you have to use opening to stop the temperature gradient you need to use circular opening all around, this may be reducing stress concentration but the other global action will be to same.

Some companies have some temperature estimates for the skirt connection. But what would happen if you consider 2 deg C / mm of temperature reduction from the source of the heat on the steel to the cooler side without using the slots? How different your un-slotted skirt temperature gradient is from this concept? Using this concept, you may use the same head material for integral skirt up to a certain elevation, and you may be able to change the material for the temperature at that elevation.

Are you trying to do design-by-analysis using FEA to be able to reduce thicknesses? If you use design by formula what would you lose?