zero defect in p91-report-2005

ZERO DEFECTS IN P-91 WELDING

AT PS-SR CONSTRUCTION SITES

INTRODUCTION:

Conventional materials like SA335P11, P12, P22 were the choice of Power

Plant Designers till the last decade. The need for higher parameters viz. the

operating pressures and temperatures in order to improve the thermal efficiency

of the system has demanded the employment of superior materials. Obviously

the available option was to go in for stainless steel.

The exorbitant price of stainless steel material for piping application made it

necessary to go in for development of new materials, which are suitable for high

temperature operations and still available at affordable prices. The development

of X20 (12Cr-1Mo-1/4V) and P91 (9Cr-1Mo) has made a great impact in the

material technology adopted for power plant design. When X20 was being

developed in Germany, parallel efforts were being made in US for developing a

steel having better properties than available austenitic steel.

X20 MATERIAL:

The first use of X20 in India for high temperature steam piping was made some

times around 1970 in Neyveli Lignite Corporation Stage-II. Next application was

in Trombay unit-6. The use of X20 in preference to P22 allowed wall thickness

reduction by about 50%.

Advantages of X20 over P22:

Saving in cost of piping materials including hangers and supports,

structural steel & thermal insulation.

Easier handling of the piping components at site

Saving in weld metal.

Saving in the energy needed for pre-heating, welding and post weld heat

treatment

Saving in installation time

Faster start-up, load changes and shut down for the unit.

Zero defects in P91 welding BHEL Gunvatta Puraskar PSSR-Chennai

Disadvantages of X20:

The weld should be kept dry after welding until post weld heat treatment is

carried out to avoid threat of stress corrosion cracking due to presence of any

humidity.

The evolution of material is given below:

* 1950* 1950 - 2 - 211//44Cr-1Mo (A335P22, 10CrMo910)Cr-1Mo (A335P22, 10CrMo910)

* 1960* 1960 - - 11//22Cr-Cr-11//22Mo-Mo-11//44V (14MoV63) V (14MoV63)

* 1970 - 12Cr-1Mo-1/4V (X20CrMoV12 1) * 1970 - 12Cr-1Mo-1/4V (X20CrMoV12 1)

* 1980 - 5Cr-* 1980 - 5Cr-11//22Mo (P5) & 9Cr-1Mo (P9) Mo (P5) & 9Cr-1Mo (P9)

* 1984 - 9Cr-1Mo with Vanadium & Niobium (P91) * 1984 - 9Cr-1Mo with Vanadium & Niobium (P91)

P22 X20

P91

(21/4 Cr, 1Mo) (12Cr, 1Mo) (9Cr, 1Mo)

P91 MATERIAL:

In India P91 was introduced in the nineties by BHEL.

Advantages of P91 Material:

High temperature strength.

Considerable Wall Thickness Reduction

Significant weight savings, particularly in the case of components

More flexible Piping system.

Lower Support Loads.

Faster Start-up Rate.

Excellent oxidation resistance.

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Comparison of pipe sizes used in 500 MW projects and savings in thickness of Comparison of pipe sizes used in 500 MW projects and savings in thickness of

pipes for P22 / X20 / P91 material is given below:pipes for P22 / X20 / P91 material is given below:

Comparison of Pipe Sizes (500 Mw)

Description Singrauli Farakka SimhadriMaterial SA335P22 X20 SA335P91

Main steam (Double)

ID 345..44 x85.3 min

ID345.44 x42.42 min

OD 406.4 x42.0

Main steam (Single)

ID467.36 x114.86 min

ID467.4 x56.95 min

OD 564.0 x57.0

HPBP upstream piping

ID 248.92 x61.9 min

ID 248.92 x30.91 min

OD 323.9 x35

Relative Thickness - P22 / X20 / P91

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Comparison of welding process

Parameter P22 X20 P91Filler Wire (GTAW)

TGS 2CM (ER 90S-G)

CM2.1G (Bohler) ER90S-G (BOHLER CM 2-1G)

Electrode (SMAW)

E9018-B3 FOX20MVW (Bohler)

E9018-B9 BOHLER FOX C9MV

Preheat Temp. C 150 250 220Inter pass Temp. C

350 450 350

Post heat (Preheat Maintenance) C

80-100(1 hr. Min.)

80-100(1 HR.)

PWHT Temp. C 700-760 740-760 700-760Minimum Soaking time Minutes

15 120 MTS 120 MTS

Shielding gas Argon Argon ArgonPurging Gas Nil Argon Argon

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BACKGROUND FOR ADOPTING P91 WELDING:

Carbon content mainly dictates the highest hardness achievable in any steel.

Alloying elements help in reducing the cooling rate at which such high hardness

can be achieved, or for better understanding it can be said that alloying

elements only make it easier to develop the full hardness what steel is capable

of. However its carbon content decides the absolute hardness value achieved

by steel. Thus even though both P91 and X20 have enough alloy content to

achieve the highest hardness at normal weld cooling rate, the hardness values

of P91 steel is lower.

Material Typical hardness values achieved in HAZ (assuming a 15 minutes cooling time to 100C from welding

temperature)-HVP22(.11% C) 348P91(.11% C) 409X20 (.18% C) 540

Note: It may be noted that P22 even though having same carbon content as

P91 (.11%) does not develop the same hardness, in view of its lower alloy

content.

The susceptibility to hydrogen induced stress corrosion cracking is more with

higher hardness values, whenever the hardness values are more than about

350 HV. In such a situation X20 is most susceptible, with P91 coming next and

P22 being not susceptible.

NTPC, Vindhyachal project experienced a weld failure in X20 material in main

steam line during pre commissioning activities. When members of the Central

Boiler Board carried out investigation after visiting site, it was committed from

BHEL that adequate care will be exercised to maintain quality during welding for

future projects. NTPC / Simhadri site was the next site where P91 welding was

involved. Keeping this in mind, Power Sector-Quality, Chennai decided that, the

P91 welding in view of the stringent quality requirement, be given proper care

so that, we could achieve better quality welds.

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NEED FOR AWARENESS ON P91 AND ITS WELDING:

Even though P91 is less susceptible to such cracking as already noted, the

hardness values achieved in this material are in the susceptible range. Besides

M/s Mannessmann, Germany who had visited India, in connection with supply

of P91 pipes and X20 welding problems strongly recommended the following

precautions for P91 materials also based on their experience.

1. Post Weld Heat Treatment (PWHT) using induction method only

2. Immediate cooling to 80-100C, and not allowing the temperature

to fall below 80C before PWHT particularly for site welds and beyond

80mm thickness for shop welds.

These requirements have also been put in their WPS for welding of this steel.

The above requirements are critical for site welds, since these welds are

exposed to open atmosphere.

ACQUIRING KNOWLEDGE ON P91 WELDING:

With the objective of maintaining good Quality welds, Power Sector-Quality,

Chennai took a lead role in acquiring more information on P91 welding by

interacting with Welding Research Institute (WRI), Welding Technology Centre,

Engg. And Quality groups of BHEL-Tiruchy and Piping Centre, Chennai. Series

of meeting were attended by PS-Quality, Chennai organised by Piping Centre,

Chennai and Tiruchy unit, where M/s Mannesmann representative was also

present and clarified on many points related to P91 welding.

DISSEMINATION OF KNOWLEDGE TO TASK PERFORMERS:

Meet On P91:

The knowledge gained from these meetings were imparted to Power Sector

Engineers by organizing a “MEET ON P91” at BHEL, Tiruchy unit, which was

attended by site welding & Quality engineers from all the four Power Sector

regions. A comprehensive exposure was given to them during the meet. Also by

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directly interacting with WRI, Welding Technology Centre (WTC) & Design

engineers from Tiruchy unit, the site engineers gained good knowledge about

the P91 material, which was very much useful to them in the site activities.

During the meet a booklet on “P91 Material” containing information about

Metallurgy of the material, Fabrication, Welding and Heat treatment, Non-

Destructive Examination and Induction Heating machine was issued to all

participants.

Awareness Programmes:

Before commencing the erection of P91 piping, awareness programmes were

arranged at Simhadri, Talcher, NALCO, Angul & Ramagundam projects for the

benefit of customer, BHEL & Sub-contractor engineers. Faculty members from

WRI, NDTL, Tiruchy, EDN, Bangalore & Power Sector-Quality, Chennai

explained in detail the various aspects of P91 welding to the participants.

The field experience from Simhadri site was shared with Talcher & NALCO,

Angul site engineers. The entire process was photographed sequentially and

presented through Power Point presentation. This has helped them understand

the procedure easily and implement it during actual operation. This has helped

to clarify the doubts raised by task performers on the procedure, equipments &

instruments used in P91 welding.

Process Procedure for P91 Welding:

For achieving defects free weld, a detailed process procedure for site operation

was prepared in consultation with WRI, WTC & QA of Tiruchy unit and Piping

Centre, Chennai. This procedure deals with all the aspects of P91 welding like

right from material identification, edge preparation, fit-up, fixing of

thermocouples, purging arrangements, welders’ qualification, preheating,

welding, post weld heat treatment, Non-Destructive Examinations and welding

consumables, hardness survey, ultrasonic testing of completed welds. This

procedure also deals about measures to be taken during power failure and non-

functioning of equipments. The entire welding cycle was explained in detail with

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time-temperature chart (Fig.1) in the procedure. A typical chart is shown below.

Customer before use at site approved this procedure.

Fig.1

DEVELOPMENT OF INDUCTION HEATING MACHINE:

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Since the process of welding and heat treatment is susceptible to cracking and

lower ductility associated with P91 material, exact control of heat flow during

welding and post weld heat treatment is required. Sophisticated induction

heating machines thus become a necessity for the above purpose.

A versatile high precision induction heating system rated for 120 KW, 3KHz has

been developed as an import substitute of rotary sets for heat treatment of P91

pipes used in superheated steam pipeline of 500 MW Thermal Power Stations.

A series resonant inverter has been developed which controls the power fed to

an induction coil wound around P91 pipe by changing the frequency in 1.5 KHz

to 3 KHz range; a microprocessor based PID regulator controls the pipe

temperature automatically within 10C. Temperature cycle for the entire

welding operation can be pre-programmed for up to 96 hours. To ease the

commissioning, PID regulator can be auto tuned. Four machines, using fast

thyristor technology, have been built to be used in BHEL Power Sector.

These machines replaced imported sets at the time when there was a severe

foreign exchange crunch in the country. Subsequently, a very much-improved

version of these machines, using air-cooled Insulated Gate Bipolar Transistor

(IGBT) based inverter, was developed with a simplified topology, resulting in

reduction in size, weight and higher reliability.

A number of improvements had been carried out based on feedback from sites

to make these machine highly accurate and user friendly for outdoor

application. The new type of temperature controller ensures more accurate

temperature control within 0.5C as compared to 5C

PROCESS MONITORING:

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Cross-Functional Team (CFT):

As directed by General Manager / Corporate Quality, New Delhi, a Cross

Functional Team (CFT) may be formed. Accordingly, a CFT comprising of

Corporate Quality, Power Sector-Quality, Chennai, PSSR-Quality & BHEL-

Tiruchy & Piping Centre Quality was formed for continuous supervision of P91

welding at Simhadri, the first 500MW project where P91 is introduced. The CFT

members visited Simhadri site continuously for monitoring the welding

operations at site till the completion of all the welds. The CFT members at site

ensured the effective implementation of welding procedure by supervising the

operation meticulously in round-the-clock shifts. Each and every activity of the

welding were inspected by the CFT members and cleared for further operation.

Role Of CFT Members:

CFT members will play advisory role to the task performers at site in all

matters of welding, heat treatment, NDE & hardness measurements of P91

welding.

CFT members will monitor and control the process and generate

feedback.

CFT members will assist the task performers in maintaining the quality

of work and record quality issues.

CFT members will assist the task performers in sorting out any quality

issues in communication with Head-Quality.

CFT members will be analysing the records and generate statistical

facts for review by the experts and to implement process improvements.

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Documentation:

We started documenting the necessary requirements for defect free welds. The

following documents are made available to customer, BHEL Engineers &

Associate’s Engineers.

Drawings

Welding Procedure Specification (WPS) / Procedure Qualification Record

(PQR)

Booklet on P91

Field Quality Plan (FQP) for P91 Piping

Piping Erection Manual

Procedure for welding of P91

Field Welding Schedule (FWS)

Welding, Heat Treatment & NDE Manuals

Implementation:

The basic Plan-Do-Check-Act (PDCA) cycle was followed. The cycle represents

these four stages – Plan – Do – Check – Act arranged in a circular fashion to

show the continuing cycle. Thus a never-ending circular management process

is envisaged. Each step of the PDCA cycle was gone through carefully.

Based on the above PDCA cycle, the following were ensured by CFT members

at sites:

Qualification and training of welders

Job card system

Ensured uninterrupted Power Supply by maintaining a Diesel Generator set

by BHEL exclusively to take care of power failure during welding/heat

treatment process

Readiness of Induction heating system including spare cables

Fit-up inspection tools (root mismatch as per Log sheet)

Identifying & measuring thermocouple of the pipe

Fixing of thermocouple

Continuous logging of events in log book

100% inspection of root weld through windows

Completion of heat treatment

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Surface preparation for hardness survey

Surface preparation for Ultrasonic Testing and completion of UT and

maintaining records

Regular communication / interaction with HQ

Apart from CFT, stage inspection and audits were conducted by welding / QA

Engineers, Engineers from Quality-Piping Centre, NDTL, Tiruchy & Collaborator

and feedback obtained for subsequent correction. Based on the feedback,

ensured continual improvement in the total process.

QUALITY RECORDS

The following quality records were maintained:

Weld Joint fit-up details

Welding job card

PWHT job card

Welding log

Drying / Holding temperatures of Welding electrodes

Issue of electrodes / filler wires

Welders’ performance test

Heat treatment chart

Personnel Qualification Records (NDE)

UT / MPI / Hardness survey Reports

Calibration records of measuring & test requirements

ACHIEVEMENTS:

zero defects in welds:

As a result of the above, Simhadri unit had the distinction of first 500MW project

achieving defect free (zero defects) weld for all the 254 joints (for both units i.e.

127x2=254). Similarly all the four units of Talcher project (4x500 MW), one unit

of Ramagundam (1x500 MW), two units of Nalco CPP (2x120 MW) and one unit

of Kuttalam CCPP achieved defect free welds. In all a total of 939 welds were

completed so far without any defect. The following table gives project wise

details of welds completed.

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P 91 WELDS JOINT DETAILS & DEFECT STATUS

PROJECT: SIMHADRI 2 x 500 MW UNIT No.: 1

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT

JOINTS TESTED BY HARDNESS

JOINTS TESTED BY MPI

DEFECTIVE JOINTS

MS PIPING Dia 323.9 x 35 mm

2 2 2 2 NIL


62 62 62 62 NIL

MS PIPING Dia 564 x 57 mm

22 22 22 22 NIL

HP BYPASS Dia 323.9 x 35 mm

41 41 41 41 NIL

TOTAL 127 127 127 127 NIL

PROJECT: SIMHADRI 2 x 500 MW UNIT No.: 2

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


62 62 62 62 NIL


22 22 22 22 NIL


41 41 41 41 NIL

TOTAL 127 127 127 127 NIL

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P 91 WELD JOINT DETAILS & DEFECT STATUS

PROJECT: TALCHER 4 x 500 MW UNIT No.: 3

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


50 50 50 50 NIL


18 18 18 18 NIL


37 37 37 37 NIL

TOTAL 107 107 107 107 NIL


DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


52 52 52 52 NIL


18 18 18 18 NIL


39 39 39 39 NIL

TOTAL 111 111 111 111 NIL

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DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


56 56 56 56 NIL


22 22 22 22 NIL


40 40 40 40 NIL

TOTAL 120 120 120 120 NIL


DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


56 56 56 56 NIL


22 22 22 22 NIL


40 40 40 40 NIL

TOTAL 120 120 120 120 NIL

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PROJECT: NALCO 2 x 120 MW UNIT No.: 7

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS

MS PIPING ID 323 x 29 mm

40 40 40 40 NIL

PROJECT: NALCO 2 x 120 MW UNIT No.: 8

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS

MS PIPINGID 323 x 29 mm

40 40 40 40 NIL

PROJECT: KUTTALAM CCPP 1 x 101 MW

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS

MS PIPING Dia 158 x 14.6 mm

21 21 21 21 NIL

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PROJECT: RAMAGUNDAM 1 x 500 MW UNIT No.: 7

DESCRIPTION OF LINE

JOINTS WELDED

JOINTS TESTED BY UT



DEFECTIVE JOINTS


2 2 2 2 NIL


60 60 60 60 NIL


24 24 24 24 NIL


40 40 40 40 NIL

TOTAL 126 126 126 126 NIL

TOTAL NUMBER OF P-91 WELDS SO FAR COMPLETED IN PSSR SITES IS 939.

By achieving zero defects in P91 welds from Simhadri onwards, we have

honoured the commitment given to Central Boiler Board. Also, we have brought

laurels to our esteemed organisation by maintaining zero defects consistently

from Simhadri onwards.

Cycle Time Reduction:

The Achievement of zero defects in P91 welding has aided in compression of

the project schedule on account of no rework, early availability of fronts for other

works. Simhadri had the distinction of synchronizing the 500MW unit in a record

time of 39 months, which is ahead of schedule.

Similarly all the four units of Talcher (4x500MW) also achieved zero defects in

P91 welds and one unit of Ramagundam 500MW. Ramagundam 500MW unit-7

has created a benchmark of achieving Boiler Light Up in 32 months. Zero

defects in P91 welding also contributed to this achievement.

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Cost Reduction:

(A) Repair cost per joint

Sl.No. Description Amount (Rs.)

1 Removal of defect by grinding - 8 man-hours 2002 Cost of imported electrodes 20003 Electrical power consumption (2 KV X 400Amps X 36

Hours = 28 800 units @ Rs.2 per unit )57600

4 Cost of annealing cables (Rs. 75000 / 15) 50005 Surface grinding - 8 man-hours 2006 Cost of NDE 30007 Repair cost per joint 68000

(B) TOTAL REPAIR COST FOR 100 JOINTS : Rs. 68 00 000

Note: As on date, better acceptable defects in heavy walled pipe welds is to the

tune of 1% of length welded in PSSR sites against maximum allowable defect of

2.5% of length welded as per standards. In normal circumstances 1% of repair

in P91 pipe welding amounts to 1/100 X 939 X 1.274 (939 joints completed X

1.274 metre average length of weld per joint) ie 11.963 metres length of repair.

Assuming maximum length of defect in one joint as 120 mm, we could have

ended up with repair of 11.963/0.120 = 99.962 ~ 100 joints.

LONG TERM PROCESS IMPROVEMENT:

Use Of Eurotherme Controller

EDN / Bangalore has supplied ‘ATHENA ‘controller along with Induction Heating

Machine. The seven segments of welding & post weld heat treatment need to

be programmed one segment at a time on completion of each segment of the

total cycle. An experienced Engineer must be available at site for 24 hours to

take care of controller programming till the completion of welding and heat

treatment.

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EDN, Bangalore conducted two trails successfully using a new controller

EUROTHERME in induction heating machine at Simhadri site in place of

Athena controller supplied along with induction heating machine. Eurotherme

controller having ability to store the entire 7 segments program in its memory.

Suitable programmes were developed and fine-tuned to suit P91 welding &

PWHT operation of entire cycle. In addition, Eurotherme controller capability

was further explored to have two more advantages for P91 welding & PWHT as

detailed below.

Eurotherme controller ‘power failure sensor‘ was activated to take corrective

action automatically in case of power failure.

During cooling segment, ‘ holdback ‘ feature is activated which holds the

timer intermittently whenever process value exceeds 2C than the controller

programmed value.

Based on Simhadri experience and recommendation from PSSR, EDN

Bangalore has replaced all Athena controllers with Eurotherme controller

in induction heating machine.

Stage Inspection During Welding

Welders are informed to keep window opening during root welding at two places

180 apart for visual inspection of root welding. Any imperfection observed in

root welding, is corrected immediately. This practice eliminates root defect and

repair work after completion of entire welding & PWHT of weld. Moreover due to

stage inspection welders are taking enough care to give defect free welds.

The above improvements have resulted in cycle time reduction and also in

establishing better control on process.

Interaction and Brainstorming:

Periodical interaction with welders, operators of Induction Heating Machine,

operators in UT, hardness survey, fitters, grinders and helpers and with the

participation of experts brainstorming to share results and improving process

and appreciating being part of winning team had helped in continued

outstanding performance in their respective areas.

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CONCLUSION:

The five activities viz. acquiring & imparting of knowledge on P91,

implementation & evaluation helped in achieving defects free welds in five

different projects. The acceptable norm of welding performance in pipe welding

is a repair rate of 2.5% of length welded as per BHEL standard. PSSR has been

achieving a repair rate of about 1% in pipe welding other than P91, which we

are in process of approaching 6 sigma level.

Considering the above, the achievement of “ZERO DEFECTS” consistently in

939 welds of heavy wall thick pipes of manual arc welding at construction sites

in 7 units of 500MW projects at Simhadri (2X500MW), Talcher (4X500MW),

Ramagundam (1X500MW) and 2 units of 120 MW projects at NALCO

(2X120MW) in Power Sector-Southern Region is excellent job. By achieving

zero defects in P91 welds during the past three years of operation, BHEL-PSSR

has set a “BENCHMARK” in P91 welding process.

This achievement was possible because of systematic quality measures

ensured by the dedicated team by way of establishing a welding & heat

treatment procedure and also by implementing at sites through continuous

monitoring in successive process improvements by CFT members and active

involvement of all concerned.

New process design, exploiting existing technology, education and

implementation of process improvements for corrective / preventive measures

has really helped us to meet the satisfaction of customer.

Future applications requiring piping system for high temperature and high

pressure would derive advantage from the use of P91 material from the point of

view of welding, heat treatment & NDE. While the thermal power generation in

India enters in to the era of super critical parameters, obviously P91 WOULD

BE THE NATURAL CHOICE FOR CRITICAL PIPING.

Hence, we request management to kindly encourage the team for having

introduced the new process in professional manner and obtained the

confidence of customer with “zero” defects for an Excel award under “Gunvatta

Puraskar” (Award for excellence in Quality).

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zero defect in p91-report-2005

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