Appendix 41 - external pressure

[Mechanimal]

Hi,

I was wondering if anyone knew if ASME Section VIII Division 1 Appendix 41 can be used for external pressure calculations?

The definition of P in Appendix 41 is design pressure and does not refer to internal or external as it does in UG-34 (UG-34 is only for internal pressure, see interpretation VIII-1-89-206).

From my understanding Appendix 41 is really just UHX 12 U-tube tubesheet design procedure with Pt=0, and configuration d. The interesting thing is that UHX 12.3 defines Ps as the shell side design pressure, for vacuum service use a negative value for Ps.

Would that be the case with Appendix 41?

The confusion for me is in the external pressure restriction in UG-34 for blind flanges. I don't understand from the Roark equation derivation how external pressure would result in a different thickness than internal pressure. If you absolute the moments similarly to UHX the results are same.

 

[SnTMan]

Mechanimal, I think you'd be justified using Apx 41 for negative pressure design, if also following Apx 2-11 to insure consistency for W, Wm1, etc, etc. You could call this a U-2(g) approach if necessary.

I'm like you, I don't see how a flat plate cares which way the pressure acts.

Regards,

Mike

 

[Mechanimal]

Thanks for the reply!

A little off track but for a external pressure thickness calculation for a blind flange, you could use UG-34(c)(2) eq. 2 modified by 2-11? I think the below would be correct, you only modify the gasket seating load by the W value in 2-11.

t= G x sqrt((0.3 x Pe)/(S x E) + (1.9 x (3.14 / 4 * G^2 * Pe + 2b x 3.14 x m x Pe) x hg) / (So x E x G^3)) (operating)
t = G x sqrt(1.9 x ((((3.14 x b x G x y)+Ab x Satm)/2) x hg) / (Satm x E x G)) (gasketing seating)

Pe= external pressure
G= diameter of gasket load reaction
S = allowable stress (at operating (So) & gasket seating (Satm))
E= joint efficiency
hg= gasket moment arm
b= effective gasket width
m= gasket factor
Ab= total bolt area
y = gasket unit seating load

 

[Mechanimal]

I was reviewing my taylor forge documents and realized that for the operating case the bolt load in an external pressure case would be written Wm1e=Hp-H = 2 x b x 3.14 x G x m x Pe - 3.14 / 4 x G^2 x Pe for G <= 8 x m x b, for G > 8 x m x b Wm1e = 0.

t= G x sqrt((0.3 x Pe)/(S x E) + (1.9 x (Wm1e) x hg) / (So x E x G^3)) (operating)