aumid.comaumid.com/Fluid Mechanics/PUMPS updated.docx · Web viewThe word pump is a general term for any fluid machine that adds energy to a fluid. Some authors call pumps energy

TURBOMACHINERYCLASSIFICATIONS AND TERMINOLOGYThere are two broad categories of turbo machinery, pumps and turbines. The word pump is a general term for any fluid machine that adds energy to a fluid. Some authors call pumps energy absorbing devices since energy is supplied to them, and they transfer most of that energy to the fluid, usually via a rotating shaft (Fig. 14–1a). The increase in fluid energy is usually felt as an increase in the pressure of the fluid. Turbines, on the other hand, are energy producing devices—they extract energy from the fluid and transfer most of that energy to some form of mechanical energy output, typically in the form of a rotating shaft (Fig. 14–1b). The fluid at the outlet of a turbine suffers an energy loss, typically in the form of a loss of pressure.

The purpose of a pump is to add energy to a fluid, resulting in an increase in fluid pressure, not necessarily an increase of fluid speed across the pump.

An analogous statement is made about the purpose of a turbine:

The purpose of a turbine is to extract energy from a fluid, resulting in a decrease of fluid pressure, not necessarily a decrease of fluid speed across the turbine.

Fluid machines that move liquids are called pumps but there are several other names for machines that move gases (Fig. 14–3).

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A fan is a gas pump with relatively low pressure rise and high flow rate. Examples include ceiling fans, house fans, and propellers. A blower is a gas pump with relatively moderate to high pressure rise and moderate to high flow rate. Examples include centrifugal blowers and squirrel cage blowers in automobile ventilation systems, furnaces, and leaf blowers. A compressor is a gas pump designed to deliver a very high pressure rise, typically at low to moderate flow rates. Examples include air compressors that run pneumatic tools and inflate tires at automobile service stations, and refrigerant compressors used in heat pumps, refrigerators, and air conditioners.

PUMPS

Definition of PUMPS and Introduction :

To transport water through pipes energy has to be fed to the water. The energy is needed to overcome the dynamic friction losses in the pipe. Also energy is needed to compensate differences in level between the beginning and the end of a pipe (lift energy).

Basically a pump is a piece of equipment to feed energy to a water flow.

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Two types of pumps can be distinguished:

Pumps capable of lifting water from one free surface to another: open pumps or Archimedean screws

Pumps capable of feeding energy to water in combination with a closed pipe: centrifugal or impeller pumps

Pumps are used for instance to

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1. pump water out of the ground,2. to overcome level differences in treatment processes, 3. to transport drinking or sewerage water over large distances in combination with pipes 4. or to dispose of rain water from polders. 5. Numerous other applications of pumps can be given, but they won’t be dealt with in this

lecture.

Classifications of Pumps

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Question could be asked as follows: Classify Pumps into three categories and their sub divisions. Then draw the Centrifugal type of pumps showing the main components of the pump.

Centrifugal Pump

This machine consists of an IMPELLER rotating within a case (diffuser). Liquid directed into the center of the rotating impeller is picked up by the impeller’s vanes and accelerated to a higher velocity by the rotation of the impeller and discharged by centrifugal force into the case (diffuser).A collection chamber in the casing converts much of the Kinetic Energy (energy due to velocity) into Head or Pressure.

Pump characteristics (Performance)

The hydraulic properties of a pump can be described by some characteristics:1) Q-H curve2) Efficiency curve3) Power curve4) Net Positive Suction Head (NPSH) curve.

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Q-H curve The Q-H curve is the relation between the volume flow and the pressure at a constant speed of the pump crank. The H in the curve is the difference in energy level between the suction and the pressure side of the pump. Q-H curves will be given by the manufacturer of the pump and can normally be considered as a simple quadratic curve.An example of a pump curve is given in figure

System Performance Curve is a mapping of the head required to produce flow in a given system. A system includes all the pipe, fittings and devices the fluid must flow through, and represents the friction loss the fluid experiences

Efficiency curve the hydraulic efficiency of the pump with the motor is given with the efficiency curve. The hydraulic efficiency is the relation between the absorbed hydraulic energy (pressure and velocity) and the provided mechanical energy at the pump crank including the power efficiency of the motor.

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How we avoid Cavitation??

Where we take the datum through the centerline of the pump impeller inlet (eye). This difference is called the Net Positive Suction Head (NPSH), so that

NPSH curve The Net Positive Suction Head curve is the relation between the volume flow Q and the needed margin between the energy level at the suction side of the pump and the vapour pressure of the water to prevent too much cavitation in the pump. At the suction side of a pump negative pressures, i.e. pressures below the atmospheric pressure, can occur, especially when the actual weir of the pumpis above the level of the reservoir the water is drawn from. This negative pressure islimited to the actual vapour pressure of the fluid at the current temperature. If this allowable negative pressure is subsided, cavitations will take place in the pump. Although a small amount of cavitations within a pump cannot be avoided, this should be limited. The NPSH requirements of a pump give these limitations.

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NPSH=Psγ +

V s2

2g−

Pvaporγ

To avoid unacceptable cavitations the available NPSH should be larger or equal to the needed NPSH. The available NPSH is defined as:

with h the pressure at the impeller entrance, v the velocity of the water at the impeller entrance. The various pump curves are provided by the pump manufacturer.

Classification of pumpsFluid machines may also be broadly classified as either positive-displacement machines or dynamic machines, based on the manner in which energy transfer occurs. In positive-displacement machines, fluid is directed into a closed volume. Energy transfer to the fluid is accomplished by movement of the boundary of the closed volume, causing the volume to expand or contract, thereby sucking fluid in or squeezing fluid out, respectively. Your heart is a good example of a positive-displacement pump (Fig. 14–5a).

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It is designed with one-way valves that open to let blood in as heart chambers expand, and other one-way valves that open as blood is pushed out of those chambers when they contract. An example of a positive-displacement turbine is the common water meter in your house (Fig. 14–5b), in which water forces itself into a closed chamber of expanding volume connected to an output shaft that turns as water enters the chamber. The boundary of the volume then collapses, turning the output shaft some more, and letting the water continue on its way to your sink, shower, etc. The water meter records each 360° rotation of the output shaft, and the meter is precisely calibrated to the known volume of fluid in the chamber.

Pumps in Series and ParallelWhen faced with the need to increase volume flow rate or pressure rise by a small amount, you might consider adding an additional smaller pump in series or in parallel with the original pump. While series or parallel arrangement is acceptable for some applications, arranging dissimilar pumps in series or in parallel may lead to problems, especially if one pump is much larger than the other (Fig. 14–22).

A better course of action is to increase the original pump’s speed and/or input power (larger electric motor), replace the impeller with a larger one, or replace the entire pump with a larger one. The logic for this decision can be seen from the pump performance curves, realizing that pressure rise and volume flow rate are related Arranging dis-similar pumps in series may create problems because the volume flow rate through each pump must be the same, but the overall pressure rise is equalto the pressure rise of one pump plus that of the other. If the pumps have widely different performance curves, the smaller pump may be forced to operate beyond its free delivery flow rate, whereupon it acts like a head loss, reducing the total volume flow rate. Arranging dissimilar pumps in parallel may create problems because the overall pressure rise must be the same, but the net volume

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flow rate is the sum of that through each branch. If the pumps are not sized properly, the smaller pump may not be able to handle the large head imposed on it, and the flow in its branch could actually be reversed; this would inadvertently reduce the overall pressure rise. In either case, the power supplied to the smaller pump would be wasted. Keeping these cautions in mind, there are many applications where two or more similar (usually identical) pumps are operated in series or in parallel. When operated in series, the combined net head is simply the sum of the net heads of each pump (at a given volume flow rate),

When two or more identical (or similar) pumps are operated in parallel, their individual volume flow rates (rather than net heads) are summed,

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Positive-Displacement Pumps

People have designed numerous positive-displacement pumps throughout the centuries. In each design, fluid is sucked into an expanding volume and thenpushed along as that volume contracts, but the mechanism that causes this change in volume differs greatly among the various designs. Some designs are very simple, like the flexible-tube peristaltic pump (Fig. 14–26a) that compresses a tube by small wheels, pushing the fluid along. (This mechanism is somewhat similar to peristalsis in your esophagus or intestines, where muscles rather than wheels compress the tube.) Others are more complex, using rotating cams with synchronized lobes (Fig. 14–26b), interlocking gears (Fig. 14–26c), or screws (Fig. 14–26d). Positive displacement pumps are ideal for high-pressure applications like pumping viscous liquids or thick slurries, and for applications where precise amounts of liquid are to be dispensed or metered, as in medical applications.

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To illustrate the operation of a positive-displacement pump, we sketch four phases of half of a cycle of a simple rotary pump with two lobes on each rotor (Fig. 14–27). The two rotors are synchronized by an external gear box so as to rotate at the same angular speed, but in opposite directions. In the diagram, the top rotor turns clockwise and the bottom rotor turns counterclockwise, sucking in fluid from the left and discharging it to the right. A white dot is drawn on one lobe of each rotor to help you visualize the rotation.

Pumping powerThe power imparted into a fluid will increase the energy of the fluid per unit volume. Thus the power relationship is

between the conversion of the mechanical energy of the pump mechanism and the fluid elements within the pump. In general, this is governed by a series of simultaneous differential equations, known as the Navier-Stokes equations. However a more simple equation relating only the different energies in the fluid, known as Bernoulli's equation can be used. Hence the power, P, required by the pump:where ΔP is the change in total pressure between the inlet and outlet (in Pa), and Q, the fluid flow rate is given in m3/s. The total pressure may have gravitational, static pressure and kinetic energy components; i.e. energy is distributed between change in the fluid's gravitational potential energy (going up or down hill), change in velocity, or change in static pressure. η is the pump efficiency, and may be given by the manufacturer's information, such as in the form of a pump curve, and is typically derived from either fluid dynamics simulation (i.e. solutions to the Navier-stokes for the particular pump geometry), or by testing. The efficiency of the pump will depend upon the pump's configuration and operating conditions (such as rotational speed, fluid density and viscosity etc.).

For a typical "pumping" configuration, the work is imparted on the fluid, and is thus positive. For the fluid imparting the work on the pump (i.e. a turbine), the work is negative power required to drive the pump is determined by dividing the output power by the pump efficiency. Furthermore, this definition encompasses pumps with no moving parts, such as a siphon.

Pump efficiencyPump efficiency is defined as the ratio of the power imparted on the fluid by the pump in relation to the power supplied to drive the pump. Its value is not fixed for a given pump, efficiency is a function of the discharge and therefore also operating head. For centrifugal pumps, the efficiency tends to increase with flow rate up to a point midway through the operating range (peak efficiency) and then declines as flow rates rise further. Pump performance data such as this is usually supplied by the manufacturer before pump selection. Pump efficiencies tend to decline over time due to wear (e.g. increasing clearances as impellers reduce in size). One important part of system design involves matching the pipeline headloss-flow characteristic with the appropriate pump or pumps which will operate at or close to the point of maximum efficiency. There are free tools that help calculate head needed and show pump curves including their Best Efficiency Points (BEP).[13] Pump efficiency is an important aspect and pumps should be regularly tested. Thermodynamic pump testing is one method.

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Pump selection is done by performance curve which is curve between pressure head and flow rate. And also power supply is also taken care of. Pumps are normally available that run at 50 hz or 60 hz.

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