HYDROPOWER ENGINEERING  ·  POWERHOUSE

Inside a Hydropower Powerhouse: How the Building Around the Turbine Is Put Together

A turbine does not simply sit inside a shed. The powerhouse is a load-bearing machine structure, cast in two separate concrete campaigns with steel welded into it in between — and the order in which that happens decides how well the unit will ever run. This walkthrough follows one axial-flow unit, stage by stage.

Technical explainer  ·  Hydrotu Engineering Blog

The five stages at a glance
1.  First-stage concrete — foundation mass, with the tailwater passage formed inside it.
2.  Crane and roof — lifting capacity has to exist before the heavy lifts.
3.  Embedded steel — spiral case, runner chamber and draft-tube liner welded into place.
4.  Second-stage concrete — the steel is encased, the machine pedestal and floor slabs are cast.
5.  Installation — turbine, then generator, then the auxiliary systems around them.

A station normally houses several generating units side by side. To explain the structure, a scale model is enough: it reproduces only the part of the powerhouse that serves one axial-flow unit — effectively a single bay of the building. The model is cut on a plane that contains the shaft axis and runs parallel to the direction of flow, so the entire water path, from spiral-case inlet to draft-tube outlet, lies open in one view. Everything that follows should be read in that section.

Stage 1 — First-stage concrete: the foundation and the water path

Powerhouse construction is completed in campaigns, and the concrete is placed in two distinct stages. The first stage builds the base of the whole building: the part that carries every load above it and transfers it into the rock. Figure 1 shows those first-stage works — the foundation mass, with the water passages already formed inside it.

The first obligation of that base is to leave a road for the water. Leaving the runner, the water drops into the draft tube and passes down through a passage cast inside the foundation, which turns from vertical to horizontal and grows in cross-section towards the outlet: a straight cone under the runner, a bend — the elbow — and then a diffuser. That passage has to be formed in the first pour, and a gate slot is left at its outlet so that a draft-tube gate can be dropped in for dewatering. Figure 2 follows the tailwater through it.

Along with those passages, the first-stage concrete also includes the seats and supporting piers for the spiral case, the runner chamber and the draft tube, plus the structural frame of the building itself — partition walls, columns and the crane girders that will carry the travelling crane.

Figure 1 — First-stage concrete: the foundation mass of the powerhouse, with the water passages already formed inside it.

Figure 2 — The draft-tube passage cast into the first-stage concrete; the blue arrows trace the tailwater.

Stage 2 — The crane goes in before the equipment it will lift

Nothing heavy can be installed before something heavy can lift it. The overhead travelling crane is erected, and the roof is closed over it (Figure 3). At very large stations the roof is deliberately left open at this point, so that an outdoor heavy-lift crane can set the spiral case and penstock sections from outside; the roof follows once the embedded steel is in.

Figure 3 — Crane and roof in place, ready for the heavy lifts.

Stage 3 — Setting the embedded steel

The spiral case and inlet pipe, the runner chamber and the draft-tube liner are welded and assembled on the first-stage foundation (Figure 4). In the model the stay ring is omitted, so that the internal geometry stays legible in section.

Figure 4 — Spiral case and related steel components assembled on the first-stage foundation.

Stage 4 — Second-stage concrete: locking the steel into the structure

Once the embedded components are aligned and welded, the second-stage concrete is poured around them, encasing the steel in the structure; maintenance and access galleries are kept open, though the model does not show them. This is also the campaign that casts the machine pedestal — the block the generator will stand on — and the floor slabs that give the installation crew and the crane a working level (Figure 5).

Figure 5 — Second-stage concrete encasing the embedded steel and forming the machine pedestal.

Stage 5 — Mechanical and electrical installation

With the structure closed, the main plant goes in. For an axial-flow (Kaplan) unit that means the distributor with its guide vanes, the runner, the head cover and the vane control mechanism, together with the draft-tube gantry crane and the draft-tube gates (Figure 6). The generator follows (Figure 7), and the unit is complete.

Figure 6 — Installing the turbine on the second-stage concrete structure.

Figure 7 — The completed powerhouse, with the generating unit fully installed.

What the model leaves out

Real installation is considerably more involved, and the turbine and generator are erected in an interleaved sequence rather than one strictly after the other. A station also carries lubrication and high-pressure oil systems, compressed air, auxiliary water supply and drainage, control and protection, and the step-up equipment. Every one of those needs conduits, sleeves and anchor plates embedded during the two concrete campaigns, with the equipment itself installed progressively afterwards. The model does not show that layer, and the actual order of work varies with station layout, unit size and site logistics.

Figure 8 shows an axial-flow unit Kaplan turbine as it stands in service.

Figure 8 — An installed axial-flow kaplan turbine and generator.

From one bay to a whole station

Everything above describes a single unit bay. A real station simply repeats the bay. Three Gorges, with 32 large units, remains the largest installation of its kind in the world. Figures 9 and 10 show a three-unit powerhouse seen from the dam side and from the downstream side respectively.

Figures 9 and 10 — A powerhouse with three generating units, seen from the dam side (left) and from downstream (right).

Why the sequence matters

• The building is a machine support, not a shelter. Shaft alignment is only ever as good as the pedestal and the embedded plates cast underneath it.

• The water path is fixed in concrete. The elbow and the diffuser cannot be corrected afterwards — a badly formed draft tube is a permanent efficiency loss.

• Two pours, one structure. The first gives a foundation accurate enough to weld on; the second locks the steel in and carries the load down into the rock.

• Lifting capacity sets the schedule. Crane and roof before equipment is not a detail — it is the critical path.

A powerhouse is therefore best read as a sequence rather than as a building: foundation, water path, lifting, steel, concrete, machine. Get the order right and the unit goes together cleanly; get it wrong, and no amount of commissioning will win the difference back.

The construction sequence described here is also available as a short animated walkthrough of the same model.

TERMS USED IN THIS ARTICLE

Draft tube — the passage that carries water from the runner to the tailwater. Its cone, elbow and diffuser recover pressure, and its shape is fixed in the first pour.

Spiral case — the steel casing that wraps around the runner and distributes water evenly over its full circumference.

Stay ring — the steel ring between spiral case and distributor that carries the weight of the unit and of the structure above. Omitted from the model so the section stays readable.

Runner chamber — the steel-lined space in which the runner turns, cast into the first-stage concrete and completed with the second pour.

Machine pedestal — the concrete block on which the generator and the thrust bearing stand; its level and centring set the shaft alignment.

Hydrotu designs, manufactures and supplies complete hydro generating sets, along with the balance of plant around them.