Differentiating Hydraulic Turbines: Reaction, Impulse and Propeller

Hydroelectric plants are one of the major sources of renewable energy. They work by utilizing water stored in dams or as runoff to produce clean, low-cost electricity by spinning the hydraulic turbines using the energy of falling or flowing water streams.

In the real world, electric power is produced by synchronized generators or alternators driving through turbines.

Hydraulic pumping equipment. Image used courtesy of Adobe Stock

The turbine converts the kinetic or potential energy of water (i.e., a water storage dam) into mechanical energy. The generators, coupled with the runner through the shaft, convert the mechanical energy into electrical. The dams are the water reservoirs for water storage, which provide the water head with potential energy. The water head is the difference between the water level stored in the reservoir and the tailrace water level.

Figure 1. Hydropower Generation Model. Image used courtesy of Munir Ahmad

Turbine design and selection depend on water flow and the head at the site, ranging from low to high head. An understanding of turbine types, along with their applications and uses, is important for maximizing the performance and efficiency of hydroelectric facilities.

Some big turbine manufacturers in the market today are:

• Voith Hydro
• Hitachi
• General Electric
• Andritz AG
• Harbin Electric Corp.

Turbine Types

The turbine is coupled directly with the generator, resulting in a generating unit. There are three main types of hydraulic turbines: impulse, reaction, and propeller.

Selecting a turbine for a particular application depends on its output and the head under which it will operate. The Pelton wheel is generally suitable for the highest head, the Francis turbine for a medium head, and the propeller type for a low head.

Figure 2. Nameplate data of Francis Hydraulic Turbine, Type: Vertical with single runner coupled with AC Generator rated speed of 136.4 R.P.M. Image used courtesy of Munir Ahmad

Reaction Turbine

Francis is a type of reaction turbine where the runner and the draft tube are filled with water. Francis turbines are built with either horizontal or vertical shafts. The larger turbines usually have vertical shafts. The scroll case forms a complete water passage around the turbine and allows the water to enter the runner from all sides. The draft tube conveys the water discharged from the runner into the tailrace.

Several types of draft tubes are used: vertical tube, moody spreading tube, and elbow tube. The design of the elbow tube, which is simple to construct, has increased its efficiency to that of other types. The elbow draft tube, as its name implies, turns the water at a right angle and provides a continually expanding passage for the discharged water. This construction requires less depth of excavation than some of the other types and is consequently less expensive.

With large turbines, the horizontal discharge end of the draft tube is generally subdivided into two more water passages by vertical or horizontal splitters, which straighten the flow before it is finally discharged.

Impulse Turbine

The principle of the peloton wheel is illustrated in Figure 3. The turbine usually has a horizontal shaft. The water is transported to the turbine by a steel penstock and enters a nozzle, which permits the water to be issued in the form of a high-velocity nozzle jet. The jet sprays into the wheel housing, which is at practical atmospheric pressure, and is directed into the buckets, which are mounted around the rim of the wheel disc. The water flows around the buckets. The jet is divided into two equal parts by a central portion or splitter and travels around the curved bucket without filling it. The wheel will be rotating, and the buckets will be moving away from the jet with a velocity a little less than half of that of the jet. The energy represented by the velocity of the water leaving the bucket is wanted, but a greater part of the original energy is just as effective at turning the wheel.

Figure 3. Pelton turbine, nozzle, and buckets. Image used courtesy of Munir Ahmad

Sometimes, the jet deflector is used to divert the jet from the wheel in case of load rejection, thus performing the function of a relief valve in preventing a dangerous pressure rise in the penstock. However, relief valves are also used.

The buckets are usually replaceable and bolted solidly to the wheel. Cast steel is generally used for large buckets, while smaller buckets are of bronze or stainless steel. The inner surfaces are smooth or polished to minimize the frictional loss of water while passing around the bucket.

Propeller Turbine

The propeller turbine type was developed to provide a higher speed and a higher capacity turbine for use where the available head is low. The propeller turbine derives its name from its ship propeller-like runner. Austrian professor Viktor Kaplan, in 1913, developed the propeller turbine.

The runner is located in the throat of the draft tube. The above runner is known as the whirl chamber. The water enters this region from the scroll case through the wicket gates and is turned downwards towards the runner. The individual runner blades are pivoted to the runner hub. The blades move together, and the blade angle is automatically adjustable according to load changes. The use of the correct blade angle for each load results in a high turbine efficiency over most of the operating range. Therefore, due to automatically adjusting blades, the turbine is best suited for operation over a wide range of loading.

Relief Valve and Surge Tank for Turbine Safety

When the turbines are connected with the long penstocks, it is necessary to provide some means of preventing excessive pressure rise in case of sudden load rejection. Following are the main applications when a machine suddenly rejects the load.

1. When a large area like a 550 kV line trips or big power equipment fails, the large amount of load is disconnected as a result, and the huge dip in the system frequently causes the power generation plants to trip (load rejection). The speed of generators also increases at a dangerous level.
2. Disturbance at the grid level, like fluctuations in voltage and frequency, causes load rejection.
3. Faults in control systems like the governor and excitation system initiate the generator load rejection.

Especially in hydraulic turbines, when there is a sudden load change, the motion of turbine gates or wicket gates must be sufficiently rapid to prevent excessive turbine speed and voltage variation. If the wicket gate closure is too fast and no pressure relief is provided, the destructive water pressure in the penstock may cause destruction. A surge tank or relief valve can be used to relieve the line's excessive pressure. To be more effective in preventing pressure variations, the surge tank must be located close to the turbine; a pressure regulator or a relief valve may be used instead of a surge tank.

Turbine Performance and Plant Efficiency

Tests made to determine the output and efficiency of a new turbine are termed acceptance tests, and these tests normally require accurate measurement of the quantity of water used by the turbine. The tests are also performed to measure the performance of the turbine under different heads and to determine the condition of the unit after considerable service.

It is convenient to determine the overall performance of the unit, which consists of penstock, turbine, and generator. The output of the unit can be measured by the generator kilowatt-hour meter. If the turbine discharge is determined accurately, the overall efficiency of the unit for a particular head at the time of the test may be readily determined. As we know, efficiency is the ratio of output to input. The output is the measured output of the generator in kilowatts, while the input is proportional to the product of the quantity of water used and the gross head. The input is expressed in kilowatts whereby:

Input in kW = 0.0845 QH

Where

Q = quantity of turbine discharge in cubic feet per second (cfs)

H = gross head in feet (difference between forebay and tailrace elevations)

Therefore,

e = Output/input= Generator kW/0.0845 QH

The generator output depends on the amount of water converted into power by the turbine, the available head, and the unit's efficiency. The efficient utilization of water is still a matter of primary importance at all plants that do not customarily operate with an abundance of water and must receive additional consideration like reservoir storage and sedimentation management, pressure losses, and maintenance schedule for the prevention of cavitation, etc., because it is not a quantity that can be indicated directly by the plant meters.

Hydroelectric Takeaways

A hydroelectric power plant is an environment-friendly, emission-free, and low-cost electrical energy source for industrial and domestic loads. The water power is used to spin the turbine which converts kinetic or potential energy stored in the water into electrical energy through generators.