Turbine Selection Chart — Head vs Flow Calculator | Hydel Energy
Every hydropower scheme reaches the same question once the survey work is done: the site gives you a head and a flow, so which turbine actually suits it?The answer has been drawn as a turbine selection chart for decades.
Head on one axis, flow on the other, both logarithmic, with an envelope marking the working range of each runner type. Every manufacturer publishes a version. Most of them are static PDFs that you squint at, trying to locate your site somewhere between the gridlines.
This is that chart, made interactive. Enter your net head and design flow, or drag the point, and every envelope covering that duty lights up — along with the electrical output, the estimated annual energy, and a note on what each runner type would mean in practice.
How to read the turbine selection chart
Both axes are logarithmic, so each gridline is a multiple rather than an addition. That is what lets a single chart hold everything from a 2 m weir to a 1,800 m alpine scheme.
The diagonal guide lines are constant electrical output — 10 kW through to 1 GW. Slide along one of them and your power stays the same while head trades against flow. It is a useful thing to see directly: 100 m at 1 m³/s and 10 m at 10 m³/s produce identical power, and demand completely different machines.
The envelopes overlap deliberately. Where two or three cover your point, all of them are technically viable and the decision moves to specific speed, cavitation setting, part-flow behaviour, silt load and cost. A chart narrows the field; it does not close the argument.
Enter flow per unit, not station discharge
This is the mistake worth avoiding. The envelopes describe one machine, not a whole powerhouse. Divide the station discharge by the number of units before entering it.
Get this wrong and the chart will tell you your duty is impossible when it is entirely ordinary.
Take Suki Kinari, in Kaghan valley. The station runs roughly 114.6 m³/s at about 922 m of net head for 884 MW. Enter those figures and the chart correctly refuses them — no single turbine on earth passes that flow at that head.
Divide by the four units and enter 28.65 m³/s instead, and it returns Pelton, near the upper edge of the range. Which is exactly what is installed there. The computed output at 85 percent efficiency comes to 220.3 MW against a 221 MW nameplate.
What the chart cannot tell you
Two things, and both matter more than the runner type.
It cannot pick the number of units
Hydraulic similarity is scale-free — halve the flow, shrink the runner, raise the speed, and you have the same machine in a different size. Run the specific speed calculations for a station like Suki Kinari and one unit through eight all come back hydraulically valid. What actually bounds the count is the largest machine anyone can build (the record for a single Pelton is 423 MW, at Bieudron in Switzerland), the shape of the flow duration curve, and the need to keep generating when one unit is out for maintenance.
It cannot choose your design flow
That comes from hydrology. The flow duration curve, built from years of gauged daily discharge, is what tells you how often each flow is actually available — and therefore what capacity is worth installing. Head and flow arrive at this chart as inputs. Producing them is the larger part of the work.
If you have your output figure and want the power calculation, use the hydropower calculator. This chart answers the next question: which machine.
Where the boundaries come from
The envelopes are compiled from ESHA small hydro guidance and published manufacturer application charts from Voith, Andritz and Ossberger. They are indicative. Every manufacturer draws the corners slightly differently, and a supplier will happily quote outside the published range when a project justifies it.
They have been checked against real plants — Bieudron, Itaipu, Three Gorges, Tarbela, Neelum-Jhelum, Iron Gates and Suki Kinari all land where they should. Walchensee, which runs Pelton and Francis machines at the same head, correctly returns both.
If you work with a duty that the chart places wrongly, tell us. Field correction is how a reference like this earns trust.
Related: Hydropower Calculator · Hydropower Commissioning
