Chapter 5 1. Hydraulic Pumps (pp. 47 90, Gorla & Khan; Wiki) 1. Two Basic Categories of Pumps Positive Displacement (PD) Pumps A positive displacement pump causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe. For a positive displacement pump, energy is added intermittently to the fluid. A positive displacement pump can be further classified according to the mechanism used to move the fluid into: Reciprocating action pumps Rotary action pumps Kinetic Pumps For a kinetic pump, energy is added continuously to the fluid. Kinetic pumps include: Centrifugal pumps Axial pumps Jet pumps 2. Rotary Pumps (A General Introduction) Source of information: http://www.pumpschool.com Gear pumps Lobe pumps Vane pumps Screw pumps 3. Centrifugal Pumps 3.1 Basic Parts: There are three important parts: (1) the impeller, (2) volute casing, and (3) diffuser. There are two types of diffuser designs: (1) vaneless diffuser (volute), and (2) vaned diffuser. For the diffusion process, the vaneless diffuser is reasonably efficient and is best suited for a wide range of operations. It consists simply of an annular passage without vanes surrounding the impeller. The vaned diffuser is advantageous where small size is important. In this type of diffuser, vanes are used to diffuse the outlet kinetic energy of the fluid at a much higher rate than is possible by a simple increase in radius, and hence it is possible to reduce the length of flow path and diameter. Example of A Centrifugal Pump with volute Example of A Reciprocating Pump Example of Jet Pumps Example of A Centrifugal Pump with volute 77
Example of A Centrifugal Pump (Engineering Graph) Example of Centrifugal Pumps (with a vaned diffuser ring) Example of A Centrifugal Pump (with a vaneless diffuser) Question: for the vaneless diffuser shown in the left figure, what causes the change in the cross sectional area? Your answer is:. 78
4. Cavitation in Hydraulic Machinery (Pumps, Propellers and Turbines) 4.1 What is Cavitation? What are the Undesirable Effects of Cavitation? Cavitation on a Propeller (an axial pump, used on, e.g. a boat/ship) http://www.youtube.com/watch?v=gpklbs3s7iu&feature=related http://www.youtube.com/watch?v=kexsxt lo5c&feature=related http://www.youtube.com/watch?v=r LAwH5wKpg&NR=1 What causes cavitation? Local vaporization of the fluid when the local static pressure falls below the vapour pressure of the fluid. Small bubbles and cavities are formed. Cavitation on a Propeller (e.g., on a boat/ship) Effect of Cavitation on a Propeller Cavitation damage to a Francis turbine Undesirable Effects of Cavitation: Local pitting and erosion in the impeller; Vibrations and noises (sharp cracking sound) when cavitation takes place; Drop of efficiency due to vapour formation, which reduces the effective flow areas. 79
Chapter 5 2. Hydraulic Turbines (Gorla & Khan, pp.91 141; Wiki) 1. Terminologies Related to A Hydropower Plant Hoover Dam generates more than 4 billion kilowatt-hours of electricity each year, enough to serve 1.3 million people. Reference: http://www.greennews.com/hydropower.asp Draft (Tailrace) tube Draft tube Tailrace Tailrace 2. Impulse and Reaction Turbines Reading/watching materials (Hydraulic turbines): http://en.wikipedia.org/wiki/hydraulic_turbine http://en.wikipedia.org/wiki/pelton_wheel http://en.wikipedia.org/wiki/kaplan_turbine Reading/watching materials (Ocean hydroelectricity): 2.1 Impulse Hydraulic Turbines http://www.youtube.com/watch?v=hdwwtgb0k8u http://www.youtube.com/watch?v=lzc9 V9DSew&feature=related http://www.youtube.com/watch?v=tsbaczre3gw&feature=related Reading/watching materials (River hydroelectricity): http://www.youtube.com/watch?v=wvxuzf4lvgw&feature=related http://www.youtube.com/watch?v=fvyactjpmvk&feature=related http://www.youtube.com/watch?v=hh2l_tlvzq0&feature=related http://www.youtube.com/watch?v=yrx7 8izmbE&feature=related 80
Impulse turbines change the velocity of a water jet flow. The jet impinges on the turbine's curved blades which change the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy. Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation. Impulse turbines are most often used in very high head (>300m/984ft) applications. Consequently, the size of an impulse turbine can be compact. Examples of Impulse Hydraulic Turbines (Pelton Turbines/Wheels) 81
Lester Allan Pelton (Sept. 5, 1829 Mar. 14, 1908) Pelton made his living as a carpenter and a millwright. He created the most efficient form of impulse water turbine. Viktor Kaplan (Nov. 27, 1876 Aug 23, 1934) Kaplan was an Austrian engineer and the inventor of the Kaplan turbine. James Bicheno Francis (May 18, 1815 Sept. 18, 1892) Francis was a British-American engineer. 2.2 Reaction Hydraulic Turbines Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow. In reaction turbines, pressure drop occurs in both fixed and moving blades. Most water turbines in use are reaction turbines and are used in low (<30m/98ft) and medium (30 300m/98 984ft) head applications. For this reason, the size of a reaction turbine is usually larger than an impulse turbine (the latter of which is used for high head applications). Typical reaction hydraulic turbines include: Kaplan turbines and Francis turbines. The Francis turbine is a type of water turbine that was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts. Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to several hundred meters and are primarily used for electrical power production. The Kaplan turbine is a propeller type water turbine which has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan. Kaplan turbines are now widely used throughout the world in high flow, low head power production. The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low head applications that was not possible with Francis turbines. A Kaplan turbine is used where a large quantity of water is available at low heads and hence the blades must be long and have large chords so that they are strong enough to transmit the very high torque that arises. Reading/watching materials: http://www.youtube.com/watch?v=2wq2va1iica&feature=related http://www.youtube.com/watch?v=4vgeumbvcdk&feature=related http://www.youtube.com/watch?v=hzqpnpp55xq http://www.youtube.com/watch?v=nwpdc2dylrg http://www.youtube.com/watch?v=hgi5c8jft9y 82
Examples of Reaction Hydraulic Turbine (Francis Turbines/Wheels) Francis runner, Three Gorges Dam Guide vanes at full flow setting Francis Inlet Scroll, Grand Coulee Dam Small Swiss made Francis turbine 83
Examples of Reaction Hydraulic Turbine (Kaplan Turbines/Wheels) 84
Comparison the Pelton, Francis and Kaplan Turbines 85