STRUCTURAL ERECTION OF STEAM GENERATOR PART-IV.

Structural erection of steam Generator /boiler can be divided into few parts according to the importance .

a. Preparation of soil inside the cavity for heavy duty crane movement  in spite of columns located 

inside the cavity. 

b. Secondary grouting of the columns  by  ready mix concrete.

c.  Procedure of torque tightening of HSFG bolts .

d. Most important item is fabrication and erection of Ceiling Girders . This is heaviest structural items

 to be lifted at site. 

e. Structural items like beam ,bracing , welded beams ,rolled beams , angles ,channel , fabricated piece

etc are erected as per drawing as routine erection but a planning needs to be done before hand so that

clear passage to be kept at ground level so that crane can move inside and also passage to be made 

for pressure parts material movement. It may require a few bracing to be kept on hold for erection.

The above picture shows a heavy duty crane is standing on platform . Platforms are made to size by 

20 mm steel plates and ISMC 100/ISMC 150 channel. These platforms made such a way that crawler

of the crane can stand on it. Platforms can be easily removed from one place to another while the 

crane is on move. In some place 12" square and 10' -12' long  wooden logs are used instead of steel 

platform. However  arranging wooden logs  is very difficult now a days therefore every body tries to

make steel platforms. The soil need to be compacted by running roller if required boulder pitching

has to be carried out. Laying of platform should be done such a way that there should not be any

load on the foundation columns located inside the cavity. This can be understood if some one can

looked into the boiler plan given on earlier submission. Any heavy duty crane of 600 MT or 1200 MT

needs to stand  firmly on platform within a limit of undulation . Crane manufacturer set half a degree

is the max limit. A crane sizing ( capacity) depends on lifting of ceiling girder ( heaviest one ) to 

required height. In our 660 MW we choose 600 MT Manitowac crane with luffing jib at 67 M and 

jib can lift the heaviest girder. This was done to keep in mind the erection of welded beams , rolled 

beams and roof structure including the silencers. Our erection method i.e erecting boiler structure

from the cavity does not suit the heavy lift crane with super lift arrangement as crane will not have

radius to move. Above picture shows the crane platform from other angle.

we will discuss secondary grouting of columns and HSFG bolting at the end.

We will now discuss about the ceiling Girder fabrication and erection.

The limits set out for ceiling Girder pre-fabrication is given below.

- Root gap at the erection joint of the ceiling Girder. Flange 4 mm to 6 mm , web 6 to 10 mm.

- Camber and sweep of individual ceiling girder 1 mm/ M , max. 15 mm

- Assembled ceiling girders for sweep and camber  1 mm / M max 15 mm.

- Position of Bottom bolt hole with reference to centre of the ceiling girder  + /- 3 mm

- Spacing between adjacent ceiling girder +/- 5 mm ( taken during Girder alignment)

- Diagonal difference between to adjacent girders is  25 mm Max .

-ceiling girders centre line variation w.r.t. Boiler axis  12 mm Max.

continued to part-v

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STRUCTURAL ERECTION WORK PART -III OF STEAM GENERATOR/ BOILER.

Spacing between reference axis and pedestal          1 mm/ M or  + /- 5mm 

( Reference axis is the line which is centre  line some times  calls as Boiler axis. It is transferred from 

the Bench Mark having marked latitude , longitude and elevation. This point is transferred column 

number 13 and 17 . )

Spacing between two foundations ( adjacent)         1 mm / M  or + /- 5 mm     

Spacing between first row and last row foundation  1 mm / M  or +/-  15 mm 

Diagonals between foundations                                 1 mm / M  or +/- 10 mm

( this is most important measurement )

Pitch difference between two foundation bolts          + / -  3 mm

Diagonal difference between  foundation bolts            + / - 5 mm

(All foundations are having template made  OF steel plates with holes pitched as per drawings . 

Bolts are placed there and bottom channel has been locked . The bolts cannot be moved during

casting of foundations . This checks has to be carried out before casting of columns.)  

The above picture shows that Boiler column erection has started . Formation of Box between Main

column and Auxiliary Column is under progress. Some of the manufacturer columns are joined by 

HSFG Bolts . In this picture main column is bolted but auxiliary columns are welded and also bolted.

The above picture shows boiler erection ( structural) is under progress. Loose columns are tied with 

Guy rope . End of the Guy rope is either terminated to max puller or a hand winch . Both the cases 

the equipment is securely anchored. 

Most of the foreign manufacturer supplies material tier wise so that this type of erection is not 

 done . This type of erection is having safety issues and can be termed as unsafe . But due to 

non sequential material such erection was carried out.

  250 MT crawler crane assembly is going on.         

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STRUCTURAL ERECTION PART -II OF STEAM GENERATION

We will now discussing the erection of Steam Generator structures . Our discussion will be mainly on super critical boiler which is now being introduced in India. The unit capacity is 660 MW onwards. Capacity less than 660 MW units are all sub critical and available in plenty in India. 

Structures for Sub critical and super critical do not have major difference in construction. Structure can be divided in to following parts .

1. Structure which is holding the furnace                                                                6780 MT

2. Structure which is holding platforms ,Air heaters and auxiliaries                       3600 MT

3. Structure which is holding ducts of boiler , BOF to ESP and ESP to chimney   2340 MT

4. Misc. inter connecting structures ,platforms ,handling ,elevator etc                     1200 MT

The above tonnages are approximate tonnage of a 660 MW boiler . Quantity of  smaller unit will be less than what it is indicated. However the tolerances followed will be the same. Some of the common tolerances followed not only for Boiler but also civil structure is given below.

he surface1. Individual column piece sweep and camber                                      1 mm / M max up to 10 mm

( Individual columns are placed on the pre-assembly bed to check the sweep and camber. Pre-assembly

bed need to be prepared carefully so there is no undulation on top of  the surface. Beds are prepared 

by using concrete blocks and beams . Dimension check ,cleat angle welding , Marker fixing , Bolt 

Hole cleaning etc are  done at  Pre - assembly bed.)

2. Camber and Sweep for assembled column                                        1 mm / M max up to 10 mm

3. Total length of column                                                                      +  / -  15 mm

4. Top elevation of column with reference point                                    + / -  5 mm

5. Position with reference axis                                                               + / -   5 mm.

( Measurement of the total length was done at manufacturing time . However during erection same

check is being carried out after erecting the piece of column and dropping plumb from the top to the

reference point at regular  interval ( thrice up to the 7 th tier ). Final check has been done after 6th tier

column erection and before seventh tier erection. Seventh tier measurement done at ground. )

Grouting Gap below the column base                                    minimum 20 mm to 70 mm max.

 ( Ideal gap is 35 mm to 45 mm ) for easy free flow secondary grouting)

Elevation of Top of pedestal                                                                + 30 mm to - 20 mm.

( All the pedestal need to be cleaned from loose concreting on the surface. At least 10 mm /15 mm

to be chipped of to position the packer plate below the column base plate . If the pedestal is more

on positive side then we have to chip more and doing it manually as it is being done in India will be

taking extra time. It is all time better to have pedestal elevation on negative side.)  . 

Typical floor plan ( foundation detail ) of Steam Generator is given above. Column no 7 to 17 L & R ( 

except 17 all other columns having two numbers i.e . one in right side and other is on left side)  are the 

main columns and column number one to six ( L& R) auxiliary column. Distance between 7R and  7L

will give an idea of space availability in Boiler cavity. This area inside the boiler is very important for 

erection of steam Generators. Inside the Boiler Cavities there are column no (14 ,15 and 16 ) L &R . 

These columns are shorter columns in height and need to be erected afterwards i.e. only after erecting 

the ceiling Girders. However these column foundations need to be protected until and unless we start

erection on these columns../

Continued to part III

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STEAM GENERATOR ERECTION PART- I

We have covered important civil works in thermal Power Station. We have discussed major works in civil and structural work . How they have been carried out , the requirement of tools and tackles for the job, Limits and tolerances to be kept for the job. Last 50 episodes will give a total picture of civil and structural work to the newcomers.

Now we will start discussing major equipment of thermal power plant. Power Plant can be divided into a few major groups . 

1. Steam Generator  or popularly known as Boiler and its auxiliaries.

2. Turbine Generator and auxiliaries .

3. Coal Handling Plant 

4. Ash Handling Plant.

5. Switch yard 

6. Plant electrical and C&I system.

7. Balance of Plant ( Beside above major packages other packages like water system etc)

We will discuss it all one by one starting with steam generator. However our discussion will be confined to erection and basic design input required. Definition of steam Generator can be obtained from any standard book or from Internet.

Size of steam Generator will be different for different size of unit e.g 60 MW ,120 MW , 210 MW , 250 MW , 300 MW, 500 MW ,600 MW  , 660 MW and 800 MW. Mostly these sizes units are available in India  . The sizes will be different for same unit if fuels are different in composition. However construction features will be same for all the units.

- Steam generators comprises of following components .

- Structure ( columns , bracing ,beams , angles ,platforms , Ceiling Girders , welded beams ,rolled beams , HSFG nuts and bolts , Gusset Plates  etc.

- Pressure Parts ( water wall panel , economiser , super heater Panels , HP piping , Re heater , Suspension , headers , Link tubes , Tie rod etc.

- Ducting - Air and flue gas Pass through it. Plates, beams , pipes etc are required to manufacture the ducts.

- Rotating Machinery - FD ,ID ,PA , SEAL AIR FANS , MILLS and other small rotating equipment.

- Insulation - Glass wool or rock wool , refractories etc.

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  SOME OF THE ERECTION TOLERANCE FOR STRUCTURAL WORK IN A POWER HOUSE                   (TYPICAL)

-Foundation for columns ( Top elevation)                                                  - 20 mm / + 10 mm.

-Dimension between two adjacent foundations for columns                 1mm/metre max. up to + 5 mm

-Adjacent pedestal diagonal difference                                                 1mm/metre max. up to + 15 mm

- Foundation Bolt pitch difference                                                          + 3 mm

- Foundation bolt diagonal difference                                                    + / - 5 mm

- Grouting Gap for secondary Grout                                                        + 25 to + 70 mm (max)

- Individual Column sweep and Camber                                    1mm/metre max. up to + 10 mm

-Total length after pre-assembly                                                    + / - 15 mm

- Sweep and camber for total length                                               max 15 mm.

- Out of vertical                                                                      1mm/metre max. up to 25 M

- Elevation Tolerance at reference level                                           + 5 mm

- Level difference of supporting joints                                              5 mm

- Deviation difference between purlins                                              5 mm

- Crane Girder centre to center spacing                                              5 mm

- Crane girder top level                                                                      5 mm

- platform level difference                                                                  5 mm

- HSFG bolt tightening                                           ( as per size of the bolt with torque wrench )

- when truss span <  25  metre                                                            + /- 7 mm

- truu span is > 25 metre                                                                     + / - 10 mm

- Beam span is < 25 metre                                                                    + / - 10 mm

-Beam span is > 25 metre                                                                      + /- 15 mm

SOME OF THE FABRICATION TOLERANCES

Name of Deviation                             column          Beams           Crane girder

Base plate and column                          0.1 mm

Frame Joints                                         2.0 mm

Web and flange                                    1.5 mm         1.5 mm               0.1 mm

Web & stiffeners                                   1.5 mm        1.5 mm               0.1 mm

Flange & stiffeners Intermediate

 stiffeners & Bearing stiffeners:              1.5 mm       1.5 mm               0.1 mm

Cap plate & column                               0.1 mm

Crane girder seat                                   0.1 mm

Beam brackets                                      2.0 mm     

          PORTABLE HOLDING OVENS FOR WELDER 'S USE  AT WORK SITE.

  
 
           A TYPICAL SKETCH OF COLUMN FOUNDATION SHOWING MEASUREMENT PTS.

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STRUCTURAL WORK CONTINUED WITH WELDING

- Suitable electrodes are to be used for the structural steel.

- Generally low Hydrogen electrodes are used for plates more than 20 mm thickness.

- All low hydrogen electrodes shall be baked and stored before use as per manufacturer recommendations.The electrodes shall be re baked at 250 C - 300 degree C for one hour and later on cooled in the same oven to 100 degree C. It shall be transferred to an holding oven maintained at 60 degree C -70 degree C. The electrodes shall be drawn from this oven for use.

-  Minimum pre-heating and inter pass temperature for welding is given below.

-  Thickness of thicker part at the point of welding     Welding with low hydrogen electrode or SAW 

    Up to and equal to 20 mm thickness                                            none 

    Over 20 mm and Up to and equal to 40 mm                                 20 degree C

    Over 40 mm and Up to and equal to 63 mm                                 66 degree C

    Over and Above 63 mm                                                               110 degree C

-  Pre -heating can be done by LPG heating , electric resistance heating or by electric induction heating.

- Temperature can be checked by thermal chalk or putting Thermocouple ( generally Nickel-cromium-Nickel ) . The relation between temperature and voltage is 4.12 mv /100 deg c rise (approximately).

- Sequence of welding to be followed as per WPS and drawings .However a few basics may be noted.

- The direction of welding should be away from restraint point and should be towards open end.

- Butt weld in flange plates and/or web plates shall be completed before the flanges and webs are welded together.

- Generally The beam and column stiffeners shall preferably be welded to the webs before the web and flanges are assembled.

- All welders including tack welders to be tested as per IS codes and identity card to be issued for verification time to time during actual job.

- Carry out Non Destructive examination  RT / UT /LPI  as per requirement.

 A general view of fabrication yard where plates are getting cut into pieces for further fabrication.

 A general view of electrode drying oven . The electrodes are dried here and transported to holding oven.

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