Fluid Machines

The machine that either add energy to the fluid or extract energy from fluid are called fluid machines.

They are classified into two categories

1. Positive displacement machine
2. Turbomachines or dynamic machines

POSITIVE DISPLACEMENT MACHINE:- are those machine in which energy transfer is accompolished by volume changes that takes place due to movement of the boundaries in which the fluid is confined.

TURBOMACHINES:- Fluid device which direct the flow with blades or vanes attached to a rotating member are termed as dynamic or turbo machine.

Eg Francis turbine, Centrifugal pump etc

Each of these two category or sub classification as pumps and turbines

                Pump are those machine which add energy to the fluid

                Turbines are those machines which extract energy from the fluid

Some example of positive displacement pump are Reciprocating pump single or double acting, External gear pump, Double screw, sliding vane pumps.

RECIPROCATING PUMP:- They are sub classified into two categories

                                a) Piston or plunger type

                                b) Diaphragm type

POSITIVE TYPE RECIPROCATING PUMP:- This type of pump consists of a piston or a plunger reciprocating inside a close fitting cylinder. The increase in energy  to the pumping fluid in this type is provided by the mechanical motion of the piston , suction and delivery pipe are connected to non-return valves so that unidirectional flow of water is achieved.

WORKING

1. First suction pipe and clearance volume are filled with water to replace the air trapped inside it this is known as the priming of pump
2. Backward movement of piston creates a vacuum in the cylinder. Hence suction valve open and water from suction pipe rushes into the cylinder. As a result the cylinder is filled with water in backward is stroke while suction remains open.
3. In forward stroke the suction valve closes, delivery valve open and water is pushed upwards in the delivery pipe.

This is single acting reciprocating pump and it delivers water in forward stroke of piston. So the flow obtain is pulsating flow. This can be taken care of in double acting reciprocating pump in which Piston moves between two sets of suction and delivery pipes.

        The purpose of priming is to closely connect the piston to the water. Once this is done the Piston moves, water in the sump follows it due to the cohension between sump, priming water and adhesion forces between priming water and piston

ROTODYNAMIC PUMP:- These are those person which add Momentum to fluid by means of fast moving blades or vanes or certain special design. There is no close volume.

        The fluid increase momentum while moving through open passage and then convert it's high velocity to a pressure increased by exciting into diffusion section.

CENTRIFUGAL PUMP (RADIAL EXIT FLOW PUMP)

It is a type of rotadynamic pump which is very commonly used today.

A centrifugal pump consists of an impeller rotating with a casing.

Main parts of centrifugal pump 

• Impeller
• Casing
• Suction pipe with foot valve and strainer
• Delivery pipe

        Impeller is the rotating part of centrifugal pump with series of backward curved vanes. This is usually made of steel, bronze, brass or plastic. Casing is the airtight passage surrounding the impeller and is designed in such a way that kinetic energy of water discharge is at outlet of impeller is converted into pressure energy before the water leaves the casing.

        Suction pipe with a foot valve and strainer. A pipe connecting inlet of pump to water in sump is known as suction pipe. Foot valve is a one way valve at lower end of suction pipe it opens in upward direction only and not allow water downward. Strainer is also fitted at the lower end of the suction pipe and prevent foreign material from entering the pipe pump. Delivery pipe connects outlet of the pump to the required height at which water is to be delivered.

Working

1. Delivery wall is closed and the pump is primed i.e. suction pipe, casing and piston of delivery pipe up to the delivery valve are completely filled with the liquid so no air pockets is left
2. Now pump is started keeping the delivery valve close. The rotation of impeller blades creates a strong section or vacuum at the eye of impeller.
3. After the impeller attends normal speed the delivery is opened. Fluid enter axially through eye of casing, is caught up in the impeller blades and whirled tangentially and radially outward until it leaves through all circumferential part of impeller into diffuse part of casing.
4. The fluid and velocity head and pressure head both through impeller but drought shaped diffuser or scroll section of casing decelerate the flow and further increase the pressure head.
5. From casing the liquid passes into pipe and is lifted to required height.
6. When the pump is to be stop the delivery wall should be first close, otherwise they may be soon back flow from reservoir.

        Cavitation when liquid pressure in pipe fall below vapour pressure then the liquid vaporizes and the flow will no longer remain continous. This operation of liquid appear in the form of bubbles. These bubble collapses suddenly in the high pressure region.

DIMENTIONLESS PUMP PERFORMANCE PARAMETER

        SPECIFIC SPEED as a speed of geometrical similar pump which would deliver 1 cubic metre of liquid per second against a head of 1 m. It is denoted by Ns

TURBINES

    Turbines are machine which extract energy from fluid that possesses high head. In Hydraulic turbines working fluid is water so that the flow is incompressible. In gas turbine and steam turbine the density of working fluid may change considerably.

        Turbine are classified based on the working principle as 

1.  Impulse turbine eg. Pelton wheel, Turgo wheel
2. Reaction turbine eg. Francis turbine, propeller turbine, Kaplan turbine.

    IMPULSE TURBINE:- In impluse turbine all available energy of water is converted into kinetic energy or velocity head by its through nozzle provided at the end of the penstock.

(Impulse turbine impluse force acts on blades) High head and low flow rate turbine, low specific speed, water coming from nozzle is formed into free jet which impinges into series of buckets of runner thus causing it to revolve.

Casing is provided on runner to prevent splashing and to guide water discharge from buckets to tail race. Eg: Pelton turbine

It consist of a router at Periphery of which are mounted equally spaced buckets.

MAIN COMPONENT OF PELTON TURBINE 

(i) BUCKET FITTED TO RUNNER:- Each bucket is divided vertically into two parts by a splitter which is as shaped edge at the centre, giving the shape of double hemispherical cup. Splitter help the jet to be divided without shock into two parts moving sideways in opposite direction.

(ii) FLOW GOVERNING MECHANISM:- This control the quantity of water passing through nozzle and strike the bucket does meeting variable demands. It consist of spear fixed to the end of the shaft, which is operated by governers. when the speed of wheel increases, the spear is pushed into the nozzle thereby reducing the quantity of water strike bucket and vice versa sometimes sudden reduction off load will causes spear to close the nozzle this way cause pipe to bust so a bypass nozzle or deflector is used which deflect the water jet from buckets. Casing is prevented splashing and lead water to tail race and also safeguard against accident

HYDRAULIC BRAKES when nozzle is completely closed by moving the moving the sphere in forward direction, the amount of water striking the pelton turbine is zero but due to the inertia of runner keeps revolves. To bring runner to rest in short time nozzle is provided which directs the jet of water on the back of bucket and stop the bucket . This is called break Jet.

PENSTOCK It is a large diameter pipe to carry water from reservoir to turbine casing 

WORKING

        Water is transfer from high head source through penstock which is fitted with nozzle. Through which water flows out at high speed jet.

All potential energy is converted into kinetic energy before jet strike the bucket of runner. The pressure all over the wheel is constant and equal to the atmosphere so energy transfer occur due to impulse action.

The high speed water strikes the bucket in succession which turns the bucket as a result the bucket gains the relative velocity. Thus runner revolves freely in air spend water thus falls in the tail race.

REACTION TURBINE

        In a reaction at the entrance to the runner only a part of available energy of water is converted into kinetic energy and a substaintial part remains in the form of pressure energy.

They are low head, High flow rate devices.

The flow is opposite to that of pump, entering at larger diameter section and discharging through the eye after giving a most of the energy to the impeller.

FRANCIS TURBINE

        In Francis turbine when water enters the runner radially at its outer periphery and leaves axially at its centre. Francis turbine operates under medium head and also require medium flow rate.

Main parts of modern Francis turbine

• penstock
• spiral or scroll casing,
• Guide mechanism
• Runner and Main shaft
• Draft tube

PENSTOCK It is the large diameter pipe to carry water from reservoir to turbine casing.

SCROLL CASING The water from the penstock enter a scroll casing which completely surround the runner. Scroll casing are design with a cross-sectional area reducing uniformly around circumference, maximum at the entrance and nearly zero at tip. The design facilities uniform distribution of water around guide rings with approx. constant velocity.

GUIDE MECHANISM Guide vane has two functions

        (a) To regulate the quantity of water supply to the runner 

        (b) To get the water in the direction desirable to the rotor blade.

Guide vanes are fixed between two rings in a form of wheel known as guide wheel. Each guide vane can rotate about it pivot centre which is regulated by ring having links and levers by rotating the regulating of shaft the guide vanes can be closed or open thus eyes allowing available quantity of water.

RUNNER AND MAIN SHAFT:- The runner width depend upon the specific speed. The runner is key to the shaft which may be vertical or horizontal. The shaft is generally made of steel and is forged. It is provided with a collar for transmitting the axial thrust to bearing.

DRAFT TUBE:- The water after doing work on runner passes on the tailrace through a Draft tube. Draft tube is a tube which connects the runner exits to tailrace and its cross sectional area increase gradually towards outlet.

FUNCTION OF DRAFT TUBE 

    It convert the large portion of kinetic energy into pressure energy as its area increases so from continuity equation velocity decreases and pressure increases.

    Thus kinetic head saved by draft tube adds to effect head of turbine. Draft tube is submerged deep in tail water and entire water passage is totally enclosed.

WORKING

    Francis turbine is an invert mixed flow turbine. Water has pressure energy and kinetic energy before entering the turbine and both energy provides movement on the wheel. Water from Reservoir enter the turbine casing after passing through penstock. This incoming water flows circumferential through the turbine casing. It enters periphery of stationary guide vanes and flow towards runner.

    Water enter the runner nearly radially and is turned downward to leave nearly axially.(Reverse of centrifugal pump).

       The water provide both impulse force and reactive force at the impeller. Therefore Francis turbine is known as mixed type turbine. Thus water rotates the blade on runner which in turns rotate the shaft connecting generators to (pressure drop occur at impeller ) provide electricity.

KAPLAN TURBINE   (Reaction turbine)

    All parts spiral casing, guide mechanism and Draft tube of Kaplan turbine are same except runner.

The water enters nearly axially to the blades of the runner. Then the flow leaves the runner passes through the Draft tube as in Francis turbine. They operates at lower head and high flow rate.

Propeller turbine have fixed blades on runner

 

Kaplan have adjustable runner blades.

PERFORMANCE CHARACTERISTIC:- Turbine performance parameter are same as those of a  pump but the dependent variable is output brake horsepower which depends upon flow rate, discharge Q, Head H, speed of impeller n,

SPECIFIC SPEED Ns

    It is the speed of a geometrical similar turbine which will develop unit power when working under a unit head. Each type of turbine will have its own value for the specific speed. Hence this is also known as type of characteristics of turbine By known Ns type of turbine and performance can be predicted.

N is speed of turbine in rpm

P is power in KW

H is Head in meters

TURBINE EFFICIENCY (n):- It is defined as output brake horsepower divided by available water horsepower.

UNIT QUANTITIES:- It various quantities are reduced to a theoretical 1 metre head the comparison of performance data and computation of experiment value are simplified.

HYDRAULIC RAM

    Hydraulic ram is a pump which raises the water without any external power from its operation. When large quantity of water is available at a small height, a small quantity of water can be raised to greater height with the help of hydraulic ram. It works on the principle of water hammer.

Main components of hydraulic ram

1. Supply tank
2. Supply pipe 
3. Chamber
4. Waste valve
5. Delivery valve

Chamber is empty initially. The outside pressure is more than pressure in chamber. So waste wall valve remain open in download position. As the inlet valve is opened, Water it starts flowing from supply tank to chamber. As the water is coming into the chamber from supply tank, the level of water rises in the chamber and waste valve B start moving upward. A stage will come when the waste wall B closes suddenly. This creates high pressure in chamber. This high pressure forces open the delivery valve C. The water from chamber enters the air vessel and compresses the air inside the vessel. This compressed exerts force on water in the air vessel and small quantity of water is raised to a greater height. When water in chamber loses is momentum, the waste valve B opens in downward direction and the flow of water from supply tank start flowing to chamber and cycle is repeated. 

Efficiency of hydraulic ram (n)

HYDRAULIC PRESS

It is a device used for lifting heavy weights by application of a much smaller force. It is based on Pascal Law which state that "the intensity of pressure in a static fluid is transmitted equally in all direction" 

It consists of two cylinder of different diameter. One of cylinder is of large diameter and contain ram while other cylinder is a small diameter and contain plunger. The two cylinder are connected by a pipe the cylinder and pipe contains a liquid through which pressure is transmitted. When small forces is applied on plunger in downward direction a pressure is produced on liquid in contact with plunger. This pressure is transmitted equal in all direction and acts as ram in upward direction. The heavier weight placed on ram is lifted up.

HYDRAULIC ACCUMULATOR

    It is a device used to store energy of fliuid under pressure and make this energy available to Hydraulic Machines such as cranes, lift or press etc. This is analogous to fly wheel of an engine.

CONSTRUCTION

    It consists of a fix vertical cylinder containing a sliding ram/plunger. A load/ weight is placed on the top of the sliding ram to create a pressure in the cylinder chamber. One side of the cylinder is connected to the pump and the other side is machine.

WORKING

In beginning the ram is at lower most position. During ideal period the driven machine (crane or lift) high pressure liquid supplied by pump is admitted in the hollow space of the cylinder. It raises the ram on which heavy load is placed. Flow of liquid continues till the ram is at its upper most position. At this position the cylinder is full of fluid and max amount of energy is accumulated. This accumulated energy is later discharge at the driven machine during its working period.