ACU-FLAP (Check Element)

The ACU-FLAP Check Element System is designed to prevent flooding and backflows caused by high downstream water levels. The ACU-FLAP can and has been installed at combined sewer overflow (CSO) locations, sanitary sewer overflow (SSO) locations, retention tank overflows and basin overflows. Overloading of sewer systems and sewage treatment plants is avoided and energy costs for pumping systems and sewage treatment are reduced. The ACU-FLAP also controls odor, as it is normally in the closed position.


  • Up to 7 feet (2 m) of head
  • Modular construction up to 13 feet (4 m) long
  • Elastomeric flap
  • Opens with minimal water level rise
  • Design ensures airtight seal for odor control
  • Low hydraulic resistance
  • Easy to retrofit into existing installations
  • Stainless steel construction

Range of Application

The ACU-FLAP Check Element can vary from a model GAF-40-* to GAF-100-*. The GAF-40-* is designed to fit over a concrete opening measuring 16 inches (40 cm) high and up to a maximum width of 13 feet (4 m) and the GAF-100-* is designed to fit over a concrete opening measuring 40 inches (100 cm) high and up to a maximum width of 13 feet (4 m). The ACU-FLAP’s model selection is based solely on the concrete opening size. The units are currently designated with asterix due to the high variety of model options.

System Operation

The ACU-FLAP Check Element is an elastomeric flap mounted over a rectangular concrete opening. In the rest position the elastomeric flap is bolted along the top edge of its stainless steel mounting frame, thereby closing and sealing the opening. If the water level in the sewer or tank overflow rises, the hydrostatic pressure against the ACU-FLAP Check Element causes it to open thereby allowing the water to flow over the sill. If the downstream water level rises, the ACU-FLAP is pressed against its stainless steel mounting frame so that no water can flow back into the sewer system, tank or basin. The ACU-FLAP’s lightweight design makes it highly responsive to minimal water level changes while ensuring structural stability under medium downstream water level conditions.

The ACU-FLAP Check Element is equipped with an integral stainless steel frame for anchoring to the concrete structure. This flat and smooth sealing-surface increases the tightness of the flap and avoids the potential problems caused by irregular concrete openings and surfaces.

The ACU-FLAP Check Element can be fitted with a stainless steel plate for extra rigidity and weight to improve odor control and accommodate higher back water conditions.