Mechanical Engineering

Comparison between Impulse and Reaction Steam Turbines

2012-12-11T03:23:00.000-08:00

In Impulse Turbine, steam completely expands in the nozzle itself. Hence its pressure remains constant on both ends of the moving blades.

In Reaction Turbine, Fixed blades act as nozzles. Hence steam expands both in fixed and moving blades continuously as it passes over them. Thus the pressure drop occurs gradually and continuously over both the fixed and moving blades.

In Impulse Turbine, Blades Passage is of constant cross section area, as there is no expansion of steam.

In Reaction Turbine, Blade passage is of variable cross-sectional area (converging type) due to expansion of steam.

In Impulse Turbine, As pressure remains constant in moving blades, the relative velocity of steam passing over the moving blades remains constant

In Reaction Turbine, Continuous expansion of steam means relative velocity in the moving blades increases.

In Impulse Turbine, Blades are of symmetrical profile types : hence, manufacturing of blade is simple.

In Reaction Turbine, The blade shapes are of aerofoil and non-symmetrical type; hence manufacturing is difficult.

In Impulse Turbine, Because of large pressure drop, the steam speed and the running speed are high.

In Reaction Turbine, Due to small pressure drop, the steam speed and the running speed are low.

In Impulse Turbine, Because of large pressure drop in the nozzles, the number of stages are less. The size of an impulse turbine for power output is comparatively small.

In Reaction Turbine, Because of small pressure drop in each stage, the number of stages are more for the same pressure drop. Hence the size of the reaction turbine for the same power output is large. Reaction turbines are multi-stage turbines only.

Impulse Turbine Occupies less space per unit power

Reaction Turbine Occupies more space per unit power

Impulse Turbine is Suitable for small powers

Reaction Turbine is Suitable for medium and higher powers.

Reaction turbine

2012-11-19T07:03:00.000-08:00

Description of reaction turbine
It consists of a wheel or rotor, casing, fixed and moving blades. In this type, equal number of fixed and moving blades are attached alternately to the casing and the wheel respectively. The fixed blades is similar to a nozzle where velocity increases with decrease of pressure.

Working Principle

In reaction turbine, the steam is not expanded in the nozzle, but expands as it flows over the blades.

The steam passes over the fixed blade F. The fixed Blade changes the direction of steam and at the same time allows it expand to a higher velocity, with decrease of pressure.
Then the steam passes over the moving blade M. The moving blade converts the kinetic energy into mechanical work with decrease of velocity; but at the same time steam expands as it flows over the moving blade and there is a fall of pressure. This produces a reaction on the blades, by the expanding steam.

Thus in the reaction turbine the steam expands both in fixed and moving blades continuously as the steam passes over them. Therefore, the pressure drop occurs gradually and continuously over both fixed and moving blades. Parson turbine is an example of reaction turbine.

Compounding of Impulse Turbines

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The disadvantage of De Laval type of turbine is that it’s extremely high speed, of the order of 30,000 rpm, cannot be employed for practical purposes. To reduce the high speed, more than one set of blades are used. This is called “compounding of impulse turbine”.

In the compounding method, the steam jet velocity or the steam pressure is absorbed in stages as it flows over the rotor blades. When “steam velocity” is absorbed in stages, it is called “Velocity compound impulse turbine”. When “steam pressure” is absorbed in stages it is known as “pressure compound impulse turbine”.

Velocity Compound Impulse Turbine:

In velocity compound impulse turbine, moving and fixed blades are placed alternately. Moving blades are fitted with the wheel while the fixed blades are fitted with the casing.

The steam is expanded in the nozzle from the boiler pressure to condenser pressure, to a high velocity. It is then passed over the first ring of moving blades. Only a portion of the high velocity of steam jet is absorbed by this blade ring, the remainder being exhausted on to the next ring of fixed or guide blades. These fixed blades change the direction of steam jet.

The jet is then passed on to the next ring of moving blades. A further portion of the steam velocity is now absorbed by this second moving blade ring. The process is then repeated as the steam flows over the remaining pairs of blades until practically all the velocity of the jet has been absorbed and the kinetic energy is converted into mechanical work.

It should be noted that the entire pressure drop takes place in the nozzle itself, the pressure remaining constant, as the steam flows over the blades. Hence the turbine is an impulse turbine. The Curtis turbine is an example of velocity compound impulse turbine.

Pressure compound impulse turbine:

In this type the expansion of steam takes place in more than one set of nozzles and each set of nozzles is followed by a set of moving blades. The total pressure drop of the steam does not take place in the first set of nozzles, but is divided up equally between all the sets of nozzles.

Impulse Turbine

2012-10-09T03:47:00.000-07:00

Description of impulse turbine:

Wheel or Rotor :
The wheel or rotor is fitted over a shaft from which the useful power is available. It is a rotating element of the turbine on which moving blades are fixed.

Nozzle:
The nozzle is a passage for the flow of steam where pressure energy is converted into kinetic energy. Its main function is to produce a jet of steam with a high velocity.

Blades :
De Laval turbine shown in the image below is an example of simple impulse turbine.
In this, only one set of impulse type blades is rigidly fixed to the rim of the rotor or wheel. It converts the kinetic energy of steam into mechanical work.

Casing :
The casing is the outside cover of the steam turbine fixed over a frame. It is fitted with nozzle.

Working Principle of Impulse turbine :

If a jet of steam is discharged from a fixed nozzle at a high speed over a flat stationary plate, a steady force will be exerted over this plate. This force is nothing but an impulse. No work is done as the plate is fixed. But, if a number of such plates are fixed on the rim of a wheel, the wheel may be rotated due to the impulse of steam. Curved plates are used instead of flat plates to utilize greater amount of energy.

In the impulse turbine, steam is expanded in the fixed nozzle only. In the nozzle the velocity of steam increases with decrease of pressure. As the steam passes over the blades, the pressure remains constant with a decrease of velocity.

As the high velocity steam impinges against the baldes, it changes the momentum of jet causing impulsive force on the blades. The wheel is thus made to rotate in a definite direction.
Here the kinetic energy is converted into mechanical work, only by one set of blades. It is simplest type of impulse turbine.

Steam Turbine | Prime Movers

2012-09-30T00:05:00.000-07:00

The steam turbine is universally used as prime mover in steam power plants.

Flow over Blades:

The steam turbine obtains its motive power from the change of momentum of a jet of steam flowing over a curved blade. The steam jet, in moving over the curved surface of the blade, exerts a pressure on the blade owing to its centrifugal force. This centrifugal force is exerted normal to the blade surface as shown in figure and acts along the whole length of the blade.

The resultant of these centrifugal forces plus the effect of change of velocity is the motive force on the blade. It should be realized that the blade obtains no motive form any impact of the jet, because the blade is so designed that the steam jet will glide on and off the blade without any tendency to strike it. In principle, it is analogous to a train passing around a railway curve. The train exerts a radially outward force on the line due to the centrifugal force.

Moving and Fixed Blades

In a steam turbine, a number of small blades are fixed to the ring of a revolving wheel or rotor. Jets of steam of a high velocity are obtained by expansion through nozzles and are directed on to the blades. The effective force of these jets, acting on the blades, rotates the wheel.

In modern turbines several of the wheel of moving blades are keyed to the same shaft, having a ring of fixed blades between each wheel of moving blades. These fixed blades are fixed to the turbine casing. Their object is to receive the steam jet from the moving blade ring and to divert it on to the next ring of moving blades by changing its direction as shown. This diversion may continue over several rings of moving and fixed blades until the whole of the kinetic energy of the steam jet is expended.

In the next post, I will explain about the impulse turbine and reaction turbine.

Combined Cycles

2012-07-25T00:23:00.000-07:00

Combined cycle are obtained by coupling a steam power plant (or sometimes a diesel engine plant) with a gas turbine installation. These systems are used to increase the overall efficiency of the gas turbine cycle, as the efficiency of the basic gas turbine plant is as low as 20 to 25%.

There are a number of combinations of combined cycles. However, we shall discuss the following two important combined cycle systems:

Combined cycle with reheat of exhaust:

The image above shows a gas turbine plant couple with a steam plant.

Gas Turbine :
Atmospheric air is drawn into the air compressor, when it is compressed to a high pressure. Fuel (Natural gas or Gasified Coal) is injected into the combustion chamber, CC and burns in the steam of compressed air. The products of combustion, comprising a mixture of gases at high temperature and pressure, are passed to the turbine. Products of combustion are expanded in the gas turbine and electric power is generated in the generator G1.

It should be remembered that the exhaust gases from the gas turbine have a temperature of 400 to 500’C and contain about 40 to 50% of the initial heat energy.

Steam Plant with Reheat Arrangement:

As seen in the image above, the exhaust gases from the gas turbine are heated in a reheater (RH). The hot flue gases then pass through the boiler to generate steam. This high pressure steam from the boiler is used to drive a steam turbine. Thus electric power is generated in the generator G2.

This combined cycle recovers much of the exhaust heat energy by reheating the high temperature exhaust gases from the gas turbine and passing to the heat recovery boiler for power generation.

Combined cycle with cogeneration arrangement:

The topping cycle system shown in the above image is another type of combined cycle with cogeneration arrangement. In this arrangement, the exhaust heat from the gas turbine is used for steam generation in a waste heat recovery boiler. Steam from the boiler is used for process plant.

Advantages of combined cycles

The overall efficiency of the combined cycle plant is nearly 47 to 42% which is much higher than a simple gas turbine or stem plant.
The capital cost of combined plant with supplementary firing is less than the combined cost of separate units of gas turbine and steam turbine plants for the same power capacity.
The cooling water requirement of a combined cycle is much less than a pure stem plant having the same output. The reason is no cooling water is required for the gas turbine.
The combined plant is more suitable for quick start and shut down than steam plant. Whether natural gas or gasified coal is the fuel, combined cycle greatly reduces CO2 emissions. Thus environmental pollution is minimized.
By using combined cycle plants, energy resources could be used efficiently so that they last longer. Thus the concept of combined cycle systems will promote energy conservation.

Applications of Cogeneration

2012-06-25T00:37:00.000-07:00

Industrial cogeneration systems have received an impetus in recent years. The potential savings of cogeneration are significant. A gas turbine cogeneration system has been installed at ONGC, Uran with a capacity of 40 MW.

Gas turbine with heat recovery, steam turbine with heat recovery and diesel engine with heat recovery is the different cogeneration schemes used in traditional power plants.
Cogeneration systems are eminently suitable for various process industries like rayon, pulp and paper, chemical process, textile and fertilizer where both power and process system are used.

A few typical examples of industries where cogeneration systems could be utilized are:

In industries, such as rayon, pulp and paper, chemical processing, and textile, which require simultaneous steam and power, it is possible to meet either part or full heat and power requirements using steam turbine, gas turbine with heat recovery boiler.
Cement kilns and brick kilns require a large amount of high temperature process heat. The gas turbine exhaust, with or without supplementary firing, can supply this heat and produce electric power for the factory.
In glass melting furnaces, heat from the exhaust gases can be recovered in waste heat boiler to produce steam. The steam can be expanded in steam turbines to produce electrical power.

Benefits of cogeneration

2012-06-20T04:17:00.000-07:00

This Post is a continouation of my previous post on Types of cogeneration systems. I have listed the benefits and advantages of using cogeneration system in your factory.

By product
The power produced by na industrial cogeneration system is a by-product of the process which may or may not match the power demands of the industry.

Fuel economy
Electricity abotained by industries through cogeneration consumes only about a half of the fuel (coal, oil or gas) that a convetional power station needs to generate it. The cogeneration system has a fuel efficiency of 60% to 70%, whereas the latter has no more that 30% to 35%.

Important indirect benefits flowing from fuel economy include savings in transportation cost of the fuel as well as reduced pollution.

Fuel conversation
Cogeneration, if exploited from every possible source and in every possible way, is not only an economical source of energy, but also a source of conservation of fuel.

Energy efficiency
Cogeneration is a convenient way of improving overall energy efficiency. Maximum energy efficiency is realized when energy loss is minimized. By capturing and using otherwise wasted thermal energy, the overall energy efficiency of a cogenerating system can be twice that of a conventional power plant producing electricity.

Quality of supply
Cogeneration entiles simpler equipment due to usage of lower temperatures and pressures, and almost completely obviates transmission and distribution. Hence it output is far less vulnerable to interruption than the grid. Voltage of supply can also be much better controlled.

Types of cogeneration systems

2012-06-14T02:04:00.000-07:00

There are two types of cogeneration systems: Topping cycle and Bottoming cycle.

Topping cycle system

Hot gases expand in the gas turbine to generate electricity. The still-hot rejected waste heat from the turbine is sent through a waste heat recovery boiler to produce low pressure process steam. The steam is sent to an industrial process requiring large quantities of hot steam (textile, paper and pulp, sugar, food processing, etc.).

An approach that circumvents the problem of distance is to bring the industry to the power plants; to create an industrial park in which the power plant provides both electricity and steam.

Bottoming cycle system

See the above image. In this the primary energy source is applied to an useful industrial process. The reject heat emerging from the process is then used for power generation.

In the next post I will explain the benefits of cogeneration.

Cogeneration with Definition and Principle

2012-06-11T07:34:00.000-07:00

In the generation of electricity (except hydro-electric power), a large amount of heat energy remains in the exhaust steam from the steam turbine or hot gases from the gas turbine. On the other hand, many industrial processes use high – temperature heat: textile, cement, pulp and paper, food processing, fertilizer, petroleum refining, steel and glass industries are examples. There is often much heat energy left in the exhaust from such industries.
The waste heat in both the above cases in energetic, and is, therefore, potentially capable of producing useful in the form of process steam or electricity. Industrial managers are being urged to look at systems that “cogenerate” both heat and electric power.

Definition of cogeneration:

Cogeneration is an energy conversation technology. It is defined as the sequential production of electricity and steam (or) heat) energy from the same fuel source. It is employed to capture the heat energy availability which would otherwise be lost in the normal operation of a traditional power plant or an industrial process. For instance, heat from the exhaust gases of a gas turbine power plant could be made use of in the waste heat recovery boiler to produce process steam.

Principle of cogeneration:

See the image 1. In the Conventional power plant, heat energy must be added to the boiler feed water in an amount sufficient to bring it up to point A. However, the turbine which drives the generator can utilize the amount of energy between points A and C only. A large amount of heat energy from point C back down to the feed water level E is rejected to the environment as shown in image 2.

See the image 3. In this industrial cogeneration system, the energy added from the feed water level up to point B is needed to generate process steam. On top of this, an extra amount of energy is added to bring the steam up to point A. the energy from A to B is now available for power generation and that from B down to D for process use as steam. A relatively small amount is rejected as waste heat as shown in image 4.

Benson Boiler

2012-04-22T23:07:00.000-07:00

The presence of steam bubbles in contact with the surface of tubes seriously impairs heat transmission from the flue gases to water. By rising the boiler pressure to the critical pressure of steam (225 kgf/sq.cm.), this difficulty is overcome, as suggested by Mark Benson in 1922. At the critical pressure water and steam have the same density and no bubbles are formed.

The first modern high pressure drumless boiler developed by benson was put into operation in 1927 in west Germany power station.

Working principle of Benson Boiler:
This boiler has a unique characteristic of absence of steam separating drum. The entire process of heating, steam generation and superheating is done in a single continuous tube.

Economiser
The feed water by means of the feed pump is circulated through the economiser tubes. Hot flue gases pass over the economiser tubes and the feed water is preheated.

Radiant evaporator
The feed water from the economiser flows into the radiant evaporator with radiant parallel tube sections. The radiant evaporator receives heat from the burning fuel through radiation process and majority of water is converted into steam in it.

Convection Evaporator
The remaining water is evaporated in the convection evaporator, absorbing the heat from the hot gases by convection. Thus the saturated high pressure steam at a pressure of 210 kg/sq.cm is produced.

Convection superheater
The saturated steam is now passed through the convection superheater where the saturated steam os superheated to 650’C. The radiant evaporator, the convection evaporater and the convection superheater are all arranged in the path of the flue gases.

Steam outlet
The superheated steam is supplied to the steam turbine through the steam outlet.

Capacity
Capacity of benson boiler is about 150 tonnnes/hr at a pressure of 210 kgf/sq.cm. and at a temperature of 650’C. (Efficiency may be improved by running the boiler at a pressure slightly lower than the critical pressure).

Salient features of Benson Boiler

As there are no drums, the total weight of benson bolier is 20% less than other boilers. This also reduces the cost of the boilers.
As no drums are required, the transfer of the benson parts is easy. Majority of the parts may be carried to the site without pre-assembly.
Since no drum is used, this is an once-through boiler and the feed water entering at one end is discharged as superheated steam at the other end.
Circulating pump and downcomers are dispensed with.

Loeffler Boiler

2012-04-15T22:39:00.000-07:00

This is also a modern high pressure water tube boiler using the forced circulation principle and named after Prof.Loeffler.

Salient features of Loeffler Boiler
The novel feature of the Loeffler Boiler is to evaporate water solely by means of superheated steam. The furnace heat is supplied only to economiser and superheater. In other words, steam is used as a heat absorbing medium.

The major difficulty experienced in La-Mont boiler is deposition of salt and sediment on the inner surfaces of water tubes. The deposition reduces the heat transfer, ultimately, the generating capacity. This difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes. Feed water is evaporated in the drum using part of the superheated steam coming out from the water-heater. Thus only the dry saturated steam passes through the tubes. Poor feed water can, therefore, be used without any difficulty in the boiler, which is great advantage of this boiler.

Working principle of Loeffler Boiler
The image shows the outline diagram of Loeffler Boiler.

Economiser
The feed water from the feed tank is supplied to the economiser by feed pump. In the economiser the feed water is made to flow through a number of tubes surrounding which the hot gases leaving the furnace pass over. There is a heat exchange from the hot gases to the feed water, which is preheated in the economiser.

Evaporated Drum
It is housed away from the furnace. It contains a mixture of steam and water. The feed water from the economiser tubes enters the evaporator drum into which is also passed two-thirds of the superheated steam generated by the boiler. The superheated steam gives its superheat to the water in the drum and evaporates it to saturated steam.

Mixing Nozzles
The nozzles distribute and mix the superheated steam throughout the water in the evaporator drum.

Steam circulating pump
A steam circulating pump forces this saturated steam from the evaporator drum to the radiant superheater through the tube of the furnace wall.

Radiant superheater
The radiant superheater is placed in the furnace. The hot gases in the furnace are used for superheating the saturated steam from the drum. The radiant superheater receives heat from the burning fuel through radiation process.

Convection superheater
Steam from the radiant superheater enters the convection superheater where it is finally heated to the desired temperature of 500’C. The convection superheater receives heat from the flue gases entirely by convective heat transfer. Both radiant and convection superheater are arranged in series in the path of the flue gases.

Steam outlet
About one-third of the superheated steam from the convection superheater passes to the steam turbine while the remaining two-thirds is passed on to evaporator drum to evaporated the feed water to saturated steam.

Capacity
Capacity of the Loeffler boiler is about 100 Tonnes/Hr of superheated steam generated at a pressure of 140 kgf/sq.cm and at a temperature of 500’C.

La Mont Boiler Working and Construction

2012-03-27T08:06:00.000-07:00

A forced circulation boiler was first introduced by La-Mont in the year 1925 which is used in Europe and America. This is a modern high pressure boiler (water tube type steam boilers) working on forced circulation system.

Working principle of La Mont Boiler
The image shows the flow circuit of La Mont Boiler.

I will explain working of each and every part in La Mont boiler one by one.

Steam separator drum
The la Mont boiler consists of a steam separator drum which is placed wholly outside the boiler setting . The drum receives a mixture of steam and water from the evaporator tubes and feed water from the economizer. The steam is separated from water in the drum.

Circulating pump
The water from the drum is then drawn to the circulating (centrifugal) pump through the down-comer. The pump circulates water (“forced circulation”) equal to 8 to 10 times the weight of steam evaporated. This prevents the tubes from being overheated.

Distributing header
The circulating pump delivers the feed water to the distributing header with orifices at a pressure above the drum pressure.

Evaporator
The header distributes water through orifices into the evaporator tubes acting in parallel. Orifice in the header controls the flow of water to the evaporator tubes. Here part of the water is evaporated and a mixture of steam and water from these tubes enters the drum.

Convection superheater
The steam produced in the boiler is nearly saturated. This steam as such should not be used in the steam turbine. The presence of moisture in it will cause corrosion of turbine blades, etc. to raise the temperature of steam and thereby to increase the turbine efficiency, superheater is used.

The principle of convection superheater is similar to steam generating tubes of the boiler. The hot flue gases at high temperature sweep over convection superheated tubes and raise the temperature of steam. Convection superheater thus receives heat from the flue gases flowing from the combustion chamber, entirely by convective heat transfer. Such a superheater may be more conveniently located since it is not necessary for it to “see” the furnace.

Saturated steam from the top of the drum enters the convection superheater placed in the path of the flue gases and is superheated.

Steam outlet
Superheated steam from the superheater passes out to the steam turbine through the steam outlet.

Economizer
The quantity of superheated steam thus delivered to turbine is continuously made up in the form of feed water. Feed water supplied by the feed pump is heated in the economizer on its way to the steam separator drum.

The economizer is a device used to preheat the feed water using the hot gases leaving the boiler. Before the gases are let off to the atmosphere, they are made to flow in a definite passage in the economizer so that some of the heat in the hot gases, which otherwise gets wasted, can be used to preheat the feed water. The preheated water requires only a small amount of heat to be supplied in the boiler, resulting in some saving of the fuel burnt. This results in an increase in the boiler efficiency.

Air preheater
Since the heat of the exit gases cannot be fully extracted through the economizer, the air preheater is employed to recover some of the heat escaping in these gases. These exit gases preheat the air from the blower in the air preheater. The preheated air is supplied to the furnace for combustion.

Capacity
The capacity of la-mont boiler is about 50 Tonnes/hr of superheated steam at a pressure of 170 kgf/sq.cm. and at a temperature of 500’C.

Modern High Pressure Boilers

2012-03-12T23:04:00.001-07:00

A boiler which generates steam at a pressure of 85 kgf/sq.cm or above is termed as a “high pressure boiler”. The present tendency is towards the use of high pressure boilers in power plants. The modern high pressure boilers used for power generation have capacities of 40 to 1600 tonnes/hr of superheated steam with a pressure upto 210 kgf/sq.cm and a temperature of about 650’C. One of the largest modern steam power plants in the world is in japan with a steam capacity of 1600 Tonnes/hr. In India, the trombay power plant has a steam generating capacity of 550 tonnes/hr, Ramagundampower power plant with 320 tonnes/hr and bokaro plant with 160 tonnes/hr.

Water tube boilers are generally preferred for high pressure and high output whereas fire tube boilers for low pressure and low output.

Advantages of high pressure boilers:

Method of water circulation

Water circulation through the boiler may be either natural circulation due to density difference or by forced circulation. In high pressure boilers, water circulation is made with the help of a centrifugal pump which forces water through the boiler tubes. This is called “forced circulation of water”. The use of natural circulation is limited upto 120 kgf/sq.cm. Steam pressure and forced circulation upto 210 kgf/sq.cm. Forced circulation increases the rate of heat transfer and hence increases the steam generating capacity of boilers.

Size of drums

The high pressure boilers are characterized by the use of very small steam separating drums or by the complete absence of any drum.

Type and arrangement of tubes

The heat of combustion is utilized more efficiently by the use of small diameter and light weight tubes in large numbers. To avoid large resistance to the flow of water , the high pressure boilers have a parallel set of arrangement of tubes.

Compactness

The boiler components can be arranged horizontally, giving greater accessibility and operational convenience as high head required for natural circulation is eliminated by using forced circulation. The space required is hence less and arrangement is compact.

Foundation cost

Due to the light weight tubes and small size drums required and the arrangement being compact, the cost of foundation is reduced.

Efficiency

The efficiency of the power plant is increased upto 40%, by using high pressure superheated steam. Also steam can be raised quickly after the boiler is fired.

Cost of electricity

Since efficiency of the plant is increased by using high pressure boilers, the cost of electricity production is reduced.

Overheating

All the parts are uniformly heated; therefore the danger of overheating is reduced. Also thermal stress probelm is avoided.

Scale formation

The tendency of scale formation is eliminated due to the high velocity of water through the boiler tubes.

Types of high pressure boilers

In the upcoming posts, I will write about the types of high pressure boilers in breif. If you have any comments please let me know.

Fusible plug

2012-03-08T21:30:00.000-08:00

The function of the fusible plug is to put-off the fire in the furnace of the boiler when the water level falls below an unsafe level and thus avoid the explosion which may take place due to overhearing of the tubes and the shell.

Description of fusible plug:

It consists of a hollow gun metal body screwed into the fire box crown plate. A hollow gun metal plug is screwed into the gun metal body by tightening the hexagonal flange in it. There is another copper plug locked with the gun metal plug by pouring a low melting point metal (lead) into the groove provided for the same.

Working of fusible plug:

During the normal operation, the fusible plug is submerged in water which keeps the temperature of the fusible metal below its melting point.

But when the water level falls below the top of the fusible plug, it is uncovered by the water. The fusible plug therefore melts by the heat of the furnace. Thus the copper plug drops down and is held within the gun metal body by the ribs. The opening so made allows the steam rush into the furnace and extinguish the fire. The damage to the fire box which could burn up, is avoided.

Spring loaded safety valve (Ramsbotom Safety Valve):

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Description of spring loaded safety valve:

It is loaded with a spring instead of weights. Hence it is called spring loaded safety valve. It consists of a cast iron body having two branch pipes P1 and P2. Two separate valves are placed over the valve seatings, which are fixed to the top of the branch pipes. A lever is placed over the valves by means of two conical pivots.
The lever is attached to a spring at its middle. The spring pulls the lever in downward direction. The lower end of the spring is attached to the valve body by means of a shackle. Thus the valves are held tight to their seats by the spring force.

Working of spring loaded safety valve:

When the steam pressure exceeds the normal working pressure , the valves rise up against the action of the spring and allow the steam to escape from the boiler till the pressure in the boiler reaches its working pressure.

The spring loaded safety valve is much lighter and compact compared with other safety valves. For locomotive or marine service, the safety valve should be such that it is unaffected by jerks and vibration likely to occur in such device. Hence spring loaded safety valve is preferred for locomotive and marine services, in addition to stationary boilers.

Lever safety valve:

2012-03-03T01:59:00.000-08:00

The advantage of level safety valve over the dead weight safety valve is that the heavy dead weight is replaced by a lever with a smaller weight.

Description of Lever safety valve:

It consists of a valve resting over a gun metal seat. The valve seat is fixed on a mounting block, fitted over the boiler shell. One end of the level is hinged to a rod of the mounting block, while the other end carries a weight. A short strut is placed over the valve.

Working of Lever safety valve:

The thrust of the lever with its weight is transmitted to the valve by the strut. When the steam pressure exceeds the safe limit, the upward thrust of steam lifts the valve from its seat and the lever with its weight. The excess steam escapes till the pressure falls back to the normal value. The valve then returns back to its original closed position.
The required weight W at the end of the lever for maintaining the pressure P in the boiler is obtained by taking moments about the hinged point.

i.e., PaL1 = WL2

Where a = area of the valve exposed to steam,
L1 = distance of valve centre from the hinged point, and
L2 = distance of the centre of the weight to the hinged point.
The lever safety valve is used in stationary boilers only.

Dead Weight safety valve:

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The Dead Weight safety valve consists of a valve V which is made of gun metal to prevent rusting. It rests on the gun metal seat S and is fixed to the top of a vertical steam pipe P. The pipe has a flange F at the bottom for fixing at the top of the boiler shell.

A weight carrier C is suspended from the top of the boiler. It carries cast iron rings (i.e., weight W). the total weight must be sufficient to the keep the valve on its seat against the normal working pressure.

Working of Dead Weight safety valve:

When the steam pressure in the boiler exceeds the normal working pressure, it lifts the valve with its weight. The excess steam therefore escapes through the pipe to the atmosphere, until the pressure reaches its normal value.

It is the simplest type of safety valve; it is suitable for stationary boilers only, because it cannot withstand the jerks and vibration of mobile (marine) boilers. Another disadvantage of this valve is the heavy weight required to balance the steam pressure. Hence, it is not suitable for high pressure boilers.

Safety valves

2012-02-14T00:00:00.001-08:00

Safety valves are devices used to maintain a constant safe pressure inside the boiler. When the pressure inside the boiler increases, the excess steam will escape to the atmosphere through the valve automatically. Generally, a boiler is provided with two safety valves.

There are four different types of safety valves, viz.,

In the upcoming posts, I will write post about the four different types of safety valves.

Pressure gauge

2012-02-09T22:48:00.000-08:00

A pressure gauge is used to indicate the steam pressure of the boiler. It is also called as steam gauge. It is usually mounted in the front top of the steam drum.

Figure shows a commonly used pressure gauge known as bourdon type. It consists of an elastic metallic bourdon spring tube S of elliptical cross section and bent in the form of circular arc. One end of the tube is fixed at the block B. it is connected to the steam space of the boiler by menas of cock C. The other end is connected to a toothed sector T through a link L hinged at the point H. The sector is in mesh with a pinion P fixed on a spindle. An indicating pointer N is attached to the spindle to read the pressure on a dial gauge D.

Working:
When steam enters the elliptical tube, the tube section tries to become circular, which causes the other end of the tube to move outward. The movement of the closed end of the tube is transmitted and magnified by the link and the tooth sector. The magnitude of the movement of the sector is indicated by the pointer on the dial.

Note: Since the spring tube is surrounded by the atmospheric air, the pressure in the interior of the tube is above that of the atmosphere.
Hence absolute pressure = gauge pressure + atmospheric pressure.

Use the link to know more about Bourdon tube pressure gauge

Water gauge or Water Level Indicator

2012-02-06T23:45:00.001-08:00

Water gauge indicates the water level inside the boiler and is hence called as water level indicator. Usually two water gauges are fitted in front of boiler.

Description of water gauge:

It consists of a glass tube, two gun metal tubes and three cocks. The steam cock C1 is provided on the gun metal tube M1 which connects the glass tube with the steam space in the boiler. The water cock C2 is provided on the gun metal tube M2 which connects the glass tube with the water space. The gun metal tubes M1 and M2 are bolted to the boiler shell.

The drain cock C3 is used to drain the water from the glass tube at intervals to ascertain whether the gauge is in proper order or not. The glass tube is protected by means of a cover, made of specially toughened glass which will prevent any accident that may happen due to the breaking of glass tube.

Working of Water gauge:

The water gauge shows the level of water in the boiler drum. It warns the operator if the water level goes below a fixed mark, so that corrective action may be taken in time to avoid any accident.

For the observation of the water level in the boiler, the water and steam cocks are opened and drain cock is closed. The steam enters from the upper metal tube M1 into the glass tube and water enters from the lower metal tube M2 into the glass tube. Hence, water stands in the glass tube at the same level as in the boiler.

The junctions of the metal tubes and the glass tube are provided with two balls. In case the glass tube is broken, the balls are pushed to the top nd bottom ends of the glass tube. Thus the flow of both water and steam out of the boiler is prevented.

When the boiler is not working, the water gauge can be taken out from the boiler for cleaning purposes by removing the bolts.

List of Boiler Mounting Valves and Gauges

2012-02-02T23:06:00.000-08:00

To provide for intelligent and safe operation of a boiler, a full complement of gauges and safety devices should be provided.

These include:

Water gauge (water level indicator)
Pressure gauges.
Safety valves:
- Dead weight safety valve:
- Spring loaded safety valve
- Lever safety valve
- High steam low water safety valve
Fusible plug
Feed check valve
Stop valve
Blow – off cock

In the upcoming posts, I will write about the above listed topics in detail.

Babcock and Wilcox Boiler

2012-02-01T05:38:00.000-08:00

It is a water tube boiler used in steam power plants. In this, water is circulated inside the tubes and hot gases flow over the tubes.

Construction of Babcock and Wilcox Boiler

The Babcock and Wilcox Boiler consists of

Steam and water drum (boiler shell)
Water tubes
Uptake-header and down corner
Grate
Furnace
Baffles
Super heater
Mud box
Inspection door
Damper

Steam and water drum (boiler shell):
One half of the drum which is horizontal is filled up with water and steam remains on the other half. It is about 8 meters in length and 2 meter in diameter.

Water tubes:
Water tubes are placed between the drum and furnace in an inclined position (at an angle of 10 to 15 degree) to promote water circulation. These tubes are connected to the uptake-header and the down-comer as shown.

Uptake-header and down-corner (or downtake-header)
The drum is connected at one end to the uptake-header by short tubes and at the other end to the down-corner by long tubes.

Grate: Coal is fed to the grate through the fire door.

Furnace : Furnace is kept below the uptake-header.

Baffles: The fire-brick baffles, two in number, are provided to deflect the hot flue gases.

Superheater: The boiler is fitted with a superheater tube which is placed just under the drum and above the water tubes

Mud box: Mud box is provided at the bottom end of the down comer. The mud or sediments in the water are collected in the mud box and it is blown-off time to time by means of a blow –off cock.

Inspection doors: Inspection doors are provided for cleaning and inspection of the boiler.

Working Babcock and Wilcox Boiler:

Coal is fed to the grate through the fire door and is burnt.

Flow of flue gases:

The hot flue gases rise upward and pass across the left-side portion of the water tubes. The baffles deflect the flue gases and hence the flue gases travel in the zig-zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the superheater. The flue gases finally escape to atmosphere through chimney.

Water circulation:

That portion of water tubes which is just above the furnace is heated comparatively at a higher temperature than the rest of it. Water, its density being decreased, rises into the drum through the uptake-header. Here the steam and water are separated in the drum. Steam being lighter is collected in the upper part of the drum. The water from the drum comes down through the down –comer into the water tubes.

A continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained. The circulation of water is maintained by convective currents and is known as “natural circulation”.

A damper is fitted as shown to regulate the flue gas outlet and hence the draught.

The boiler is fitted with necessary mountings. Pressure gauge and water level indicator are mounted on the boiler at its left end. Steam safety valve and stop valve are mounted on the top of the drum. Blow-off cock is provided for the periodical removed of mud and sediments collected in the mud box.

Salient features of Babcock and Wilcox Boiler:

Its overall efficiency is higher than a fire tube boiler.
The defective tubes can be replaced easily.
All the components are accessible for inspection even during the operation.
The draught loss is minimum compared with other boiler.
Steam generation capacity and operating pressure are high compared with other boilers.
The boiler rests over a steel structure independent of brick work so that the boiler may expand or contract freely.
The water tubes are kept inclined at an angle of 10 to 15 degree to promote water circulation.

Advantages and disadvantages of water tube boilers over fire tube boilers:

Advantages water tube boilers

Steam can be generated at very high pressures.
Heating surface is more in comparison with the space occupied, in the case of water tube boilers.
Steam can be raised more quickly than is possible with a fire tube boiler of large water capacity. Hence, it can be more easily used for variation of load.
The hot gases flow almost at right angles to the direction of water flow. Hence maximum amount of heat is transferred to water.
A good and rapid circulation of water can be made.
Bursting of one or two tubes does not affect the boiler very much with regard to its working. Hence water tube boilers are sometimes called as safety boilers.
The different parts of a water tube boiler can be separated. Hence it is easier to transport.
It is suitable for use in steam power plants (because of the various advantages listed above).

Disadvantages of water tube boilers

It is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tubes. Hence, water treatment is very essential for water tube boilers.
Maintenance cost is high.
Failure in feed water supply even for a short period is liable to make the boiler overheated. Hence the water level must be watched very carefully during operation of a water tube boiler.

Lancashire Boiler

2012-01-31T00:48:00.000-08:00

It is a stationary, fire tube, internally fired boiler. The size is approximately from 7-9 meters in length and 2-3 meters in diameter.

Construction of Lancashire Boiler:

It consists of

Cylindrical shell
Furnace tubes, bottom flue and side flues
Grate
Fire bridge
Dampers

Cylindrical shell

It is placed in horizontal position over a brick work. It is partly filled up with water. The water level inside the shell is well above the furnace tubes.

Furnace tubes, bottom flue and side flues:

Two large internal furnace tubes (flue tubes) extend from one end to the other end of the shell. The flues are built-up of ordinary brick lined with fire bricks. One bottom flue and two side flues are formed by brick setting, as shown in the figure.

Grate

The grate is provided at the front end of the main flue tubes. Coal is fed to the grate through the fire hole.

Fire bridge:

A brickwork fire bridge is provided at the end of the grate to prevent the flow of coal and ash particles into the interior of the furnace (flue) tubes. Otherwise the coal and ash particles carried with gases form deposits on the interior of the tubes and prevent the heat transfer to the water.

Dampers:

Dampers is in the form of sliding doors are placed at the end of the side flues to control the flow of gases from side flues to the chimney flue.

Working of Lancashire boiler

Coal is fed to the grate through the fire hole and is burnt. The hot gases leaving the grate move along the furnace (flue) tubes upto the back end of the shell and then in the downward direction to the bottom flue. The bottom of the shell is thus first heated.

The hot gases, passing through the bottom flue, travel upto the front end of the boiler, where they divide into two streams and pass to the side flues. This makes the two sides of the boiler shell to become heated. Passing along the two side flues, the hot gases travel upto the back end of the boiler to the chimney flue. They are then discharged into the atmosphere through the chimney.

With the help of this arrangement of flow passages of hot gases, the bottom of the shell is first heated and then its sides. The heat is transferred to water through the surface of the two flue tubes (which remain in water) and bottom and sides of the shell.

The arrangement of flues increases the heating surface of the boiler to a large extent.

Dampers control the flow of hot gases and regulate the combustion rate as well as steam generation rate.

The boiler is fitted with necessary mountings. Pressure gauge and water level indicator provided at the front. Safety valve, steam stop valve, low water and high steam safety valve and man-hole are provided on the top of the shell.

High steam low water safety valve:

It is a combination of two valves. One is lever safety valve, which blows-off steam when the working pressure of steam exceeds. The second valve operates by blowing-off the steam when the water level falls below the normal level.

Blow-off clock:

It is situated beneath the front portion of the shell for the removal of mud and sediments. It is also used to empty the water in the boiler during inspection.

Fusible plug:

It is provided on the top of the main flues just above the grate. It prevents the overheating of the boiler tubes by extinguishing the fire when the water level falls below a particular level. A low water level alarm is mounted in the boiler to give a warning when the water level falls below the preset value.

Salient features of Lancashire Boiler

The arrangement of flues in this boiler increases the heating surface of shell to a large extent.
It is suitable where a large reserve of steam and hot water is needed.
Its maintenance is easy.
Superheated can be easily incorporated into the system at the end of the main flue tubes. Thus overall efficiency of the boiler can be increased.

Note : The simple vertical Boiler, Cochran and Lancashire Boilers discussed till this post are Fire tube boilers. In the upcoming posts, I will write about water tube boilers namely Babcock and Wilcox Boiler.

Cochran boiler

2012-01-25T23:46:00.000-08:00

It is a multi-tubular vertical fire tube boiler having a number of horizontal fire tubes. T is the modification of a simple vertical boiler where the heating surface has been increased by means of a number of fire tubes.

It consists of

Shell
Crate
Fire box
Flue pipe
Fire tubes
Combustion chamber
Chimney
Man-hole

Shell
It is hemispherical on the top, where space is provided for steam.

Grate
It is placed at the bottom of the furnace where coal is burnt.

Fire box (furnace )
It is also dome-shaped like the shell so that the gases can be deflected back till they are passed out through the flue pipe to the combustion chamber.

Flue pipe:
It is a short passage connecting the fire box with the combustion chamber.

Fire tubes:
A number of horizontal fire tubes are provided, thereby the heating surface is increased.

Combustion chamber:
It is lined with fire bricks on the side of the shell to prevent overheating of the boiler. Hot gases enter the fire tubes from the flue pipe through the combustion chamber.

Chimney:
It is provided for the exit of the flue gases to the atmosphere from the smoke box.

Manhole:
It is provided for inspection and repair of the interior of the boiler shell.

Normal size of a Cochran boiler:
Shell diameter – 2.75 meters:
Height of the shell – 6 meters.

Working of the Cochran boiler:

Coal is fed into the grate through the fire hole and burnt. Ash formed during burning is collected in the ashpit provided just below the grate and then it is removed manually.

The host gases from the grate pass through the flue pipe to the combustion chamber. The hot gases from the combustion chamber flow through the horizontal fire tubes and transfer the heat to the water by convection.

The flue gases coming out of fire tubes pass through the smoke box and are exhausted to the atmosphere through the chimney.
Smoke box is provided with a door for cleaning the fire tubes and smoke box.

The following mountings are fitted to the boiler:

Pressure gauge: this indicates the pressure of the steam inside the boiler.

Water gauge: this indicates the water level in the boiler. The water level in the boiler should not fall below a particular level, otherwise the boiler will be over heated and the tubes may burn out.

Safety valve: the function of the safety valve is to prevent an increase of steam pressure in the boiler above its normal working pressure.

Steam stop valve: it regulates the flow of steam supply to requirements.

Blow-off cock: it is located at the bottom of the boiler. When the blow-off cock is opened during the running of the boiler, the high pressure steam pushes (drains) out the impurities like mud, sand, etc., in the water collected at the bottom.

Fusible plug: it protects the fire tubes from burning when the water level in the boiler falls abnormally low.

Salient features of Cochran boiler:

The dome shape of the furnace causes the hot gases to deflect back and pass through the flue. The un-burnt fuel if any will also be deflected back.
Spherical shape of the top of the shell and the fire box gives higher area by volume ratio.
It occupies comparatively less floor area and is very compact.
It is well suited for small capacity requirements.