Circulation Mech stress Control

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Control calculations

Heat Engines & Boilers


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Contents

Control calculations:

Heat transfer calculation by radiation and convection

has already been discussed

Fluid flow calculations- pressure drop

- natural circulation

Mechanical and theral stress calculations


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!enerally you have to a"e control calculation:

- at the a#iu load $full load% and

- at the iniu load of the boiler

But you always have to ta"e into account if there

is other critical situation at operation 'ou have to control every possibility of critical

situations to avoid crash and industrial accident

(t is necessary to ta"e into account every situationduring the wor" of the boiler) fro start till stop

*top of boiler operation can be- noral stop- eergency stop


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+he e#act calculation you can a"e if you "now the e#actconstruction

,t the design you can a"e appro#iate calculations todeterine sies) considering

- cobustion process

- heat transfer

- flue gas flow- water.stea flow

- pressure drop

- echanical stress

- theral stress

- soot/ash coating and deposit foration) etc

,fter the deterination of the construction you have to a"econtrol calculations

(f the result of the control calculation does not acceptable) youcan change the construction and you can a"e the controlcalculations again


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Hydraulic fluid flow calculations

Hydraulic fluid flow calculations can bedone after heat transfer calculation) because

results of it are needed) eg velocity)

teperature) density etc

Main calculation groups:

0ressure drop calculation

Boiler circulation


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Pressure drop calculation

Fluid flow in boilers in ost of the cases turbulent0ressure drop calculation can be done according toBernoullie1uation:

+= d

lwp 2

2

30a4

where: density 3"g/54w velocity 3/s4

l length 34

d e1uivalent diaeter 34

dynaic loss coefficient 3-4 resistance factor 3-4

(n case of turbulent flowBlasius rule can be used:

6 7589 / e72;

e 6 wd/ 3-4Reynoldsnuber %


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BOILER CIRCULATION

+o reove heat fro the boiler surfaces) it is necessary

that ade1uate and positive water and stea circulation beprovided $in a predeterined direction% throughout theboiler circuits +he flow of water) stea) or other fluidwithin the boiler is called circulation


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Flowtypes in !oilers

a =atural circulation

b Forced circulationc >nce through $Benson type%

d >nce through with separator $*uler type%


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Natural circulation

=atural circulation curve and specific weight variation


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"ensity relations#ip of stea$ and water $i%ture


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"ifferent for$s of !oilin& in #ori'ontal tu!es


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"ifferent for$s of !oilin& in (ertical tu!es


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"ifferent

for$s of

!oilin&

#eattransfer

and wall

te$perature(ariation


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Circulation ratio

+he ratio ass flow of the water fed to the stea-generating tubesto the stea actually generated by the boiler is

called the circulation ratio ?6 cir"/steaout

(n a once-through positive circulation boiler thecirculation ratio is 8:8

(n a recirculation boiler) the circulation ratio canbe fro 5:8 up to 27:8

+he typical circulating rate is ;:8 *tea content of return i#ture is reciprocate of

circulation ratioFor e#aple in case of ?6;) # 6 8/; 6 72


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Pressure !alance in circulation cur(e

0ressure at botto distributor chaber:

pdc6 pdru@ water g H - plossdowncoer 6

6 pdru

@ i#ture

g H @ plossriser

earranging this e1uation

$water- i#ture%g H 6 plossriser @plossdowncoer

pforce 6 presistance


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Circulation nu$!er for$ation


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Circulation nu$!er (ariation


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)ec#anical and t#er$al stress calculations

Materials

For boiler constructions only that aterial can be used which hasproofed strength at elevated teperature

Calculation method

+here are standardied calculation ethod is available for generalcases

But in cople# cases further calculations could be needed) egwith soe finite eleent ethod

Calculation procedure

Fro heat transfer and hydraulic calculations there shall beavailable wall teperature and pressure values

+han ta"ing into account operation teperature and strength atthis teperature allowable stress needs to be calculated

,fter it follows stress analysis of different parts ,ctual stress hasto be lower than allowable in each case

realA allowed


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*tress types

0riary stress: 0riary stress is caused by ain load) eg

inner pressure) bending) etc considering

regular shape of different parts*econdary stress:

*econdary stress is caused by cuttings)

outings) supports) etc) which odifynoral stress distribution


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T#er$al stress

Because of operation at

high teperature different

parts of the boiler would

li"e to e#pand in differentlevel


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T#er$al stress calculation $et#od

For the first tie diensions of

free e#pansion needs to becalculated

h6 le t +hen this part is pressed bac" to

original diension (n this waystress and reaction forces can be

calculated

Mechanical stress in plate fi#ed

in two direction:

t1

Ele2,1

=


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Fati&ue

Boiler parts can have different stress level atdifferent operation conditions *tressfluctuation can cause fatigue for that part,plitude of stress can be calculated as

follows

For a certain stress aplitude cycle nuberliit can be deterined

na#6 f$a#%

a

R R= a# in

2


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Cu$ulati(e de&radation $et#od


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Creeprupture stren&t#


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*uary

'ou are already failiar with

Control calculations:

Fluid flow calculations- pressure drop

- natural circulation

Mechanical and theral stress calculations


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+han" 'ou for 'our ,ttention D

Date post:	02-Mar-2018
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Circulation Mech stress Control

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