Structural components and the surge tank

The structural (mainly civil) components of a hydropower plant collect, store, clean and carry water to the turbine and return it to the river.

Schematic

 Reservoir/River
  ==|== Dam/weir + spillway
     |
   Intake (trash rack, gates)
     |
   Desander
     |
   Headrace tunnel ================\
                                   |
                            [SURGE TANK]
                                   |
                              Penstock
                                   |
                        [POWERHOUSE: turbine,
                         generator]
                                   |
                              Tailrace -> River

Structural components

1. Dam or weir with spillway: creates head/storage and passes floods.
2. Intake: admits water; trash rack keeps out debris.
3. Gravel trap and desander: remove sediment.
4. Headrace canal or tunnel: carries water at a gentle slope.
5. Forebay or surge tank: free water surface at the head of the penstock.
6. Penstock with anchor blocks and saddle supports.
7. Powerhouse: foundation, superstructure, crane beams.
8. Tailrace channel.

Role of surge tank

A surge tank is an open vertical shaft or chamber placed at the junction of the long low-pressure headrace tunnel and the steep penstock.

• Protects against water hammer: when load is rejected, the governor closes the gates quickly. The moving water column cannot stop instantly; without a surge tank the pressure rise would travel through the whole tunnel. The surge tank lets the water rise in it, so the high pressure is limited to the short penstock.
• Supplies water on load increase: when gates open suddenly, water is drawn from the tank until the slow tunnel flow speeds up, avoiding a drop in pressure (and vacuum) in the penstock.
• Reduces penstock and tunnel thickness: lower design pressure means cheaper tunnel lining.
• Improves governing: reduces the effective water inertia time constant $T_w = \dfrac{LV}{gH}$, since only the penstock length counts, making speed regulation stable.
• Types: simple, restricted orifice, differential and chamber types.