EG9700194HEB 97, Alexandria, Egypt April 5-6, 1997

Quality Assurance in Fabrication of Boilersand Pressure Vessels

B.S.C. RaoHead Quality Assurance Section (Projects)Indira Gandhi Center for Atomic Research,

Kalpakkam, India

Assurance for safety and reliability of boilers and pressurevessels is a systematic approach involving various stages rightfrom material identification to final stages of testing, transportationand storage before commissioning. This paper brings out variousQuality Assurance aspects to be implemented by themanufacturers of boilers and pressure vessels.

QUALITY ASSURANCE INj FABRICATION OF BOILERS AND PRESSURE VESSELS

B.S.C.RAO.Head Quality Assurance Section (Projects),

Indira Gandhi Centre for Atomic Research,Kalpakkam,INDIA-603102.

ABSTRACTQuality Assurance -for safety and reliability of boilers

and pressure vessels is a systematic approach involving variousstages right from material identification to final stages oftesting, transportation and storage before commissioning. Thispaper brings out various Quality Assurance aspects to beimplemented by the manufacturers of boilers and pressure vessels.

INTRODUCTION

The concept of Quality Assurance has emerged sincenineteen fifties. In 1950 when Americans had the the task ofensuring safety and reliability of Aerospace and Nuclearcomponents, they started a new concept of training and qualifyingthe inspectors of manufacturers to meticulously inspect thesophisticated components. This is one of the steps towards theQuality Assurance by the manufacturers. The Aerospace and NuclearProgrammes in U.S.A. have imposed certain conditions on themanufacturers to satisfy with regard to organisation, workingmethods and procedures, training and qualification of men,qualitycontrol and documentation. All these conditions are aimed towardsassurance by manufacturer to meet the quality standards.

Thus quality assurance is a term related to the efforts of themanufacturer to give confidence to the client about the products.

QUALITY ASSURANCE (Q.A.)

Quality Assurance is the well planned and systematicactions taken by the manufacturer to provide adequate confidencein the minds of the buying organisation that the manufacturedproduct would perform the intended service satisfactorily overthe period of design life. Quality Assurance comprises of twoaspects namely Quality Control and Quality Administration.

QUALITY CONTROL

The term quality control can be defined as the processof inspection to ensure that the material or finished productmeets the specifications and drawings.

QUALITY ADMINISTRATION

Quality Administration is the systematic organisation toeffectively implement the quality standards through writtenprocedures by trained and qualified personnel and establishdocumentation as proof of quality achieved.

SUBJECT

In the light o-f the concepts explained above, the qualityassurance in fabrication of boilers and pressure vesselscomprises of certain activities to be carried out by themanufacturer to assure the quality of design and construction -forthe safety and reliability of the boilers and pressure vesselsin the plant they are intended to serve.

The activities which form the quality assurance programme forboilers and pressure vessels hereinafter expressed as"pressure vessels" are given below and explained.

1. Approval of detailed designs by the client.2. Approval of raw materials and consumables by the client.3. Calibration of testing equipment and inspection tools.4. Qualification of welding procedures and welder performance.5. Qualification of welding inspectors and Non-Destructive

Testing (NDT) personnel.6. Quality control during forming and shaping of components.7. Quality control during assembly of parts.8. Quality control during production welding.9. Non-Destructive testing of welds.10. Quality control during post weld heat treatment(PWHT)11. Non-destructive testing after PWHT.12. Hydraulic testing or load testing of pressure vessels.13. Leak testing of pressure vessels.14. Documentation.15. Pre-planning for quality assurance in pressure vessels.

Approval o-f detailed designs by the clienti

After receiving a firm purchase order from the client, themanufacturer has to perform detailed design calculations for thepressure vessel based on the design input and specificationsprovided by the client and prepare detailed design drawings. Indetailed design, the design of individual parts from the point ofview of primary stresses, temperature and thermal stresses,fatigue, creep, etc. are considered and the thickness ofmaterials and welds are calculated. As a measure of QualityAssurance in design, the detailed designs are to be scrutinisedby a separate design group of the manufacturer's organisation orby an independent design organisation. After verification andscrutiny of the detailed design, the detailed drawings aresubmitted along with design calculations to the client forapproval. The client will check the detailed designs and verifythe design adequacy for the intended service and approve thedetailed drawings.

Approval of raw materials and welding consumables!

Material selection for a pressure vessel is one of thedesign criteria. The material is selected on the basis of its

physical properties such as strength, ductility, fatigueresistance, service temperature, creep resistance etc. and itscompatibility and corrosion resistance to the fluids handled.

After specifying a particular material for a particularservice, inadvertent use of a different material or a differentwelding consumable in the fabrication can lead to premature-failure of the pressure vessel in service. This is to beavoided. Hence the materials and consumables conforming to thespecification are to be identified by the manufacturer beforefabrication. This is done by identifying the materials andconsumables with linkable mill test certificates. In the absenceof linkable mill test certificate the material or weldingconsumable is tested for chemical composition and mechanicalproperties, and then accepted if the test results conform to thespecification.

Calibration of the testing machines and inspection toolsi

The manufacturer has to prove certain minimum physical andmechanical properties which form the design basis, on thematerial and welds by way of tensile testing, bend testing,impact testing, drop weight testing etc. If the testing machinehas an error on positive side in reporting the properties of thematerial, an unacceptable material may be accepted. This is notdesirable. Hence the testing machines are verified by usingcalibration devices. During calibration, the error for loadswithin the range of the testing machine shall not exceed certainpercent of the load applied. After calibration, over a period oftime, errors exceeding the limit may develop due to usage of thetesting machine. Hence the testing machines are to be calibratedperiodically once in one or two years and certified for accuracy.Similarly the inspection tools used in the shop should also becalibrated periodically for accuracy. This is one of theactivities of quality assurance programme.

Qualification of welding procedure and welders performances

The manufacturer has to prepare a welding procedurespecification in writing envisaging the materials to be joined,the consumables to be used, welding process to be used, weldingparameters to be used and the precautions and treatments to begiven to the weld. The intent of the welding procedurequalification is to demonstrate that welds made by a specificprocedure under fabricating conditions can meet prescribedphysical characteristics. In procedure qualification tensileproperty, ductility and impact strength of the weld is checked.Fig. 1A and IB show the procedure test pad of plate welding andpipe welding respectively and how to locate the test pieces forthe mechanical tests as specified by American Society forMechanical Engineers

than minimum strength of base metals joined. The 180 deg. bendtest should not show cracks or openings on tension side o-f bendspecimen. The impact strength o-f the weld when speci-fied shouldcon-form to welding consumable specification.

The manufacturer has to use, in procedure qualification andin production welding, electrodes and welding consumables whicharB conforming to relevant standards. The electrodes andconsumables shall be qualified by the respective manufacturersand supplied with conformance certificate. After procuring theelectrodes/consumables with test certificate, the fabricatorshall ensure before use, proper storage and baking conditionsrecommended by the supplier.

The welder who performed the procedure qualification test pad isqualified along with the procedure. The other welders to beengaged on the job are qualified separately by a performancequalifications test. The intent of the performance qualificationtest is to verify that the welder can produce sound welds byfollowing the qualified welding procedure. The performancequalification test pad is subjected to either radiography or 2Nos. 180- deg. bend tests < 1 root bend, 1 face bend) to checkthe soundness.

The welding procedure and welder performance qualifications arethe most important stages of the quality assurance in thefabrication of pressure vessels and thus form good base forachieving the required weld quality in production.

After the welding qualifications are successfully completed, thefabricator shall make welding qualification records as a proof ofthe welding qualification attained. The fabricator need notrepeat a welding procedure qualification unless weldingparameters for a new pressure vessel vary beyond the limitsspecified in the welding codes. There ars nine parameters inwelding which influence the quality of the weld produced.They are

1. Joint design2. Base materials3. Welding consumables4. Melding Process5. Gas6. Polarity7. Pre-heating8. Post-heating and9. Electrical characteristics of power source used.

In each of the above parameters there are differentvariables. The variables which call for requalification ofwelding procedure are called essential variables. Thesupplementary essential variables call for requalification of thewelding procedure only when the materials to be joined are ofimpact tested quality. The non-essential variables do not callfor requalification of welding procedure.

Qualification of welding inspectors and NOT Personnel!

In the introduction o-f the subject "Quality Assurance" itmentioned that training and qualification of inspectors of themanufacturer is one of the steps towards the quality assurance bythe manufacture.

The personnel employed by the manufacturer for inspection andnon-destructive examination of pressure vessels should beknowledgeable and competent to characterize the defects in weldsand to assess their acceptability Dr otherwise. They should beable to interpret the intent of the specification and strive toachieve the desired quality of the welds. They should havethorough knowledge of pressure vessel codes and criteria fordesign of pressure vessels. It is seen in practice thatinspectors who do not have such knowledge and those who are freshfrom college and work in the area of quality control of pressurevessels make much fuss about unimportant aspects ignoring theimportant ones. Hence the welding inspectors of the manufacturershould be trained in the areas of welding metallurgy of variousmaterials, welding procedure and performance qualifications,causes and control of various defects in welds, interpretation ofNDT results and assessment of weld quality, and testing ofpressure vessels. Similarly the NDT supervisors and engineersshould be trained in the theory and practice of respective NDTtechniques, manufacturing processes, causes and control ofdefects in welds, interpretation of NDT signals with respect tonon-relevant and relevant indications and defects in welds andassessment of weld soundness. Trained and qualified inspectorsand NDT personnel of the manufacturer contribute to a greatextent to the quality in fabrication of pressure vessels.

Quality control during -forming and shaping o-f componentsi

A pressure vessel is built-up by joining shells, dished ends,nozzles, flanges etc. The shells and dished ends are formed byeither cold forming or by hot forming, provided the formingprocess will not unduly impair the physical properties of thematerial. If the plates are to be rolled, the adjoining edges oflongitudinal joints of cylindrical vessels shall first be shapedto the proper curvature by preliminary rolling or forming inorder to avoid having objectionable flat spots along thecompleted joints.

The foregoing norms are applied for forming of coded vessel partsin order to preserve the physical properties of the materialduring and after forming.

1. Carbon and low-alloy steel plates shall not be formed cold byblows.

2, Carbon and low alloy steel plates may be formed by blows at aforging temperature provided the blows do not unduly deformthe plate and it is subsequently post weld heat treated.

3. Vessel shell sections, heads and other pressure boundaryparts of all carbon and low alloy steels fabricated by coldforming, shall be stress relieved subsequently when theresulting extreme fiber elongation is more than 57. from theas rolled condition and any of the following conditions exist

i) The vessel will contain lethal substances

ii) The material requires impact testing.

iii) The thickness of the part before cold forming exceeds15mm.

iv) The reduction by cold forming from the as rolledthickness is more that 107.

v) The temperature of the material during the forming isin the range of 250 Deg.F to 900 Deg.F.

The extreme fiber elongation is to be calculated usingthe following formula:

7. extreme fiber elongation for cylindrical shell= 50t/Rf x (1 - Rf/Ro)

V. extreme fiber elongation for dished ends

= 75t/Rf x (1-Rf/Ro)

Where t = Plate thickness in inchesRf = Final center line radius in inchesRo ~ Original center line radius in inches

(equals infinity for flat plate)

The code give some exceptions for stress relief heattreatment for certain materials with cold work up to maximum 407.extreme fibre elongation.

In the case of pressure vessels fabricated from high alloysteels such as stainless steels, the shells and dished ends' canbe formed by either cold forming or hot forming. Many times,differential strain caused due to these operations will lead toresidual stresses. Many failures of these parts laden withresidual stresses, due to hostile shop floor environment orstorage environment, or service environment are reported asstress corrosion cracking (SCO of the material or intergranularstress corrosion cracking (IGSCC) on Heat Affected Zone (HAZ) ofwelds of such materials. For materials which are prone to SCC orIGSCC due to forming, it is better to prescribe stress relievingor annealing treatment with due cars to avoid sensitizationduring the heat treatment.

After forming the shells and dished ends, and other pressureretaining parts the manufacturer shall examine to ensure theyconform to the prescribed shape and meet the thickness

requirements after forming. The codes specify the tolerance onthe shape of the cylindrical shells, and the shape of crownradius, knuckle radius and flange of the dished ends.

Quality Control during assembly of parts:

During assembly of shells, dished ends, nozzles, manholeframes, nozzle reinforcement pads and supports etc., themanufacturer has to take certain precautions and exercisecontrols to ensure that the assembly of parts should not resultin unacceptable mismatch in weld fit-ups, improper orientation ofparts in assembly, unacceptable dimensions of the pressure vesseland its appurtenances and should ensure that all nozzles, manholeframes, nozzle reinforcement and other appurtenances to theinside and outside of the vessel properly fit the vesselcurvature. The codes specify the tolerance on weld jointconfigurations finished weld shape and various dimensions of thevessel.

Many a time, the shells of the vessels are rolled and weldedbefore the dished ends are received from the sub-contractor. Theshells would have been made with negative tolerance on diameterand dished ends with positive tolerance on diameter resulting inmismatch in the joint between dished end to shell leading toundesirable stress concentration at the joint and difficulty inwelding. Before sizing and forming of the shells, themanufacturer should first get the dished ends formed, inorder toget the circumference data of the dished ends. Using thesevalues the manufacturer should decide the sizes of the shellcourse in order to keep the mismatch between shells and dishedends within the tolerances.

The manufacturer should generate records of the dimensionsof each shell, dished end, nozzles and the appurtenances as aproof of achieving the dimensions and shape within the limits.

quality Control during production weldingt

The following welding precautions and controls are to beexercised by the manufacturer to achieve the desired quality.

I. Weldments fit-up configuration control.

II. Cleanliness of the Weldments and welding consumables to befree from rust, oil, moisture etc.

III. Proper storage and baking of electrodes.

IV. Purging and shielding gas quality and gas flow control inthe Gas Tungsten' Arc Welding (GTAW) process.

V. Pre-heating and interpass temperature control.

VI. Welding parameters control such as welding current, arcvoltage and heat input rate.

8

VII. Weld pro-file control.

VIII. Weld spatter control.

All the above mentioned controls during production welding willgreatly help achieving the desired quality of the welds in apressure vessel.

Non-Destructive Examination (NDE)i «.

After production welding is completed, the welds are to besubjected to non-destructive examination such as radiography orultrasonic examination for volumetric evaluation and dyepenetrant or magnetic particle testing for surface examination.The welds are evaluated as per the standards specified by thecustomer and repairs carried out, if required. The NDE reportson all the welds from part of documentation as a proof of qualityachieved on welds.

Quality Control during post weld heat treatment (PWHT)i

The post weld heat treatment is aimed at bringing down theresidual stresses generated in the pressure vessel material dueto weld metal contraction during solidification and cooling.This post weld heat treatment is done by lowering the yieldstress of the material by raising the temperature to a pointwhere the residual stress exceed the yield stress, thereby thematerial will yield and the residual stresses come downconsiderably.

The heating, soaking and cooling of the material in PWHT isdone as per a regulated thermal cycle to avoid any damage to thematerial by undue thermal shocks. During the PWHT, themanufacturer should take care that the welded structure is heateduniformly from all sides. The furnace should have an uniformdistribution of heat. The thermocouples used to monitor thetemperature o-f the pressure vessel must be calibrated ones andthe thermocouples are attached to the pressure vessel in such away that they represent the temperature of the pressure vesselglobally. The -fabrication codes recommend the rules andprecautions to be taken by the manufacturer during the post weldheat treatment. The strip chart recorder records the varioustemperatures the pressure vessel sees verses time duration duringheat treatment. From the strip chart one can perceive thethermal cycle given to the vessel during PWHT and judge whetherthe PWHT operation was carried our satis-factorily.

Non-D*structiv» Examination after PWHT operation!

Certain materials like high strength alloy steels are proneto cracking during PWHT operation in the coarse grained heataffected zone which is called as stress relief cracking or reheatcracking. Precipitation of carbides during stress relaxation atelevated temperature causes this intergranular cracking. Heat

affected zones of welds of such materials are to be subjected tonon-destructive testing after post weld heat treatment. This NDTafter PWHT is required for pressure vessels made of only certainhigh strength low alloy steels which are prone to reheat cracking.

hydraulic testing or load testing of pressure vesselsi

In the design of the pressure vessel the thickness of thematerial and welds are worked out with a factor of safetyincorporated. The ides is that even if the working pressureexceed the envisaged value by 507. , accidentally or otherwise, thepressure vessel offers resistance within the yield stress and donot deform plastically and fail. The factor o,f safetyincorporated in design of pressure vessel is verified byhydraulic pressure test.

The pressure vessel is subjected to hydraulic pressure of 1.5times design pressure. Under test conditions, it can deformelastically and come back to original size after pressure isreleased. In hydraulic pressure test, the vessel should notdeform plastically or should not fail by a fast fracture. In thehydraulic pressure test of the vessels, the welds ars looked for

-3 3any possible gross leaks of the order of 10 pascal M/sec. Ifwater cannot be introduced into the pressure vessel due tointended service, the vessel can be tested with air at a reducedtest pressure of 1.25 times the design pressure with dueprecautions taken at the time of test, as failure of vessel underpneumatic test is dangerous.

Leak Testing of welds:

In certain applications like process systems for chemical,fertilizer and nuclear plants, leakage of contents of the systemcan be hazardous or the contents may be costly to be lost byleakage. In such cases, the pressure vessels arB tested for leaktightness to a micro leak level. The choice of leak test methoddepends on the sensitivity of leak detection method and thepermissible leak rate from the vessel. Among various methods ofleak testing, three methods are generally used in the industry.They ares

a. Pneumatic leak test with soap solution applied on the welds-5 3

with a leaktightness of 10 pascal M /sec.

b. Halogen diode detector method with a leaktightness of the-6 3

order of 10 pascal M/sec.-7

c. Helium leak detection methods with leaktightness from 10 to-11 310 pascal M/sec.

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Documentationi

Documentation of the quality control activities is the mostimportant aspect of the quality assurance programme. Thedocumentation serves as a proof of the activities carried out andthe quality achieved by the manufacturer. The documentation forthe fabrication of pressure vessels should cover activities suchas

1. materials and welding consumables identification2. welding qualifications.3. quality control personnel qualification.4. forming of parts.5. assembly of parts6. production welding quality control7. non—destru^ t.ive testing.8. post weid heat treatment.7. pressure testing and10. leak testing.

Documentation of quality control functions guides the shopfloor people to act and ensure the quality checks at appropriatestages.

The documents of each activity cf quality control andtesting are to be &ndcrbed by the manufacturer's, quality controlpersonnel and the customers quality surveyor. These endorsementsmake the manufacturer and the inspecting agency responsible forthe quality achieved. The documentation gives the user goodconfidence that ths> prsjssure vessel will give optimum servicewith safety during the design life of the plant in which it isinstalled. A few typical documents which Bre generated in afabrication shop as part of quality assurance documentation areappended to this article.

Pre-planning for quality assurance on pressure vessels!

In quality assurance of pressure vessels, inspite of the best ofefforts by the engineers of the manufacturer, surprises do cropup. This is because of long production cycle involving manycomponents, machines, operations, processes and testing in thefabrication and certification of pressure vessels.

It was observed that even with a good quality assurance system,many a time unforeseen problems in manufacture and testing ofpressure vessels baa ised to deviations resulting in costlyrepairs and loss of time in production schedule or occasionallyeven in total rejection of the vessel leading to shift in projectschedules costing a lot more money.

In order to prevent these unforeseen problems in the fabricationof pressure vessels, a sound pre-planning to foresee the problemsis to be undertaken much before the fabrication starts. This ispossible by quality assurance engineers who have considerableknowledge and experience in the fabrication and inspection of

II

pressure vessels and who have seen a number of deviations andrejections. In the -fabrication o-f pressure vessels, unforeseensituations may be arising out o-f the -following areas:

1. Selection of materials and consumables.2. Drawings and specifications.3. Manufacturing processes such as,

i) Dished end to shell fit-upii) welding distortion and shrinkageiii) Exotic metals weldingiv) Cold working, hot working, and residual stresses.

4. Heat treatment after forming and welding.5. Hydrostatic pressure testing.6. Leak testing.7. Surface treatments.8. Transportation.9. Storage conditions at site and10. Suitability to site conditions.

In each of these areas the quality assurance engineers beforestarting the fabrication should identify the possible problemsand document procedures and methods to prevent these problemsduring fabrication and testing.

The quality assurance system established in a company will not becomplete and effective without this pre-planning for qualityassurance of pressure vessels. By a systematic approach of pre-planning for quality assurance of pressure vessels, the qualityassurance engineers can ensure that the pressure vessels arefabricated expeditious meeting the quality standards. Theintegrity and reliability of the pressure vessel is high if it isbuilt in a shop with a quality assurance system.

References

1. ASME Code Sec.VI11 Div.I, 1989 for pressure vessels.

2. ASME Code Sec.111, 1989 for nuclear components.

3. ASME Code Sec.IX, 1989 for welding and brazing qualification

4. Technical paper entitled "Quality Assurance on WeldedStructures" by Mr,B-S.C.Rao, presented in the NationalWelding Seminar, 1989 at Delhi.

5. Technical paper entitled "Pre-planning for Quality Assurancein the fabrication of pressure vessels" by M/s.B.S.C.Rao,K.Shanmugam, M.Bopalakrishna & B.Mai 1ikarjunan, presented inthe National Welding Seminar 1990 at Bombay.

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12

Discard

Reduced Section

Root Bend

Face Bend

Root Bend

Face Bend

Reduced Section

Discard

This Piece

Tensile Specimen

Specimen

Specimen

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Specimen

Tensile Specimen

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LFigure 1 A.

Plates 1-6 to 19 mm Thick

Root Bend

Reduced SectionTensile

Face Bend

Root Bend

Reduced Section Tensile

Figure 1 BOrder of Removal and Types of Tests for

Welding Procedure QualificationDRAWN SJ

13

PRIVATE LIMITED

ME AS U REM E NT B ECORJD SHEET

W. O. No.

p.o. No. i t ,Drg. No.

BOILER No.

DESCRIPTION: | _ £ l O ~ - ' ° 3 > 7

INSPECTION OF BOILER / FURNACE / SHELL No.INSPECTION (1) 1-1 & 1-2 After Marking before cutting

(2) 2-1 to 2-6 After cutting Rolling and Tack welding but before welding.(3) 3-1 to 3-3.3 After welding & Rerolllng.(4) 4 to 4-3 After Radiography (.. V C i »

SI. No.

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1.2

2.1

2-2

2-3-1

2-3-2

2-4

2-5

2-6

3-1

2-1

3-2-2

3-3-1

3-3-2

3-3-3

4

4-1

4-2

4-3

DESCRIPTION

Developed length

Marked width

Length oi Bhell

Circumference before welding

Ovallty . Near end

Ovallty . Far end

Vee groove size

Root gap

Overlap

Circumference after welding

Template profile near and

Template profile far end

Ovallty . Near end

Ovallty . Far end

Weld reinforcement

Radiography

Spot 100%

Joint Identification

DRGDIMENSION

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REMARKS

QUALITY CONTROL

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CUSTOMER

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U S.SAKTHI CONSTRUCTIONS

BRAVING NO: 1-810-1037 RST.04 REPORT NO: 035H

DESCRIPTION: CD 1004 DATS: 30-04-1991

FITUP REPORT

SL. NO. PART NO.LENGTH

API) ACT.CLEANINGREPORTNO,

01. P01 to P06 2450 2450 to 2452 025H dt,

18.4.91.

02. P06 to 029 803 803 025H dt,

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FITUP CLEARED FOR WELDING.

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SAKTHI CONSTRUCTIONS

15

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PRIVATE LIMITEDRADIOGRAPHIC REPORT

Report No. O*D' I H

Date:

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Maker No.:

Work Order No. :

Drawing No.: / - £ /

Component: C •£> "

Material Specification :

Single Wall thickness:

Diameter 3 5 ^

— (C>37

/ -

Procedure No.:,

Radiation Source:

X-ray: KV.. Jit5L....FFD....3.Q.IR-192: Source

StrengthSFD....

Exposure time:

Type of film:Sensitivity:Penetrameter:Screen :

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• too..Density

Customer/Inspection Agency/Date

16

M/s. SAKTHI CONSTRUCTIONS/MADRAS

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Drg.No.: 1-610-10^7 Jiev.07

Description: SMJMg CD 7004

'sport No. •

Date: l-$-6-3)

HYDROSTATIC TEST

Test Pressure

Te3t Medium

No* of Gauge

Range

Holding Time

1.5 Kg/en

DM Vater

0 to 4 Kg/cm"

1 Hour

The pressure was held at 1.5Kg/cm (G) for a period of 1 Hour and tie

weld joints were checked for any leakage. No leakge was found and

no pressure drop was noticed.

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FINAL. DIMENSIONAL REPORT

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