By The Pool & Hot Tub Association
Understanding the difference between blockable and unblockable drain covers/SOFAs is required to determine whether or not a suction system is compliant with the VGB Act. The reason is that single drain systems/SOFAs that are designated as blockable require an additional device or system designed to prevent [body] suction entrapment in order for the installation to qualify as VGB Act compliant.
Blockable suction outlet openings (sumps) are those which can be fully blocked by a mathematical representation of the 18” x 23” Body Blocking Element used for VGB2008 and VGBA-2017 certification testing. Visualize a floor-mounted drain sump with the Body Blocking Element centered directly over it in the mid-day sun. If the sump opening is fully shadowed, it must be classified as blockable.
If a portion of the sump protrudes into the sunlight, it can be classified as unblockable, provided there are unblocked flow openings in sufficient quantity to keep the suction levels within expectable limits. This two-part prerequisite (physical sump measurement and flow testing) was clarified as part of the VGBA2017 revision. Product certified in conformance with the VGBA2017 standard can be identified as blockable or unblockable by the applicable mark engraved on the cover/grate and the product’s specifications and instructions.
Originally, the VGBAct’s language defined an unblockable drain/SOFA as all components, including the sump, cover/grate, and hardware, such that its perforated (open) area could not be shadowed by the VGB2008 Body Blocking Element and that the rated flow through the remaining open area (beyond the shadowed portion) could not create a suction force in excess of the removal force values in Table 1 of the standard. This policy did not evaluate the size of the sump, allowing larger grates over mathematically blockable sumps to qualify as unblockable.
The CPSC voted to change this policy effective October 11, 2011. The result is that the size of a cover/ grate cannot be used as the sole means of determining if a pool has a single, blockable drain or not.
The Body Blocking Element test was introduced in the VGB-2008 standard, where the original version, ASME/ANSI A112.19.8M – 1987 (R1996) had no such test. For this reason, it was possible for blockable drain covers that were flat and flush mount to be certified as conforming to the standard. Virginia Graeme Baker lost her life on one of these flat, blockable drain covers despite it being properly certified. Experts recognized it was too easily sealed, driving the creation and addition of the body block test.
They also identified another significant risk factor from this case: the drain cover was approximately 10 years old, allowing the plastic to become weak and brittle, breaking just when it was needed the most. This was a driving factor in the service life requirement being added to the standard, and it being enforced by the CPSC.