Underwriters Laboratories UL-58 Type II construction is defined as -- An outer tank physically separated from the inner primary containment tank by standoffs; where, the inner tank is completely contained within the outer tank. Standoffs shall provide a defined annular space between the primary and secondary tank and can consist of mesh material, lap welds or channels. Construction consists of a steel inner shell and a steel outer shell.

The channel standoffs create an outer shell that has a 2" space between the separator shells and a 6" space between the separator heads. Therefore, the overall separator diameter increases by 4" and the overall separator length increases by 12".

The lap weld standoffs create a space similar to a Type I double wall. The overall separator dimensions increase by two times the thickness of the outer shell (about 1/4" to 1" depending upon separator size). The interstitial monitoring ports on a McTighe Type I double wall separator are located at the ends of the separator and must be taken into consideration in the overall length of the vessel.

If a UL-58 Type II double wall is desired, McTighe recommends the lap weld standoffs because it provides the same funtion as the channel standoffs and is more economical.

For a UL-58 Type II double wall with lap weld standoffs, add a "-DWII+" to the end of the separator model. (For example: OWS-1000-DWII+). For channel standoffs, add a "-DWII". The drawing below shows a typical detail of a UL-58 Type II+ double wall separator with lap weld standoff.