Choosing the Right System

AS 3996 Clause 1.1 Scope
“This standard specifies requirements for access covers and grates for use in vehicular and pedestrian areas. It applies to access covers & grates having a clear opening of up to 1300mm…”

ACO believes that EN 1433: Drainage Channels for Vehicular and Pedestrian Areas, also provides a good measure of performance.

The load classes of both codes are shown in the table below.

A channel’s hydraulic capacity is calculated by the amount of liquid the channel is able to collect and drain in a given time period. This determines the size of channel required.

One way to alter a channels capacity is by changing its physical cross sectional size (width x depth). The other is by changing its hydraulic run length. This is defined as the distance water needs to travel before being discharged through an outlet. Changing this can dramatically alter a channel’s run capacity. With all other factors equal, the shorter a hydraulic run length, the higher a channels capacity to drain.

The volume of liquid a channel system needs to collect and remove in a given time period determines its size. Slab restrictions may limit the depth of the channel leaving its width as the usual variable for correct sizing.

Typical factors that affect the size of a channel:

• number of, and discharge rate (generally measured in litres per second) of wash down hoses in a room
• volume of spill containment
• hydraulic capacity of waste pipe beneath the floor connecting to the channel (for liquid evacuation)
• quantity, size(s) and location (along its length) of trough outlet(s)
• desired trough invert fall along the length of the system
• speed of liquid across the floor during service and/or wash down operations. In this instance the selection of the grate must also be considered
• for external areas: rainfall intensity,size of catchment, ground falls etc

ACO offers different sized channels and a broad selection of grates to meet most hydraulic requirements and offers technical support to customers to help ensure correct system specification.

A grate fails hydraulically when liquid bypass occurs. Consideration must also be given to the inlet size of the grate to ensure it adequately removes surface liquid, but not at the expense of introducing litter/waste into the drainage system, or compromise the heel safety of users.

A grate’s hydraulic capacity is calculated by the amount of liquid it will allow to pass through it in a given time period. Failure to allow passage of liquid into the underlying channel will result in bypass, regardless of how much capacity the channel has.

There are three factors that affect the hydraulic capacity of a grate, its size, its collective intake area and the design of its inlets.

For instance, a longitudinal grate (slots parallel to the channel) can have a large cumulative intake area. Between the bars, each slot acts as an individual elongated orifice and the grate will only reach its capacity once each slot has flooded. The bars also have the effect of slowing down the speed of the liquid, ensuring gravity has enough time to maximise the evacuation of the water between the bars.

In comparison, the intake areas (and therefore capacity) of only single or double slotted grates are much smaller than multi slotted design. A double slotted grate has only two openings along the length of the drain. There are fewer opportunities for the liquid to be evacuated through the design.

Where there is a substantial amount of surface liquid on the floor or where there is a heavy concentrated pour, water bridging is common. This can result in liquid not entering the channel and splashing back. This should be avoided in vulnerable areas.

Furthermore, single slotted grates are the most compromised design as there is only one slot for water to be interrupted and slowed.

Ensure the correct size and location of outlet is selected to meet the underground pipework requirements