Boilers by Asad Hafeez

8/10/2019 Boilers by Asad Hafeez

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Power PlantLab Report

5/3/2013

Submitted By: 2009-ME-357

Submitted to: Mr. Saqib


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ASME Standards


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BoilerA boiler, as the name suggests, is a device that boils (water to produce steam). More scientifically, a

boiler converts the chemical energy in fuel into the heat energy in steam, or the heat energy of hot

gases into the heat energy of steam when no firing is involved. The earliest form of boiler is the kettle.

Boiler ClassificationThe boilers can be classified according to the following criteria.

According to flow of water and hot gases

1.

Water tube

2.

Fire tube

In water tube boilers, water circulates through the tubes and hot products of combustion flow over

these tubes. In fire tube boiler the hot products of combustion pass through the tubes, which are

surrounded, by water.

Based on Operating Pressure

1.

Subcritical boilers

2.

Supercritical (SC) boilers

with 200 bar serving as the dividing line.

Based on Circulation

1.

Natural circulation or drum-type boilers

2.

Forced circulation boilers

3.

Once-through (OT) or no-drum boilers

Types of Boiling:

Boiling is the addition of latent heat at constant temperature. It is of two types;

1.

Nucleate Boiling

2.

Film Boiling

Consider heating water in a pan. When the heating is relatively low, bubbles form at the bottom and rise

up, carrying heat with them. The bottom surface is always covered by the water film, which keeps the

surface cool and close to the liquid temperature. This is nucleate boiling, and all efforts are made to stayin this regime in a boiler as the water absorbs its latent heat.

When the heating is too high and heat flux is large, the water film at the bottom quickly evaporates and

is replaced by a steam film with low conductance. As a result, the surface is no longer cooled adequately

and tends to attain a temperature close to the heat source, becoming severely overheated in no time.

This is film boiling, which must be avoided in boiler practice under all conditions. Water moving at


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velocities of 23 m/s at the entry to a tube can effectively prevent i lm boiling, such as in the evaporator

surfaces of boilers.

Working of Water Tube Boiler

It consists of mainly two drums, one is upper drum called steam drum other is lower drum called muddrum. These upper drum and lower drum are connected with two tubes namely down-comer and riser

tubes as shown in the picture. Water in the lower drum and in the riser connected to it, is heated and

steam is produced in them which comes to the upper drums naturally. In the upper drum the steam is

separated from water naturally and stored above the water surface. The colder water is fed from feed

water inlet at upper drum and as this water is heavier than the hotter water of lower drum and that in

the riser, the colder water push the hotter water upwards through the riser. So there is one

convectional flow of water in the boiler system.


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Working of Fire Tube Boiler

In fire tube boiler, the fuel is burnt inside a furnace. The hot gases produced in the furnace then passes

through the fire tubes. The fire tubes are immersed in water inside the main vessel of the boiler. As the

hot gases are passed through these tubes, the heat energy of the gasses is transferred to the water

surrounds them. As a result steam is generated in the water and naturally comes up and is stored upon

the water in the same vessel of fire tube boiler. This steam is then taken out from the steam outlet for

utilizing for required purpose. The water is fed into the boiler through the feed water inlet.

Cochran boiler is a type of fire tube boiler. Its major auxiliaries are discussed below;

1. Pressure Gauge: This indicates the pressure of the steam in the boiler.

2.

Water Level Indicator: 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 overheated and the tubes

may burn out.

3.

Safety Valve:The function of the safety valve is to prevent the increase of steam pressure in

the boiler above its design pressure. When the pressure increases above design pressure, the

valve opens and discharges the steam to the atmosphere. When this pressure falls just below

design pressure, the valve closes automatically. Usually the valve is spring controlled.

4.

Fusible Plug: If the water level in the boiler falls below a predetermined level, the boiler shell

and tubes will be overheated. And if it is continued, the tubes may burn, as the water cover will

be removed. It can be prevented by stopping the burning of fuel on the grate. When the

temperature of the shell increases above a particular level, the fusible plug, which is mounted


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over the grate as shown in the figure melts and forms an opening. The high-pressure steam

pushes the remaining water through this hole on the grate and the fire is extinguished.

5.

Blow-off Cock: The water supplied to the boiler always contains impurities like mud, sand and,

salt due to heating, these are deposited at the bottom of the boiler, and if they are not

removed, they are accumulated at the bottom of the boiler and reduce its capacity and heat

transfer rates. Also the salt content will goes on increasing due to evaporation of water. These

deposited salts are removed with the help of blow off cock. The blow-off cock is located at the

bottom of the boiler as shown in the figure and is operated only when the boiler is running.

When the blow-off cock is opened during the running of the boiler, the high-pressure steam

pushes the water and the collected material at the bottom is blown out. Blowing some water

out also reduces the concentration of the salt. The blow-off cock is operated afterevery 5 to 6

hours of working for few minutes. This keeps the boiler clean.

6. Steam Stop Valve: It regulates the flow of steam supply outside. The steam from the boilerfirst

enters into an ant-priming pipe where most of the water particles associated with steam are re-

moved.

7.

Feed Check Valve: The high pressure feed water is supplied to the boiler through this valve.This

valve opens towards the boiler only and feeds the water to the boiler. If the feed water pressure

isless than the boiler steam pressure then this valve remains closed and prevents the back flow

of steamthrough the valve

Once Through Type Boilers:In an OTU type boiler, all water entering the furnace enclosure tubes is

evaporated into steam as the fluid makes its pass through the furnace circuitry as shown in the figure.

Thus, the circuitry is referred to asonce-through. The steam flow rate is established by the feed water

pump and the superheat steam temperature is determined primarily by the fuel firing rate.


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Pros and Cons of Once Through Boiler

1.

The once through boiler has high load response characteristics due to the fact that it does not

have a drum and hasa much lower water inventory.

2.

Once through boilers of super-critical pressure boilers have higher efficiency. However in the

sub-critical range there is no difference in efficiency when compared to the drum type design.

3.

Generally the power consumption is higher by 5 to 8 % for the same capacity boilers of drum

type.

4.

Once through boilers have a separate start-up loop along with all its controls.

5.

As the load demand is met by varying both fuel and feed water flow simultaneously, the

controls are more sophisticated and have to be more reliable.

6.

The water quality level is much more stringent than drum type boilers.

7.

A closer regime of operation is expected in once through boilers.

8.

Once through boilers life time cost is expected to be more than the drum type units.

Types of CirculationIn water tube boilers the water is circulated in the water tubes in either Natural Circulation or Forced

Circulation. Both cases are discussed below;

Natural Circulation:Natural circulation relies only on the density difference between the saturated

water in the down comer and the steamwater mixture in the heated tubes.For drum pressures up to

211 bar (3000 psia), where the density differential between water and steam is still 2.5 times, natural

circulation has been employed. Natural circulation is the most common type adopted for subcritical

pressures because of its simplicity; no pump or associated equipment are required moreover they

exhibit self-limiting characteristics.

Forced Circulation:In this type of circulation a pump is used to circulate the feed water in the water

tubes of a boiler. Forced circulation is employed mostly in waste heat boilers to ensure adequate flow at

all conditions with low heat flux, rapidly fluctuating heating/cooling requirements and horizontal

evaporators.

Boiler Auxiliaries

Steam Drum

A steam drum is a standard feature of awater-tube boiler.It is a reservoir of water/steam at the top end

of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase

separator for the steam/water mixture.The difference in densities between hot and cold water helps in

the accumulation of the "hotter"-water/and saturated-steam into the steam-drum.
http://en.wikipedia.org/wiki/Water-tube_boilerhttp://en.wikipedia.org/wiki/Vapor-liquid_separatorhttp://en.wikipedia.org/wiki/Vapor-liquid_separatorhttp://en.wikipedia.org/wiki/Water-tube_boiler


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Types of Steam Drum:

Baffle Plate Separators: Deflection baffles or baffle-type separators as shown in the figure are inertial

separators that utilize the change of direction to separate steam and water.

Cyclone Separators: Cyclone separators in contrast, work on the principle of centrifugal separation and

are used for all pressures because the action is more positive. The steamwater mixture enters the

cyclone tangentially at68 m/s. The density difference throws the water particles to the periphery

and keeps the steam at the center. The steam travels upward, passing through a perforated baffle

pyramid, where it is decelerated and spread across the whole area of the steam drier, which provides

additional fine separation.


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Super Heater:

The steam that is produced in the boiler has a certain percentage of moisture content. This moisture

content is referred to as the quality of the steam. Due to the high velocities of the steam inside a

turbine, the moisture content of the steam can shear the turbine blades. A super-heater is utilized to

remove the moisture content in the steam. This is done by raising the temperature above while keeping

the pressure constant. Steam which undergoes this process is referred to as superheated steam. The

degree of superheat is a term which is used to describe the temperature difference between the raised

temperature and the temperature at constant pressure.

Types of Super Heater

On the basis of Design Super Heater has following three types;

1.

Parallel and counter flow

These flows are based on the overall direction of steam flow with respect to gas flow.

The section is in parallel flow when the two flows are in the same direction and in counterflow

when in opposite direction.

2.

Radiant and convective

This function is based on the type of heat transfer that is predominant. Superheaters in a

furnace are radiant. They are in the form of platens or wing walls. With increasing load, the

temperature of the steam in a radiant surface reduces unlike on a convective surface, where the

steam temperature rises with load. This is because the radiation incident is not much lower than

that at full load, whereas the steam low reduces proportionally, thereby absorbing greater heat

and becoming hotter at low loads. The converse is true for convective surfaces. Both radiant and


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convective Super heaters are necessary to attain the high steam temperatures needed. Their

characteristics can complement each other to obtain a fairly constant temperature at all loads.

3.

Single and Multi-pass

Steam should also be viewed as a coolant for Super heater coils. Adequate velocity of steam is

essential to remove the heat and keep the metal temperature in control. In other words, the

steam film on the inside of the tube has to be as thin as possible so that the tube metal stays

close to the bulk steam temperature. At the same time high steam flow contributes to an

excessive steam pressure drop. 2030 psi pressure drop between headers is recommended to

prevent flow reversal at low loads and to avoid unnecessary load on feed pumps. To achieve this

end, tubes may have to be arranged in parallel in several loops along the gas flow as shown in

Figure with a two-loop arrangement. In addition, across the gas flow, steam may be carried in

one or several parallel passes.


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Attemperator

Attemperation is the desuperheating of main or reheat steam to control the final temperatures to the

set limits. It has two types;

1.

Spray Allemperator: In a spray attemperator demineralized deaerated water of high purity is

sprayed in a fine mist which evaporates and cools down the temperature of the steam but all

the impurities of the spray water are introduced into the final steam.

2. Surface/Drum Type Attemperator: A surface attemperator is a shell and tube heat exchanger

(HX) with overheated steam passing through the body and Feed Water through the coils.

Another popular surface type is the drum-type attemperator where steam flows through the

coils immersed in the water drum.


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Economizers

Economizer surfaces transfer heat from flue gases to

pressurized and sub-cooled Feed Water on its way to the

drum. Water in ECONs should always flow upward to allow

the steam bubbles to ascend smoothly. In the downward

flow, steam bubbles can cause flow stagnation and disrupt

the operation. In cases when downward flow is compulsory

due to layout, adequate pressure drop is created by inserting

ferrules for flow equalization. Its a heat exchanger as shown

in figure.

Air Pre-Heater

The air-heater is usually the last heat trap in the boiler. Air-

heater exit gas temperature should be higher than the

corrosion limit of the Air-Heaters internals and the

downstream equipment. Air-heaters generally operate in a

temperature range of 450120C on the gas side and 150

450C on air side.


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Fire Tube Boiler VS Water Tube Boiler- which one to choose in High

Pressure RequirementTwo types of stress act on pressure vessels; one is radial stress and the other one in hoop stress.

Experimental and Theoretical data shows that hoop stress is greater than radial stress. So we are

considering hoop stress in our selection of type of boiler for high pressure utility. Moreover, we know

that magnitude of hoop stress in much larger in case of external pressurization as compared to themagnitude of hoop stress in result of internal pressurization.

We know that fire tube boilers are externally pressurized and the water surrounding the fire tubes

exerts pressure on the tubes. That pressure may cause huge hoop stress in the tubes which may exceed

the compressive strength of the tube, thus causing failure. On the other hand, water tube boilers are

internally pressurized. Water exerts pressure on the inside surface of the tubes that causes hoop stress

which are less in magnitude as compared to stress caused by external pressurization. Thus there is less

likelihood of failure of water tube boiler as compared to fire tube boiler for same value of pressure

applied.

There is a limitation in fire tube boiler that the fire tubes must be submerged all the time in the water.

To accomplish this, we need a bigger radius boiler shell. But if we increase the radius of shell, the value

to hoop stress also increases keeping value of pressure constant. To counter that increase in hoop stress

value, we can increase the shell thickness. But there are manufacturing limitation to fabricate thick wall

vessel. On the other hand, we do not face any limitation regarding placement of water tubes in water

tube boilers.

Thirdly, we need to support the fire tubes in the fire tube boiler through flat plate tubesheets. But the

immense pressure of surrounding water will deform the flat plate tubesheet into hemispherical shape.

This can cause failure, which is un-acceptable. On the other hand, we can support the water tubes inwater tube boilers in any manner we can, because the supporting tubesheets will not face any pressure

on them, as water tubes are outside the water drum.

Thats why we will choose water tube boiler for high pressure steam generation.

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