Protect a hydro turbine with three stages in order: a coarse trash rack of bars 2 to 3 cm apart at the intake, a settling basin that holds the water for 2 to 3 minutes at design flow so sand drops out, and a fine screen of 2 to 5 mm before the penstock. For a flow of 20 L/s the basin holds 2.4 to 3.6 m³, and the rack needs about 0.056 m² of gross area so the water through it stays slow enough that leaves do not pin against the bars.
Part of Micro-Hydro Turbine
The intake diverts a share of the stream into the system, and its screens and basin remove the sticks, leaves and sand that would jam or wear the turbine.
Different Debris Needs Different Screens
The carrier calls for a small weir or dam to divert water, a trash rack of metal bars at 2-3 cm spacing to stop leaves, sticks and debris, an intake above the streambed to avoid silt, and a settling basin where sand drops out. The Installation page adds a screen with openings smaller than the smallest clearance in the turbine, usually 2-5 mm for small turbines. The two openings, 2-3 cm and 2-5 mm, are not a disagreement. A coarse rack at the intake stops the large debris, and a fine screen after the basin stops what is small enough to pass the rack.
The fine screen matters more as the turbine gets smaller. The 3 L/s Pelton in Pelton Wheel (High Head) has a nozzle of about 13 mm. A 2-3 cm rack gap passes a stone larger than that nozzle; only the 2-5 mm screen protects the nozzle.
Method: Lay Out The Intake
Step 1: Place the opening above the bed. Set the intake sill above the bottom of the stream so bedload gravel and silt pass the opening instead of entering it.
Step 2: Fit the coarse rack. Weld or lash bars of about 10 mm thickness with 25 mm gaps; this is within the carrier’s 2-3 cm. Lean the rack back so a rake can pull debris up the bars to a platform.
Step 3: Size the rack area. For 20 L/s (0.020 m³/s), choose an approach speed through the open gaps of 0.5 m/s, an assumed value from my recall of intake practice, not a corpus figure. Open area = 0.020 ÷ 0.5 = 0.04 m². With 10 mm bars and 25 mm gaps the open fraction is 25 ÷ (25 + 10) = 0.71, so the gross area is 0.04 ÷ 0.71 = 0.056 m². A slower speed through the rack gives less pinning of leaves to the bars and a lower head loss.
Step 4: Dig the settling basin. Retention time × flow = volume. At 20 L/s, 2 minutes is 120 × 0.020 = 2.4 m³ and 3 minutes is 180 × 0.020 = 3.6 m³. A basin 2 m long, 1.2 m wide and 1.5 m deep holds 3.6 m³. Add an overflow weir so a rising stream cannot overfill it and a flush drain at the bottom to wash out the sand (Installation).
Step 5: Fit the fine screen and gates. Put the 2-5 mm screen where water leaves the basin for the penstock. Add an intake gate to shut the flow off, and a bypass that sends the whole stream past the system for maintenance, as the carrier requires.
Worked Example: The Rack Area And Basin For A Small Plant
| Quantity | Working | Result |
|---|---|---|
| Design flow | 20 L/s | 0.020 m³/s |
| Net open area at 0.5 m/s | 0.020 ÷ 0.5 | 0.04 m² |
| Open fraction, 10 mm bars, 25 mm gaps | 25 ÷ 35 | 0.71 |
| Gross rack area | 0.04 ÷ 0.71 | 0.056 m² |
| Basin volume, 2 to 3 min | 0.020 × 120 to 180 | 2.4 to 3.6 m³ |
Keep The Rack Clean
The carrier’s maintenance schedule starts with a daily check of the rack, especially after rain. A blocked rack starves the penstock, so the turbine runs on part flow and the output drops. Clean the rack with a long rake with tines the same spacing as the bars, from the bank or a platform. See Debris & Ice Management.
Common Mistakes
- Relying on the coarse rack alone — gravel and twigs smaller than 2-3 cm reach the nozzle.
- An intake on the bed — it fills with sand within a season.
- No overflow or flush — a flood fills the basin and a full basin passes sand to the turbine.
Safety
The rack is cleaned from outside the water
Never step into the flow to clear a rack. Water pressing on a blocked rack can hold a person against it, and a swollen stream rises quickly. Close the intake gate first, work with a second person on the bank, and use a rake on a long handle. The gate is the stop: it should be reachable from the cleaning position.
What’s Next
- Learn next: Penstock Design & Materials for the pipe that starts at the basin
- Combine with: Mill Race and Headgate for a channel and headgate if the intake is far from the turbine