La Mont Boiler Working and Construction

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.

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Modern High Pressure Boilers

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

1. La-Mont Boiler
2. Loeffler Boiler
3. Benson Boiler

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.

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Fusible plug

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.

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Spring loaded safety valve (Ramsbotom Safety Valve):

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.

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Lever safety valve:

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.

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