Intake screen calculator for through-slot velocity

How the intake screen calculator works

A passive intake screen is sized so that water passes through the slots slowly enough to protect fish and to keep debris off the screen face. The calculator works back from the design flow and the chosen through-slot velocity to the screen area, and then to the length needed at each standard diameter.

1. Open area fraction: OA = S / (S + W), from the slot S and the profile wire width W.
2. Open area required: the flow Q divided by the maximum through-slot velocity v. A value of 0.15 m/s is commonly used, subject to the regulator or the design basis.
3. Gross screen area: the open area required divided by OA, increased by any blockage allowance entered.
4. Length at each diameter: gross area / (pi x D), for diameters from 300 mm to 800 mm. For a tee intake with two drums, the length per drum is half.

Lengths over 6,000 mm are flagged, because a screen of that length is made in sections.

Worked example

A flow of 540 cubic metres an hour (0.15 cubic metres a second) at 0.15 m/s needs 1.0 square metre of open area. With a 3 mm slot on 35SB wire (3.0 mm wide), OA is 50 percent, so the gross screen area is 2.0 square metres. A single 500 mm drum would be about 1,273 mm long, while a tee with two 500 mm drums would be about 637 mm per drum.

Reading the result

The table gives a starting geometry. The final selection also depends on the water depth over the screen, the clearance from the bed and the surface, the current in the river and the material for the water quality, which SIMO engineering confirms on the general arrangement drawing.

The intake screen design guide explains the method in full, and the passive intake screens and river intake screens pages describe the construction.