How is the clear opening area of a grate inlet determined?


Product(s):StormCAD, CivilStorm, SewerGEMS
Version(s):CONNECT Edition, V8i
Area:Layout and Data Input

Problem

How is the clear opening area of a grate inlet determined?

Solution

The clear opening area of the grate is based on the grate length, width, opening ratio and clogging factor. These are multiplied together to achieve the effective opening area (see example further below) which is used when a grate is operating at higher depths where orifice flow controls.

The opening ratio is an adjustment based on the type of grate, which reduces the effective clear opening area. HEC-22 defines an opening ratio for each grate type, and the grate type is selected in a dropdown in the Inlet Catalog. See Chart 9A and Chart 9B near the end of the HEC-22 manual for the ratios used for each of the grate types.

You can see more information about the HEC-22 inlet equations in the "Theory" section of the Help documentation.

Grate TypeOpening RatioDescription
30° & 45° tilt bar0.34Tilt-bar grate with transverse bar spacing 102 mm (4 in.) on center.
Curved vane0.35Curved-vane grate with longitudinal bar spacing 82 mm (3.25 in.) on center and transverse bar spacing 114 mm (4.5 in.) on center.
Reticuline0.80“Honeycomb” pattern of lateral and longitudinal bearing bars
P-30 mm (P-1-1/8")0.60Parallel bar grate with bar spacing 29 mm (1.125 in.) on center
P-50 mm (P-1-7/8")0.90Parallel bar grate with bar spacing 48 mm (1.875 in.) on center
P-50 X 100 mm (P-1-7/8"-4")0.80Parallel bar grate with bar spacing 48 mm (1.875 in.) on center, and with 10-mm (0.375-in.) diameter lateral rods spaced at 102 mm (4 in.) on center


See pages 381-385 of our Stormwater book for illustrations of these standard types.

Furthermore, the Clogging Factor property is an additional adjustment to the effective open area. See more here: 

For example if you have a curved vane grate inlet with a length of two feet, a width of two feet and clogging factor of 20%, the effective open area would be 2 * 2 * 0.35 * 0.8 = 1.12 ft^2

What if I need to use a different Grate Type (opening ratio) that is not in the list?

  1. Choose the next closest Grate Type. If you have a grate type that is not included in the available pull-down options (does not conform to the standard Grate Types from HEC-22), you can either choose one that is close to one of the available options or do your own "Sensitivity Analysis" which means making adjustments to the structure dimensions in order to get an opening area which is accurate for your requirements.

  2. Adjust the inlet length/width to achieve the same effective open area. For example say you have a grate inlet with a grate that has an opening ration of 0.5, which is not in the list above. If the actual width and length are each two feet and the clogging factor is 20%, you would expect the effective open area to be: 2 * 2 * 0.5 * 0.8 = 1.6 ft^2    Now, to model an inlet with that same effective area of 1.6 ft^2, you would need to back-calculate the grate length that would result in that same open area, using one of the available grate types. For example if you choose the grate type "P-30 mm (P-1-1/8")" which has an opening ratio of 0.6, you can re-write the equation as:

    2 * L * 0.6 * 0.8 = 1.6
    L * 0.96 = 1.6
    L = 1.6 / 0.96
    L = 1.6667

    So, you would choose the grate type "P-30 mm (P-1-1/8")" and change the Grate Length from 2 feet to 1.6667 ft, in order to achieve the same effective area as the desired 0.5 opening ratio.

Troubleshooting

Note that as of version 10.04.00.158, grate, ditch and combination inlets assume a clear opening ratio of 0.90 (the P-50 grate type) when operating in-sag with orifice flow (higher gutter depths). So, changing the grate type will not change the results in such cases. This is planned to be fixed for the next release (reference #1177196) In the meantime, you can use the second option above to achieve the same effective hydraulic results.

See Also

Which dimensions are used when inputting grate information for inlets in StormCAD ?