Positive Displacement Turbine

3 Key excerpts on "Positive Displacement Turbine"

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  eBook - ePub

  Fluid Mechanics, Hydraulics, Hydrology and Water Resources for Civil Engineers

  • Amithirigala Widhanelage Jayawardena(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 12 Fluid machinery

  12.1 Introduction

  Fluid machinery can be broadly classified into two types; namely, pumps and turbines. Pumps convert mechanical energy into fluid energy, thereby increasing the energy of the fluid (increase of pressure). Turbines convert the fluid energy into mechanical energy, thereby decreasing the energy of the fluid.

  Compressor is also a pump. Its primary function is to increase the pressure of the gas. Fan and Blower are machines used only for causing the movement of gas.

  12.2 Types of pumps

  There are various types of pumps; some have only historical importance. Examples include.

  12.2.1 Chain or bucket pumps

  This is the simplest type of pump that uses gravity lift. It has a valve which opens when it goes below the water line and closes when lifted above the water line.

  12.2.2 Static or positive displacement pumps

  Positive displacement type functions by changes of the volume occupied by the fluid within the machine. They create an expanding cavity on the suction side and a contracting cavity on the delivery side. The operation of a positive displacement type machine depends only on mechanical and hydrostatic principles. Only a few principles of fluid dynamics are involved. Positive displacement pumps are more suitable for low flow high-pressure situations, and their efficiency is relatively independent of the pressure. They are preferred for moving viscous fluids such as oil, asphalt, etc. There are several types of positive displacement pumps.

  12.2.2.1 Reciprocating type

  They are usually of piston−cylinder combination with inlet and exit valves involving suction and compression. Work done by pump is ∫pdv

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  eBook - ePub

  Practical Hydraulics Handbook

  • Barbara Hauser(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 14

  Positive Displacement Pumps

  Whereas centrifugal pumps are meant for low pressure, high flow applications, positive displacement pumps can achieve greater pressures, but are slower moving, low flow pumps. They are used for pumping sludges and slurries and other high density or viscous fluids for which other types of pumps are inappropriate. Tolerances are close between stationary and moving parts, and flow is constant. A given volume of liquid is positively displaced with each pump cycle; there is no backwards slippage of fluid, and high shutoff pressures are developed. When pumping head increases from a blockage, or when system head increases, the pump will still move the same amount of fluid, but will draw more horsepower, and create high discharge pressure in overcoming that head.

  This is quite different from the operation of the centrifugal pump, which delivers less flow and uses less horsepower under high head conditions. With a positive displacement pump, total blockage of pump or line, whether caused by foreign material or a closed valve, will cause failure of pump, line or motor.

  Note: Never start a positive displacement pump with the discharge valve closed!

  These pumps operate at much slower speeds than centrifugal pumps. They are much larger, heavier, and more costly. The large number of moving parts (valves, cams, reciprocating components) make them more complex, and the close running clearances make them more subject to wear, with resulting parts replacements necessary. Efficiency is lower and maintenance cost higher than in the centrifugal pump.

  There are two basic categories of positive displacement pump: the reciprocating pumps, and the rotary pumps. Under reciprocating pumps are generally included the plunger, or piston pump, the diaphragm pump, and the peristaltic pump. Under rotary pumps are included the rotary gear pump, the vane pump, the progressive cavity pump, and the Archimedes screw.

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  eBook - ePub

  Practical Hydraulics and Water Resources Engineering

  • Melvyn Kay(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 7

  Pumps

  7.1 INTRODUCTION

  Few water supply systems have the benefit of a gravity supply, most require some kind of pumping. Pumps are a means of adding energy to water. They convert fuel energy, such as petrol or diesel, into useful water energy using combustion engines or electric motors. In the typical pipe flow problem in Chapter 5 , the energy to drive water along the pipeline to the town was from a reservoir located high above the town. The energy line drawn from the reservoir to the town indicated the amount of energy available. Adding a pump to this system increases the available energy and raises the energy line so that the discharge from the reservoir to the town can be increased (Figure 7.1 ).

  Pumps have been used for thousands of years. Early examples were largely small hand or animal-powered pumps for lifting small quantities of water. It was not until the advent of the steam engine, only two centuries ago, that the larger rotating pumps were developed and became an important part of the study of hydraulics. Consequently, there are two main types of pump: positive displacement pumps , which are mostly small hand and animal-powered pumps still in common use in developing countries; and roto-dynamic pumps , those driven by diesel or electric motors and used in all modern water supply and irrigation systems.

  Because of the importance of pumping, most of this chapter is devoted to pumps, but mention is also made of turbines which are hydraulic machines for generating energy.

  7.2 POSITIVE DISPLACEMENT PUMPS

  Positive displacement pumps usually deliver small discharges over a wide range of pumping heads. Typical examples are hand-piston pumps, rotary pumps, airlift pumps and Archimedean screws (Figure 7.2 ).

  Figure 7.1    Pumps add energy to pipe systems.

  7.2.1 Typical pumps

  Hand-piston pumps (Figure 7.2a and e

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