I'm trying to understand the intent of UG-37(a) requirement for tr:
"
tr  =  required thickness of a seamless shell based on the circumferential stress, or of a formed head, computed by the rules of this Division for the designated pressure, using E = 1, except that...
"

What E value shall be used when one of cases (a), (b) or (c) exists?

Thank you very much in advance.
 

E = 1.0 for all cases.

The point of the definition of "tr" is that it is the required thickness of a seamless shell or head (even if the actual shell or head is welded). Note that the case of a nozzle extending  through a welded joint is handled through the definition of E1.

The definition of "tr" relieves the requirement even further: when the nozzle and all reinforcing area are contained within the spherically dished portion of a torispherical head, or the equivalent region of an ellipsoidal head, then "tr" may be calculated as per subparagraphs (a) and (c), which will result in lesser values of "tr" than otherwise.

 

Thank you TomBarsh.

For a Torispherical Head, I understand that when the nozzle and all reinforcing area are contained within the spherically dished portion – you get the credit of using M = 1

For Ellipsoidal Head: what is the meaning of the credit you gain by using RADIUS FACTOR K1?
Namely: when opening and its reinforcement in an Ellipsoidal Head exceed the circle of 80% of the shell diameter – what then is "tr" required thickness calculation basis?

"For Ellipsoidal Head: what is the meaning of the credit you gain by using RADIUS FACTOR K1?"

   If the opening and all its reinforcement are within the region specified then the value "tr" may be calculated for an equivalent spherical shell of the radius K1*D, where K1 is from Table UG-37

"Namely: when opening and its reinforcement in an Ellipsoidal Head exceed the circle of 80% of the shell diameter – what then is "tr" required thickness calculation basis? "

   When any part of the opening or its reinforcement extends beyond this circle then "tr" cannot be calculated as per subparagraph "c" and you must refer back to the basic definition of "tr". So for this case "tr" is that for a seamless ellipsoidal head.