Underwriters Laboratories UL-58 Type I construction is defined as: a primary tank wrapped by an exterior steel shell that is in direct contact with the primary tank; where, the exterior shell is wrapped from 300 to 360 degrees of the primary tank circumference.

All McTighe Type I double wall separators are wrapped a full 360 degrees. The 300 degree wrap mainly applies to storage tanks that are not meant to operate liquid full.

Since the outer shell is a tight wrap of the primary tank, the separator shell dimensions will increase by two times the thickness of the outer shell (about 1/4" to 1" depending upon separator size). The interstitial monitoring port(s) on a Type I double wall separator are located at the end(s) of the separator and must be taken into consideration in the overall length of the vessel.

For a Type I double wall oil-water separator, add a "-DWI" to the end of the separator model. (For example: OWS-1000-DWI). The drawing below shows a typical detail of a UL-58 Type I double wall separator.