Hydraulic machine

 Hydraulic machine

Introduction :-
The subject ' Hydraulic Machines may be defined as that branch of Engineering - science which deals with the machines run by water under some head or raising the water to higher levels.
Impact of water jets  :-
What is the impact of jets of water in hydraulic machines and fluid dynamics? What force does a water jet imposes on different surfaces like plane surface, curved surface and moving curved vanes etc.
Hydraulic Turbines :-

A hydraulic turbine is a machine which converts the hydraulic energy into mechanical energy. The hydraulic turbines are also known as water turbines. Following two types of hydraulic turbines are important.

1.     Impulse turbine

2.     Reaction turbine

In an impulse turbine, the total energy at the inlet of a turbine is only kinetic energy. The pressure of water both at entering and leaving the vanes is atmospheric. It is used for high head of water. A Pelton wheel is a tangential flow impulse turbine.

In a reaction turbine, the total energy at the inlet of a turbine is kinetic energy as well as pressure energy. It is used for low head of water. The Francis and Kaplan turbines are inward flow and axial flow reaction turbines respectively.

1. Impulse Turbines :-

What is an impulse turbine? How do we calculate hydraulic efficiency and mechanical efficiency of an impulse turbine? The following important points may be noted for impulse turbines:

(a) The hydraulic efficiency of an impulse turbine is the ratio of the work done on the wheel to the energy of the jet.

(b) The hydraulic efficiency of an impulse turbine is maximum when the velocity of wheel is one half the velocity of jet of water at inlet.

The mechanical efficiency of an impulse turbine is the ratio of the actual work available at the turbine to the energy imparted to the wheel.

2. Reaction Turbines:-

What is a reaction turbine? How to calculate hydraulic and overall efficiency of reaction turbine? What is difference between a Kaplan turbine and Francis turbine?

The following important points may be noted for reaction turbines :

(a) In a reaction turbine, the water enters the wheel under pressure and flows over the vanes.

(b) The hydraulic efficiency of a reaction turbine is the ratio of the workdone on the wheel to the energy (or head of water) actually supplied to the turbine.

(c) The overall efficiency of a reaction turbine is the ratio of the power produced by the turbine to the energy actually supplied by the turbine.

(d) A Kaplan turbine is an axial flow reaction turbine. The number of blades are generally 4 to 8 in a Kaplan turbine runner.

(e) A Francis turbine is an outward flow reaction turbine. The number of blades are generally 16 to 24 in a Francis turbine runner.

Draft Tube in Turbine :-

The draft tube is a pipe of gradually increasing area used for discharging water from the exit of a reaction turbine. It is an integral part of mixed and axial flow turbines. Because of the draft tube, it is possible to have the pressure at runner outlet much below the atmospheric pressure.

The efficiency of a draft tube is defined as the ratio of net gain in pressure head to the velocity head at entrance of draft tube.

Specific Speed of turbine :-

The specific speed of a turbine is defined as the speed of an imaginary turbine, identical with the given turbine, which develops unit power under unit head. Mathematically, specific Speed. 

The specific speed plays an important role in the selection of a type of turbine. By knowing the specific speed of a turbine, the performance of the turbine can also be predicted.

Unit Speed, Unit Discharge and Unit Power :-

What is Unit Speed, Unit Discharge and Unit Power of a turbine? How do we calculate these using formula?


The unit speed is the speed of the turbine operating under one meter head. Mathematically, unit speed. 

The unit discharge is the discharge through a turbine when the head on the turbine is unity. Mathematically, unit Discharge. 
The unit power is the power developed by a turbine when the head on the turbine is unity. Mathematically, unit power. 

Cavitation :-

What is cavitation and how is it formed and how to prevent/ avoid cavitation in a hydraulic machine? The formation, growth and collapse of vapour filled cavities or bubbles in a flowing liquid due to local fall in fluid pressure is called cavitation. The cavitation in a hydraulic machine affects in the following ways:

(a) It causes noise and vibration of various parts.

(b) It makes surface rough.

(c) It reduces the discharge of a turbine.

(d) It causes sudden drop in power output and efficiency.

"The cavitation in reaction turbines can be avoided to a great extent by using the following methods"

a) By installing the turbine below the tail race level.

(b) By using stainless steel runner of the turbine.

(c) By providing highly polished blades to the runner.

(d) By running the turbine runner to the designed speed.

Centrifugal Pumps :-

What is a centrifugal pump and its manometrichead? How to calculate discharge, efficiency and power required for a centrifugal pump using formula?

A centrifugal pump is a machine which converts the kinetic energy of the water into pressure energy before the water leaves its casing. The flow of water leaving the impeller is free vortex. The impeller of a centrifugal pump may have volute casing, vortex casing and volute casing with guide blades.

blades.

The following important points may be noted for centrifugal pumps:

(a) The manometric head is the actual head of water against which a centrifugal pump has to work.

Manometric head = Workdone per kg of water - Losses within the impeller

 = Energy per kg at outlet of impeller - Energy per kg at inlet of impeller

 = Suction lift + Loss of head in suction pipe due to friction + Delivery lift + Loss of head in delivery pipe due to friction + Velocity head in the delivery pipe.

"The mano metric efficiency of a centrifugal pump is defined as the ratio of the mano metric head to the energy supplied by the impeller"
"The mechanical efficiency of a centrifugal pump is defined as the ratio of energy available at the impeller to the energy supplied to the pump by the prime mover"
"The overall efficiency of a centrifugal pump is defined as the energy supplied to the pump to the energy available at the impeller"
"The efficiency of a centrifugal pump will be maximum when the blades are bent backward"

Multistage Centrifugal Pumps :-

The multistage centrifugal pumps are those which have two or more identical impellers mounted on the same shaft or on different shafts. They are used to produce high heads or to discharge a large quantity of liquid. In order to obtain a high head, a number of impellers are mounted in series or on the same shaft while to discharge a large quantity of liquid, the impellers are connected in parallel.

Reciprocating Pump :-

The reciprocating pump is a positive displacement pump as it discharges a definite quantity of liquid during the displacement of its piston or plunger which executes a reciprocating motion in a closely fitting cylinder. It is best suited for less discharge and higher heads.
The difference between the theoretical discharge and the actual discharge is called the slip of the pump.

Air Vessels:-

What is an air vessel in a reciprocating pump What is purpose of air vessel and why is it used. How it increases saving of work and power (mechanical)?
The air vessel, in a reciprocating pump, is a cast iron closed chamber having an opening at its base. These are fitted to the suction pipe and delivery pipe close to the cylinder of the pump. The vessels are used for the following purposes:

(a) To get continuous supply of liquid at a uniform rate.

(b) To save the power required to drive the pump. This is due to the fact that by using air vessels, the acceleration and friction heads are reduced. Thus the work is also reduced.


It may be noted that by fitting an air vessel to the reciprocating pump, the saving of work and subsequently the power is about 84.8 % in case of a single acting reciprocating pump and 39.2 % in case of double acting reciprocating pump.

Miscellaneous Hydraulic Machines:-

Information about different hydraulic machines like hydraulic press, hydraulic ram, hydraulic accumulator, hydraulic intensifier, hydraulic crane and hydraulic lift. The following are the miscellaneous hydraulic machines:


(a) Hydraulic press. It is a device used to lift larger load by the application of a comparatively much smaller force. It is based on Pascal's law.

(b) Hydraulic ram. It is a device used to lift small quantity of water to a greater height when a large quantity of water is available at a smaller height. It works on the principle of water hammer.

(c) Hydraulic accumulator. It is a device used to store pressure energy which may be supplied to hydraulic machines such as presses, lifts and cranes.

(d) Hydraulic intensifier. It is device used to increase the intensity of pressure of water by means of energy available from a large quantity of water at a low pressure.

(e) Hydraulic crane. It is a device used to lift heavy loads. It is widely used in docks for loading and unloading ships, ware houses, foundry workshops and heavy industries.

(f) Hydraulic lift. It is a device used for carrying persons and loads from one floor to another, in a multistory building.

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