spiral wall system.pdf

SPIRAL WALL SYSTEM OF 2 X 660 MW

1 | P a g e B y R o h i t

A

REPORT

ON


(BARH STPP STAGE-II)

AT

BHEL :PSER:BARH SITE

Prepared By: Guided By:

RohitManjhi Mr. S Chatterjee Engineer SDGM BHEL PSER Barh site BHEL PSER Barh site



Acknowledgement

First and foremost, I would like to thank my guide for this report, Mr. Sumanta Chatterjee,

Senior. Deputy General Manager, BHEL:PSER:BARH Site for his valuable guidance and

wholehearted support in completing my report as well as enriching my knowledge. His advice

from time to time has been very helpful; without which this report would not have been

possible.

Besides, I would like to thank my esteemed organization , BHEL for providing me with a good

environment and facilities to complete this report. It was am opportunity to study and learn

about various welding & NDE work , which are inevitable while erecting spiral wall system.

Finally, I am grateful for the support which I have received from Mr. D Guha GM

/Projects(Barh), Mr. M Prasad AGM, Our Technical associate ASTOM India Barh site &

colleagues during this total exercise.

Rohit Manjhi



Contents

SL NO DESCRIPTION PAGE NO.

1. Introduction 1-3

2. Why do we use Spiral wall system 4- 4

3. Detail of spiral-wall system 4-10

4. Critical checks on spiral wall system 11-18

5. References 19-19



1. Introduction Supercritical Boiler technology is gaining acceptance worldwide as Clean Coal Technology

due to its significant advantages like higher overall plant efficiency, reduced coal

consumption, reduced gaseous emissions like SO2, NO2 and CO2 and particulate

emission. Large capacity units of size 600 to 1000 MW are now built up with supercritical

parameters. In India, the trend is clearly towards large capacity Mega / Ultra Mega Power

Projects based on supercritical technology. Evaporators of once through supercritical

boilers are designed with either high mass flux spiral/vertical wall or low mass flux vertical

wall.

The major difference between a drum type boiler and once through supercritical boiler is the

furnace wall design. An once through furnace wall design needs to take care of the

temperature difference between tube-to-tube at furnace wall outlet due to the variation in

furnace heat absorption. Also the occurrence of boiling crisis, associated metal

temperatures need to be critically analyzed and taken care in the furnace wall design. There

are basically two types of furnace wall designs used namely helically wound spiral wall and

vertical wall. The design aspects of these two types of furnace wall are discussed in this

paper.

TEMPERATURE ENTROPY DIAGRAM OF SUPERCRITICAL RANKINE CYCLE



Key parameters of Super Critical Boiler at Barh

Main steam flow 2120 T/hr

SH steam pressure 255 kg/cm2

SH steam temp. 568 deg centigrade

Feedwater temp. 294 deg centigrade

RH steam flow 1708 T/hr

RH steam pressure 54.7 kg/cm2

RH steam temp. 596 deg centigrade

What are the key differences between the subcritical units and the Supercritical units?

Efficiency

The main advantage and the reason for a higher pressure operation is the increase in the

thermodynamic efficiency of the Rankine cycle.

Large Subcritical thermal power plants with 170 bar and 540 / 540 C (SH / RH) operate at

an efficiency of 38 %. Supercritical units operating at approx 250 bar and 600/610 C can

have efficiencies in the range of 42 %.

Ultra supercritical units at 300 bar and 615 / 630 C will still increase the efficiency up to 44

%.

Increase in efficiency directly lead to reductions in unit cost of power and CO2 emissions.



Operational Flexibility

Most of the Supercritical units use the once-through technology. This is ideal for sliding

pressure operation which has much more flexibility in load changes and controlling the

power grid.

However this also requires more sensitive and quick responding control systems.

Evaporation End Point

In subcritical units the drum acts as a fixed evaporation end point. The furnace water walls

act as the evaporator. Not so, in the case of a supercritical unit. The evaporation end point

can occur in various levels of the furnace depending on the boiler load. The percentage of

Superheat in supercritical units is higher than subcritical units. Because of this the furnace

tubes act more as superheaters than waterwalls. This necessitates the use of higher grade

of materials like alloy steels in the furnace.

Heat transfer Area

Higher steam temperatures in supercritical units results in a lesser differential temperature

for heat transfer. Because of this, heat transfer areas required are higher than subcritical

units.

Higher Superheat steam temperatures entering the HP turbine also mean higher reheater

inlet temperatures which again results in a higher heat transfer areas.

Materials

Supercritical power plants use special high grade materials for the boiler tubes. The turbine

blades are also of improved design and materials. In fact, the very increase in higher

pressure and temperature designs are dependent on the development of newer and newer

alloys and tube materials.

The aim of the industry is to achieve power plant efficiencies in the range of 50 %.



2. Why do we use spiral wall system:- i) Smaller OD tubes.

ii) Helically wound spiral wall construction in which the tubes are inclined (15 to

25 deg.) and furnace tubes pass through the circumference of the furnace

more than one time and connected to a transition header above the burner

zone. Above the transition header the furnace enclosure is made up of

vertical water wall tubes. The spiral wall concept reduces the number of

parallel tubes and hence increases the mass flux through the tubes. As all

tubes pass through all the furnace walls, any variation in heat absorption is

applicable to all these tubes and hence the temperature difference between

these tubes is minimized.

iii) To achieve reliable cooling, the mass flux generally adopted is around 2000

kg/(m2s) at full load. It may be chosen higher for other reasons, e.g. to lower

the minimum load for once through operation.

iv) Smooth tubes are adequate, as the mass flux is high.

v) By using spiral wall system total height of the boiler get reduced.

3. Details of spiral wall system in 660 MW Barh stage II project:-

Joints detail of spiral wall system as per FWS:-

SL NO.

MATERIAL SPEC.

DIM(mm) PROCESS OF WELD

NO. OF JOINTS

ELECTRODE WPS NO.

TIG ARC

1 SA213 T22 + SA 213 T22

Dia 41.3 + 41.3

TIG & ARC 9498 ER90S B3

E 9018-B3

1031/01

2 SA213 T22+ SA182 F12 CL2

Dia 38.1 + 38.1

TIG & ARC 1032 ER80S B2

E 8018-B2

1011/01

3 SA106 GR.C+SA234 WPC

Dia 406.4+ 406.4

TIG & ARC 4 ER70S A1

E 7018-A1

1005/05



Sequence wise snapshot of spiral wall system at 2X660 MW Barh stage-II project.

Assembly of transition header with terminal tubes at pre-assembly area

Pre-assembly/Fit-up of spiral wall panel for matching confirmation

Transition

Header

Term-Tube



Tube welding of transition header term tube with vertical water wall panel

Pre-assembly of tension bar with spiral wall panel



Pre-assembled spiral wall panel ready for lifting after DP & sponge test

Lifting of assembled right side spiral wall panel

Tension Bar,

Material-

SA387 Gr 22

Cl 2

Thickness-

30mm

Scallop

Bar,

Material-

SA387 Gr

22 Cl 2

Thickness-

12 mm



Corner tube joints

Assembly of sofa panel

Buck stay

fitted with

stir-up

system &

spiral panel

Tension bar



Burner panel fit-up at boiler-IV corner-1

A view of bottom side Spiral wall system



Assembled bottom ring header with term tube

Assembled bottom ring header with term tube



4. Critical checking of spiral wall panel SL

NO.

CHECKING DESCRIPTION CLASS OF

CHECK

TOLERENCE

i Physical verification of header, panels,

Tension bar, scallop bar etc.

A As per FQP/Drawing

Ii Measurement of materials as per

FQP/drawing

B Header(1mm/M),

Panel(+2mm or -5mm)

Iii Fit-up checking of tube joints, panel fin to

fin joints, tension bar, Scallop bar.

B Mismatch(2.5 mm)

Iv Root gap checking of tube joints, panel fin

to fin joints, TB & SB .

B Tube(2 to 5mm),fin(3 mm),

TB(3 to 4 mm)

V Visual inspection of weld joints A As per ASME code & FQP,

Reinforcement(0.5 to 4 mm)

, weave(3X dia of electrode)

Vi Nomenclature of weld joints. A As per FQP/drawing

Vii DP test of weld joints as per FWS &FQP B As per FWS/FQP, 10% for

TB,SB & fin (T-22 & T-12 )

Viii Radiography test of weld joints as per FWS &FQP

A Asper FQP/FWS, 20% RT

Ix If found some discrepancy in the weld segment in RT/DP Test, required to do repair for the same & take RT/DP (as per requirement) for confirmation

A As per ASME code & NDE MANUAL

x PWHT/SR of applicable weld joints as per FWS

A As per WPS (+ or 15 deg centigrade)

Material Test Report of Tension Bar, Scallop Bar & Fins,

Nomenclature of weld joints and Protocols of

Radiography Test, Dye Penetration Test & Post Weld

Heat Treatment are attached below:-



Nomenclature of weld joints



5. References:

FWS of 2X660 MW Barh stage-II

Approved drawing of Barh stage-II , Boiler unit-IV

spiral wall system.pdf

Documents