By Anzar Hasan, Chief Inspector
Since we service several utilities that include Hydropower plants, following is the first part of information that I think most inspectors would find informative. Next time, I will discuss the protection, testing and maintenance systems and programs.
Hydroelectric plants relative to their “HEAD” (The vertical difference in elevation between the plant intake or dam water level and the discharge level) utilize two main types of hydro turbines: impulse and reaction. Other deciding factors include how deep the turbine must be set, efficiency and cost.
Type of Turbines (in operation in USA):
Impulse Turbine – The impulse turbine generally uses the velocity of the water to move the runner and discharges to atmospheric pressure. The water stream hits each bucket on the runner. There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner. An impulse turbine is generally suitable for high head, low flow applications.
- Pelton Turbine – These machines are limited to high head applications. The number of jet nozzles per wheel can range from one to six. The pelton wheel is a low specific speed. The specific speed increases as the square root of the number of nozzles used. Draft tubes are not required for impulse turbine since the runner must be located above the maximum tailwater to permit operation at atmospheric pressure. The jet nozzle is usually equipped with a jet deflector which serve to force the jet stream away from the buckets whenever the generator is unloaded suddenly. Machines without jet deflectors may utilize a system incorporating reverse jets or some relief device to redirect the flow of water.
- Turgo Turbine: A Turgo Wheel is a variation on the Pelton and is made exclusively by Gilkes in England. The Turgo runner is a cast wheel whose shape generally resembles a fan blade that is closed on the outer edges. The water stream is applied on one side, goes across the blades and exits on the other side. These turbines operate in the medium to high head range.
- Cross-Flow Turbine: A cross-flow turbine is drum-shaped and uses an elongated, rectangular-section nozzle directed against curved vanes on a cylindrically shaped runner. It resembles a “squirrel cage” blower. The cross-flow turbine allows the water to flow through the blades twice. The first pass is when the water flows from the outside of the blades to the inside; the second pass is from the inside back out. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross-flow was developed to accommodate larger water flows and lower heads than the Pelton.
Reaction Turbines: A reaction turbine develops power from the combined action of pressure and moving water. The runner is placed directly in the water stream flowing over the blades rather than striking each individually. Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines.
- Francis Turbine – A Francis turbine has a runner with fixed buckets (vanes), usually nine or more. Water is introduced just above the runner and all around it and then falls through, causing it to spin. Besides the runner, the other major components are the scroll case, wicket gates, and draft tube. Water flows into the spiral case, through fixed stay vanes, through adjustable wicket gates, and then through the runner vanes. The wicket gates can be used to shut off the flow of water in an emergency. This machine can be used for low to high heads.
- Kaplan Turbines – These have adjustable runner blades and adjustable wicket gates. The shaft operation could be vertical.
- Semi-Kaplan Turbines – These may have adjustable runner blades and non-adjustable wicket gates (guide vanes or distributors).
- Kinetic Energy Turbines – These types generate electricity from the kinetic energy present in flowing water rather than the potential energy from the head. The systems may operate in rivers, man-made channels, tidal waters, or ocean currents. Kinetic systems utilize the water stream’s natural pathway. They do not require the diversion of water through manmade channels, riverbeds, or pipes, although they might have applications in such conduits. Kinetic systems do not require large civil works; however, they can use existing structures such as bridges, tailraces and channels
- Propeller Type Turbine – The turbines in this classification include propeller, Kaplan, Semi Kaplan, S-turbine, Upstream, Elbow, Bulb, Pit and Rim Turbine. A propeller turbine generally has a runner with three to six blades in which the water contacts all of the blades constantly. The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube. The propeller turbines have non-adjustable runner blades and non-adjustable guide vanes.
- Bulb Type Turbine – The turbine and generator are a sealed unit placed directly in the water stream. The generator is attached directly to the perimeter of the turbine.
Types of hydropower applications: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not.
Impoundment – The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.
Diversion – A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.
Pumped Storage – This operates by storing the energy by pumping water uphill to a reservoir at higher elevation from a second reservoir at a lower elevation. When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir and turns a turbine, generating electricity which may be worth three times as off peak power.
Run-Of-River – A dam with or without a short penstock directs the water to a turbine. The natural flow of the river is used with very little alteration to the terrain stream channel at the site and little if any, impound (Storage) of the water.
Tidal Power – In some estuaries tidal power can be economically harnessed to develop power. These installations use the water flowing back and forth as the result of tidal action and the fact that there is significant difference in elevation of the water surface in the estuary from one stage of tide to another.