(1015) Hydraulic Machines

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Mohinder Pal GillEnroll. No. - NIMT/T/MPG/3508773

Branch CODL001

Hydraulic Machines.II Year.

1. (a). Draft Tube is a tube or a pipe which connects the outlets of reaction turbine withthe tail race.Draft tube is required to perform the following two main functions:

(i) If water is discharged freely from the runner then turbine will workunder a head equal to the height of head race water level above the runner outlet.But by providing an air tight draft tube, which connects runner with the tailrace, it is made possible to establish a negative head at the outlet by an amountequal to the height of the runner outlet above the tail race.

(ii) When water leaves the runner, it possess high velocity and thuskinetic energy of the turbine is lost, if it discharges freely. By using a draft tube ofincreasing cross- sectional area, the velocity is largely reduced at the exit of drafttube of increasing cross- sectional area, the velocity is gained, reduced at the exitof draft tube. Thus kinetic head is gained, but at the outlet of turbine onlyatmospheric pressure is required. Therefore, a further negative head at runner exitcan be established by an amount equal the gain in velocity head.

Thus with the help of above two functions, negative head at the runner exit isincreased, which result in the increase of net working head on the turbine and alsooutput increases. In this way with the help of draft tube nearly 10% increase inefficiency is obtained.

1. (b). When water is discharged freely from the runner then turbine will work under ahead equal to the height of head race water level above the runner outlet. But byproviding an airtight draft tube, which connects runner with the tail race, anegative head at the outlet get established by an amount equal to the height of therunner outlet above the tail race.Thus, by providing a draft tube, it is possible to install the turbine above the tail

race without loss of head. This makes the inspection and maintenance of turbine

easy.

2. (a). Uses ofDraft tube:

The draft tube is a conduit which connects the runner exit to the tail race wherethe water is being finally discharged from the turbine.

i. The primary use of the draft tube is to reduce velocity/ kinetic energy ofwater making exit thereby converting kinetic energy to pressure headallowing turbine to be installed above tail pool level.

ii. To create negative suction head.iii. Provide added head.iv. It makes possible the installation of the turbine above the tail race level

without the loss of head.v. The velocity of water at the runner outlet is very high. By employing a draft

tube of increasing cross sectional area, the discharge takes place at a muchlower velocity and thus, a part of the kinetic energy that was going as a

waste is recovered as a gain in the pressure head, and this increases theefficiency of the turbine.

vi. The draft tube prevents the splashing of water coming out of the runnerand guides the water to the tail race.

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2. (b). Types of Draft TubesThe following are the four types of draft tubes used in turbines:

(i). Straight Divergent tube.(ii). Moody spreading tube.(iii). Simple elbow tube.(iv). Elbow type with circular cross-section at inlet and rectangular at outlet.

i. Straight Divergent tube.

The shape of this type of tube is similar to the frustum of a cone. The maximumcone angle of such tube is 8 ( = 4). If the cone angle is more than 8 waterremains away from the inner walls of the tube and cavitation occurs, causingloss of head. This tube should be submerged by nearly 1 m in the tail race. Thishas an efficiency up to 90 % and is most suitable for Francis turbine.

As shown in the figure, this type of tube is a short vertical tube bifurcating intotwo long horizontal tubes. It is helpful in reducing whirling action of the watercoming tubes. It is helpful in reducing whirling action of the water coming outfrom the runner and thus reduces eddy losses and increases the life of the tube.

Efficiency of such draft tubes is about 80 %.

ii. Simple elbow tubeAs shown in figure above it has circular cross- section at the top as well as atbottom. This requires lesser excavation for the installation. its efficiency is onlyabout 60% and, therefore, not used.

iii. Elbow type with circular cross-section at inlet and rectangular atoutlet.

This type of tube is shown in the figure below and is useful where largerexcavation is necessary. This has a larger cross section at outlet, which recoversmore kinetic head at outlet. This is generally used with Kaplan turbine and has

efficiency of about 70%.

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3. (a). Pump is a mechanical device to increase the pressure energy of a fluid.Generally the pump is used for raising the fluids from lower level to a higher level.

This is achieved by creating a vacuum pressure at the inlet or suction end andhigh pressure at the outlet or delivery and of the pump. Vacuum pressure raisesthe fluid from a depth where it is available and the high pressure at outlet forcesit up to the height where it is required. Broadly pumps can be classified as

centrifugal or positive displacements pumps. Centrifugal pumps are vertical orhorizontal; multistage or single stage, submersible or dry type. Whereaspositive displacement pumps are reciprocating or rotary pumps. Rotary pumpsinclude, gear type, screw type, lobe type etc.

3. (b). Reciprocating pumps are also known as a positive displacement pumpsbecause in it, liquid is pushed out of the cylinder by the actual displacement of thepiston or plunger. These are now-a-days out of date.

Classification of Reciprocating Pumps(a). According to the action of

water:(i). Single acting.

The intensifier has two mainparts the low pressure,double-acting cylinder and thehigh pressure cylinder. Thepistons in these cylinders aremechanically attached thelow pressure piston pushes thehigh pressure piston in adelivery stroke and pulls it inthe return stroke.

(ii). Double acting.A continuous, uniform flow maybe attained with the double-acting arrangement. While one oft he double-actingpistons is delivering, the other cylinder is being charged during its intake stroke.

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This apparatus is a neat, compact assembly. While these units could be made tostroke with a manual four-way directional valve, they are almost invariablyassembled for fully automatic operation. Meeting requirements for explosionproofing is conventional and the application of remote control is simple.

(b). According to the number of cylinders:(i). Single cylinder.(ii). Double cylinder.

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(iii). Triple cylinder.

As the name suggests, a reciprocatingpump having only one cylinder isknown as single-cylinder pump. It may

be either single acting or double actingas shown in figures, respectively.Pumps having more than one cylinderare known as multi-cylinder pumps.

(c). According to the existence of airvessel.

i. With air vessel.ii. Without air vessel.

4. (a). Suction Head is the head on the suction side of the pump. This issubtracted from the discharge head to determine the head being produced by the

pump. It is a sum of the static, pressure and friction heads.Associated with each H-Q location on the pump curve is a quantity that can be

read called NPSH.An energy balance on the suction side of the fluid system (point 1 to pump inlet) withpinlet set to the vapor pressure of the fluid being pumped gives a quantity calledNPSHA (net positive suction head available).

The requirement is that:NPSHA > NPSH

Otherwise (if NPSHA < NPSHpump), the pressure at the pump inlet will drop to that

of the vapor pressure of the fluid being moved and the fluid will boil.The resulting gas bubbles will collapse inside the pump as the pressure rises again.These implosions occur at the impeller and can lead to pump damage and decreasedefficiency.

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Delivery headThe delivery head is a theoretical value which is used within the pumpcharacteristics (see pump characteristic) instead of the feed pressure (see feedpressure). The advantage is that the delivery head is valid for any given liquid. The

feed pressure is then calculated for the respective feed medium by multiplying the

density with the delivery head (see feed pressure).

4. (b). Rotary PumpsRotary pumps are the positive displacement type pumps having circular motion,

and are without valves and can deliver a constant capacity against variable pressure.These are generally used to produce 25 kg/cm. these are suited for handling

viscous fluids, maintain high volumetric efficiency, require very less clearancebetween rubbing surfaces. Although used successfully for clean water and gasoline,their field of application is primarily in pumping oils or other liquids having samelubricating qualities and sufficient viscosity to prevent excessive leakages.

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Classification.Mainly these are of two types:

1. Constant Delivery Pumps.These type includes:

(a). Gear pump.

AGear pump uses the meshing of gears to pump fluid by displacement. They are

one of the most common types ofpumpsforhydraulic fluid powerapplications. Gearpumps however are also widely used in chemical installations to pump fluid with acertain viscosity.There are two main variations;external gear pumpswhich use twoexternalspur gears, andinternal gear pumps which use anexternal and an internal spur gear.Gear pumps are fixed displacement,

meaning they pump a constant amount of fluid foreach revolution. Some gear pumps aredesigned to function as either a motor orpump.

(b). Screw pump.The complete assembly and the usual flow path are shown in Figure. Liquid istrapped at the outer end of each pair of screws.

The three-screw, high-pitch, screw pump, shown in Figure, has many of the sameelements as the two-screw, low-pitch, screw pump, and their operations are similar.Three screws, oppositely threaded on each end, are employed. They rotate in a triplecylinder, the two outer bores of which overlap the center bore.

(c).Vane pump.Arotary vane pump is a positive-displacement pumpthat consists of vanes

mounted to arotorthat rotates inside of a cavity. In some cases these vanes can bevariable length and/or tensioned to maintain contact with the walls as the pumprotates.The simplest vane pump is a circular rotor rotating inside of a larger circular cavity.The centers of these two circles are offset, causing eccentricity. Vanes are allowed toslide into and out of the rotor and seal on all edges, creating vane chambers that do

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the pumping work. The action of the vane drives out the same volume of fluid witheach rotation.Multistage rotary vane vacuum pumps can attain pressures as low as 10-3

mbar(0.1Pa).(d). Radial piston pump.

Aradial piston pump is an element of thehydraulic. At this pump the working

pistons are in radial direction and symmetrical arranged around the drive shaft - incontrast to the axial piston pump. The stroke of each piston is caused by an eccentricdrive shaft or an external eccentric tappet (e. g. stroke ring).

When filling the workspace of the pumping pistons from "inside" (e. g. over a hollowshaft) so its called a inside impinged (but outside braced) radial piston pump.

If the workspace is filled from, outside its called an outside impinged radial pistonpump (but inside braced).

2. Variable Delivery Pumps.These type include:

(a). Axial piston pumps

Axial piston pumps using the swash plateprinciple (fixed and adjustable displacement)

have a quality that is almost the same as the bent axis model. They have the advantage

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of being more compact in design. The pumps are easier and more economical to

manufacture; the disadvantage is that they are more sensitive to oil contamination.

(b). Radial or seated valve piston pump.Radial Piston Pumps can produce a very smooth flow under extreme pressure. Figureabove illustrates the operation of the radial piston pump. The pump consists of a

pintle, which remains stationary and acts as a valve. The slide block does not revolve but houses and supports the rotor, which does revolve due to the friction setupby the sliding action between the piston head sand the reaction ring. The cylinderblock is attached to the drive shaft.

5. (a). Deepwell or Borehole pumps.

Asubmersible pump is a device which has a hermetically sealedmotorclose-

coupled to the pump body. The whole assembly is submerged in the fluid to bepumped. The main advantage of this type of pump is that it preventspump

cavitation, a problem associated with a high elevation difference between pump and

the fluid surface. Submersible pumps push water to the surface as opposed to jetand centrifugal pumps having to pull water.

Submersibles are more efficient.These pumps are used where water or any other liquid like oils etc. are required to

be raised from a larger depth. These are generally used for pumping from depthsmore than 6 to 7 m. special type of pumps can lift the oils even from a depth of 3000m but generally ordinary deepwell can lift upto 300 m. these have capacities upto500 litre/sec and have pipe dia upto0.6 m nearly.

5. (b). External gear pumps are a popular pumpingprinciple and are often used as lubrication pumpsin machine tools, in fluid power transfer units,and as oil pumps in engines.

External gear pumps can come in single ordouble (two sets of gears) pump configurations

with spur (shown), helical, and herringbonegears. Helical and herringbone gears typicallyoffer a smoother flow than spur gears, althoughall gear types are relatively smooth. Large-capacity external gear pumps typically use helical or herringbone gears. Smallexternal gear pumps usually operate at 1750 or 3450 rpm and larger modelsoperate at speeds up to 640 rpm. External gear pumps have close tolerances andshaft support on both sides of the gears. This allows them to run to pressures

beyond 3,000 PSI / 200 BAR, making them well suited for use in hydraulics. Withfour bearings in the liquid and tight tolerances, they are not well suited to handling

abrasive or extreme high temperature applications.Tighter internal clearances provide for a more reliable measure of liquid passingthrough a pump and for greater flow control. Because of this, external gear pumpsare popular for precise transfer and metering applications involving polymers, fuels,and chemical additives.

Construction

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External gear pumps in particular can be engineered to handle even the most aggressivecorrosive liquids. While external gear pumps are commonly found in cast iron, newermaterials are allowing these pumps to handle liquids such as sulphuric acid, sodiumhypochlorite, ferric chloride, sodium hydroxide, and hundreds of other corrosive liquids.

Externals (head, casing, bracket) - Iron, ductile iron, steel, stainless steel, high

alloys, composites (PPS, ETFE) Internals (shafts) - Steel, stainless steel, high alloys, alumina ceramic Internals (gears) - Steel, stainless steel, PTFE, composite (PPS) Bushing - Carbon, bronze, silicon carbide, needle bearings Shaft Seal - Packing, lip seal, component mechanical seal, magnetically-driven

pump.

Working of External Gear PumpsExternal gear pumps are similar inpumping action to internal gear pumps inthat two gears come into and out of meshto produce flow. However, the external

gear pump uses two identical gearsrotating against each other -- one gear isdriven by a motor and it in turn drives the other gear. Each gear is supported by a shaft

with bearings on both sides of the gear.

1. As the gears come out of mesh, they create expanding volume on the inlet side of thepump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate.

2. Liquid travels around the interior of the casing in the pockets between the teeth andthe casing -- it does not pass between the gears.

3. Finally, the meshing of the gears forces liquid through the outlet port under pressure.

Because the gears are supported on both sides, external gear pumps are quiet-running

and are routinely used for high-pressure applications such as hydraulic applications.With no overhung bearing loads, the rotor shaft can't deflect and cause premature wear.

Applications

Common external gear pump applications include, but are not limited to:

Various fuel oils and lube oils Chemical additive and polymer metering Chemical mixing and blending (double pump) Industrial and mobile hydraulic applications (log splitters, lifts, etc.) Acids and caustic (stainless steel or composite construction) Low volume transfer or application.

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(1015) Hydraulic Machines