Misc. Fluid Machines

8/3/2019 Misc. Fluid Machines

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Miscellaneous Fluid Machines

1.Hydraulic RamA hydraulic ram is a cyclic water pump powered by hydropower. It functions as a hydraulictransformer that takes in water at one "hydraulic head" (pressure) and flow-rate, and outputswater at a higher hydraulic-head and lower flow-rate. The device utilizes the water hammer

effect to develop pressure that allows a portion of the input water that powers the pump to belifted to a point higher than where the water originally started. The hydraulic ram is sometimes

used in remote areas, where there is both a source of low-head hydropower, and a need forpumping water to a destination higher in elevation than the source. In this situation, the ram is

often useful, since it requires no outside source of power other than the kinetic energy of water.

Fig. 1(a) Hydraulic ram pump Fig. 1 (b) A hydraulic ram that drives a fountain

Construction and principle of operation

A hydraulic ram has only two moving parts, a spring or weight loaded "waste" valve sometimes

known as the "clack" valve and a "delivery" check valve, making it cheap to build, easy tomaintain, and very reliable. In addition, there is a drive pipe supplying water from an elevated

source, and a delivery pipe, taking a portion of the water that comes through the drive pipe to anelevation higher than the source.

Sequence of operation

A simplified hydraulic ram is shown in Figure 2. Initially, the waste valve [4] is open, and thedelivery valve [5] is closed. The water in the drive pipe [1] starts to flow under the force of

gravity and picks up speed and kinetic energy until it forces the waste valve closed. Themomentum of the water flow in the supply pipe against the now closed waste valve causes a

water hammer that raises the pressure in the pump, opens the delivery valve [5], and forces somewater to flow into the delivery pipe [3]. Because this water is being forced uphill through the


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delivery pipe farther than it is falling downhill from the source, the flow slows; when the flowreverses, the delivery check valve closes. If all water flow has stopped, the loaded waste valve

reopens against the now static head, which allows the process to begin again.

Fig. 2 Basic components of a hydraulic ram.

1. Inlet drive pipe

2. Free flow at waste valve3. Outlet delivery pipe

4. Waste valve5. Delivery check valve

6. Pressure vessel

A pressure vessel [6] containing air cushions the hydraulic pressure shock when the waste valve

closes, and it also improves the pumping efficiency by allowing a more constant flow through

the delivery pipe. Although, in theory, the pump could work without it, the efficiency woulddrop drastically and the pump would be subject to extraordinary stresses that could shorten its

life considerably. One problem is that the pressurized air will gradually dissolve into the wateruntil none remains. One solution to this problem is to have the air separated from the water by an

elastic diaphragm (similar to an expansion tank). The optimum length of the drive pipe is five-to-

twelve times the vertical distance between the source and the pump, or 500-to-1000 times thediameter of the delivery pipe, whichever is less. This length of drive pipe typically results in aperiod between pulses of one-to-two seconds. A typical efficiency is 60%, but up to 80% is

possible. The drive pipe is ordinarily straight but can be curved or even wound in a spiral. Themain requirement is that it be inelastic, strong, and rigid; otherwise, it would greatly diminish the

efficiency.

2.Fluid couplingA fluid coupling is a hydrodynamic device used to transmit rotating mechanical power.

[1]It has

been used in automobile transmissions as an alternative to a mechanical clutch. It also has

widespread application in marine and industrial machine drives, where variable speed operationand/or controlled start-up without shock loading of the power transmission system is essential.

Overview

A fluid coupling consists of three components, plus the hydraulic fluid:

The housing, also known as the shell (which must have an oil tight seal around the driveshafts), contains the fluid and turbines.

Two turbines (fan like components):


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o One connected to the input shaft; known as the pump or impellor,[4]primarywheelinput turbine

o The other connected to the output shaft, known as the turbine, output turbine,secondary wheel or runner.

Fig. 3 (a) Fluid Coupling

The driving turbine, known as the 'pump', (or driving torus) is rotated by the prime mover, which

is typically an internal combustion engine or electric motor. The impellor's motion imparts bothoutwards linear and rotational motion to the fluid.

The hydraulic fluid is directed by the 'pump' whose shape forces the flow in the direction of the'output turbine' (or driven torque). Here, any difference in the angular velocities of input stage

and output stage results in a net force on the 'output turbine' causing a torque; thus causing it torotate in the same direction as the pump.

The motion of the fluid is effectively toroidal - travelling in one direction on paths that can be

visualised as being on the surface of a torus:

If there is a difference between input and output angular velocities the motion has acomponent which is circular (i.e. round the rings formed by sections of the torus)

If the input and output stages have identical angular velocities there is no net centripetalforce - and the motion of the fluid is circular and co-axial with the axis of rotation (i.e.round the edges of a torus), there is no flow of fluid from one turbine to the other.

Stall speed

An important characteristic of a fluid coupling is its stall speed. The stall speed is defined as the

highest speed at which the pump can turn when the output turbine is locked and maximum inputpower is applied. Under stall conditions all of the engine's power would be dissipated in the fluid

coupling as heat, possibly leading to damage.

Slip

A fluid coupling cannot develop output torque when the input and output angular velocities are

identical. Hence a fluid coupling cannot achieve 100 percent power transmission efficiency. Dueto slippage that will occur in any fluid coupling under load, some power will always be lost in

fluid friction and turbulence, and dissipated as heat.


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The very best efficiency a fluid coupling can achieve is 94%, that is for every 100 revolutionsinput, there will be 94 revolutions output. Like other fluid dynamical devices, its efficiency tends

to increase gradually with increasing scale, as measured by the Reynolds number.

Hydraulic fluid

As a fluid coupling operates kinetically, low viscosity fluids are preferred.[6]

Generally speaking,

multi-grade motor oils or automatic transmission fluids are used. Increasing density of the fluidincreases the amount of torque that can be transmitted at a given input speed.[7]

Hydrodynamic braking

Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as heat

through frictional forces (both viscous and fluid/container). When a fluid coupling is used forbraking it is also known as a retarder.

Applications

Industrial: Fluid couplings are used in many industrial application involving rotational power,especially in machine drives that involve high-inertia starts or constant cyclic loading.

Rail transportation: Fluid couplings are found in some Diesel locomotives as part of the power

transmission system. Self-Changing Gears made semi-automatic transmissions for British Rail,and Voith manufacture turbo-transmissions for railcars and diesel multiple units which contain

various combinations of fluid couplings and torque converters.

Automotive: Fluid couplings were used in a variety of early semi-automatic transmissions andautomatic transmissions. Since the late 1940s, the hydrodynamic torque converter has replaced

the fluid coupling in automotive applications.

In automotive applications, the pump typically is connected to the flywheel of the enginein

fact, the coupling's enclosure may be part of the flywheel proper, and thus is turned by theengine's crankshaft. The turbine is connected to the input shaft of the transmission. While the

transmission is in gear, as engine speed increases torque is transferred from the engine to theinput shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior of the

fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.

Fluid flywheels, as distinct from torque converters, are best known for their use in Daimler carsin conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of

luxury cars, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alviswere both also known for their military vehicles and armored cars, some of which also used the

combination of pre-selector gearbox and fluid flywheel.

Aviation: The most prominent use of fluid couplings in aeronautical applications was in theWright turbo-compound reciprocating engine, in which three power recovery turbines extracted

approximately 20 percent of the energy or about 500 horsepower (370 kW) from the engine'sexhaust gases and then, using three fluid couplings and gearing, converted low-torque high-speedturbine rotation to low-speed, high-torque output to drive the propeller.

Calculations


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Generally speaking, the power transmitting capability of a given fluid coupling is stronglyrelated to pump speed, a characteristic that generally works well with applications where the

applied load doesn't fluctuate to a great degree. The torque transmitting capacity of anyhydrodynamic coupling can be described by the expression r(N

2)(D

5), where r is the mass

density of the fluid, N is the impeller speed, and D is the impeller diameter.[10]

In the case ofautomotive applications, where loading can vary to considerable extremes, r(N

2)(D

5) is only an

approximation. Stop-and-go driving will tend to operate the coupling in its least efficient range,

causing an adverse effect on fuel economy.

3.Hydraulic Torque ConvertorA torque converter is a fluid coupling that is used to transfer rotating power from a prime mover,

such as an internal combustion engine or electric motor, to a rotating driven load. Like a basicfluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the

load to be separated from the power source. However, a torque converter is able to multiplytorque when there is a substantial difference between input and output rotational speed, thus

providing the equivalent of a reduction gear.

Usage:

Automatic transmissions on automobiles, such as cars, buses, and on/off highway trucks. Forwarders and other heavy duty vehicles. Marine propulsion systems. Industrial power transmission such as conveyor drives, almost all modern forklifts,

winches, drilling rigs, construction equipment, and railway locomotives.

Function:

A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque

converter has at least one extra elementthe statorwhich alters the drive's characteristicsduring periods of high slippage, producing an increase in output torque.

Fig. 3 (b) Torque convertor elements


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In a torque converter there are at least three rotating elements: the impeller, which ismechanically driven by the prime mover; the turbine, which drives the load; and the stator,

which is interposed between the impeller and turbine so that it can alter oil flow returning fromthe turbine to the impeller. The classic torque converter design dictates that the stator be

prevented from rotating under any condition, hence the term stator. In practice, however, thestator is mounted on an overrunning clutch, which prevents the stator from counter-rotating with

respect to the prime mover but allows forward rotation.

Fig. 4 Location of Torque convertor in Cars

Operational phases:

A torque converter has three stages of operation:

Stall: The prime mover is applying power to the impeller but the turbine cannot rotate.For example, in an automobile, this stage of operation would occur when the driver has

placed the transmission in gear but is preventing the vehicle from moving by continuingto apply the brakes. At stall, the torque converter can produce maximum torque

multiplication if sufficient input power is applied (the resulting multiplication is calledthe stall ratio). The stall phase actually lasts for a brief period when the load (e.g.,

vehicle) initially starts to move, as there will be a very large difference between pumpand turbine speed.

Acceleration: The load is accelerating but there still is a relatively large differencebetween impeller and turbine speed. Under this condition, the converter will producetorque multiplication that is less than what could be achieved under stall conditions. The

amount of multiplication will depend upon the actual difference between pump andturbine speed, as well as various other design factors.

Coupling: The turbine has reached approximately 90 percent of the speed of theimpeller. Torque multiplication has essentially ceased and the torque converter isbehaving in a manner similar to a simple fluid coupling. In modern automotiveapplications, it is usually at this stage of operation where the lock-up clutch is applied, a

procedure that tends to improve fuel efficiency.

The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluidcoupling design, periods of high slippage cause the fluid flow returning from the turbine to the

impellor to oppose the direction of impeller rotation, leading to a significant loss of efficiencyand the generation of considerable waste heat. Under the same condition in a torque converter,

the returning fluid will be redirected by the stator so that it aids the rotation of the impeller,instead of impeding it. The result is that much of the energy in the returning fluid is recovered

and added to the energy being applied to the impeller by the prime mover. This action causes asubstantial increase in the mass of fluid being directed to the turbine, producing an increase in


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output torque. Since the returning fluid is initially travelling in a direction opposite to impellerrotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change

direction, an effect that is prevented by the one-way stator clutch.

Unlike the radially straight blades used in a plain fluid coupling, a torque converter's turbine andstator use angled and curved blades. The blade shape of the stator is what alters the path of the

fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades

helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape ofthe blades is important as minor variations can result in significant changes to the converter'sperformance.

During the stall and acceleration phases, in which torque multiplication occurs, the stator

remains stationary due to the action of its one-way clutch. However, as the torque converterapproaches the coupling phase, the energy and volume of the fluid returning from the turbine

will gradually decrease, causing pressure on the stator to likewise decrease. Once in the couplingphase, the returning fluid will reverse direction and now rotate in the direction of the impellor

and turbine, an effect which will attempt to forward-rotate the stator. At this point, the statorclutch will release and the impeller, turbine and stator will all (more or less) turn as a unit.

Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence,causing the converter to generate waste heat (dissipated in many applications by water cooling).

This effect, often referred to as pumping loss, will be most pronounced at or near stallconditions. In modern designs, the blade geometry minimizes oil velocity at low impeller speeds,

which allows the turbine to be stalled for long periods with little danger of overheating.

Efficiency:

A torque converter cannot achieve 100 percent coupling efficiency. The classic three elementtorque converter has an efficiency curve that resembles: zero efficiency at stall, generally

increasing efficiency during the acceleration phase and low efficiency in the coupling phase. Theloss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid

flow interference generated by the stator, and as previously mentioned, is commonly overcomeby mounting the stator on a one-way clutch.

4.Submersible PumpA submersible pump (or electric submersible pump (ESP)) is a device which has a hermeticallysealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to

be pumped. The main advantage of this type of pump is that it prevents pump cavitation, aproblem associated with a high elevation difference between pump and the fluid surface.

Submersible pumps push fluid to the surface as opposed to jet pumps having to pull fluids.Submersibles are more efficient than jet pumps.


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Fig. 5 Submersible pump

Working principle

The submersible pumps used in ESP installations are multistage centrifugal pumps operating in a

vertical position. Although their constructional and operational features underwent a continuousevolution over the years, their basic operational principle remained the same. Produced liquids,

after being subjected to great centrifugal forces caused by the high rotational speed of theimpeller, lose their kinetic energy in the diffuser where a conversion of kinetic to pressure energy

takes place. This is the main operational mechanism of radial and mixed flow pumps.

The pump shaft is connected to the gas separator or the protector by a mechanical coupling at thebottom of the pump. Well fluids enter the pump through an intake screen and are lifted by the

pump stages.Other parts include the radial bearings (bushings) distributed along the length of theshaft providing radial support to the pump shaft turning at high rotational speeds. An optional

thrust bearing takes up part of the axial forces arising in the pump but most of those forces areabsorbed by the protectors thrust bearing.

Fig. 6 ESP systems are effective for pumping produced fluids to surface.


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Applications

Submersible pumps are found in many applications. Single stage pumps are used for drainage,sewage pumping, general industrial pumping and slurry pumping. They are also popular with

aquarium filters. Multiple stage submersible pumps are typically lowered down a borehole andused for water abstraction, water wells and in oil wells.

Special attention to the type of ESP is required when using certain types of liquids. ESP'scommonly used on board naval vessels cannot be used to dewater contaminated flooded spaces.These use a 440 volt A/C motor that operates a small centrifugal pump. It can also be used out of

the water, taking suction with a 2-1/2 inch non-collapsible hose. The pumped liquid is circulatedaround the motor for cooling purposes. There is a possibility that the gasoline will leak into the

pump causing a fire or destroying the pump, so hot water and flammable liquids should beavoided.

5.Gear PumpA gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most

common types of pumps for hydraulic fluid power applications. Gear pumps are also widelyused in chemical installations to pump fluid with a certain viscosity. There are two mainvariations; external gear pumps which use two external spur gears, and internal gear pumps

which use an external and an internal spur gear. Gear pumps are positive displacement(orfixeddisplacement), meaning they pump a constant amount of fluid for each revolution. Some gear

pumps are designed to function as either a motor or a pump.

Fig. 7 Schematic of Gear pump

Theory of operation

As the gears rotate they separate on the intake side of the pump, creating a void and suction

which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, wherethe meshing of the gears displaces the fluid. The mechanical clearances are small in the order

of 10 m. The tight clearances, along with the speed of rotation, effectively prevent the fluidfrom leaking backwards.


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The rigid design of the gears and houses allow for very high pressures and the ability to pumphighly viscous fluids.

Many variations exist, including; helical and herringbone gear sets (instead of spur gears), lobe

shaped rotors similar to Roots Blowers (commonly used as superchargers), and mechanicaldesigns that allow the stacking of pumps. The most common variations are shown below (the

drive gear is shown blue and the idler is shown purple).

Fig. 8 (a) External gear pump

design for hydraulic powerapplications.

Fig. 8 (b) Internal gear

(Gerotor) pump design forautomotive oil pumps.

Fig. 8 (c) Internal gear

(Gerotor) pump design forhigh viscosity fluids.

Suction and pressure ports need to interface where the gears mesh (shown as dim gray lines in

the internal pump images). Some internal gear pumps have an additional, crescent shaped seal(shown above, right).

Usage:

1. PETROCHEMICALS: Pure or filled bitumen, pitch, diesel oil, crude oil, lube oil etc.2. CHEMICALS: Sodium silicate, acids, plastics, mixed chemicals, isocyanates etc.

3. PAINT & INK.4. RESINS & ADHESIVES.

5. PULP & PAPER: acid, soap, lye, black liquor, kaolin, lime, latex, sludge etc.6. FOOD: Chocolate, cacao butter, fillers, sugar, vegetable fats and oils, molasses, animal food

etc.

6.Axial Piston PumpAn axial piston pump is a positive displacement pump that has a number of pistons in a circulararray within a cylinder block. It can be used as a stand-alone pump, a hydraulic motor or an

automotive air conditioning compressor.

Fig. 9 Axial Piston Pump


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Description:

An axial piston pump has a number of pistons (usually an odd number) arranged in a circulararray within a housing which is commonly referred to as a cylinder block, rotoror barrel. This

cylinder block is driven to rotate about its axis of symmetry by an integral shaft that is, more orless, aligned with the pumping pistons (usually parallel but not necessarily).

Fig. 10 Axial Piston Pump

Mating surfaces. One end of the cylinder block is convex and wears against a matingsurface on a stationary valve plate. The inlet and outlet fluid of the pump pass through

different parts of the sliding interface between the cylinder block and valve plate. Thevalve plate has two semi-circular ports that allow inlet of the operating fluid and exhaust

of the outlet fluid respectively. Protruding pistons. The pumping pistons protrude from the opposite end of the cylinder

block. There are numerous configurations used for the exposed ends of the pistons but inall cases they bear against a cam. In variable displacement units, the cam is movable and

commonly referred to as a swash plate,yoke or hanger. For conceptual purposes, the camcan be represented by a plane, the orientation of which, in combination with shaft

rotation, provides the cam action that leads to piston reciprocation and thus pumping. Theangle between a vector normal to the cam plane and the cylinder block axis of rotation,

called the cam angle, is one variable that determines the displacement of the pump or theamount of fluid pumped per shaft revolution. Variable displacement units have the ability

to vary the cam angle during operation whereas fixed displacement units do not.

Reciprocating pistons. As the cylinder block rotates, the exposed ends of the pistons areconstrained to follow the surface of the cam plane. Since the cam plane is at an angle tothe axis of rotation, the pistons must reciprocate axially as they precess about the cylinder

block axis. The axial motion of the pistons is sinusoidal. During the rising portion of thepiston's reciprocation cycle, the piston moves toward the valve plate. Also, during this

time, the fluid trapped between the buriedend of the piston and the valve plate is ventedto the pump's discharge port through one of the valve plate's semi-circular ports - the

discharge port. As the piston moves toward the valve plate, fluid is pushed or displacedthrough the discharge port of the valve plate.

Effect of precession. When the piston is at the top of the reciprocation cycle (commonlyreferred to as top-dead-center or just TDC), the connection between the trapped fluid

chamber and the pump's discharge port is closed. Shortly thereafter, that same chamberbecomes open to the pump's inlet port. As the piston continues to precess about thecylinder block axis, it moves away from the valve plate thereby increasing the volume of

the trapped chamber. As this occurs, fluid enters the chamber from the pump's inlet to fillthe void. This process continues until the piston reaches the bottom of the reciprocation

cycle - commonly referred to as bottom-dead-center or BDC. At BDC, the connectionbetween the pumping chamber and inlet port is closed. Shortly thereafter, the chamber

becomes open to the discharge port again and the pumping cycle starts over.

Variable displacement. In a variable displacement unit, if the vector normal to the camplane (swash plate) is set parallel to the axis of rotation, there is no movement of the


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pistons in their cylinders. Thus there is no output. Movement of the swash plate controlspump output from zero to maximum.

Pressure. In a typical pressure-compensated pump, the swash plate angle is adjustedthrough the action of a valve which uses pressure feedback so that the instantaneous

pump output flow is exactly enough to maintain a designated pressure. If the load flowincreases, pressure will momentarily decrease but the pressure-compensation valve will

sense the decrease and then increase the swash plate angle to increase pump output flow

so that the desired pressure is restored. In reality most systems use pressure as a controlfor this type of pump. The operating pressure reaches, say, 200 bar (20 MPa or 2900 psi)and the swash plate is driven towards zero angle (piston stroke nearly zero) and with the

inherent leaks in the system allows the pump to stabilise at the delivery volume thatmaintains the set pressure. As demand increases the swash plate is moved to a greater

angle, piston stroke increases and the volume of fluid increases; if the demand slackensthe pressure will rise, and the pumped volume diminishes as the pressure rises. At

maximum system pressure the output is once again almost zero. If the fluid demandincreases beyond the capacity of the pump to deliver, the system pressure will drop to

near zero. The swash plate angle will remain at the maximum allowed, and the pistonswill operate at full stroke. This continues until system flow-demand eases and the pump's

capacity is greater than demand. As the pressure rises the swash-plate angle modulates totry to not exceed the maximum pressure while meeting the flow demand.

Uses

Despite the problems indicated above this type of pump can contain most of the necessary circuit

controls integrally (the swash-plate angle control) to regulate flow and pressure, be very reliableand allow the rest of the hydraulic system to be very simple and inexpensive.

Axial reciprocating motors are also used to power many machines. They operate on the sameprinciple as described above, except that the circulating fluid is provided under considerable

pressure and the piston housing is made to rotate and provide shaft power to another machine. Acommon use of an axial reciprocating motor is to power small earthmoving plant such as skid

loader machines. Another use is to drive the screws of torpedoes.

7.Axial Flow PumpAn axial flow pump, or AFP, is a common type of pump that essentially consists of a propellerin a pipe. The propeller can be driven directly by a sealed motor in the pipe or mounted to the

pipe from the outside or by a right-angle drive shaft that pierces the pipe.

Fig. 11 Axial flow pump for industrial use


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The main advantage of an AFP is that it can easily be adjusted to run at peak efficiency at low-flow/high-pressure and high-flow/low-pressure by changing the pitch on the propeller (some

models only).

These pumps have the smallest of the dimensions among many of the conventional pumps and

are more suited for low heads and higher discharges.

An application example of an AFP would be transfer pumps used for sailing ballast. In chemicalindustry, they are used for the circulation of large masses of liquid, such as in evaporators and

crystallizers. In sewage treatment an AFP is often used for internal mixed liquor recirculation(i.e. transferring nitrified mixed liquor from aeration zone to denitrification zone).

8.Air Jet PumpIf water is speeded up through a jet, it causes a drop in pressure. Here the pump is fitted into a

secondary casing which contains water at discharge pressure, (see Fig. 12). A proportion of the

water from this chamber is bled back to a nozzle fitted into the suction end of the pump casingand directed into the eye of the impeller. Once the pump has been used once (having beenmanually primed initially) it remains full of water so that on start up the pump circulates water

from the discharge through the jet and back into the suction side. As before, air is sucked throughand bubbles out of the discharge, while (until the pump primes) the water falls back and

recirculates. The jet causes low pressure in the suction line and entrains air which goes throughthe impeller and is discharged, hence water is gradually drawn up the suction line. As soon as all

the air is expelled from the system, most of the discharge goes up the discharge line, but aproportion is fed back to the nozzle and increases the suction considerably compared with the

effect of a centrifugal impeller on its own. Therefore, this kind of pump not only pulls a highersuction lift than normal, but the pump can reliably run on "snore" (i.e. sucking a mixture of air

and water without losing its prime). This makes it useful in situations where shallow water isbeing suction pumped and it is difficult to obtain sufficient submergence of the footvalve, or

where a water source may occasionally be pumped dry.

This jet pump principle can also be applied to boreholes as indicated in Fig. 75. An arrangementlike this allows a surface-mounted pump and motor to "suck" water from depths of around 10-

20m; the diffuser after the jet serves to raise the pressure in the rising main and preventcavitation. Although the jet circuit commonly needs 1.5-2 times the flow being delivered, and is

consequently a source of significant power loss, pumps like this are sometimes useful for liftingsandy or muddy water as they are not so easily clogged as a submerged pump. In such cases a

settling tank is provided on the surface between the pump suction and the jet pump discharge to

allow the pump to draw clearer water.

Jet pumps are commonly used to extract water from water wells. A powered pump, often acentrifugal pump, is installed at ground level. Its discharge is split, with the greater part of the

flow leaving the system, while a portion of the flow is returned to the jet pump installed belowground in the well. This recirculated part of the pumped fluid is used to power the jet. At the jet

pump, the high-energy, low-mass returned flow drives more fluid from the well, becoming alow-energy, high-mass flow which is then piped to the inlet of the main pump.


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The disadvantages of jet pumps are, first, greater complexity and therefore cost, and second,reduced efficiency since power is used in pumping water through the jet, (although some of this

power is recovered by the pumping effect of the jet). Obviously it is better to use a conventionalCentrifugal pump in a situation with little or no suction lift, but where Suction pumping is

essential, then a self-priming pump of this kind can offer a successful solution.

Fig. 12 Surface suction jet pump Fig. 13 Borehole jet pump

9.Air Lift PumpThe primary virtue of air lift pumps is that they are extremely simple. A rising main, which is

submerged in a well so that more of it is below the water level than above it, has compressed airblown into it at its lowest point (see Fig. 14). The compressed air produces a froth of air and

water, which has a lower density than water and consequently rises to the surface. Thecompressed air is usually produced by an engine driven air compressor, but windmill powered

air compressors are also used. The principle of it is that an air/water froth, having as little as halfthe density of water, will rise to a height above the water level in the well approximately equal to

the immersed depth of the rising main. The greater the ratio of the submergance of the risingmain to the static head, the more froth will be discharged for a given supply of air and hence the

more efficient an air lift pump will be. Therefore, when used in a borehole, the borehole needs to

be drilled to a depth more than twice the depth of the static water level to allow adequatesubmergence.


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Fig. 14 Air lift pump

The main advantage of the air lift pump is that there are no mechanical below-ground

components, so it is essentially simple and reliable and can easily handle sandy or gritty water.

The disadvantages are rather severe; first, it is inefficient as a pump, probably no better, at best,than 20-30% in terms of compressed air energy to hydraulic output energy, and this iscompounded by the fact that air compressors are also generally inefficient. Therefore the running

costs of an air lift pump will be very high in energy terms. Second, it usually requires a boreholeto be drilled considerably deeper than otherwise would be necessary in order to obtain enough[submergence, and this is generally a costly exercise. This problem is obviously less serious for

low head applications where the extra depth [required would be small, or where a borehole needsto be drilled to a considerable depth below the static water level anyway to obtain sufficient

inflow of water.

10. Self-priming Rotodynamic PumpRotodynamic pumps, of any kind, will only start to pump if their impellers are flooded withwater prior to start-up. Obviously the one certain way to avoid any problem is to submerge the

pump in the water source, but this is not always practical or convenient. This applies especiallyto portable pump sets, which are often important for irrigation, but which obviously need to be

drained and re-primed every time they are moved to a new site.

Sometimes the most reliable arrangement is to use a special "self-priming" centrifugal pump(Fig. 15). Here, the pump has an enlarged upper casing with a baffle in it. When the pump and

suction line are empty, the pump casing has to be filled with water from a bucket through thefiller plug visible on top. Then when the pump is started, the water in the casing is thrown up

towards the discharge and an eye is formed at the hub of the impeller which is at low pressure;until water is drawn up the suction pipe the water discharged from the top of the pump tends to

fall back around the baffle and some of the entrained air carries on up the empty discharge pipe.The air which is discharged is replaced by water drawn up the suction pipe, until eventually the

suction pipe fills completely and the air bubble in the eye of the impeller is blown out of thedischarge pipe. Once all the air has been expelled, water ceases to circulate within the pump and

both channels act as discharge channels. A check valve is fitted to the inlet of the pump so thatwhen the pump is stopped it remains full of water. Then even if the foot valve on the suction line

leaks and the suction line empties, the water trapped in the casing of the pump will allow thesame self-priming function as described earlier to suck water up the suction line. Hence, pumps


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of this kind only need to be manually filled with water when first starting up after the entiresystem has been drained.

Fig. 15 Self-priming centrifugal pump

11. Screw PumpScrew pump is a positive displacement pump that use one or several screws to move fluids orsolids along the screw(s) axis. In its simplest form (the Archimedes' screw pump), a single screw

rotates in a cylindrical cavity, thereby moving the material along the screw's spindle. Thisancient construction is still used in many low-tech applications, such as irrigation systems and in

agriculturural machinery for transporting grain and other solids.

Fig. 16 Screw Pumps

Development of the screw pump has led to a variety of multi-axis technologies where carefullycrafted screws rotate in opposite directions or remains stationary within a cavity. The cavity can

be profiled, thereby creating cavities where the pumped material is "trapped".


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In offshore and marine installations, a three spindle screw pump is often used to pump highpressure viscous fluids. Three screws drive the pumped liquid forth in a closed chamber. As the

screws rotate in opposite directions, the pumped liquid moves aling the screws spindles.

Three-Spindle screw pumps are used for transport of viscous fluids with lubricating properties.They are suited for a variety of applications such as fuel-injection, oil burners, boosting,

hydraulics, fuel, lubrication, circulating, feed and so on.

Compared to centrifugal pumps, positive displacements (PD) pumps have several advantages.The pumped fluid is moving axially without turbulence which eliminates foaming that would

otherwise occur in viscous fluids. They are also able to pump fluids of higher viscosity withoutlosing flow rate. Also, changes in the pressure difference have little impact on PD pumps

compared to centrifugal pumps.

Misc. Fluid Machines

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