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Drum Minimum Thickness Calculations Sect VIII vs. Sect I

Drum Minimum Thickness Calculations Sect VIII vs. Sect I

Drum Minimum Thickness Calculations Sect VIII vs. Sect I

(OP)
Good morning all,

In calculating minimum required thickness on a 2:1 Elliptical head for a code stamped "S" drum in a power boiler, I have noticed a difference in the value given by the Sect VIII formula and the Sec I formula.  

In the past these drums were calculated using the Sect VIII code but I am not comfortable reporting this thickness to our client before gaining more knowledgable input from experienced sources.

The following are the formulas being used:

Sect VIII t=P(O.D.)K/2SE+2P(K-0.1)

Sect I t=5PL/4.8S

P=703
O.D.=54"
S=19850
K=1
E=1
L=27"

Sect VIII t=0.927
Sect I t=0.996

Please advise if I am mistaken in these calculations and the best method to solve.

Thanks,
Alice

P.S. The shell calculations from both Sect VIII and Sect I gave me the same result.  I was pleased with this.

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

The difference is BPV I is  its own standards committee, as well as BPV VIII, with members having different personalities, methodology for use of equations and experience levels.

Since BPV I is considered the "mother code" of ASME because it evolved well before Section VIII, I would use the more conservative approach for elliptical heads. Now, I probably upset my BPV VIII brothers, but so what.

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

alicbergtate,

Quote:

Sect I t=5PL/4.8S
This formula applies to a dished head when it is a segment of a sphere. It does not apply to a 2:1 ellipsoidal head. Use PG-29.7.
 

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

(OP)
Thank you for your responses.

metengr,
I hope none of your BPV Sect VIII brothers are upset over your statement.  We should all respect being conservative for the sake of safety.

doct9960,
You stated to use PG-29.7.  In review of this section, I discovered the following statement:

"If a flanged-in manhole that
meets the Code requirements is placed in an ellipsoidal
head, the thickness of the head shall be the same as
for a head dished to a segment of a sphere (see PG-
29.1 and PG-29.5) with a dish radius equal to eighttenths
the diameter of the shell and with the added
thickness for the manhole as specified in PG-29.3"

Both heads of these drums do in fact have manholes so does that constitute using the formula from PG-29.1?

My comfort areas are Sect VIII and 653 so working with Sect I is a learning experience for me.   

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

alicbergtate,

Do you have flanged-in manholes? (similar to flued-in openings in Fig. UG-38)

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

(OP)
doct9960,

The manholes are flanged-in manholes.   

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

alicbergtate,

Are you sure you have flanged-in (flued-in) manholes and not welded manholes with repads?

Anyway, if you have a flanged-in manhole in a 2:1 ellipsoidal head, then you need to follow the 2nd sentence in PG-29.7.

I have not done any head calcs with a flanged-in manhole, but this is how I interpret the 2nd sentence in PG-29.7:
1. Determine thickness using the formula in PG-29.1 with L = 0.8 x O.D.
2. Increase the thickness in accordance with PG-29.3

So your head thickness would be 1.833".

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

(OP)
doct9960,

The manholes are probably not flanged-in as their thicknesses are not above the value that you recommended.  My field inspector must have been mistaken.

I will use the rules in PG-29.7 that state that the minimum thickness of the heads should equal the minimum thickness of the shell.

Please advise of this is the method that you would recommend.

Again thanks for all of your help.

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

You mentioned a field inspector. Has this drum been in service or is it new? What is the purpose of your calculations?

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

(OP)
This drum has been in service.  I am calculating a minimum thickness to provide corrosion rates and retirement dates based on ASME code.

RE: Drum Minimum Thickness Calculations Sect VIII vs. Sect I

alicbergtate

Find the final data book of this drum and see the original design calculations.

Regards

rhg

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