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What's important for water power designer to make the right choice of a turbine
What’s important for water power designer to make the right choice of a turbine (on photo: Pelton water turbine wheel runner; credit: francishydroturbines.com)

Right choice of a turbine

The water power designer has to make a choice on the type of turbine that can be adopted for a particular project.

After the range of head to be handled by a turbine has been evaluated by stream flow analysis and the installed capacity determined from the analysis of the power-generating capacity of the proposed plant, the task of the designer is to choose:

  1. An optimum turbine type and series,
  2. The number of power generating units,
  3. The runner diameter,
  4. Rotational speed, and
  5. Runner axis elevation.
Knowing the total installation at the power station, the number of units can be decided. The capacity of the plant should be fixed as high as possible with adequate care on efficient running and low initial costs, and available transport and shipping facilities.

The following Bureau of Indian Standard Codes of practices may be referred to for the detailed design of hydraulic turbines and their selections:

  1. IS:12837-1989 “Hydraulic turbines for mediumand large power houses – guidelines for selection
  2. IS: 12800-1993 “ Guidelines for selection of turbines, preliminary dimensioning and layout of surface hydro-electric power houses”:
    • Part 1: Medium and large power houses
    • Part 2: Pumped storage power houses
    • Part 3: Small, mini and micro hydroelectric power houses

One of the important parameters of a turbine is the Specific Speed denoted as ns, which and defined as the speed in r.p.m. at which a turbine of homologous design would operate, if the runner were to reduce to a size which would develop one metric horse power under one metre head.

It is given by the following relation:

Specific speed of turbine in r.p.m.

where:

ns = Specific speed of turbine in revolutions per minute (r.p.m.)
n = Rated speed of turbine in revolutions per minute
P = Turbine output in kW, and
H = Rated head in metres.

Once the specific speed (ns) is determined, the chart given in IS: 12837-1989 and reproduced in Figure 1 may be used to determine the type of turbine that may be adopted for the particular project.

Chart for determining the selection of turbine
Figure 1 – Chart for determining the selection of turbine

The type of turbine selected also depends upon techno-economic considerations of the generating equipment, power house cast and relative benefits of power generation.

The factors given in the following table determine the type of turbine to be used depending upon site conditions.

Type of MachineHead variation percent of rated head (m)Load variation percent of rated outletSpecific speed (m-mhp)Peak Efficiency in percent
Pelton120 to 8050 to 10015 to 6590
Francis125 to 6550 to 10060 to 40093
Deriaz125 to 6550 to 100200 to 40092
Kaplan125 to 6540 to 100300 to 80092
Propeller110 to 9090 to 100300 to 80092
Bulb125 to 6540 to 100600 to 120092

Additional Notes

The following points may additionally be noted:

1. The performance of a turbine is ideal at the design head.

Fall of efficiency in case of Pelton, Kaplan and Bulb turbinesis much less in comparison to Francis-and Propeller types.

Therefore in overlapping head ranges selection of type of turbine should consider the head variation existing at site.

2. Turbine efficiency varies with load. Fall of efficiency at part load for Francis and Propeller is much steeper in comparison to that for Kaplan and Pelton turbines. Therefore, necessity of operating turbinesat part loads for longer timeinfluences the choice of turbines in the overlapping head ranges.

Thus in the head ranges where both Kaplan and Francis are suitable.

The requirement of large pressure head and electrical load variation dictates, Kaplan turbine to be superior to Francis turbine from considerations of higher power generation on account of better overall efficiency.

Similarly, in the overlapping head ranges where both Francis and Pelton could be used, Pelton has advantages over Francis in overall performance level when variation of load and head is higher.

3. Highest specific speed of turbine resulting in higher speed of rotation for generator with consequent reduction in cost of generator. This criteria is very important for selecting-type of turbine from cost consideration in the overlapping head ranges.


Kaplan turbine / Run-of-the-river hydroelectricity – How it works!

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Working of Francis Turbine

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715 Kw Pelton turbine

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Reference: Hydropower Engineering – Version 2 CE IIT, Kharagpur

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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