Thickness of Torispherical Head 2

[jaminn1]

Hi, I am working with a vessel having torispherical top head. the vessel has some thinning in the head and when I went to calculate the MAWP it came out very low. in fact the original thickness from data report according to my calculations are not meeting the design MAWP. we operate the vessel lower than the MAWP so that's a good thing. but I want to make sure I am calculating the right way before de-rating the vessel. below are the calculations:

Given as per data report: Tori spherical Head
Material: SA-516 Gr. 70;
OD= 54”
Crown radius= 54"
Knuckle radius= 3.24"
E=0.85
Min thickness= 0.24”
MAWP= 100 Psi @ max temperature 500 F
From Sec II stress tables: S= 20.0 Ksi (SA516 Gr 70 @ 500 F)
From UG-32: t= 0.885PL/(SE-0.1P)= 0.885*100*54/(20000*.85-0.1*100)= 0.28"
P= SEt/(0.885L+0.1*t)= 20000*.85*.24/(0.885*54+0.1*0.24)= 85.33 Psi as opposed to design MAWP of 100 mentioned in data report.

I forced myself to assume that the manufacturer may have assumed a higher stress value but as per UG-32e limits the max stress value of 20000 psi for torispherical heads..
so I would appreciate any comments as to what is happening here or anything can be done? otherwise I will have no option but to de-rate the vessel. thanks in advance.

 

[pdiculous963]

I get about the same values. Are you sure the head joint efficiency is 0.85? What was the radiography performed? Seamless heads can have a joint efficiency of 1.0 if the spot radiography requirements of UW-11(a)(5)(b) are met. If a 1.0 is used the calculated MAWP is 100 psi.

 

[jaminn1]

Thanks pdiculous963.

That could be the mistake I am making. Data report shows the joint efficiency for type 1 (long) @ 70% and type 2 (Girth)@ 85%, but could it be only for the shell? not sure what kind of radiography was done for the head

. please see attached.

 

[pdiculous963]

Odd that the longitudinal seams were not radiographed, while the circumferential seams were. I'd wager that this was done to be able to use a 1.0 on the head.

Also, if the thinning is limited to the crown portion (away from the knuckle) you can always use the spherical formula to calculate minimum thickness.

 

[TGS4]

What do the formulae in VIII-2 say? (using the VIII-1 allowable stresses)

 

[pdiculous963]

Those result in a lower MAWP...

 

[c8829]

As there is no external pressure consider App 1-4(d).

 

[prex]

Given the diameter of the head, I assume there are (of course) no welds in the head, hence E=1 (UG-32(c))

prex
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[PlantIntegrityEngine]

Is the thinning due to corrosion? Where about in the head is the thinning? If it's within the 80% central portion consider the API 510 override and use the Tmin calc for a hemi-head. Will give you lots more corrosion allowance.

 

[TGS4]

pdiculous963 I ran the VIII-2 calculations just now. Inputting the numbers from the OP produce a maximum allowable internal pressure of 113.9psi. The maximum allowable internal based on rupture of the crown is 177.4psi.

If this were a new vessel, I would apply Code Case 2695 and use the VIII-2 rules. Even if there were a weld joint efficiency factor of 0.85, this would reduce the crown allowable pressure to 150.8psi.

I don't see the problem here.

 

[pdiculous963]

TGS4 - Double check the temperature you ran the calculations. I believe the wrong modulus or yield was used. I get similar values to you at ambient for the permissible pressure based on knuckle buckling, however, at the design of 500°F, it drops to the mid to low 90 psi range. Crown pressure values clearly are not changed as they are based on allowable stress.

I do however tend to agree that CC2965 is the right approach for something new.

 

[c8829]

Have a look this attached file.

 

[prex]

c8829, head thickness is 0.24" per OP data, so your calculation adds nothing to the discussion. And of course, in calculating MAWP you use the actual (corroded) thickness, not the required minimum!

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[c8829]

Oh Yes, My mistake. Sorry PREX