Flow Velocity and Properly Sizing Drains & Outlets

April 11, 2017 | Categories: Pool Equipment & Systems

VGB compliant safety drain cover

When sizing drains, outlets and other openings that provide exiting flow from a swimming pool, spa or water feature, care must be taken to avoid suction entrapment. Excessive velocity and improperly installed or poorly maintained drains can cause serious injury or death.

In general, best design practice requires that the velocity of the flow of a grating or opening must not exceed 1 foot per sec (fps).  All openings must be covered with a properly sized and permanently (tamper-proof) secured grating, which must have an adequate open area that limits the maximum flow velocity (1 fps).

Additionally, exiting flows from a pool, spa or water feature should leave the pool through two or more openings that are separated far enough apart to prevent a person from sealing off the opening. Main drains that are accessible to swimmers should be covered with safety covers that prevent a person or persons from blocking or significantly reducing the flow through the drain. Such covers are generally called anti-vortex covers or safety covers and are configured to provide proper flow while preventing the openings from being sealed off.

Drains and other outlets can also be provided with an alternate suction port or vacuum limit control that will prevent excessive vacuum. Such devices should be designed and installed by a professional engineer or pool professional.

Most codes require that the grating provide an open area of at least 4 times the pipe diameter of the pipe stub or drain box connection or a discharge velocity in the pipe of less than 1.5 fps (whichever is less).


The flow through a grating can be calculated using the following formula:

Velocity (fps) = .321 x flow (in gpm)/open area (in²)

The equivalent pipe diameter of an open can be calculated from the following formula:

Equivalent pipe diameter (inches) = 2 x

Flow and sizing for typical Natare drains, gratings and outlets

Max Velocity through Grating =                        1              ft./s

Max Velocity through Stub =                             3              ft./s

Opening in gratings =                          0.315   in.


Have questions about your local codes or grating requirements?

Contact Natare

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