INTRODUCTION;-2x250 MW NSPCL Expansion Project, Bhilai, Chattisgarh

Coal Bin: --- coal bin is the storage for coal used as fuel for furnace. Shape of bins may

be circular, polygonal, square, or rectangular in plan. Bottom hopper portion is generally

conical-cum-hyperbolic or any other profile shape. Bins are termed as bunkers or silos

according to their shape and plane of rupture of coal. The bins are fabricated and

erected in segments and are made of mild steel plates joined together with full strength

butt weld and provided with stiffeners at regular intervals. Stiffeners are also provided on

external face of the coal bunkers.

The bending of plates and rolled sections to the required shape for fabrication is done by

plate bending machine without resorting to heating, hammering, angle smithy or black

smithy process.

Poking holes (manual or pneumatic ) and striking plate shall be provided to facilitate coal

flow .Poking holes have circular MS pipe and cover cap as detailed in attached drawing

below.

AT Bhilai project we have circular type coal bunker with conical type hoppers and a ring

beam in between that connects the cylindrical shells with hopper below. The ring beam

rests at 29.00 m with the support of main support beams.

The coal bunkers are made up of IS 2062 Grade B MS plates of thickness 12 mm and 8

mm with provision of lining by SS plates of grade SS 304M

Reasons to use strand jack method for erection of silos instead of conventional method

of erection by crane:-

Customer was aggressively asking BHEL to complete the tripper floor suggesting that it is

required for erection of conveyer and tripper, due to this change in sequence of erection

and floor completion, TRIPPER FLOOR was casted .Now it became impossible to get

erected all bunker by conventional method of erection by crane, as we have to feed the

various shells in parts from top.

It was then decided, after protracted discussions within BHEL as well as with our

associate for structural fabrication and erection M/s L&T, to finalize strand jack method

for erection of coal bunkers using the FEEDER FLOOR. M/s L & T then proposed a

suitable erection scheme in line with BHEL set targets and keeping in mind safety

aspects as well as quality aspects.

This Work involved FABRICATION OF VARIOUS PARTS OF BUNKER AND ERECTION BY

CRANE UPTO EL (+) 17.00 MTR (FEEDER FLOOR).

FABRICATION OF SHELLS, RING BEAM AND HOPPER

Supply of structural steel is in the scope of BHEL and transportation from BHEL storage

yard to the fabrication yard is in the scope of agency i.e. M/s L&T.

For fabrication of shell we bring MS plate of full size from BHEL yard. After properly

assessing the size of plate and requirement as per drawing considering the mark nos. a

cutting plan is prepared, and then marking on plate is done by using punch press. While

making cutting plan for fabrication, we should take care that it allows for minimum

wastage. Wastage allowed will not be more than 5%.and it is calculated by formula as

below:-

Wastage = theoretical consumption –actual consumption < 5%

Actual consumption

Wastage in structural steel is of two types:-

(A) Visible wastage <4.0%

(B) Invisible wastage <0.5 %

Total sum of above wastages should not exceed more than 4.5%

It is the responsibility of Engineer-in-charge to take care of scrap generated during

fabrication by judiciously making/reviewing cutting plan by iteration and finalizing the

minimum scrap generating plan.

For making shell/ring beam/hopper we required a rolling machine of capacity 10 MT.

First we fix the guide roll for required diameter to generate the shell. Shell part is made

here by rolling two nos. full length plates and welding them together. After rolling one

plate it should be locked by welding by at least two angle sections (equal angles of 50x6

are enough)to avoid the buckling/opening up of rolled plate due to springing action.

Again the second plate is rolled and fit up with the first one to complete the cylindrical

portion of bunker. The fit up is done by making a template by way of punch presses on

fabrication bed and getting the two rolled plates fixed on these marking which is

subsequently checked by L square, vertical pin-o- wire or any such suitable method.

Proper gap (5-6 mm) should be maintained between the two shell edges .After that MS

welding is done on outside surface. After this back chipping, DP and such other tests as

called for by Approved quality plan are performed in presence of customer

representatives. Once outside welding is complete, we go for inside MS welding and then

subsequently SS inside welding.

Similar process is followed for fabrication of Ring beam and hopper. Although hopper

fabrication requires more attention and expertise; this is so since the shape of the

hopper is not exactly conical instead it is hyperbolic.

TECHNICAL BRIEFS FOR BUNKER AT THIS PROJECT: ---

There are six shells, one ring beam, and one hopper portion to complete one set of

bunker. Bunker shells are reinforced with stiffener plates. (See ATTACHED Photograph)

.Each shell has two nos. LS joints(longitudinal seam joint ).Shell is named as

shell01,shell 02,shell 03,shell 04,shell 05,shell 06, and ring beam is also taken as

RB(Piece 01 and Piece 02),hopper in four pieces Named H01,H02,H03,H04.

DIMENSIONAL (height) DETAILS OF BUNKER: ---

(1) Shell 01- 1.64 m (7) Ring beam- 1.0 m

(2) Shell 02- 2.50 m (8) Hopper:-

(3) Shell 03- 2.50 m H01- 2.2 m

(4) Shell 04- 2.50 m H02- 2.2 m

(5) Shell 05- 2.50 m H03- 2.1 m

(6) Shell 06- 2.50 m H04- 1.5 m

PLATE THICKNESS DETAILS USED FOR FABRICATING DIFFERENT PORTIONS-

1. SHELL-12 mm MS plate (IS 2062 Gr B) is required as per drwg.

2. RING BEAM- 20 mm thk MS plate (IS 2062 Gr B) is required as per drwg.

3. FOR HOPPER – 8 mm thk MS plate (IS 2062 Gr B)

4. Inside SS 304 M LINER –

Shell- 3.15 mm thk

Ring beam-3.15 mm thk

Hopper - 4 mm thk

5. STIFFENERS:-

Shell-12 mm thk

Ring beam:-16 mm thk

Cover Plate thickness: - 6 mm thk

6. Fixing of SS liner with MS plate is done by PLUG welding ground smooth having

16 Ø

7. Outside dia.of shell=7.7 m

8. Total height of Bunker=EL (+) 48.512 m to EL (+) 24.5 m (TOS) =24.012 m

9. As per FQP 10 % RT and DPT are required, if not possible UT should be carried.

10.Capacity of Bunker (approx.) =850 MT

Proposed Work Break down Structure

Silo erection Work at 2X250 MW Expansion Project Bhilai

Stage I (Shifting and Lifting of silo)

Shifting involves reaching of shell upto Erection site:Lifting of silo from ground level to the rail arrangement at feeder floor (17.3 mtr.) location between Grid 12-13 and then dragging shall upto the erection position

Cost Implication: The Process of shifting of shell/ring beam/hopper takes approximately 60 mandays

a) Manpower involved:-1) Gang of ten labors for 1 man day @ 165/day = 1650

99000 Per silo Total = Rs 99000 per silo Grand Total = Rs 12870000.00 for 13 silo (A)

b) T&P Involved:-2) Two no hydra (12 T cap) = 177600 for one month

1598400 for nine months

Grand Total = Rs 1598400.00 for 13 silo (B)

Stage II (Fit up & fixing in position of shell/ring beam/hopper)

Fit up involves fixing up of shell with the places above maintaining verticality/circularity. Fit up of ring beam is in two pieces to facilitate erection. Hopper is to be Fit upcompletely at feeder floor and then lifted up for assembly to the ring beam.

Cost Implication: The Process of shifting of shell/ring beam/hopper takes approximately 108 mandays (36 Day)

a) Manpower involved:-1) A group of 4 fitters for 1 manday @ 165/day = 660 71280 per silo4) A group of 4 helpers for each fitters @ 90/day = 360

38880 per silo6) A group of 6 riggers for 1 manday @ 165/day = 990

106920 per silo7) A group of 3 khalasi for 1 manday @ 120/day = 360

38880 per siloTotal =Rs 255960 per siloGrand Total = Rs 3327480.00 for 13 silos (D)

b) T&P Involved:-

1) 4 nos chain pulley blocks (3 MT cap) = 9720 for one month 87480 for nine months

2) 8 nos chain pulley blocks (5 MT cap) = 32400 for one month 291600 for nine months

3) 1 nos electric winch (4 MT cap) = 40500 for one month 364500 for nine months

4) 6 nos Welding Generators 450/400A = 93600 for one month 842400 for nine months

Grand Total = Rs 1585980.00 (E)

Stage III (Welding of shell/ring beam/hopper)

Welding involves welding of bottom piece of silo with that of top piece, back chip and then full MS welding: later on final SS welding

Cost Implication: The process of MS/SS welding of shell/ring beam/hopper takes.

a) Manpower involved:-1) A group of 3 MS welders for 2 manday @ 150/day = 900 108000 per silo2) A group of 2 SS welders for 1 manday @ 250/day = 500 60000 per silo4) A group of one helper for each welder @ 90/day = 630

75600 per silo5) A group of 4 grinders for 1 manday @ 160/day = 640 76800 per siloTotal = 320400 per siloGrand Total = 4165200.00 for 13 silos (F)

b) T&P Involved:-

1) 12 welding Generators 450/400A= 187200 for one month 1684800 for nine months

Grand Total= Rs 1684800.00 (G)

MANPOWER INVOLVED IN STRAND JACK OPERATION:-

1) Strand jack operation @ 19121/month= Rs 57363 for one month Rs 516267 for nine months

2) Stand jack fixing arrangement for 14 mandays per silo= 4200 per silo

Rs 54600 for nine silos T&P Involved:-

a) Strand jack Hire Charges per jack = 15000 150000 per silo

b) Time period in months for jacks hiring 8 monthsC) No. of Jacks= 12 jacks

Total hiring Charges for strand jacks (a’b’c) 14400000 (J)

MANPOWER involved in supervision

1) Erection Engineers @ 25494/month= Rs 50988 for 2 engineers Rs 458892 for nine months

2) Quality Engineers @ 25494/month= Rs 50988 for 2 engineers

Grand Total = Rs 1488651.00 (H)

ADDITIONAL Fabrication/Erection for STRAND JACKS

1) lifting arrangement at shell for lifting through hooks= 59247.908 per silo Rs 770222.8 for 13 silos

2) rail arrangement at feeder floor grid 12-13 upto grid 2-3 3) MC 150used for a 14736 mm length of 84 mtr4) platform extension at 12-13 grid=

MB 600 used for 21834.222 mm length of 17 mtr5) strand jack support beam arrangement at tripper floor=184310.486) Strand jack support beam arrangement dismantling & fixing at new position=

For 5 times in a set of 1612716.5 three jacksGrand Total = Rs. 2603819.72 (I)

Total Amount = Rs. 32,141,330.72 (A+B+D+E+F+G+H+I+J)

Notes:

1. All labour rates as prevailing at site.2. All T&P rates as prevailing and as mentioned in rate schedule of BHEL’s T&P hire

charges. 3. One manday equals 8 working hours.4. Strand jack operator taken from sub contractor’s BOQ item for “providing

services of diploma engineer”.5. Erection/quality engineer rate taken from sub contractor BOQ item for “providing

services of diploma engineer”.

FIELD QUALITY PLAN

Sl. No CHARACTERISTICS / ITEM TYPE OF CHECK

CLASSQUANTUM /

FREQUENCY OF CHECK

1.01.1

Raw MaterialEnsure that raw material is as per approved specifications. Verification of material Test Certificates. Note: In absence of material test certificates check test shell be performed on sample basis as per relevant IS code.

R A 100%

1.2 Ensure that raw material is free from surface defected like cracks, seams laps, lamination, and pitting.Note: Steel plates above 25 MM thickness shall be checked for above defects if any by performing UT.

P UTM/c Lab

100%

2.02.1

Marking & Cutting:Ensure marking at convenient intervals is made by punching.

M B 100%

2.2 Ensure the cutting is performed using shearing/Cropping/Sawing only and after cutting, the edges are cleaned / dressed before taking up the fabrication up the fabrication work.

V B 100%

2.3 Ensure that the raw material after cutting & dressing as at 2.2 are identified with part Number.

V B 100%

3.03.1

Forming (As applicable)Ensure that forming is done using proper tooling free from damage.

V C 100%

4.04.1

Fit up of joints:Ensure joint fit ups before welding for the following.- Maintain appropriate gap for butt Welds. - Maintain minimum gap for fillet Welds

M B 100%

4.2 Ensure that the butt weld joint are properly aligned and the offset is not exceeding 10% of the thickness of the thinner part of the joining parts (Maximum permitted offset is 3.2 mm)

M B 100%

4.3 Ensure match marking are punched for all trail assembled components.

M B 100%

5.05.1

Preheating of weld joints:As application, ensure pre heating of joints to be welded using appropriate method.Note: Maintain uniform temperature during preheating as well as during welding.

V/M B 100%

6.06.1

Welding:Ensure availability of approved welding procedure Specifications (WPS)

R A 100%

6.2 Review Procedure Qualification test report (PQR) for acceptance if already qualified.Note: In absence of PQR, WPS shall be qualified at site by performing test.

R/T A 100%

6.3 Ensure availability of qualified welders. Review welder performance test report, if already qualified by BHEL/Customer.Note: In absence of above, welder shall be qualified by performing test at site.

R/T A 100%

6.4 Ensure availability of the approved welding consumable list, FWS. Ensure use of approved type & brand of welding consumable during the welding process.

R A 100%

6.5 Ensure Proper edge preparation & fit up prior to start of welding.

V/M B 100%

6.6 Wherever application, ensure Post heating of weld. Maintain uniform temperature during the process.

V/P B Wherever applicable

6.7 Ensure removal of temporary attachments and their grinding & Painting.

V B 100%

6.8 Ensure that welds are cleaned. Overlap/excess weld metal is removed by grinding and welds are painted.

V B 100%

6.9 Check the structure / parts correctness of dimension and matching after welding.

M B 100%

7.0

7.1

Non Destructive & Destructive Testing of welds:

DPT/MPI/LPI/RT/UT/ as applicable T A As per Annexure II

7.2 Check Trail assembly / Control assembly of fabricated structures.

P B Approved Tech.Spec.

7.3 Macro etch examination on production, ( Beams, Crane girders, and Columns):Sampling: Test coupons for main fillet welds with minimum one joint per built up for each welder.

T A ----

7.4 Mechanical Testing of butt weld on production test coupon.

T A -----

8.08.1

Cleaning & painting of Structures:Ensure that Structure surface are cleaned and painted as per approved specifications.

V B Approved Tech.Spec

8.2 Check for smooth finish and Dry Film Thickness of the painted structures.

V/M B Approved Tech.Spec

8.3 Ensure that fabricated structures are marked for following identifications:

- Part number ,- Assembly designation - Weight,- Any other important identificationsNote: All tolerance during fabrication shall be as per IS-7215 and annexure-III. However, IS-7215 will govern in case of any conflict

V/P B Drawing

Technical reasons for delay in Bunker erection at 2x250 Mw Bhilai Expansion Project

01. Delays in engineering and release of RFC drawings for silos and hopper.

Drawing for coal bunker (RFC) was received at site on 17//07/07.Normal duration for fabrication and erection of silo & hopper structure work is around 12-14 months from the date of receipt of RFC drawing including ordering and receipt of required structural material at site. Earlier fabrication for hopper was supposed to be from SS 409 M vide email from Fichtner /PEM on 27/09/06(SS 3.15 mm thick SS304 M plate for cylindrical shell liner ,8 mm thick SS 409 M plate for hopper bottom plate).However it was put on hold for the propose of re-engineering of hopper portion as there was a crack in hopper portion at simadri TPS.For the reason sited above it was decided to design hopper using both MS and SS plates ,this decision of re- engineering which includes aerodynamic flow study of coal through hopper portion took several months which led to our inability to initiate any procurement action. We could get BOQ for Hopper only after 19/07/07 with combination of 8 mm MS plate as base and 4 mm SS plate as liner for hopper portion .This means a delay of almost 10 months in engineering led to structural steel procurement delay.

02. Delays in structural material procurement.

After receiving RFC drawing on17/07/07, planning for material procurement showed special requirement of 12 mm plate (size 1250 mm length and 2500 mm width) to avoid vertical joints in shell. We had to place separate order for this special size of MS plate of 12 mm thick required for cylindrical portion of silo. Further material from BSP/SAIL also could not receive in time and in one lot. Whole material was also not received in time.

03. Use of crane for erection on sharing basis

Crane avaibility for Bunker/silo erection work has also been on shared basis with mechanical agencies which has on some occasions resulted in slowing down of progress .Initally,it was planned to erect silos by heavy lift crane only and after erection of silos ,floor beams for Tripper floor were planned to erect and do the slab casting of tripper floor subsequently however ,on great insistence/pressure from customer to give front for erection of coal handling equipments/conveyer on tripper floor ,erection of bunker was planned by strand jack method.

ERECTION SEQUENCE:-

Erection scheme proposed and implemented by M/S L&T is as follows:

It was proposed to set up four nos. strand jack at tripper floor controlled by one power pack (used to operate strand jacks). Such three sets of four nos. jack will be used for erection of three silos as parallel activities. One single jack has capacity to take load equals to 105 MT, if seven wires are used to hold the material of heavy lift. Lifting lugs and anchor plates were design for holding shell by wire coming out from strand jack. All four jacks was set up on built up beam meant of double I-section union i.e. base of jack rest on it. Four nos. holes at different angles from center point of bunker were made at tripper floor to pass on strand of jack through it, to hold anchor plate of top shell portion. Shell shall be feeded at feeder floor by use of crane and kept at built up trolley specially designed to take load more than the load of member to carry it to specified position. Then strands are fixed with anchor and lifting lug and it is lifted to height in accordance to accommodate second shell/hopper piece/Ring beam. After that union of two was completed as per FQP and again lifted to accommodate third pieces. Same will be followed for every silo. After completion of erection of six shell and Ring beam in two pieces.Erection of 8 nos 29.00 mtr beam and subsequently erection of 8 nos. stub column is executed and as per drawing along with it bracings. Locking of stub column by stitch welding with 29.00 mtr beam and its bracing is completed and after that jack can be release. Means load can be transferred through 29.00 mtr beam assembly to secondary beam of frame and then to VB and HB and columns and ground through base plate. Then we can use this set of jack for erection of other silos.

CONTROL POINTS:--

1. Proper fit up to maintain gap for welding using long cleats ,to maintain circularity.2. Proper Edge preparation.3. Maintaining plumb for control of center position to match with feeder coal pipe.5. Ensure proper supply to welding generators/rectifiers.6. Safety controlles like safety belt, life line rope, goggles for welder. Strong supports, strong, Wire ropes, slings used, pulley fixed, electric winch load capacity test and its due dates. 7. Night works with proper illumination with flood lights. Adequate safety for working at heights should be maintained at all times. Daily safety talk should be provided to workers.8. Ensure proper fabrication backup to support erection team.9. Proper housekeeping which minimizes risk of hazards. (At 17 mtr feeder floor)10. Proper planning for next day job and realization of job at the end of the day, keeping in view the last target date of its completion. Constant and continuous follow ups required and optimum utilization of all T&P‘s with proper mobilization of expertise manpower. As in this project silo supporting bracing gang got expertise in erection and their productivity becomes higher. 11. If any change in sequence of erection is required as per site requirement, we shouldkeep in mind the effect of such changes for all future projects.12. Keeping record of all dismantling and re-erection of the same such that it becomes easy to process the item claim against it in contractor’s bill.13. Constant watch on DPR (Daily Progress report) of the project.

PROBLEMS ENCOUNTERD:--

1. Mismatch of PCD bolt dia hole made to connect with coal pipe of feeder.2. Due to change in erection sequence 29.00 mtr main beam flange was required to be cut at eight positions to avoid fouling of strand jack erection hooks. It was corrected after erection completion.3. Clashes between two agencies working at feeder floor.4. Sometimes non-availability of construction power.5. Slow down of work progress during rainy season, specially welding and painting.7. Labour strikes.

Silo supporting bracings:--A net type of structure to support all shell structure to deviate laterally due to vibration (if any generated due to coal feeding in bunker or due to earthquake) requires at leastseven days for its erection.Box of ISMC 200 with gusset at the mid section is used to keep hold the bunker shell in its position (at different elevations).They are locked between two framed structures and subsequently to all structures (Photograph as enclosed).

Joining of two shells at Feeder Floor: ----

The above photograph shows:-1. Fitup of two shells using cleats.2. Platform arrangement for welding of joint CSJ013. Anchor Plate.4. Lifting Lug.

5. Strand wires

Photograph of Strand jack system at EL (+) 49.00 mtr (Tripper Floor)

Above Photograph shows (Tripper floor EL(+) 49.00 mtr.)…..1. Strand jacks.2. Rest beams (union of two I-sections)3. Guide channels.4. Power packs

Inside support system for Finishing and welding of Erection joint

A circular inside support system is prepared (three nos.) for easy and safe working while doing finishing, welding, grinding of joints etc inside the shell. Support system is generated from the scraps and its makes the working inside shell so easy. It is kept in hanging position by using slings supported from tripper floor (as shown above in photograph). It can be easily positioned at required place. After completion of all jobs from inside, it is dismantled and removed from inside.

Hopper portion and its erection:-

Final inspection protocol

Final inspection joint protocol was prepared with our customer for clearance of bunker from inside .Erection in charge (civil) from BHEL and customer and commissioning incharge will physical witnessed the completion of job and certify that, now bunker is ready for coal feeding. Protocol includes opening closing from top, chute completion, cleanliness from inside and removal of all temporary supports from inside of bunker.While checking if some of the punch point given by our customer, then it has to be attended immediately and again requisition is given to customer for re-inspection of the same.After completion of final inspection, no works inside the bunker is permitted.

Gantry crane in fabrication yard M/S L&T:-This crane is used for shifting u/p fabricated material within the fabrication yard.

Outside stiffening of shell:-Stiffeners are welded outside the shell surface, generally using 12mm plates to strengthen shell.

Silo supporting bracings: -

COAL BUNKER

Chute deflector plate

Fixation details of SS plate at MS plate

DETAILS OF SILO SUPPORTING BRACINGS AT 29.00 mtr Elevation