Powerhouse general arrangement and dimensions

General arrangement of a surface powerhouse

A surface powerhouse has a substructure and a superstructure.

• Substructure: below the generator floor. It contains the draft tube, tailrace connection, spiral (scroll) casing embedded in concrete, turbine pit, drainage and dewatering sumps, and the foundation raft.
• Intermediate part: the turbine floor, with the turbine, governor, valves (main inlet valve) and galleries for pipes and cables.
• Superstructure: above the generator floor. It contains the generator hall with the generators, the overhead travelling crane, the erection (service) bay, control room, switchgear and offices, with a roof and ventilation.

Other parts connected to it: penstock and inlet valve, tailrace channel, transformer yard and switchyard, access road and workshop.

   PLAN
 +-----------------------------+
 | Unit1  Unit2  Unit3 | Erect |
 | (G)    (G)    (G)   | bay   |
 +---------------------+ Ctrl  |
 | valves / galleries  | room  |
 +-----------------------------+
   penstocks -> | tailrace ->

Dimensions of the powerhouse

Dimensions are fixed as follows.

• Length $L$ = (number of units $\times$ unit bay width) + erection bay + end clearances. The unit bay width is about 3 to 4 times the runner diameter (governed by the spiral casing or generator size).
• Breadth $B$: width of the generator and spiral casing, plus valve/gallery space and a control/auxiliary side; usually 3.5 to 5 $D$ for Francis turbines.
• Height $H$: from the draft tube floor to the roof. It is made up of the draft tube depth (about 2.5 to 3 $D$), the distributor/turbine floor, the generator, and the crane hook clearance needed to lift the largest part (the generator rotor) with its slings.
• Elevation: the generator floor is set above the maximum tailwater flood level; the turbine level follows the setting $H_s$.