PART UW - WELDING Stress In Heads
The last E to consider is the one used to calculate thickness required or pressure allowed for formed and forged heads. Internal pressure creates stress that acts to rupture the walls of heads.
Each kind of head has a Code formula for its calculations. Two classes of heads are joined to vessels by circumferential joints. One class is joined to the shell with a Category B or C circumferential butt joint; these are heads that have a flange. Some examples are Torispherical, Ellipsoidal and forged Flat heads. Forged Flat heads are joined by Category C circumferential joints and are treated the same for determining their E as the other two. The other class is joined to the shell with a Category A butt joint; it is a Hemispherical head with out a flange.
The first examples have ellipsoidal heads that may be joined to the shell using a Type No. 1 or Type No. 2 joint. It is also representative of a torispherical head since both have a flange (skirt). The ellipsoidal head forms a Category B joint with the shell and is seamless.
The second examples have formed hemispherical heads without a flange. The joint formed by the attachment of the hemispherical head to the shell is a circumferential Category A. Hemispherical heads may be joined using either a Type No. 1 or a Type No. 2 joint provided no service restriction from UW-2 applies. If a service restriction applies the Category A butt joint must be of Type No. 1. The shell used in all examples is over 24 inches in O.D. and over 5/8 inch thick. Per Table UW-12 only Type No.1 or Type No. 2 joints are allowed for these conditions. When seamless heads, that have a flange (skirt), are attached to shells a Category B joint is created. This Category B joint will have a joint efficiency based on its Type and the amount of radiography that was applied.
Stress In Heads
This joint efficiency will not be used in the calculation of the head's required thickness or its pressure allowed. This E is used in the longitudinal stress calculations for the shell. The Category B joint may be thought of as belonging to the shell. For a seamless head which is joined by a Category B butt joint there are only two possibilities for the E used in the head calculations. The E used will either be 1.0 or .85. The E is determined based on the requirements of UW-12(d). The question then becomes has Spot RT been applied to the Category B butt joint. If it has the E is 1.0. If it has not the E is .85.
Example A: Category B butt joint of Type No. 1 or Type No. 2 has notreceived Spot RT. E = .85 for the head's thickness or pressure calculation. The shell's longitudinal stress calculation E will be .70 or .65 depending on which Type of joint was used.
Example B: Category B butt joint of Type No. 1 or Type No. 2 has received Spot RT. E = 1.0 for the head's thickness or pressure calculation. The shell's longitudinal stress calculation E will be .85 or .80 depending on which Type of joint was used.
71 Heads
The last case to consider for seamless heads that form a Category B or C joint with a shell is when the joint is of Type No.3, 4, 5 or 6 of Table UW-12. Since these types are not considered radiographicable by the Code the Spot RT cannot be applied. UW-12(d) states that the head under this condition shall always be calculated using E = .85. The shell's longitudinal calculations would use an E based on the Type No. of the joint and this E would then come directly from Table UW-12.
The most common mistake in the calculation of seamless heads attached by Category B joints is the use of the E found in table UW-12 based on the type of joint.
That E belongs in Longitudinal shell calculations. The E used for the seamless head is based only on the application of Spot RT. If Spot RT has not or cannot be performed ( as is the case for Types 3, 4, 5, or 6) an E of .85 shall be used. If it can and has E = 1.0. END OF STORY. Until they change the Code again! The last formed head of concern is the Hemispherical. A hemispherical head formed from a solid piece of plate without a flange is only seamless as long as it is lying on the shop floor; when welded to another component such as a shell it now has a Category A joint. Read UW-3(a)(1) again to confirm this statement. The Category A joint formed after welding to a shell belongs to the hemispherical head. The rules regarding seamless shells and heads in UW-12(d) specify that the spot radiography of UW- 11(a)(5)(b) must be applied to use an E of 1.0 for a seamless head's thickness or a shell's circumferential stress calculation. Since our hemispherical head will always have a Category A joint (seam) the conditions of UW-12(d) do not apply. The bottom line is; that a formed hemispherical head without a flange can never be seamless. Spot radiography on the Category A joint does have a use if the hemispherical head is welded to a seamless shell or to a shell in which all Category A&D butt joints have been fully radiographed.. The shell's circumferential stress could then be calculated using an E of 1.0 .
ATTENTION - ATTENTION
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