ENHANCING COMPETENCY IN FABRICATION OF TUBULAR

REACTOR FOR PETROCHEMICAL INDUSTRY THROUGH NDE

Pragnesh Patel, Ajit Anupam, Atul Patel, Niyant Mehta, Pemmaraju Raghavendra

Heavy Engineering Division (HED), Larsen & Toubro Ltd, Hazira, Gujarat, India

Pragneshj.patel@larsentoubro.com

Abstract:

Tubular reactor is an essential component of mono ethylene glycol plants in Petrochemical Industries. Manufacturing of such reactors is a technical challenge for capital goods manufacturing

industries, as it demands high quality and reliability throughout the product lifecycle. Non Destructive Testing (NDT) of tube to tube-sheet weld joints (approx.. 26,000 nos.), limpet coil weld

joints, tube sheet to shell joint and tube sheet long seam joint are the major challenges involved in its manufacturing. These weld joints have to restrain high stress and fatigue load (in service) due to

steam generation. This calls for a very high quality- control methodology.

The main challenges involved with the fabrication of tubular reactors include, safety & reliability

which is intertwined with quality. Radiography Testing (RT) of tube sheet weld joints having

thickness more than 400 mm requires very high energy radiation generation equipment such as

Linear Accelerator (LINAC) 12 MeV. The same requires safety, legal requirements and critical

handling of such tube sheets having weight more than 150T. Also avoidance of failure in fillet joint

of tube to tube sheet due to high pressure (during pneumatic test and in-service) is an important

aspect. To overcome these challenges it is necessary to select a reliable NDT method for the weld

joints.

Automatic UT (especially Time of Flight Diffraction -TOFD) is most reliable method to replace RT. To meet applicable ASME & specification requirements, considerable experimentation is involved

in developing and fine tuning this methodology which consists of feasibility study, developing scan plans/technique sheet using advanced simulation software and validating on similar mock up blocks.

After several trials and successful demonstration, Penetrant Testing (PT) was done at different intermediate stages of tube to tube sheet manufacturing cycle (after completion of first weld pass,

after air test, after final weld pass & after helium leak test) with best quality penetrant system with enhanced and optimized dwell time.

We have successfully developed & implemented a zero tolerance testing methodology involving the

above techniques at various stages of manufacturing. This has been a milestone achievement and the

current paper journeys through this voyage and describes the process in detail.

Keywords: Tubular Reactor, Tube sheet, in-service failure, Dwell Time, Linear Accelerator.

Introduction:

Tubular reactor is an essential component of mono ethylene glycol plants in petrochemical

industries. The reactor should be capable of flexible operation in long campaigns without

interruption and must conform to high standards of safety. These units consist of large shell and

tube heat exchangers with very large diameters up to 26 ft. (8 m) and more than 10,000 tubes. The basic ethylene oxide formation reaction is exothermic and the weld joints have to restrain high

stress and fatigue load (in service) due to steam generation. This calls for a very high quality- control methodology. Thus the manufacturing of tubular reactors is a technical challenge for capital

National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)

Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17856

goods manufacturing industries, as it demands high quality and reliability throughout the product lifecycle.

This paper discusses the critical joints involved in tubular redactors & NDT of critical joints with

Safety. The critical joints of tubular reactor includes Tube sheet weld joints having thickness more than 400 mm, Tube to tube-sheet weld joints & limpet coil joints.

RT of high thickness Tube sheet long seam joints requires very high energy radiation generation

equipment such as Linear Accelerator (LINAC) 12 MeV. Also safe handling and positioning time

will increase to do RT of 150T tube sheet long seam welds. There is probability of radiation hazard

due to very high energy radiation equipment. So it is necessary to find the alternative of this

method. To overcome limitation of RT, TOFD was done in lieu of RT in compliance with ASME

codes and standards. To withstand the high pressure in pneumatic test and helium test, it was

required to ensure the seal joint of tube to tube sheet perfectly. In order to achieve the said

requirement, PT of tube to tube sheet was performed at various stages with stringent quality levels

and best quality of consumables available.

Objective & Plan:

Our aim is to achieve the best quality levels along with safe handling and reduced cycle time. The

journey begins with the feasibility study and selection of suitable NDE methods

To achieve the same, following steps were taken.

1) Development of validation block

2) Arrangement of required equipment and accessories

3) Procedure development and validation

4) Establishment of procedure

5) Training to the NDE personnel

6) Qualification

7) Application on the equipment/welded joint in production

Selection of suitable NDE Methods:

Considerable experiments and literature study led us to the conclusion of selecting the most suitable

technique for Tube sheet long seam weld joints is Time of Flight Diffraction (TOFD).Thus TOFD was done in lieu of RT in compliance with ASME codes and standards. TOFD is a flaw sizing and

locating method based on the arrival time of flaw tip diffracted waves and the position of ultrasonic transducers. Data for each joint were captured in the form of digital images which can be stored for

ever. The handling & positioning time of the heavy thickness tube sheet can be minimized to zero due mobilization of TOFD equipment.

Faster and reliable inspection is achieved as TOFD is having higher Probability of Detection (POD) compared to other NDE methods. No additional requirements of legal permissions like in RT

AERB (Atomic Energy Regulatory Board) permission.

TOFD Procedure Development & validation:

After various trials & experiments on the validation block a detailed procedure with the entire

essential elements was prepared and demonstrated to meet the requirement of ASME code. The

development of procedure involves qualification of personnel for examination, equipment &

accessories scan plans, validation blocks, evaluation, acceptance rejection criteria & reporting.

The performance demonstration is witnessed by customer representative as well as Authorized

inspector for validation of procedure, equipment & personnel qualification.

Fig. 1 Experimental set up & Procedure validation

Arrangement of required equipment and accessories:

Calibrated Ultrasonic system “Sonoma tic Microplus-2″ Digital flaw detectors with single axis

encoded hand scanner” shall be used for the testing. Software µ Line II Version 1.0 or µ Tofd II)

shall be used for scanning and post processing of data. Special type TOFD Broad Band

Longitudinal Wave probe of diameter 12/20 mm shall be selected for higher thickness.

Development of the TOFD UT Validation Block:

In this case, block material was used for the test block as of the equipment material enabling to

evaluate flaws much accurately. This process involves wire cutting to create artificial flaws with in base material of test block. Cutting is done at various depths as per the sketch shown and

subsequently these locations are welded leaving flaws within the block. Each step involves critical

operation of fit up, controlled welding keeping in view the critical defects. . Finger print of the

block was ensured certifying the dimensions of the defects for validation of block. Close monitoring

of fabrication helped to produce the right flaws.

Same product form and material specification or equivalent P-Number grouping was used to

fabricate validation block. Prior to welding / making SDH, the block material shall be completely

examined with a straight beam search unit.

The calibration block was treated at least the minimum tempering treatment required by the

material specification for the type and grade.

Scan plan was then validated to locate the known discontinuities with the stringent tolerances. A

sample of block sketch is given in Figure-2

Fig. 2 Drawing of TOFD UT Validation Block

Scan Plan for TOFD Scan:

Procedure includes various scan plans along with the setup description. Scan plans were further

fine-tuned after the demonstration on the qualification blocks and resulted in an infallible scan plan.

Software ES Beam Tool 5 or Beam setup plotter shall be used as a tool to determine probe centering separation (PCS) and volumetric coverage of weld. Probes with different crystal sizes and

frequencies have been used at various locations in order to ensure complete coverage.

Far Surface:

2MHz with 12/20mm dia TOFD probes with 60° wedges shall be applied. A probe center

separation (PCS) shall be selected to cover the weld and HAZ region.

Bottom Surface:

2MHz with 12/20mm dia TOFD probes with 45° wedges shall be applied. A probe center

separation (PCS) shall be selected to cover the weld and HAZ region.

Near Surface: 10 MHz, 6mm dia TOFD probes with 70

0 wedges shall be applied. A probe center separation (PCS)

shall be selected to cover the near surface.

Blind zone of TOFD:

TOFD suffers from one major drawback of not detecting a discontinuity in the so called “blind

zone” creep wave UT is used to compliment the scanning process by characterizing the indications

observed by TOFD.

Zone wise Scan plan mentioned in Figure-3, all the zones were covered with minimum set of

probes.

All scans to cover the full weld volume 70Deg Scan

45Deg Scan 60 Deg scan

Fig .3 Depth Wise Zone coverage

Image Evaluation:

When indications are detected the software can easily show the TOFD image as indicated in Figure-

4 on which all kind of evaluation and sizing can be done.TOFD image, visualize in post processing

software (TOFD) for evaluation data and sizing indications. TOFD technique does not depend on

amplitude of the flaw in evaluation of indication. For this technique quality image require in terms

of contrast phase and tip resolution. Sample image of tip resolution & contrast phase is given in

figure-4

Fig. 4 Type of Tip resolution and Phase Contrast

Flaw Sizing:

Flaw shall be sized in accordance with the procedure qualified by demonstration on similar

reflectors at similar material depths. The dimension of the flaw shall be determined by the rectangle

that fully contains the area of the flaw. (Refer to fig .5)

Good Tip

resolution & contrast

Poor Tip resolution

Poor Contrast

� The length (l) of the flaw shall be drawn parallel to the inside pressure retaining surface of

the component.

� The depth of the flaw shall be drawn normal to the inside pressure retaining surface and

shall be noted as “a” for a surface flaw or “2a” for a subsurface flaw.

Fig. 5 Surface & subsurface indication

When the indication is determined to be a flaw, the acceptance criteria of PARA 7.5.5 of ASME SEC VIII Div. 2, FIG 7.8 - 7.10 shall apply (According to the thickness of the Job).

PT for tube to tube sheet joint:

Tube to tube sheet joint is critical as it is subjected to high pressure and variant temperature during

in-service. Despite the acceptance levels as mentioned in ASME code and customer specifications,

we have adopted “No indication” acceptance criteria for the entire joint. Customized block was

prepared with known defects having variation in diameter and depth (Figure-6).This variation was

necessary to study and understand the change of indication response with respect to the size.

Consumables from various manufacturers were invited for the demonstrations and accordingly the

trials are taken. A photo demonstrating the variance in response is shown in Figure- 6A & 6B.

Fig.-6: A block with known defects having variance in dimensions

Fig.-6A & 6B: Indication after the test performed on consumable A & B

Various trials were taken with different consumables on specially designed. Out of all trials,

consumables resulted with best indication were selected with the stringent dwell time. Qualification of LPT Personnel was done separately to meet the requirement. A replica of actual

joint was prepared for the purpose and accordingly allowed to work on actual job. For the entire journey of project such dedicated teams of specially qualified LPT personnel were used. Quality

check for the performance of LPT was done periodically through Reproducibility and Repeatability (R&R) technique, where a second check on randomly selected seams was performed on the joint

which were already LPT tested. Since the number of tubes to be tested with LPT was high at

various stages, monitoring of quality was a big challenge; zone wise monitoring through a printed

book was adopted to ensure the quality levels of such joints.

LPT was performed at various stages (i.e. Root run, complete weld, after pneumatic test, after

helium test), the entire setup has not only resulted in successful helium and pneumatic test with zero observation but will also ensure for its long term serviceability.

UT of half coil welds joints:

The half coil joints need to withstand high pressure fluid during service. So code demands tests like

helium leak test & air test. To withstand the high pressure test, such joint was required to be monitored strictly. To ensure this welds quality we have to perform UT in addition to other NDE

methods such as Penetrant Test, Helium Leak test and visual test. UT of this half coil weld joints is difficult due to coil shape and its smaller diameter.

Procedure made for all detail with essential variables. Procedure includes various scans with custom made small diameter probe angles with special curved shoes. Calibration piping block made as per

code demands & successfully completed of UT of coil joints. Block drawing mentioned in Figure-7.

When the indication is determined to be a flaw, the acceptance criteria of PARA 7.5.4 of ASME

SEC VIII Div. 2 will be applicable. (According to the thickness of the Job)

Fig. 7 Half coil weld calibration block drawing

Conclusion:

TOFD is relatively unaffected by the size, shape, location or orientation of flaws. TOFD has

proven to be a great tool for flaw detection and sizing with safety & less time consumption

method against RT for thickness up to 450 mm.

With the best combination of NDE method, reliability and in-service life can be enhanced for tubular reactors.

Improvement is never ending process and one has to keep stretching beyond boundaries.

References:

[1] American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, Section

VIII & Sec V,2010 Edition 2011a addenda.

[2] Sony Baby, T. Balasubramanian, R.J. Pardikar, M. Palaniappan and R. Subbaratnam, Time

of flight diffraction (TOFD) technique for accurate sizing of surface breaking cracks,

National Seminar on ISNT, Dec 2002, Chennai, India.

[3] I&T Nardoni Institute,TOFD reference collection maps as per ASME Code Sec.V UNI

EN 583-6