1 Boilers Mms

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UTILITY STEAM GENERATOR


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BOILER

Process equipment that

are use to generate

steam

Closed vessel in whichwater, under pressure, is

transformed into steam

by the application of

heat


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Three factors required to generate steam:

1) A suitable vessel or container to hold thewater, transfer the heat to the water, and

collect the steam.

2) Water is heated above its boiling point andforms steam.

3) Heat is needed to change water into

steam and the most common sources forgenerating heat are oil, gas, and coal.


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FUEL: oil, gas and coal

AIR: oxygenHEAT: ignition

Proper adjustment to

produce an efficient flame

that has a BLUE COLOR


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HEAT TRANSFER MODE IN THE BOILER:

Radiation

Convection

Conduction


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Types Of Steam Saturated Steam

Steam at temperature of boiling point of water which

corresponds to its pressure.

Superheated Steam

Steam heated top a temperature higher than the

boiling point temperature corresponds to its pressure.It can not exist in contact to water and resemble asperfect gas.


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Water-tube boilersFire-tube boilers


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Fire-tube boilers

1. Exit Gases

3. Water inlet

2. Steam Outlet

4. Combustion Chamber

5. Tubes

1 2 3

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Rely on hot gases circulating through the boiler inside tubes that are

submerged in water.

These gases usually make several passes through the tubes.


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ADVANTAGES OF FIRE-TUBE BOILERS:

Relatively inexpensive

Easy to clean

Compact in size

Easy to replace tubes

Well suited for space heating and industrial process applications

DISADVANTAGES OF FIRE-TUBE BOILERS:

Not suitable for high pressure applications 250 psig and above

Limitation for high capacity steam generation


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Fire tube boiler has an internal combustion chamberwithin a vessel containing the water.

A series of horizontal tubes pass through the vessel

and are surrounded by water to bring the hotcombustion gases in contact with the water.

The hot gases are directed back and forth through the

tubes to use as much heat as possible before venting

through an exhaust.

Steam is formed as a result of contact with the tubes

and thus rises to the top of the vessel and is ready for

designated use.


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The Steam Drum


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ADVANTAGES OF WATER-TUBE BOILER:

Available in sizes that are far greater than the firetube design.

Able to handle higher pressures up to 5,000 psig

Recover faster than the fire-tube Have the ability to reach very high temperatures

DISADVANTAGES OF WATER-TUBE BOILER:

High initial capital cost Cleaning is more difficult due to the design

No commonality between tubes

Physical size may be an issue


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Steam bubbles are formed in the heated riser.

The resulting steam water mixture is lighter than the

solid column of water in the unheated portion of the

tube and is, therefore, displaced and steam is releasedinto the steam drum.

In the drum, the bubbles rise to the waters surface and

steam is released into the vapor space.

This circuit is made over and over again as new

feedwater enters the drum at the same rate that steam

is produced from the drum.


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1Steam drum;

2Water drum;

3 Furnace;

4

Boiler bank tubes;

5 Feeders and risers tubes;

6Superheater;

7Desuperheater;

8 Economizer;

9

Insulation, lagging and refractories.

MAIN PARTS OF USG


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PO4 Dosing SH St

WATER DRUM

STEAM DRUM

SUPERHEATER

ECONOMIZER

BFW PUMP


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Some Important Boiler Problems:

SCALE LOSS OF FEED WATER

TUBE BREAKAGE SOOT


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Boiler Scale Formation

Scale is formed on boiler tube heating surfaces bythe depositing of impurities in the feed water in theform of a more or less hard adherent crust. Suchdeposits are due to the fact the water loses itssoluble power at high temperatures or becausethe concentrations become so high, due toevaporation, that the impurities crystallize andadhere to the boiler surfaces


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Effect of Boiler Scale

1/8-inch (3mm) of scale can cause a 2.0-3.0%loss in fire-tube boilers and water tube boilers.

A second but more serious effect from scale isthe overheating of boilertube metal, causing

eventual tube failure. In modern boilers with highheat-transfer rates, even extremely thin layers ofscale will cause a serious elevation in thetemperature of tube material


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Boiler Scale


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Corrosion Related Problems

Corrosion is due to the acidity/or oxidizingproperties of the feed water

Pitting Corrosion

Which consists of isolated spots of activecorrosion

caused by Oxygen attack

General CorrosionProduced by an acid attack mainly due to thepresence ofCO2 which dissolve in water

forming

carbonic acid (H2CO3)


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Pitting Corrosion

Dissolved oxygen interacts with boiler componentsurfaces, forming pits on the metal surface.

These pits may eventually grow large enough topenetrate the metal, forcing a boiler shutdown.


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Pitting Corrosion


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Treatment Processes

External treatment

Internal treatment


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Internal Treatment

1-Sodium Phosphate

It buffers the boiler water pH to minimize the

potential for boiler corrosion.

It precipitates small amounts of calcium or

magnesium into a soft deposit which can then

accumulate in mud drums or steam drums

rather than as hard scale. It helps to promote the protective oxide film

on boiler metal surfaces


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This type of program causes sludge to formand therefore requires continuousblowdown. Deposition problems are

possible in the higher heat transfer areas ofthe boiler. On the positive side, phosphate-polymer programs can essentially handleany feed water hardness and be cost

effective. This is particularly true of thenew synthetic polymers.

2-Phosphateand Polymers


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BlowdownTreatment

Blowdown is a very important part of anywater treatment program. Its purpose is tolimit the concentration of impurities in theboiler water.

The right amount of blowdown is critical: toomuch results in energy loss and excessivechemical treatment cost; too little andexcessive concentrations of impurities build

up. It can range from 1% (based on feedwater

flow) to as much as 25%.


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By definition, bottom blowdown is intermittentand designed to remove sludge from the areas ofthe boiler where it settles. The frequency of bottomblowdown is a function of experience and plantoperation. Bottom blowdown can be accomplishedmanually or electronically using automaticblowdown controllers

1-Bottom Blowdown


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Frequently used in conjunction with manualblowdown, continuous blowdown constantly removesconcentrated water from the boiler. By design, it is inthe area of highest boiler water concentration. Thispoint is determined by the design of the boiler and isgenerally the area of greatest steam release.

Continuous blowdown allows for excellent controlover boiler water solids. In addition, it can remove

significant levels of suspended solids. Anotheradvantage is that the continuous blowdown can bepassed through heat recovery equipment.

2-Continuous Blowdown


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For the sake of clarity, certain definitions arerequired:

1. Steam Purity-The amount of solid, liquid orvaporous contamination in the steam. Steampurity is normaily reported as total solids in partsper billion (ppb).

2. Steam Quality-The amount of moisture in thesteam. It is the weight of dry steam in the mixtureof steam and water droplets. It is reported as apercentage.

Carryover


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Carryover is generally considered to be anycontaminant that leaves a boiler steam drum withthe steam. It can be in solid, liquid or vaporous

form. With higher operating pressures, highersuperheat temperatures and the need for puresteam in certain processes, greater emphasis isplaced on controlling the factors that minimizecarryover.

Carryover Problem


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Some of the effects of carryover are:

1. Deposition in regulators and valving

2. Deposition in superheaters3. Deposition in control valves and turbines

4. Process contamination

Carryover Effects


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PROTECTION AND SECURITIES

BOILER START-UP \ SHUTDOWN AND PROTECTION SYSTEM IS KNOWN AS

BMS (BURNER MANAGEMENT SYSTEM)

PROTECTIONS

LOW WATER LEVEL (ALARM / TRIP).

LOW FUEL GAS PRESSURE.

HIGH FUEL GAS PRESSURE.

FLAME DETECTORS OR FLAME FAILURE.

LOW AIR FLOW.

FD FAN FAILURE (LOW SPEED)

LOW INSTT. AIR PRESSURE.

STEAM DRUM HIGH PRESSURE.

CPU OR I-O FAILURE.

SAFETY VALVES.

HIGH FURNACE PRESSURE.

OTHER INTERLOCKS ( DEPENDS UPON LIMITATION/ DESIGN FEATURES )

Documents

1 Boilers Mms