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Forebay and surge tank comparison
Short answer
Both handle sudden load changes in a hydro plant. A forebay is used at the end of an open canal to transition to pressure flow, store water, and spill excess. A surge tank is used at the end of a pressure tunnel to absorb water hammer and oscillations, with no spillway. Forebay water level is nearly constant; surge tank level oscillates.
Why are forebay and Surge Tanks provided in a hydropower plant? Mention the difference between these two.
Why they are provided
Both are provided between the low-pressure conduit and the penstock to deal with sudden changes of load on the turbines.
- Forebay is provided at the end of an open headrace canal. It changes free-surface flow into pressure flow, stores water for load increases, spills water on load rejection, traps the last sediment and debris, and gives submergence to the penstock inlet.
- Surge tank is provided at the end of a pressure tunnel or long pipeline (as close to the powerhouse as possible). When the turbine gates close or open, it gives a free water surface that absorbs the water hammer, so only the short penstock below it is designed for high water hammer. It also supplies water when load increases and stores it when load decreases, and improves speed regulation.
Canal: canal ==> [FOREBAY] ==> penstock ==> PH
Tunnel: tunnel ==> [SURGE TANK] ==> penstock ==> PH
(pressure) free surface
Differences
| Point | Forebay | Surge tank |
|---|---|---|
| Used with | Open canal (free flow) | Pressure tunnel/pipe |
| Location | End of canal, head of penstock | End of pressure tunnel, near powerhouse |
| Main purpose | Transition and short-term storage | Relieve water hammer and surges |
| Water level | Nearly constant (canal level) | Oscillates up and down (mass oscillation) |
| Shape | Open, wide, shallow pond | Tall vertical shaft or chamber |
| Spillway | Has overflow spillway | Usually none; sized for max upsurge |
| Sediment | Acts as final settling basin with flushing | No settling function |
| Design basis | Storage volume (2–3 min of flow), submergence | Max upsurge, min downsurge, Thoma stability area |
✓ Answer checked by Claude Opus 5.5 (high effort)
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