Hair Entanglement

Op Editorial by Paolo Benedetti

Op Editorial by Paolo BenedettiWhen discussing hair entanglement in the pool and how to prevent these types of incidents from occurring, we have to look at a few things. The first is line velocity. If it is low enough, hair won’t be ingested by the pipe and tangled under the drain lid. Suction entrapment wouldn’t occur either, even if a grate were missing.

The second issue pertains to the current codes and standards. Some states have state building codes that dictate maximum line velocities for energy efficiency.

California and Florida both mandate line velocities of 6 FPS suction and 8 FPS returns for energy efficiency.

Residential In-ground Pools NSPI-5/2003 (9.2) the line velocities were limited to 8 FPS suction and 10 FPS returns. (9.2.1) the limit through the drain cover was limited to 1.5 FPS.

Residential In-ground Pools NSPI-5/2013 (9.2) the line velocities were limited to 8 FPS suction and 8 FPS returns. (9.2.1) the limit through the drain cover was deferred to the APSP-7 standard.

Suction Entrapment Avoidance APSP-7 (2013) Piping line velocity is not specified, though the flow rate through the drain cover is limited to 1.5 FPS.

APSP-15 Energy Efficiency Code (2011) 6 FPS suction and 8 FPS return

ISPSC 2015 (311.3) limits the line velocities in the return piping to 8 FPS. (310)

It defers to the specifications in APSP-7 (there is no longer a suction line velocity in the standard).

Suction Outlet Fitting Assemblies APSP- 16/2017 (3.9.2) piping line velocities were eliminated, though 1.5 FPS remained as the line velocity through the RDP/SOFA.

The safety standards are not addressing energy efficiency. Most experts agree that the optimum efficiencies are achieved at 6 FPS suction and 8 FPS return line velocities. The majority of hydraulic engineers are designing at 4.5 FPS suction and 6 FPS return line velocities.

This allows for a margin of error and minor deviations that may occur during construction, while still staying below the maximums. Most codes for public pools recognize the need for low line velocities and therefore limit the velocities at the pipe connected to the main drain to 1.5 – 3 FPS. The safety standards assume two things occur. The first assumption is that the drain sump size is compliant with the SOFA requirements and the manufacturer’s specifications. Field-built sumps (divots carved out of the shotcrete) are oftentimes too shallow. Few builders thicken the shotcrete around the suction pipes to allow for a proper depth sump.

The decreased distance between the suction pipe and the underside of the drain cover increases the flow through the drain cover.

This increases the danger of hair entanglement, as the design standards and flow rate through the cover have been exceeded. Therefore the sumps are of insufficient size and volume to slow the velocity of the water.

Paolo Benedetti - Swimming Pool Expert Witness

President of Aquatic Technology