ASME VIII Div. 2 Torispherical head wall thickness in comparison to straight cylinder

[Binouli]

Pre-calculatoin of torispherical heads acc. to ASME BPVC VIII Div.2 2023 (4.3.6.1) leads to a thinner wall than a simple cylindrical shell (4.3.3.1).
Also, the minimum wall thickness of the same torispherical head acc. to ASME BPVC VIII Div.1 2023 is much thicker, whereas for a straight cylinder, Div.1 and Div.2 lead to the same wall thickness.
I think it is acceptable that Div.1 and Div.2 lead to different wall thicknesses, but i struggle with the fact that within Div.2 a torispherical head can be built thinner than a straight cylinder.
These are the inputs of the hand calculations:

Vessel inside diameter: 3000mm
Internal pressure: 6bar
Allowable stress S: 108MPa
Quality factor E: 0.85
Internal crown radius: 3000mm
Internal knuckel radius: 300mm
Allowable yield-strength at design temperature: 204MPa
Modulus of elasticity at maximum design temperature: 184256MPa

Minimum wall thickness for torispherical head Div.2: 8.6mm
Minimum wall thickness for straight cylinder Div.2: 9.9mm

Can this be intentional to build a vessel with a thinner head than the shell?

 

[TGS4]

I won't necessarily answer your question directly, but I will point out that a hemispherical head would be half the thickness of the cylinder. So, for that geometry, it is typical that the head is thinner than the shell.

The calculation bases for VIII-1 and VIII-2 are definitely different. PVP2017-65858 goes through this in detail. You can also check out the Alternate rules in Code Case 2260-3. Or you could also check out the alternate rules in 1-4(d) or 1-4(f) (if they are applicable) in VIII-1.

I am interested to better understand your situation. Which failure mode is governing for the VIII-2 calculations: plastic collapse in the crown, plastic collapse in the knuckle, or elastic buckling in the knuckle?

 

[Binouli]

Thank you for your reply!
The alternate rules in VIII-1 App. 1-4(3) for the considered torispherical head leads to 15mm wall thickness, which is much more than VIII-2 would yield. The straight cylinder acc. to VIII-1 has the same wall thickness as acc. to VIII-2 (9.9mm).

If approximately spherical heads like torispherical or ellipsoidal heads are better suited for internal pressure, i would expect that both VIII-1 and VIII-2 lead to thinner torispherical heads than straight cylinders.

My failure mode governing the VIII-2 calculations is buckling failure in the knuckle (Parameter Pak = Pck/1.5).

 

[TGS4]

If you have performed the calculations correctly (you have someone internally who checks these calculations for you, right?), and you end up with a thinner head than the cylinder, then the math is the math.

Be careful with this thinner head - nozzle reinforcement may be problematic. And keep nozzles out of the knuckle region.

 

[MJCronin]

BRAVO, TGS4 ... Your comments have always been "spot-on" regarding pressure vessels !

I would also like to offer that, any final choice of vessel head type, head thickness etc ALSO depends on:

- Nozzle number, placement and Code reinforcement selected

- Budget and availabilty

- Skills of your fabrication workforce and additional costs involved in welding a head joint between head and shell

You may find yourself in the situation where a slightly thicker head is more expensive, but immediately available

Tell us more about your final decision

(... and keep the GD nozzles out of the toro head knuckle region !!)

MJCronin
Sr. Process Engineer

 

[GD2]

OP,
After 2017 Edition, world is switching over to Div 2, that gives a thinner PV coz Part 4 DBR gives a design margin of 3.0 as opposed to Div 1 of 3.5.
Formed head Thk less than shell Thk is normal.
The cylindrical shell wall Thk could be same or near to same for both Div 1 and Part-4 of Div 2, if the allowable stress is based on Yield strength (2/3 of YS).

By the wat, what stress tables have you used for the allowable stresses?


GDD
Canada

 

[Binouli]

Thank you all for your replies!

The initial task was to design the vessel acc. to VIII-2 part 5 using a finite-element-model (design by analysis). In addition, the design rules in VIII-2 part 4 governing internal pressure should be checked. This is how I came across the fact that the vessel head design yields a thinner wall than the shell. My calculations will be checked by a senior engineer, nevertheless I should be understanding what I am doing if some day I want to become a senior engineer myself.

@GD2: I use the allowable stresses from ASME BPVC II D table 2A. For most carbon steels, the allowable stresses from table 2A are a little bit lower than in table 1A.

At this point, I figured out the following conclusions:

[ul]
[li]Because I use a combination of VIII part 4 and 5, only the design equations governing internal pressure are used. Other loads than internal pressure are only examined in the FE model, wich could lead to a thicker wall.[/li]
[/ul]

[ul]
[li]If I would only use the part 4 design equations, I would have to consider loads other than internal pressure by additional rules. [/li]
[/ul]

[ul]
[li]As long as the internal pressure equations in VIII part 4 are satisfied and the stresses from the FE-calculation are in the allowable range, the vessel design should be safe. [/li]
[/ul]

[ul]
[li]The design foruml for torispherical heads in VIII-1 quite simple, whereas in VIII-2 it is a more complex task including multiple steps. I think that this increased engineering effort is rewarded with a more accurate stress characterisation and a more economical design of the vessel. VIII-1 is more simple for the designer, but leads to an overdimensioned vessel. [/li]
[/ul]

[ul]
[li]Depending on the FE results, maybe it could make sense to make the head the same wall thickness as the shell regarding the manufacturing effort. [/li]
[/ul]

 

[GD2]

OP,
2017 Edition has classified pressure vessels into Class 1 and Class 2. It appears that your primary design option is Part 5 (DBA), the primary intention of it was to get a lighter vessel saving you cost on materials, fabrication, transportation etc. Because the Part 5 is based on FEA, output of which depends on the experience of the Analyst, you are asked another way to verify the FEA result with hand calculations given in Part 4 (DBR).

Because your starting point is Part 5, I would assume you are designing a Class 2 vessel. There are few requirements when you do this.
1. You can use Stress Table 5A or 5B (that may give you design margin of 2.4 to UTS, 1.5 on YS)- a further probability reduction in wall thk.
2. The UDS must be certified. All loadings in the UDS must be considered.
3. The MDR must be certified.

It is important that you know the difference between Class 1 and Class 2 vessels. For Class 1 vessels, if design rules are given in Part 4, Part 5 can not be used in lieu of Part 4. Following are few pointers for Class 1 vessels:
4. Allowable stress values are taken from 2A or 2B.
5. Part 4 to be used. If design rule for a component is not provided in Part 4, then Part 5 can be used. If Part 5 is used, all loadings must be considered.
6. UDS doesn't have to be certified unless fatigue analysis is required.
7. The MDR must be certified depending on certain requirements (fatigue analysis, Part 5 design of any component, quick-closing closure design per 4.8 and dynamic siesmic analysis).


GDD
Canada

 

[r6155]

Use E=1 for formed head with weld before forming the disc.
RT 100%, weld ground flush both sides before forming.
After forming RT 100% again.

Regards

 

[Binouli]

GD2,
thank you for the hint regarding class 1 and class 2 vessels. In fact, there was no specific information if the vessel should be class 1 or class 2. I assume it is class 2, because the customer wishes an FEA and only some of the design rules in VIII Div2 part 4 to be checked (cylinder, torispherical head). If I understand all of this corerctly, the vessel can only be class 1 if all of the design rules in VIII Div.2 part 4 (heads, openings, nozzles, reinforcements, supports...) are governed? I wonder what purpose the FEA would serve in this case?

Also I have noticed that I used the wrong stress table: It has to be ASME BPVC II D table 5A, not 2A. Thank you for pointing that out!

 

[GD2]

OP,
It looks you are on the track.
Yes, if it is a Class 1 vessel, the starting point will be Part 4. If there is no rule provided in Part 4 for a component, then only Part 5 rule can be applied. Example: if a Fatigue Analysis is required there is no rule in Part 4, only Part 5 provides that rule.
If you compare S (allowable stress) between Table 2A and 5A, you should see mostly higher allowable in Table 5A that will drive to a thinner vessel.
The Customer is knowledgeable. They want to verify that the person who is carrying out the FEA is knowledgeable enough and want to match the result from both FEA and manual calculation by Part 4.

GDD
Canada