Powerhouse layout with Francis turbine

A powerhouse with Francis turbines has three main levels: the substructure (draft tube and spiral casing in mass concrete), the intermediate structure (turbine and generator floor) and the superstructure (machine hall with overhead crane).

Typical layout (cross-section)

     ______________________________________
    |  roof truss / EOT crane rail         |
    |      [=== crane ===]                 |
    |                                      |
    |          _______                     |
    |         | GEN   |   generator floor  |
    |_________|_______|____________________|
    |          |shaft|    turbine floor    |
 penstock==>( spiral casing )  MIV         |
    |          \runner/                    |
    |           |    |  draft tube         |
    |__________/      \____________________|__ TWL
                \______\________~~~~~~~~~~~ tailrace

Plan: units in a row in the machine hall, with an erection (service) bay at one end at road level; control room, switchgear and transformer yard alongside.

Fixing the dimensions of the powerhouse

1. Length $= n \times$ unit spacing $+$ erection bay length $+$ end clearances.
   • Unit spacing is governed by the largest of: spiral casing width, draft tube outlet width, or generator barrel diameter, plus about 1.5–2 m for walkways and walls. Roughly 4–5 times the runner diameter $D$ for Francis units.
   • Erection bay ≈ 1 to 1.5 times the unit spacing, enough to assemble a rotor and runner.
2. Width is fixed by the spiral casing / generator size in the transverse direction (≈ 4–5 $D$) plus space for the main inlet valve (MIV) on the upstream side and governors, panels and walkways; draft tube length (≈ 4–5 $D$) fixes the substructure width.
3. Height:
   • Below the turbine centre line: draft tube depth (≈ 2.5–3 $D$) and the turbine setting $H_s$ below or above tailwater, fixed by the cavitation (Thoma) criterion.
   • Above the generator floor: height of the generator and exciter, plus the height needed for the crane to lift the rotor over other units, crane girder and roof (often 10–15 m above the generator floor).
4. Crane capacity is fixed by the heaviest part to be lifted (generator rotor or transformer).
5. Other: space for control room, auxiliaries, access, ventilation, and future units.

These empirical ratios (based on runner diameter $D$) are used at the feasibility stage; final dimensions come from manufacturer's drawings.