Intake Design Basics: Screening Debris Without Killing Your Flow
Every small hydro scheme lives or dies by what reaches the turbine. An intake that passes too much debris will hammer the runner, block the nozzles and force costly shutdowns. An intake that screens too aggressively will strangle the flow and waste the head you paid for. The art of intake design is finding the balance between protection and pressure loss.
Why the intake matters more than the turbine
Most turbine failures in low-head and medium-head schemes begin upstream of the machine. Stones, gravel, leaves and woody debris arrive with the water and must be removed before they reach the runner. A well-designed intake does this with the smallest possible drop in hydraulic grade.
The design triangle is simple: protect the machine, keep the head, and stay maintainable. Ignore any one corner and the scheme will underperform.
Coarse racks: the first line of defence
Coarse racks, sometimes called trash racks or bar screens, catch the large material that would otherwise damage downstream equipment. Bar spacing is typically 50 to 150 mm depending on the turbine type and the size of the largest expected debris.
- Bar spacing: wider bars mean lower head loss but more risk.
- Approach velocity: keep it below 1.0 m/s to reduce clogging.
- Cleaning: manual rakes suit small schemes, mechanical rakes suit larger ones.
- Material: stainless steel or coated steel resists corrosion and abrasion.
A rack that is too fine becomes a wall. A rack that is too coarse becomes a doorway for trouble.
Settling arrangements and sand traps
Gravel and sand are abrasive. They wear seals, erode runners and settle in forebays. A settling basin or sand trap slows the flow long enough for particles to drop out before they enter the penstock.
- Estimate the sediment load from the catchment.
- Size the basin for the target particle diameter, usually 0.2 to 2 mm.
- Provide a flushing gate or drain for periodic cleaning.
- Check the basin after every flood event.
Settling costs space, but it pays back in reduced turbine wear and longer service intervals.
Keeping hydraulic losses low
Head loss through a screen is governed by approach velocity, bar thickness, blockage ratio and the angle of the rack. A clean rack at 0.8 m/s may lose only a few centimetres of head, while a half-blocked rack at 1.5 m/s can lose ten times as much.
"Design for the blocked condition, not the clean one."
Use gradual transitions, avoid sharp bends before the rack, and allow a bypass or overflow path so that the scheme keeps running during cleaning.
| Element | Typical purpose | Loss concern |
|---|---|---|
| Coarse rack | Remove large debris | Blockage raises head loss |
| Fine screen | Protect small turbines | High loss if undersized |
| Sand trap | Remove abrasives | Low if velocity is controlled |
Putting the pieces together
Good intake design is a sequence: coarse rack, then settling, then a finer screen if the turbine demands it. Each stage removes what the next stage cannot tolerate, and each stage is sized for the worst realistic blockage.
Monitor head loss across the intake, schedule cleaning before performance drops, and keep the design honest about maintenance access. A scheme that screens debris without killing flow is a scheme that keeps generating.
