April 21, 2023 1

Circulation System

April 21, 2023 2

Presentation Plan• Water and Steam circuit diagram• Economiser• Super Heater• De-superheater /Attemperator• Re-heater• IBR• Super Critical Boiler

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FROM BFP DISCHARGE

DRUM

200MW WATER CIRCUIT

Bank I

Bank II

Economizer

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BOTTOM RNG HDR& Z-PANEL 1ST PASS W.W

1ST PASS W.W O/L HDRS ROOF I/L HEADER

2ND PASS UPPER C-HDR 2nd PASS LOWER C-HDRS

LTSH I/L HEADER LTSH O/L HEADER

D.P.I/L HEADER D.P.O/L HEADER

S.H. HEADER R.H.HEADER

2ND PASS ROOF O/L HDR(REAR ECONOMISER

M.S

H.R.HC.R.H

FROM F.R.S

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EconomiserBoiler Economiser are feed-water heaters in which the heat from waste gases is recovered to raise the temperature of feed-water supplied to the boiler.

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Economiser• The economizer preheats the feed water by utilizing the residual heat of the

flue gas.• It reduces the exhaust gas temperature and saves the fuel.• Modern power plants use steel-tube-type economizers.• Design Configuration: divided into several sections : 0.6 – 0.8 m gap

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Tube Bank Arrangement

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Finned Economizers

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Thermal Parameters of Economizer

• Out side diameter : 25 – 38 mm.• Tube thickness: 3 – 5 mm• The water flow velocity : 600 – 800 kg/m2 s• The waterside resistance should not exceed 5 – 8 %.

Of drum pressure.• Flue gas velocity : 7 – 13 m/s.

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6oC raise in feed water temperature, by economizers corresponds to a

1% saving in fuel consumption

Advantages of Economiser

220 C reduction in flue gas temperature increases boiler

efficiency by 1%

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Location and Arrangement

• Ahead of air-heaters • Following the primary super-heater or re-heater • Counter-flow arrangement • Horizontal placement (facilitate draining)• Supported to prevent sagging, undue deflection and

expansion .• Stop valve and non-return valve incorporated to ensure

recirculation in case of no feed-flow• Ash hopper below as flue gas takes a turn

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Type of Construction• Plain Tube : Several banks of tubes with either-in-line or

staggered type formation. Staggered arrangement induces more turbulence than the in-line arrangement. This gives a higher rate of heat transfer and requires less surface but at the expense of higher draught loss.

• Welded Fin-tube : Fin welded design is used for improving the heat transfer.

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Types of Super Heaters

• Pendant type

• Horizontal type

• Radiant Superheater

• Convection

Superheater

• Combined Superheater

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SUPER HEATER• Water is heated to raise steam to higher temperature

• Arranged in 3 stages

LTSH located above economiser.

Radiant pendent type above furnace.

Convective final super heater above furnace

in flue gas path

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Super heaters

• Super heater heats the high-pressure steam from its saturation temperature to a higher specified temperature.

• Super heaters are often divided into more than one stage.• The enthalpy rise of steam in a given section should not exceed

– 250 – 420 kJ/kg for High pressure. – < 280 kJ/kg for medium pressure. – < 170 kJ/kg for low pressure.

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Transfer of Energy

Hot Flue Gas

Thermal Structure SH

Steam

Convection &Radiation HT

Convection HT

Drop in Enthalpyof Flue Gas

Rise in Enthalpy ofSteam

Mechanism of Heat Transfer

Source/Supply Thermal Structure Sink /Demand

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Mechanism of Heat Transfer :

• Rate of heat transfer from hot gas to cold steam is proportional to:

• Surface area of heat transfer• Mean Temperature difference between Hot Gas and Cold

Steam.

Thot gas,in

Tcold steam,in

Tcold gas,out

T hot steam,out

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Thot gas,in

Tcold steam,in

Tcold gas,out

Thot steam,out

Thot gas,in

Tcold steam,in

Tcold gas,out

T hot steam,out

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Platen Superheater• Platen Superheater : Flat panels

of tubes located in the upper part of the furnace, where the gas temperature is high.

• The tubes of the platen SH receive very high radiation as well as a heavy dust burden.

• Mechanism of HT : High Radiation & Low convection

• Thermal Structure:

– No. of platens

– No. of tubes in a platen

– Dia of a tube

– Length of a tube

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Geometry of Thermal Structure : Platen SH

• The outer diameter of platen SH is in the range of 32 – 42 mm.

• The platens are usually widely spaced, S1 = 500 – 900 mm.

• The tubes within a platen are closely spaced, S2/d = 1.1.

• The number of parallel tubes in a platen is in the range of 15 – 35.

• Design Constraints: Max. allowable steam flow rates.

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Convective Superheater (Pendant)

• Convective super heaters are vertical type (Pendant ) or horizontal types.

• The Pendant SH is always arranged in the horizontal crossover duct.

• Pendant SH tubes are widely spaced due to high temperature and ash is soft.

• Transverse pitch : S1/d > 4.5

• Longitudinal pitch : S2/d > 3.5.

• The outside tube diameter : 32 – 51mm• Tube thickness : 3 – 7mmS1

S2

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Convective Superheater (Horizontal)• The horizontal SH are located in the back pass.• The tubes are arranged in the in-line configuration.• The outer diameter of the tube is 32 – 51 mm.• The tube thickness of the tube is 3 – 7 mm.• The transverse pitch : S1/d = 2 – 3.• The longitudinal pitch :S2/d = 1.6 – 2.5.• The tubes are arranged in multiple parallel sets.• The desired velocity depends on the type of SH and operating steam

pressures.• The outside tube diameter : 32 – 51mm• Tube thickness : 3 – 7mm

S1

S2

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Superheater Temperature Control

• Basically the control of temperature is to protect the superheater by preventing the metal temperatures reaching a dangerously high level reducing mechanical strength and leading to failure. Water flowing through a tube conducts heat away much more effectively than steam due to its higher specific heat capacity. This means that tubes carrying water have a metal temperature much closer to the fluid passing through it.

• For superheat temperatures alloys of chrome molybdenum steels are used (upto 560oC), difficulties in welding means that there use is restricted to only within the highest temperature zone and a transition piece fitted to connect to remaining mild steel tubing.

• Superheat temperature control is therefore fitted to ensure superheat temperature does not exceed design limits.

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Locating the desuperheater• The preferred location of

desuperheater, especially for temperature above 450 deg C is between sections of superheater. The steam is first passed through a primary superheater where it is raised to intermediate temperature. It is then passed through the desuperheater and its temperature reduction is controlled so that, after continuing through the secondary or final stage of the superheater, the required constant conditions are maintained at the outlet.

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Reheater

• Purpose: Re-heat the steam from HP turbine to 540 deg composed of

CRH header HRH header

Reheater section arranged above goose neck.

Reheating• The optimal way of increasing the steam temperature but not increase

the moisture content in the exiting vapour is to reheat the vapour after it exists the HP turbine and redirect this steam into IP or LP turbine.

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Procedure to Determine Optimum Reheat Pressure

• Calculate the efficiency of Rankine cycle, Rankine.

• Calculate mean effective temperature of heat addition, Tm,in using

• Find out pressure corresponding to Tm,in and entropy at HP turbine inlet.

• Add reheating at this pressure and calculate efficiency of the Reheat Cycle.

• Repeat above steps for few iterations.

inm

cCarnotEqRankine T

T

,. 1

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Re-heater

• The pressure drop inside re-heater tubes has an important adverse effect on the efficiency of turbine.

• Pressure drop through the re-heater should be kept as low as possible.• The tube diameter : 42 – 60mm.• The design is similar to convective super-heaters.• Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.• The arrangement and construction of a re-heater is similar to that of a super-heater. In large

modern boiler plant, the reheat sections are mixed equally with super-heater sections.

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Safety Valves

Device attached to a boiler for automatically relieving the pressure of steam before it becomes great enough to cause bursting. The common spring-loaded type is held closed by a spring designed to open the valve when the internal pressure reaches a point in excess of the calculated safe load of the boiler. Safety valves are installed on boilers according to strict safety norms and IBR recommendation

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Safety Valve Settings of a Typical 200 MW Boiler

Locacation Set Pressure (Kg/sq cm

Reliving Capacity T/Hr

Drum 175.8 177.1

179.3 180.6

181.1 182.5

Super Heater

163.3 154.0

Re-heater –inlet

45.0  

45.5

46.3

Re-heater outlet

42.70  

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Indian Boiler Regulations• History• Scope (cover regulations on):

– Electric-Resistance-Welded Steel Boiler And Super-Heater Tubes

– Boiler Tubes Subject To External Pressure – The Working Pressure To Be Allowed On Various Parts

Of Boilers – Welded And Seamless Forged Drums For Water Tube

Boilers And Super Heaters – Requisite Mountings, Fittings and Auxiliaries

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Indian Boiler Regulations (Continued)

– Boiler And Super Heater Tubes, Headers And Other Pressure Parts Tubes

– Steam-Pipes And Fittings – Registration Of Boilers And Inspection Of Boilers – Safety Of Persons Inside Boilers – Qualification Tests For Welders Engaged In

Welding Of Boilers – Feed Water For Boiler

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Why supercritical pressure

Increasing the pressure will mean increase in saturation temperature at which steam evaporates thus increasing the average temperature of heat addition.

A Boiler operating at a pressure above critical point is called ‘SUPERCRITICAL BOILER’

A point where boiling water and dry saturated lines meet so that associated latent heat is zero, this point is called Critical Point.

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Critical condition

Definition “CRITICAL” is a thermodynamic expression

describing the state of a substance beyond which there is no clear distinction between the liquid and gaseous phase.

• The critical pressure & temperature for water are • Pressure = 225.56 Kg / cm2• Temperature = 374.15 C

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T S DIAGRAM

Boilin

g W

ater

Dry Saturated Steam

Entropy KJ / Kg K

Tem

pera

ture

( 0

C )

-2730

24’0c

A

B C

D

E

F

OUTPUT INCREASEBasic Rankine Cycle

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256 Kg/cm2

0

100

200

300

400

500

600 540’C568’C

ENTROPY

TEMP

SUPER CRITICAL BOILER CYCLE WITH SH,

RH & Regeneration of SIPAT 3 x 660 MW

Steam flow :2225 T/HrSteam temp : 540 ‘cSteam Pres : 256 kg/cm2RH pre : 51.6 Kg/cm2RH Temp : 568’cFeed water Temp : 291’c

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Supercritical boiler

• Supercritical pressure boiler has no drum and heat absorbing surface being, in effect, one continuous tube, hence called

‘once through Supercritical pressure boilers.’• The water in boiler is pressurized by Boiler Feed Pump, sensible

heat is added in feed heaters, economizer and furnace tubes, until water attains saturation temperature and flashes instantaneously to dry saturated steam and super heating commences.

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Pressure Parts Material

SA210C, T12, T23ReheaterTube Final

Section

Economizer Tube SA210C

Spiral SA213T22 EvaporatorTube/Water Wall

Vertical SA213T22

Primary

SA213T23, T91

Secondary

SA213T12, T23, T91SuperheaterTube

Final SA213T23, T91, T92

Primary

SA213T23, T91 SUPER304H

Separator SA302CSH outlet SA335P91

Header RH outlet SA335P91

SIPAT

500 MW PlantMaterial Oxidation Criteria,

℃

SA-210C 454

SA-213T2 482

SA-213T12 552

SA-213T22 593

SA-213T23 593

SA-213T91 649

SA-T92 649

SUPER304H 760

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THANK YOU