According to ASME B16.9 - NFA
According to ASME B.16.25
1/2" - 48" (larger sizes on applications)
According to ASTM A 234 WPB WPC WP1 WP5 WP9 WP11 WP22 WP91 WP911 / ASTM A 420 WPL6 WPL3 / ASTM A 403 WP 304-304L - 316-316L - 321 / Special steels and alloys on application

The shape of these caps shall be ellipsoidal and shall conform to the shape requirements as given in ASME Boiler and Pressure Vessel Code.
#Length E applies for thickness not exceeding that given column "Limiting Wall Thickness for Length E".
Length E applies for thickness greater than that given column "Limiting Wall Thickness" for sizes 26 in. and larger length E. Shall be by agreement between manufacturer and purchaser.