Boiler Mountings BS-ES 3

Definition :

various valves and fittings are required for the safe and proper working of a

boiler . Those attached directly to the pressure parts of the boiler are

referred to as the boiler mountings.

Minimum requirements for boiler mountings

two safety v/v's

one stm stop

two independent feed check

two water gauge or equivalent

one pressure gauge

one salinometer v/v or cock

one blowdown/scum v/v

one low level fuel shut off device and alarm

Functions

SAFETY V/V-

protect the boiler from over pressurization. DTI require at least two safety

v/v's but normally three are fitted ,two to the drum and one to the

superheater. The superheater must be set to lift first to ensure a flow of

steam through the superheater.

These must be set to a maximum of 3% above approved boiler working

pressure.

MAIN STM STOP-

mounted on supherheater outlet header to enable boiler to be isolated

from the steam line if more than one boiler is connected. V/v must be

screw down non return type to prevent back flow of steam from other boiler

into one of the boilers which has sustained damage (burst tube etc) v/v

may be fitted with an emergency closing device.

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AUXILIARY STOP V/V-

similar to main stops but connected to the auxiliary steam line

FEED CHECK V/V'S-

a SDNR v/v so that if feed p/p stops the boiler water will be prevented

from blowing out the boiler. The main check is often fitted to the inlet flange

of the economizer if no economizer fitted then directly connected to the

boiler. The Auxiliary feed check is generally fitted directly to an inlet flange

to the drum with crossovers to the main feed line. Usually fitted with

extended spindles to allow remote operation which must have an indicator

fitted.

WATER GAUGES –

usual practice is to fit two direct reading and at least one remote for

convenient reading.

PRESSURE GAUGES-

fitted as required to steam drum and superheater header

SALINOMETER COCKS OR V/V'S-

fitted to the water drum to allow samples to be taken. Cooling coil fitted for

high pressure boilers.

BLOWDOWN COCK-

used to purge the boiler of contaminants. Usually two v/v's fitted to ensure

tightness . These v/v's lead to an overboard v/v.

SCUM V/V-

These are fitted where possibility of oil contamination exists. They are

designed to remove water and/or contaminants at or close to normal

working level.

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Smoke tube Boilers

This style of boiler still see active service were low quantities of

low quality steam are required, such as for cargo and fuel tank heating

when in port.

This style of boiler is relatively cheap, supplied as a packaged

unit and requires less stringent feed water conditioning and level control.

Design

Consists of a shell wrapper plate to which is welded (or for later designs

riveted, end plates.. Pressure is naturally container in the shell plate due to

is cylindrical design. The flat end plates, however, must be 'stayed' to

prevent buckling and distortion.

The combustion chamber is of similar section and is also

'stayed'.

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The boiler shown above is a single furnace, two pass design.

Larger boilers may have multiple furnaces and have multiple passes by

replacing the exhaust stack with a return chamber and fitting another bank

of tubes.

Fuel is combusted in the corrugated watercooled furnace. The corrugations

increase the surface area and allow a degree of flexibility to allow for

expansion and contraction.

The hot gas passes to the water cooled combustion space

though to the smoketubes. The upper portion of the combustion chamber

lies close to the water level and is therefore liable to distortion due to in

correct water level maintenance.

Access to the boiler is via a manhole door on the upper shell

plate. In addition a smaller door may be fitted below the furnace to allow

inspection and scale/sludge removal.

Modern (packaged) boiler

This style of boiler may be fitted to the vessel as a complete unit with its

own fuel and water delivery systems, control and safety equipment

mounted directly on the unit. Alarms and shut downs may be are given at

the local control panel which may be interfaced with ships alarms system to

allow UMS operation..

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The design is simialr to the scothch boiler other than the

combustion chamber which requires no stays. This design is a three pass

design.

Operation

Although the maintenance of the water level is not so critical as with water

tube designs, it should not be allowed to fall too much as overheating of

furnace and combustion spaces leads to catastrophic failure due to

component collapse. The content of the boiler is then expelled via the

furnace door.

Similarly , although water treatment is not so critical scale must

not be allowed to build up which can lead to overheating of material

WATERTUBE PACKAGE BOILERS

Watertube package boilers are classified as natural-circulation and

forced-circulation types. Natural-circulation boilers can be subdivided into basic

designs called "O," "A," and "D," according to their configuration (Fig. 4.2).

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Fig. 4.2 Natural-Circulation Watertube

Boilers

"A " type boilers are symmetrical. A longitudinal steam and water

drum located at top center runs parallel to two smaller mud drums which are

equidistantly spaced from the center line at the lower portion of the structure.

The burner is located at the centerline of the boiler and fires down the

combustion chamber. At the other end of the boiler furnace, the combustion

gases reverse their direction and pass convection banks.

"O" type boilers consist of two parallel longitudinal drums located at

the centerline of the boiler. The burner is located between the two drums at the

centerline and fires down the combustion chamber. The combustion gases

reverse their direction at the other end of the boiler and pass by convection

tubes.

"D" type boilers, do not have a symmetrical configuration. The steam

and water drum are located longitudinally along one side, directly above the

mud drum. The burner and combustion chamber are located on the side of the

convection banks. The combustion gases reverse their direction at the opposite

end of the boiler and return to the main convection tubes.

In all three designs, superheaters can be installed at the end of the

combustion chamber. In forced-circulation watertube boilers, water is pumped

under pressure into tubular coils, while combustion gases are forced over these

tubes.

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Components

Steam drum

In the early designs the drums were riveted or solid forged from a single

ingot, but for modern boilers the drum is generally fabricated from steel

plate of differing thickness and welded. The materials used are governed

by classification society rules. Test pieces must be provided.

The cylindrical drum is normally constructed from four plates.

Two dished End plates, a thick wall tube plate ( thicker to accommodate

the holes drilled in it without increased stress) and completed with a thinner

wrapper plate.

Construction takes the form of rigidly clamping the descaled,

bent wrapper and tube plates together. In addition test pieces cut from the

original material are attached to the construction in such away that the

longitudinal weld extends either sided of the join. These pieces are later

removed and shaped test shapes cut out from specified areas including

across the weld.

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The longitudinal weld is critical ( taking twice the circumferential

stress) and is normally carried out by specialized automatic machinery

using submerged arc techniques.

The dished end pieces are accurately aligned and welded.

On completion the construction is cleaned and non-destructive

testing- such as x-ray photography, carried out. Final machining is carried

out and any stub pieces and doublers attached. The now complete drum is

heat treated at 600 to 650'C.

The final process is hydraulic testing to classification

requirements. Natural circulation within a boiler is due to the differing

specific gravities of the water at the differing temperatures, the steam drum

provides a reservoir of cool water to give the gravitational head necessary

for natural circulation. Cool water entering the steam drum via the feed

lines provides the motive effect for the circulation distributing it to the

downcomers.

Also the space within the drum provides for the separation of the

steam and water emulsions formed in the water walls and the generating

tubes. Water droplets entrained with the separated steam are removed by

separating components fitted in the drum as well as the perforated baffle

plates fitted at the water line.

The space above the water line provides for a reserve steam

space needed to maintain plant stability during manoeuvring conditions.

Also fitted are the chemical injection distributing pipe and the

scuming plate.

The smaller the drum is made, the less thickness of material that

is required. However, the limitation to how small is that sufficient space

must be allowed for the separation of water from the steam before passing

out to the superheater space otherwise dryers must be used. Also, due to

the smaller reserve of water, larger fluctuations in water level occur during

manoeuvring.

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Water drum

Distributes feed water from the downcomers to the headers and generating

tubes. Provides a space for accumulating precipitates and allows them to

be blown down.

Water drum size is limited to that required to receive the

generating tubes, for modern radiant heat boilers with only a single bank of

screen tubes and no generating tubes between the drums, the water drum

has been replaced by a header and the downcomers fed straight to the

waterwall headers. With system blow down is done at the steam drum. Too

small a water drum can cause problems of maintaining ideal water level

and little steam reserve

Headers

These have a similar purpose to the water drum but are smaller in size.

Due to their reduced size they may have a square cross section without

resorting to exceptional thickness. .

Generating tubes

Consists of a large number of small diameter tubes in the gas flow, more

commonly found in boilers of an older design

For roof fired boilers the generating bank may consist of one or

two rows of close pitched tubes. For a modern radiant heat boiler the

generating bank has been omitted to allow the replacement of the water

drum by a distribution header, a bare tube economizer is fitted generating

5% of the steam capacity. The generation bank is normally heated by

convection rather than radiant heat.

For a set water circulation the tube diameter is limited to a

minimum as the ratio of steam to water can increase to a point where the

possibility of overheating could occur due to the lower heat capacity of the

steam.

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The number of tubes is limited to prevent undercooling of the gas

flow leading to dew point corrosion

Screen tubes

These are larger bore tubes receiving the radiant heat of the flame and the

convective heat of the hot gasses. The large diameter keeps the

steam/water ratio down hence preventing overheating. There main duty is

to protect the superheater from the direct radiant heat. On a modern

marine radiant heat boiler the screen wall is formed out of a membrane

wall

Downcomers

These are large diameter unheated i.e. external to the furnace, their

purpose is to feed water from the steam drum to the water drum and

bottom headers.

Riser/Return tubes

These return steam from the top water wall headers to the steam drum.

Superheater tubes

These are small diameter tubes in the gas flow after the screen tubes. Due

to the low specific heat capacity of the saturated steam they require

protection from overheating in low steam flow conditions, say when

flashing.

Superheater support tubes

These are large diameter tubes designed to support part of the weight of

the superheater bank of tubes.

Material requirements

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Tube temperatures for the water cooled sections is considered to be

saturation temperature plus 15oC. Solid drawn mild steel is generally used.

Tube temperatures for convection superheater sections is

considered to be final superheat temperatures plus 30oC. For Radiant heat

a higher temperature is considered.

For Superheater tubes operating above 455oC a Chrome

Molybdenum alloyed steel is required.

Advantages and disadvantages of watertube boilers

Advantages

Savings in weight of about 3:1 for a comparable heating

surface area

Possibility of using higher temperatures and pressures without

unduly increasing wall thickness increases plant efficiency.

More efficient combustion space allowed

Greater flexibility of the structure and rapid circulation prevents

the problems of thermal stressing in the tank boilers which

leads to grooving. In water tube boilers roof and floor tubes are

sloped at 15' to ensure circulation

thinner tube materials allow rapid steam raising and faster heat

transfer rates

Saving in space for same steaming rate

Wider safety margins- limited tube diameters and protected

drum surfaces mean failure in tubes releases a flow of steam

dependent on tube diameter

Thin tubes are easier to bend, expand and bell mouth

Disadvantages

Lower reserve of water means a more efficient water level

control is required

High quality feed required , little allowance to corrosion

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SAFETY VALVES

At least two safety valves have to be fitted to the boiler. They

may be both mounted on a common manifold with a single connection to

the boiler. C= 30 for full bore relay operated safety valves

LIFT PRESSURE

The safety v/v must be set at a pressure not exceeding 3% of the approved

boiler working pressure. It is normal to set the suphtr safety below that of

the drum to ensure an adequate flow of stm for cooling purposes under

fault conditions. Similarly the superheater should be set to close last.

10% ACCUMULATION OF PRESSURE RULE.

With all the flames in full firing the stm stop is closed, the boiler pressure

must not increase by more than 10% in 7 minutes for water tube of 15 mins

for tank boilers with the safety lifted. this is normally waivered for

superheater boilers. Instead calculations and previous experience used.

SETTING

Must be set with the surveyor present except when on the waste heat unit.

A chief engineer with three years experience may then set the safety valve

but must submit information to surveyor for issue of certificate.

Superheated steam safety valves should be set as close to operating

temperature as possible as expansion can alter the relationships between

valve trim and guide/nozzle rings which can effect the correct operation of

the valve.

a. Two safety valves- each set independently

b. Each safety valve must release entire steam flow in

pressure accumulation test

c. Surveyor uses specially checked gauge

d. One valve gagged

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e. valve initially set to approximately the correct position

then steam pressure increased to set pressure

f. adjust valve to lift

g. raise and lower pressure to check

h. fit locks to both valves on completion

Easing gear to be checked free before setting valves. Steam

should not be released as this can damage seat.

Condencer:

Air must be removed from the condenser because;

-it dissolves in water

-it destroys the vacuum

-poor conductor of heat and forms a thin film on pipes

-increases under cooling due to the following

circumstances

Deaerator

In marine water tube boilers it is essential to keep water free of dissolved

gases and impurities to prevent serious damage occurring in the boiler

however, it is recommended for boilers operating above 30 bar and essential for those operating above 42 bar that a dearator be fitted.

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Feed water Tank( observation tank)

A well designed feed tank should be designed to minimize the oxygen

within the feed system. This is especially important with open feed

systems.

The following are taken as parameters for a well designed tank

Adequate ventilation

Cold water make up enters at highest point .

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Vent Water from cond.

O/F

Drain

strainer

Water supply

Feed water to boiler

Tank to have sufficient volume of water at normal working

level to allow for 1 hours operation at maximum demand.

Take off to feed pumps to be at least 75mm from tank

bottom

Tank to be located to provide head requirements at normal

working level for feed pumps

Boiler Control

For the burner control to be fully automated transducers must be

fitted to monitor such things as fuel and air flow, steam pressure and water

level. Steam pressure is the main controlling parameter and the controller

will adjust fuel to maintain this at a set level .

The water level is monitored and kept within a certain range.

Should the level become too low then firing is ceased. Automated

restarting after this trip is not allowed and a manual reset must be

operated. A high level shut off may also be fitted by tradition tripping the

feed supply pumps.

When flashing the fan will operate for a certain period at full flow

to purge the furnace of gasses. The spark relay operates and the fuel

solenoid opens. A period is allowed for ignition. If the photocell fails to

detect a flame at the end of this set period the fuel solenoid is shut down.

Depending upon set up a second attempt may be made or an alarm output

signal will be generated.

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Boiler Water Treatments

Objective of water treatments :

1. Keep slightly Alkaline and remove of Oxygen and Carbon Dioxide

2. precipitate scale-forming salts that may be in solution

3. keep precipitate in non adherent sludge form so they can easily removed by blow down

4. maintain TDS and solids in suspension form

External treatments ;

1. Clarification

2. Filtration

3. Softening

4. Deaerating

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5. Heating

Internal treatment:

1. Use of sludge conditioning agent

2. Use of antifoaming agent

3. Use of oxygen remover

How to reduce effect of fouling on boiler:

Gas side fouling :

1. use of wide tube pitch

2. regular soot blowing

3. water washing at 60 c.

4. maintain good combustion

Water side fouling

1. maintain feed water correct temperature< 80c

2. maintain hot well temperature to prevent oxygen

3. venting while starting and continuous blow down

4. clean by chemical cleaning

Impurities found in boiler water

Dissolved solids - These are substances that will dissolve in water. The principal ones are the carbonates and sulphates of calcium and magnesium, which are scale-forming when heated.

Suspended solids - These are substances that exist in water as suspended particles.

Dissolved gases - Oxygen and carbon dioxide can be readily dissolved by water.

Scum forming substances - These are mineral impurities that foam or scum.

Effect of the impurities on boiler operation:

1- Ingress of seawater

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Mgcl+H2O =mg(OH)2 + HCL (corrosion)

2-Scale formation

1. Hard scales (prevented by treatments and maintaining feed water temperature bellow 80c )

CaSo4

Ca an Mg Silicates

2. Soft Scales (sludge , removed by blow down )

Calcium carbonate

Mg hydroxide

Result of scales:

Reduce efficiency of boiler due to poor heat transfer

Poor heat transfer causes overheating of tubes-(distortion and failure )

3. pH value

the solution will be alkaline if pH value between 8 and 14 and if between 0-6the solution acidic , if ph value 7 the solution neutral.(maintain boiler ph 10.5-11.5)

Boiler Safety and shut down devices:

Shut down and alarms due to :

extra low water level

high water level

ignition failure

high steam pressure

Alarms due to :

low water level

low steam pressure

low fuel pressure

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low feed pressure

salinity high

low feed tank level

Burners:

main burner or burners pilot burner

main burner could be one or two or more depend on the size of the boiler, and normally for big boilers operates on fuel oil and many types of burners used in marine applications using utilizing compressed air and steam jets for atomizing the fuel.

Pilot burner normally small burner used to ignite the boiler first then change over occurred after ignition maintained usually operates on diesel oil.

Boiler operation systems:

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