OISD - 128

Amended edition

FOR RESTRICTED CIRULATION

No.

INSPECTION OF

UNFIRED PRESSURE VESSELS

OISD - STANDARD-128First Edition, November 1988

Amended edition, August, 1999

OIL INDUSTRY SAFETY DIRECTORATEGovernment of India

(Department of Petroleum & Natural Gas)

I

OISD – STD- 128 First Edition, November, 1988 Amended edition, August, 1999

FOR RESTRICTED CIRCULATION

INSPECTION OF

UNFIRED PRESSURE VESSELS

Prepared by

COMMITTEE ON INSPECTION OF STATIC EQUIPMENT

OIL INDUSTRY SAFETY DIRECTORATE2nd Floor, KAILASH BUILDING,

26, KASTURBA GANDHI MARG,NEW DELHI - 110 001.

II

NOTE

OISD publications are prepared for use in the Oil and gas industry under Ministry of Petroleum and Natural Gas. These are the property of Ministry of Petroleum and Natural Gas and shall not be reproduced or copied and loaned or exhibited to others without written consent from OISD.

Though every effort has been made to assure the accuracy and reliability of the data contained in these documents, OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting from their use.

These documents are intended only to supplement and not to replace the prevailing statutory requirements.

Note 1 in superscript indicates the modification/changes/addition based on the amendments approved in the 17th Safety Council meeting held in July, 1999.

III

FOREWORD

The Oil Industry in India is nearly 100 years old. Due to various collaboration agreements, a variety of international codes, standards and practices have been in vogue. Standardisation in design philosophies and operating and maintenance practices at a national level was hardly in existence. This, coupled with feed back from some serious accidents that occurred in the recent past in India and abroad, emphasized the need for the industry to review the existing state of art in designing, operating and maintaining oil and gas installations.

With this in view, the Ministry of Petroleum & Natural Gas, in 1986, constituted a Safety Council assisted by Oil Industry Safety Directorate (OISD), staffed from within the industry, in formulating and implementing a series of self regulatory measures aimed at removing obsolescence, standardising and upgrading the existing standards to ensure safer operations. Accordingly, OISD constituted a number of Functional Committees comprising of experts nominated from the industry to draw up standards and guidelines on various subjects.

The present document on “Inspection of Unfired Pressure Vessels” has been prepared by the Functional Committee on “Inspection of Static Equipment”. This document is based on the accumulated knowledge and experience of industry members and the various national and international codes and practices. It is recognised that failure of internals of a pressure vessel may only affect its performance and at most times may not materially affect the safety of the vessel. However, it is also recognised that failure of an internal component may load to unit upsets which in turn could lead to a leak of hydrocarbons. Keeping this in view the Committee decided to include inspection of internals also as a part of this standard. This document is meant to be used as a supplement and not as a replacement for existing codes and practices. It is hoped that the provisions of this document, when adopted may go a long way to improve the safety and reduce accidents in the Oil and Gas Industry. Users are cautioned that no standard can be a substitute for the judgment of a responsible qualified inspection Engineer. Suggestions are invited from the users, after it is put into practice, to improve the document further.

This standard in no way supercedes the statutory regulations of CCE, Factory Inspectorate or any other Govt. body which must be followed as applicable.

Suggestions for amendments to this document should be addressed to

The Co-ordinator, Committee on

“Inspection of Static Equipment, Oil Industry Safety Directorate,

2ND Floor, “Kailash”26, KasturbaGandhi Road,

New Delhi – 110 001

IV

COMMITTEEON

INSPECTION OF STATIC EQUIPMENTList of Members

---------------------------------------------------------------------------------------------------------------------------------------Name Designation & Position in

Organisation Committee---------------------------------------------------------------------------------------------------------------------------------------1. Sh. R.K. Sabharwal CMNM-IOC (R & P) Leader

2. Sh.A.S. Soni DGM (P)-ONGC Member

3. Sh.R.H. Vohra DGM-IOC (Mkt.) Member

4. Sh.D.P. Dhall CH INSP & AE MGR-BPC (REF) Member

5. Sh.P. Dasgupta SIPM-IOC ( R & P) Member

6. Sh.I.M. Advani MGR INSP-(PROJ) HPC (REF) Member

7. Sh.R.M.N. Marar Jt.Director OISD Member Co-ordinator.

---------------------------------------------------------------------------------------------------------------------------------------

In addition to the above, several other experts from industry contributed in the preparation, review and

finalisation of this document.

V

INSPECTION OF UNFIRED PRESSURE VESSELS

CONTENTS

SECTION PAGE NO.

1.0 Introduction

2.0 Scope

3.0 Definition and Types of Pressure Vessels3.1 Pressure Vessel3.2 Types of Pressure Vessels

4.0 Role of Inspection

5.0 Inspection Tools

6.0 Inspection of New Pressure Vessels During Fabrication

7.0 Check List for Inspection of Pressure Vessels Prior to Erection and Commissioning.

8.0 Likely Locations of Metal Wastage8.1 Main Fractionating Towers of Crude Distillation Unit8.2 Crude Distillation Unit-Overhead Accumulators8.3 Dehydration, LP./HP. Separators8.4 Vacuum Distillation Columns8.5 Reactors in Reformers8.6 Reactors in FCCU8.7 Regenerator in FCCU8.8 Orifice Chamber in FCCU8.9 Coke Chambers8.10 Bullets and Spheres8.11 Vessels in Low Temperature Service8.12 Ammonia Storage Vessels8.13 Columns & Vessels in Diethyl Amine & Monoethyl Amine Service

9.0 Frequency of Inspection

10.0 Inspection Procedures10.1 Inspection of Columns10.1.1 External Inspection10.1.2 Internal Inspection10.1.3 Inspection of Lined Columns10.1.4 Inspection of Pressure Vessels in FCCU10.2 Inspection of Vessels 10.2.1 External Inspection 10.2.2 Internal Inspection10.2.3 Riveted Vessels 10.3 Corrosion Coupons/Probes 10.4 Safety Relief Devices

11.0 Retiring Thickness

VI

12.0 Inspection During Maintenance 12.1 Weld Build Up 12.2 Nozzles Replacement 12.3 Partial Replacement of Shell Plates and Domes 12.3.1 Hydrostatic Test 12.3.2 Pneumatic Testing 12.4 Repair of Cladding and Striplining 12.5 Repair of Painted and Rubberlined Areas 13.0 Documentation

14.0 References

ANNEXURES

I Inspection Check List for Column in Service II Inspection of Welding III Hydrogen Blisters-Inspection, Evaluation and

Repair of Pressure Vessels

VII

INSPECTION OF UNFIRED PRESSURE VESSELS

1.O INTRODUCTION

The contents of a pressure vessel are always under pressure. Any deterioration of the vessel could lead to a large release of hydrocarbon vapour and consequent creation of a flammable atmosphere. Timely inspection and Preventive Maintenance will go a long way in ensuring safe operation of pressure vessels.

2.0 SCOPE

This standard covers the minimum inspection to be carried out during operation and maintenance of pressure vessels. The standard specifies frequency of inspection, inspection procedures, areas to be inspected in the pressure vessels and inspection during and after repairs. This standard also covers in brief, fabrication and pre-commissioning inspection checks.

3.0 DEFINITION AND TYPES OF PRESSURE VESSELS

3.1 PRESSURE VESSEL

A pressure vessel is defined as a vessel designed to safely withstand an internal pressure in excess of 1.05 Kg/Sq.cm. Some vessels in a refinery may be subjected to external pressure caused by an internal vacuum or by a fluid pressure between an outer jacket and the vessel wall. Such vessels are usually inspected in the same manner as vessels with internal pressure.

3.2 TYPES OF PRESSURE VESSELS

The following four types of pressure vessels are normally used in hydrocarbon service: i) Cylindrical vessels with flat, conical,

toriconical, torispherical, semiellipsodial or hemispherical heads.

ii) Spheroids.iii) Spherical.iv) Jacketted Vessels

Cyllindrical vessels can be vertical or horizontal and may be supported in different ways.

4.0 ROLE OF INSPECTION

The following are the activities of the inspection division:

i) To inspect, measure and record the deterioration of materials and to evaluate present physical condition of the pressure vessel for its soundness to continue in service.

ii) To corelate the deterioration rate with design life for further run.

iii) To determine causes of deterioration and to advise remedial measures.

iv) To recommend/forecast short-term and long-term repairs and replacements to ensure further run length on the basis of economics and safety.

v) To advise regarding equipment/component replacement so that procurement action could be initiated.

vi) To inspect while doing the repairs and to accept after completion of repairs.

vii) To maintain uptodate maintenance & inspection records and history of pressure vessels.

viii) To keep the concerned operating and maintenance personnel fully informed as to the condition of the various pressure vessels.

ix) To advise regarding schedules of pressure vessels inspection and also statutory requirement schedules.

5.0 INSPECTION TOOLS

Tools required for Pressure Vessels Inspection are as follows:

i) Ultrasonic Thickness Gauge.ii) Ultrasonic Flaw Detector.iii) Radiography Equipment.iv) Magnetic Particle Testing Kit. (Wet

Fluorescent Type)v) Metallographic Equipment.

1

vi) Infra-red Scanner for Thermography.vii) Dye Penetrant Kit.viii) Paint Thickness Gauge.ix) Shore Hardness Meter.x) Adhesion Testing Kit.xi) Holiday Detector.xii) Spark Tester.xiii) Pit Depth Gauge.xiv) ID & OD Gauges.xv) Plumb & Bob.xvi) Magnet.xvii)Measuring Tape.xviii) Magnifying Glass.xix) Temp. Indicating Crayons.xx) Inspector’s Hammer.xxi) Straight Edge.xxii)Safety Torch.

6.0 INSPECTION OF NEW PRESSURE VESSELS DURING FABRICATION

Pressure vessels are designed as per various codes available like IS, BS, ASME etc. The design and consequently, inspection of pressure vessels shall be as per any one code only and not by a combination of different codes since factor of safety used while arriving at design stresses for different materials vary with codes. Inspection of the new pressure vessel at the time of fabrication is done as per the inspection requirements of relevant code.

Inspection shall be carried out in the following stages:

i) Study of the tender document and all the technical specifications.

ii) Identification and inspection of the materials.

iii) Check bonding of cladded plates, wherever applicable before and after forming.

iv) Approve the welding procedures in accordance with tender specifications/ code requirements.

v) Carry out welders performance qualification test as per the code.

vi) Check nozzle orientation, joints fitup and overall dimensions as per the approved drawings.

vii) Ensure that welding is carried out as per agreed welding sequence and welding procedure with approved electrodes and tested welders.

viii) Inspect the weld joints for proper quality during welding.

ix) Ensure proper post weld heat treatment whenever required.

x) Inspect weld joints by radiography as per the code.

xi) “Ensure proper heat treatment is carried out for cold formed parts (like dishends, tori cones etc.) as per relevant codes / drawings. NOTE 1

xii) Ensure all internal attachments are welded as per drawing. NOTE 1

xiii) Witness pneumatic tests of the liner welds (lined nozzles) and reinforcement pads. NOTE 1

xiv) Inspect for ferrous contamination of corrosion overlay welding/flange facings etc., as per code. NOTE 1

xv) Ensure proper welding of insulation bearing nuts wherever applicable. NOTE 1

xvi) Ensure proper welding of refractory bearing lugs/hexes etc. wherever applicable.”NOTE 1

xvii)Ensure repairs and reinspection of damaged parts, if any, are carried out before giving clearance for hydrostatic testing.

xviii) Check the procedure for various types of testing.

xix) Ensure all the tests are carried out strictly as per the approved procedures.

xx) Inspect quality of painting as specified in the tender document.

xxi) Check that vessel has been stamped as per code.

xxii)Prepare and certify the relevant documents.

7.0 CHECK LIST FOR INSPECTION OF PRESSURE VESSELS PRIOR TO ERECTION AND COMMISSIONING.

The check list format shall contain the following information.

Equipment No.PlantDutyPurchase Order No. & DateSerial No. & TypeManufacturerMain DimensionsMaterial of ConstructionMax. Allow. working pressure/VacuumMax. Allow. Working TemperatureStress-relievedRadiographyHydrostatic Test PressureErection ContractorContractor’s InspectorCompany’s Inspector

2

Date of Inspection

CHECKLIST

The following checks shall be made prior to commissioning of new pressure vessels.

CHECKS REMARKS1. Check for proper

alignment of supports.2. Check nameplate

attachment.3. Check nameplate rating.4. Check foundation bolts

and shims.

CHECKS REMARKS

5. Inspect shell wall for out of roundness, bulges and dents.

6. Inspect visually, weld joints.

7. Check alterations made during plant construction.

8. Check wall thickness of shell and nozzles.

9. Check and test reinforcement plates and test holes.

10. Check and test nozzles, facings, gaskets and bolts.

11. Check outside bolting and stiffening rings.

12. Check insulation and fire proofing.

13. Check insulation protection.

14. Check painting quality.15. Check internals.16. Check and test internal

lining of shell nozzles.17. Check for internal

cleanliness before final boxing up.

18. Check whether design of vessel and foundation allows vessel to be hydrotested in situ.

19. Test shell hydrostatically, if any alteration has been made in the shell.

20. Check whether proper relief valve is installed.

21. Check that connected pipings do not strain the vessel nozzles.

22. Check for verticality of columns and tall vessels. Note 1

8.0 LIKELY LOCATIONS OF METAL WASTAGE

8.1 MAIN FRACTIONATING TOWERS OF CRUDE DISTILLATION UNIT

The fractionating column bottom and

internals are subjected to high temperature corrosion due to presence of sulphur whereas column top is prone to low temperature acidic corrosion because of salts and H2S present in the crude. The crude containing naphthenic acid also causes the corrosion of the column shell, and the same is pronounced in the sections where temperature ranges from 200oC to 400oC. Severity of naphthenic acid attack is higher where the turbulent conditions exist. The impingement plate particularly in the columns where the feed nozzle is radial is subjected to severe erosion. Noticeable corrosion or erosion is also generally observed where the steam impinges the shell. The dislodging of the trays (particularly valve trays) is common due to steam surge.

Galvanic corrosion is also observed at the location where cladded shells and uncladded shells join together. Where the lining is bulged, the parent metal is subjected to corrosion. Liquid level corrosion is noticeable in the top tray downcomer collectors particularly at the reflux collector trays.

8.2 CRUDE DISTILLATION UNIT- OVERHEAD ACCUMULATORS

Pronounced corrosion is generally noticed at the interface level of water and hydrocarbons. Mostly the corrosion is noticed in the bottom portion of the accumulators, which are not internally protected.

8.3 DEHYDRATOR, LP/HP SEPARATORS

Dehydrators & LP/HP separators of crude stabilising units are likely to get corroded in the bottom portion from 5 to 7 O’ clock position due to presence of salt & water. 8.4 VACUUM DISTILLATION COLUMNS

The sections where the turbulent conditions exist like impingement plate/flash zone are subjected to corrosion erosion due to naphthenic acid and sulphur in the crude.

3

Columns, shells are also liable to corrode opposite to impingement plate due to rebound of fluid. Weldments and Heat Affected Zone are also susceptible to corrosion.

8.5 REACTORS IN REFORMERS

Generally the reactors are of low alloy steels like 2-1/4Cr-1 Mo or cladded with stainless steel. Due to this superior metallurgy metal wastage is generally not observed. However, the following locations give indication of deterioration/cracking. i) Cracking of weldment of grid with shell at

the bottom..ii) Baskets for collecting the catalyst dust are

also prone to corrosion.iii) Liners installed in the big diameter nozzles

are susceptible to bulging due to failure of weld joints at the end.

iv) Reactors made of low allow steel specially 2-1/4Cr-1 Mo are prone to temper embrittlement. (Temper embrittlement is defined as a loss of ductility and notch toughness due to post weld heat treatment or high temperature service above 3700 C.)

8.6 REACTORS IN FCCU

The shell, riser O.D. and portion of the cyclone dipleg O.D. are severely attacked in the riser extension type of installation. Where the grids are still used, erosion is found fairly uniform over most of the grid when high velocities are employed through cyclones. Some erosion occurs to the wall of the plenum chambers and to the top head where small plenums are in use. For those reactors with only two cyclones, high swirl of catalyst through the nozzles cause severe erosion.

The refractory lining generally stands up quite well in reactor cyclones. Normal repairs require some resurfacing of small areas or replacement of small localised sections. There are two kinds of valves at the bottom of dipleg. The one mostly in use is the flapper type with counter weight. The other is the trickle valve type, with the flapper plate hanging over the opening suspended by rings. The flapper type of valves are subjected to erosion.

8.7 REGENERATOR IN FCCU

In the plate type of distribution grid, erosion to the grid plates is a common phenomenon. Due to considerable vibration and heat differentials cracking of the grid plate can also take place.

In pipe type of grid design where seals are still in use, these seals may leak. Migration of catalyst past the seal could destroy a pipe grid in a very short time.

The branches (pipe coming off each lateral) experience metal loss, mainly to the top circumference. Occasionally the steam coming out of jet blowing directly into another branch, lateral or end plate creates erosion. Warpage of pipe grids can take place due to overheating during startup and during operation. Many 5% Cr. grids experience weld cracking.

The failure of refractory lining on the

shell is another common problem. During operation, it may cause hot spots on the shell. Erosion of cyclone dip legs, failure of cyclone welds alongwith weld of cyclone supports may also take place. 8.8 ORIFICE CHAMBER IN FCCU

Erosion of the double disc sliding valve gates, erosion at the core near the inlet and at holes/sleeves in the grid plate are common problems. Bulging or cracking on the shell adjacent to the gridplate also may take place. The erosion problems in orifice chamber are caused by the catalyst carryover from the Regenerator.

8.9 CHOKE CHAMBERS

Cracking of skirt and shell weld joints is quite common particularly in the coke chambers where the skirt is not of slotted type. In the coke chambers generally feed, stripping steam and water quench nozzles are installed at the bottom. Due to thermal cyclic shocks lower portion of the coke chamber gets bulged. At the advanced stages of bulging, circumferential welds which act as stiffeners get cracked in the axial direction. However, the effect is pronounced just opposite the feed entry nozzle at an elevation of about 1 meter. The chambers where the feed enters from the top, bulging is confined in the top portion. The conical portion of the coke chamber where the feed enters is also prone to cracking at the knuckle portion. Bottom flange to shell weld joint, weld joints of feed, water quench and steam nozzles are also likely to crack under thermal cyclic conditions.

8.10 BULLETS AND SPHERE

The corrosion and scaling is generally confined to the bottom between 5 to 7 O’ clock

4

positions probably due to the presence of corrodents like H2S and water. LPG storage vessels are also prone to stress corrosion cracking. The circumferential weld joints below the equitorial plates in the LPG Horton spheres are more prone on such cracking.

8.11 VESSELS IN LOW TEMPERATURE SERVICE

Vessels in low temperature service e.g. in KTU of refineries and propane circuit of LPG recovery units of Gas Processing Plants are prone to external corrosion due to faulty insulation, which causes condensation of the vessels. The severity of corrosion increases in case of corrosive atmosphere as in KTU.

In these vessels internal corrosion due to moist So2 where condensation can take place, also occurs. Internals and shell are affected due to this.

8.12 AMMONIA STORAGE VESSELS

Generally the storage vessels are fabricated from Carbon steel and Nickel steels. For the operating conditions prevailing at refineries, material of construction used for Ammonia storage vessels in the refineries are carbon steels. The weld joints of C.S. vessels are prone to stress corrosion cracking particularly in the vessels, which have not been stress relieved initially or after fabrication repairs.

8.13 COLUMNS & VESSELS IN DIETHYL AMINE OR MONOETHYL AMINE SERVICE

The weld joints and heat affected zone of the columns and vessels in DEA and MEA service which have not been stress relieved are also prone to cracking due to presence of H2S or H2S and H2.

Further details about corrosion in pressure vessels are available in corrosion Manual-OISD Publication No. 136.

9.0 FREQUENCY OF INSPECTION

i) All new vessels, regardless of service shall be inspected within first 2 years of operation. Thereafter, the periods of future inspection shall be scheduled on the basis of established corrosion rates, the type of service, remaining corrosion allowance and the life expectancy.

ii) The frequency of inspection shall be determined based on history, corrosiveness of the fluid handled and operating conditions. The periods between inspection shall be planned so that minimum corrosion allowance remains for the next run. In any case inspection frequencies as per statutory requirements shall be strictly adhered to.

Internal inspection of all the columns, and vessels installed in battery area should be done during scheduled turnarounds, unless inspection observations and corrosion rates dictate otherwise. Other pressure vessels installed in offsite shall be internally inspected at the time when these are due for hydrostatic test as per statutory requirements.

iii) The internal inspection of reactors is generally programmed when the catalyst is dumped or topped up. However, it is recommended that in-situ metallography be carried out once in 5 years or at the time of replacement of catalyst. The reactors shall be inspected externally in every turnaround and the internal, inspection shall be carried out within 10 years.

10.0 INSPECTION PROCEDURES

Prior to initiating the inspection of pressure vessels, the inspector should familiarise himself with the complete previous history of the vessel, design parameters, service, original thickness, corrosion allowance, corrosion rate and vulnerable locations of corrosion.

10.1 INSPECTION OF COLUMNS

10.1.1 External Inspection

Most of the external inspection can be done while the column is in operation. The following shall be checked during the external inspection.

I) FOUNDATION AND SUPPORTS

a) Foundations

Foundations for pressure vessels are mostly constructed of steel reinforced concrete or of fireproofed structural steel. These shall be checked for spalling cracking and settlement. Settlement shall be checked till it gets stabilised Note 1. If due to cracks, big gaps

5

have been formed, steel should be checked for external corrosion by removing the concrete at cracked locations.

b) Skirts

Skirts shall be inspected for corrosion, distortion and cracking from outside as well as from inside. The weather proofing on the extremities and fire proofing of structural supports shall be checked for water tightness.

The inside of the skirt is often subjected to corrosion. This is particularly true for vessels operating under cryogenic conditions. Thickness measurements shall also be done to assess the extent of deterioration. The condition of fire proofing on support beams and skirts shall be inspected for bulging and cracks. Very light taps with a hammer will indicate lack of bond between fire proofing and steel. Appearance of rust stains on the surface of fire proofing is an indication of moisture ingress and presence of corrosion on the metal underneath. If there is reason to suspect that water or moisture has seeped through to the steel, pockets of insulation should be removed to determine the extent of corrosion. however, inspection of the skirt after removal of fireproofing insulation shall be done at an interval not greater than 5 years. Skirt to shell weld joint shall be checked for cracking.

c) Support of Horizontal Vessels

Horizontal vessels resting on concrete saddle supports where water can accumulate and cause external corrosion shall also be inspected. Horizontal vessels operating at high temperatures shall be checked to ensure free thermal expansion.

ii) FOUNDATION/ANCHOR BOLTS

Foundation bolts shall be inspected for corrosion and damage. The nuts on anchor bolts may be inspected to see that these are properly tightened.

iii) LADDERS, STAIRWAYS, PLATFORMS AND STRUCTURALS

These shall be inspected visually for corrosion, cracks, paint failure etc. Visual inspection shall be supplemented by hammer testing. Corrosion is most likely to occur at points where moisture can accumulate. Crevice corrosion may exist around the heads and nuts of bolts. Ladders shall be examined

for free movement to take up expansion of the vessels.

iv) INSULATION AND PROTECTIVE COATINGS

Visual examination of insulation will

reveal its condition. Insulation shielding shall also be checked for quality and thickness. At few locations samples may be removed to determine condition of the insulation and of the metal underneath. Paint or protective coating shall be examined for peeling or rusting . Insulation shielding should be intact. If at any time insulation shielding/cladding is blown off or damaged the same shall be put back immediately after repairs to avoid corrosion. The insulation retaining rings shall be checked to see that moisture is not trapped between the rings and weldment. The pressure vessels operating at high temperature are insulated from outside and the skirt is insulated from outside as well as inside to reduce the thermal gradient between skirt and shell weld joint. Any damage to this insulation is likely to cause the cracking of this joint. Hence the insulation shall be inspected to ensure that the same is intact.

v) GROUND CONNECTIONS

Grounding connections shall be visually examined to see that good electrical contact is maintained. The cable shall be examined for broken strands. Its resistance shall be checked at intervals as outlined in OISD standard-137 (Inspection of Electrical Equipment).

vi) NOZZLES AND SMALL CONNECTIONS

The nozzles on a pressure vessel shall

be visually inspected and thickness surveyed. When vessel is out of service, carbon steel nozzles may be hammer tested. Small diameter nozzles (less than 50mm) are difficult to be thickness surveyed ultrasonically. The thickness may be determined by taking radiographs wherever possible. The tell-tale hole in the reinforcement pad shall be checked for possible leaks.

Special attention should be given to nipples used for pressure and temperature gauges etc. These nipples shall be checked thoroughly in every shutdown. Test nipples of the lined nozzles shall be checked for any leakage. Any leakage will indicate damage of the lining. If leakage is observed, pressure testing through tell-tale hole and test-nipples shall be done during internal inspection to

6

locate the leaks. Care shall be taken to keep the plugs on the tell-tale holes loose.

vii) EXTERNAL INSPECTION OF METAL SURFACE

a) Visual Inspection

The external surface of the pressure vessel shall be inspected visually. The external surface may show signs of deterioration due to atmospheric corrosion, caustic embrittlement, hydrogen blistering, thermal fatigue and mechanical damage. If caustic is stored or used in a vessel it shall be checked for caustic embrittlement. The areas around the nozzles and in or adjacent to weld seams are susceptible to this type of corrosion.

External corrosion takes place in humid areas and in areas where corrosive chemical vapours are present. External corrosion can be determined by visual inspection. Hydrogen blistering is more often found on the inside rather than outside but may be found at either place depending upon the location of the void which causes the condition. Blisters are found most easily by visual examination. The shell should be checked for buckles and bulges. These can be found and measured by placing a straight edge against the shell.

b) Weld Joints

The weld joints and heat affected zones (HAZ) shall be checked visually for cracks. In case of doubt it should be checked by dye penetrant test.

c) Hot Spots on Lined Vessels

Hot spots, which might have developed on the outer surface due to the failure of internal linings of lined vessels, shall be checked during operation. The areas, which have developed hot spots during service, shall also be checked for mechanical damage such as gouges and dents, leaks, cracks and oxidation of any external stiffeners.

d) Ultrasonic Inspection

Thickness measurement of the shell and domes may be taken from outside. Exact location of thickness measurement may be decided after internal inspection only.

e) Vessels in High Temperature Service

Pressure vessel which operates on

thermal cycle like coke chamber and at high temperature like orifice chamber in FCCU shall be thoroughly inspected from outside. In case of coke chamber if the entry of feed is at the bottom, insulation of at least 2 to 3 courses all around shall be removed. The skirt to shell weld joints shall be thoroughly checked for cracks. The bottom flange welding with the shell shall also be inspected. The weld joints, HAZ and shell of 2 to 3 courses shall be checked for cracks and apparent bulging etc. Presence of suspected cracks should be confirmed by using Dye Penetrant Kit. Welding of the nozzles shall also be checked for cracking.

f) LPG vessels

LPG Bullets and spheres having fire proofing on the outside surface shall be examined for cracks, spalling, bulging and deterioration of fire proofing. Appearance of rust stains on the surface of fire proofing is an indication of presence of corrosion of metal underneath. If above indications are apparent the fire proofing in suspected areas shall be removed and the external surface shall be inspected for any corrosion.

10.1.2 Internal Inspection

Pressure vessels entry shall be made only with an applicable work permit as detailed in OISD-STD-105 on work permit systems. The area of the column to be inspected internally shall be decided based on the past history of the equipment. Available past history shall supplement the standard inspection procedure of the equipment. The inspection of a column is divided into the top, feed and bottom zones. If installed equipment is being inspected for the first time, then all tray manways shall be opened and the complete inspection shall be carried out. Internal inspection can be divided into two parts.

i) PRELIMINARY INSPECTION

Prior to scheduled shutdown of the unit the pressure vessel shall be examined from the outside to detect any unusual condition during operation, such as leaks in nozzle welds through tell-tale holes or gaskets, the condition of the bolts and flanges, the apparent condition of insulation and any other visible defects. During shutdown, before cleaning the column from inside, preliminary internal inspection shall be done. Observations regarding internal

7

dislodging etc. shall be noted. Samples of deposits shall be collected for analysis. Preliminary inspection will also reveal the areas having deposits, scales etc. requiring thorough cleaning to detect metal wastage underneath the deposits during detailed inspection. After the preliminary inspection, clearance for internal cleaning may be given.

ii) DETAILED INSPECTION a) Top Zone

Top dome, shell and internals in the top zone shall be visually inspected. Inspection shall be done to locate corrosion, erosion, hydrogen, blistering, cracking, laminations or mechanical damage. Special attention shall be given to weld joints and surface conditions. If pits are noticed, depth of pits should be measured with depth gauge or pit gauge. Shell plates below the refux nozzle shall be inspected for any possible grooving. Reflux collector shall be checked for thinning. Spouts and counter spouts shall be checked by hammer testing for finding any possible deterioration. The trays and valves shall be checked for pitting and cracking. Thickness of dome and shell near the shell and dome welding shall be taken in all four directions (E,W,N and S). Thickness of the dome around the nozzles shall be taken. Sample thickness of column internals like downcomer, downcomer collectors and support plates should be taken. Besides this, if at certain locations of shell or dome, corrosion is observed thickness shall be measured at these locations to know the exact loss. Shell at the downcomer collector level shall be checked for any possible liquid level corrosion in the form of grooving. Reference points should be marked on shell, dome and nozzles and same should be monitored for thickness during every inspection to determine rate of metal wastage.

b) Feed Zone

While inspecting the feed zone (flash zone), the impingement plate shall be checked for any corrosion, erosion and proper attachment with shell. Shell plates shall be inspected near the impingement plate where there is a possibility of fluid impingement. The internals shall also be inspected. Thickness of the shell plates in four direction and the impingement plate shall be taken. Sample thickness should also be taken on internals.

c) Bottom Zone

In the bottom zone, bottom dome and shell shall be inspected. Special care is to be taken at the area near the bottom drawoff. If pittings are observed, pit depth should be measured. At steam injection points, the shell plate opposite to steam nozzles shall be thoroughly inspected for possible impingement. All the internal pipings etc. shall be inspected. Thickness and hammer testing wherever possible should be carried out. All the nozzles including the manhole nozzles and retractable spool piece shall be thickness surveyed. In case of insulated column, insulation around nozzles should be broken to facilitate thickness survey. Wherever, it is not possible to approach the nozzle, ID measurements from inside shall be taken to determine thickness.

d) Columns in Hydrogen Service

Column in hydrogen service shall be thoroughly inspected for possible hydrogen blistering. Hydrogen blisters shall be inspected and evaluated as outlined in Annexure III.

In catalytic Reactors and Regenerators, the supporting bars for internal equipment such as cyclones shall be closely examined for this type of attack. In these, the catalyst and air distribution facilities are particularly susceptible to erosion and shall be closely examined for this type of attack. Welding of the grid supporting rings shall also be checked for cracking and damage. Out of roundness or bulging may be evaluated by measuring the inside diameter of the vessel at the cross section of maximum deformation and comparing it with the original inside diameter. If the bulging is at intervals, the measurement can be done by dropping a plumb line and taking the measurements at selected intervals. This will also reveal the contour of the shell.

10.1.3 Inspection of Lined Columns

Austenitic stainless steel columns and columns lined with austentic SS plates shall be passivated as per the procedure given NACE RP-01-70 before opening them in order to protect them against stress corrosion cracking.

i) STRIPLINED COLUMN

Procedure for inspection of striplined column is similar to the unlined column as explained above with certain critical locations as outlined below needing special attention.

8

Strips shall be visually inspected. Special attentions shall be given to the weld joints and HAZ of welds, where cracking can take place. If cracks are suspected, dye penetrant test should be carried out. The strips shall also be inspected for bulging. The striplining should be checked by air and soap solution. The pneumatic pressure should be around 0.2 kg/sq. cm. but in no case should it exceed 0.2 kg/sq. cm. The area where the striplining ends shall be checked carefully as corrosion may take place due to galvanic action of striplining and Carbon steel shell. Thickness on the CS portion shall be measured. Nozzle liner should be tested by pressurising the area between the liner and nozzle by air through test nipple. The pressure shall not exceed 0.5 kg/sq. cm. After testing, the test nipple shall be kept open and capped only when the plant/equipment is commissioned. Otherwise bulging of the liner may take place. Thickness of the strips shall be measured at places to ascertain whether strips have been subjected to any corrosion. Thickness survey of the column shall be done from outside to check the parent material thickness.

ii) CLADDED COLUMNS

Cladding shall be visually checked for any deterioration like corrosion/erosion, pitting, bulging etc. Thickness at designated locations shall be measured to check the bonding of the cladding metal with parent metal. The portion where the cladding ends, shall be checked for corrosion which may take place due to galvanic action. The weld joints and HAZ shall be checked for cracks. Nozzle liners should be checked in a similar way as explained in para (i) above.

iii) INSPECTION OF INTERNALLY PAINTED AREA

The temperature limitations of the

painting systems used inside the vessel should be known. While shutting down a unit, water flushing shall be resorted to instead of steam flushing. If steam flushing is necessary, care should be taken that flushing steam temperature should not go beyond 100oC or as recommended by the paint manufacturer. The painted surface shall be cleaned by water washing and then mopping with cotton or jute. Cleaning with wire brush shall not be resorted to. Man entry shall be by wearing soft-shoes or bare foot. Painting shall be visually inspected and thickness should be measured with paint thickness gauge and the same shall be compared with original DFT of painting system.

The paint should be checked for holidays FRE/FRP linings shall be visually checked for mechanical damage and cracks. Thickness of the pressure vessel shall be measured from outside (Inspection checklist for column is given in Annexure-I)

10.1.4 INSPECTION OF PRESSURE VESSELS IN FCCU

i) REACTORS

Shell of the Reactor shall be visually inspected and thickness surveyed. Thermowells shall be inspected for oxidation, cracking or distortion. Linings for the primary cyclone shall be visually inspected at locations like inlet horns, barrel and helix cones etc. Dipleg shall be inspected for perforations by lowering a light through the cyclones. Lining of the secondary cyclones shall be visually inspected at the locations like barrel, cone etc. Dipleg should be inspected for perforation/plugging by lowering a light through the cyclones after cutting a window in seal port. Aeration points in the secondary dipleg shall be hammer tested. Grid holes shall be inspected for erosion and plugging. Condition of deflector plate lining, grid cone lining and riser pipe shall be checked.

In the stripping section, stripper shell and steam nozzle shall be inspected for erosion. Thickness measurements of stripper shell shall be done. Condition of the lining in the fixed and removable sections in feed riser pipe shall be checked. Inverted V-type bellows at the expansion joint shall be inspected for perforation. Inspection of steam and catalyst feed injections piping and nozzles shall be carried out.

In the plenum chamber, shell shall be inspected for erosion and thickness measurements shall be taken. Safety valve inlet nozzles and vapour outlet line shall be inspected for thinning and plugging.

In the anticoking chamber, peripheral holes shall be checked for plugging. Cyclone supports should be hammer tested. Shell and weld joints between shell and bottom plate shall be inspected. Thickness measurements of the shell should be made.

9

The internal lining of riser pipe shall be inspected with the help of a cage lowered from the reactor. The bud bayonet shall be removed and the condition of Y section shall be critically examined for erosion and cracking. Whenever, dissimilar weld joints exist in the riser pipe, these should be checked for cracks. The Reaction and Regeneration stand pipes shall be examined for failure of internal lining and metal deterioration. The convolutions of expansion bellows shall be checked for deposits. The slide valves shall be examined for erosion and proper operation.

ii) REGENERATOR

The shell lining shall be checked for deterioration. Particular attention should be given just near manway and in areas behind the grid seal. Aeration connections, thermowells and trickle valves shall be inspected.

Lining for primary cyclones shall be checked. Special care should be taken in the areas at inlet horn, barrel, helix and cones. Diplegs shall be inspected for perforations. Hangers and supports, spray shields and support lugs for cyclones shall be inspected. In the secondary cyclones, barrel, cones and dipleg shall be inspected. Lining of the plenum chamber and stack above the plenum chamber shall be inspected. Emergency steam sprays shall be inspected for oxidation. If the plenum chamber is of SS material, the bimetal weld joint between chamber and shell shall be examined from inside.

Grid plates shall be checked for bulges and thickness. Grid seal shall be inspected for cracks or perforations. Overflow well and seal boxes shall be checked for erosion and perforations. Lining of the cone below grid shall be inspected. Auxiliary burner tips, air door and the pilot shall be inspected visually. Inspection of aux-burner dome and the lining of the dome on the inside shall be carried out. Lugs and supports for the auxiliary burner dome shall also be inspected.

iii) ORIFICE CHAMBER

The orifice chamber shell shall be inspected for erosion. The areas just after the Double Disc Slide Valve (DDSV) shall be inspected critically. The holes/sleeves on the grid plate shall also be examined for increased diameter due to erosion. The shell adjacent to grid plate should be examined for any deformation, cracks or deterioration after

removing the insulation at random from outside. The discs of DDSV shall be examined for erosion and proper operation.

10.2 INSPECTION OF VESSELS 10.2.1 External Inspection

External inspection of the vessel is carried out in a manner similar to the external inspection of column. Various steps detailed in the previous chapters shall be followed. In addition to above, attention should be given to metal surface in contact with concrete saddles. Vessels which are partially or completely underground are subject to soil corrosion where they are in contact with ground. Therefore, inspection of the external surface should be done, after cleaning of the surface.

10.2.2 Internal Inspection

Normally there are no trays inside the vessels. Inspection of the vessel is done similar to internal inspection of the column. Care should be taken to inspect the liquid level corrosion. Nozzle weldings, internal stiffners and area around them shall be checked thoroughly from inside.

Pressure vessels which can not be internally inspected due to mechanical restrictions shall be inspected using ultrasonic equipments. In addition they shall be pressure tested as per statutory requirements.

All weld joints in Ammonia and LPG storage vessels shall be checked internally by wet fluorescent magnetic particle examination to detect cracks due to stress corrosion cracking, once in ten years, besides the normal inspection.

Similarly, for vessels in DEA/MEA service spot checking of T-weld joints shall be carried out by radiography/ultrasonic testing. If defects or cracks are detected 100% weld joints shall be checked by Radiography/Ultrasonic Testing.

i) CONCRETE, GUNITE AND REFRACTORY LININGS

Concrete, Gunite and Refractory

Linings inside a pressure vessel shall be visually checked for mechanical damage such as spalling and cracks.

Particular attention should be given at locations where hot spots have been noticed

10

during operation. Minor cracks and areas of porosity are more difficult to find. Light scrapping will sometimes reveal such conditions. Bulging which can be located visually is usually accompanied by cracking in most cases. If corrosion occurs behind a concrete lining, the lining will lose its bond with steel. The sound and feel of light hammer tapping will usually make such looseness evident. If corrosion behind a lining is suspected, small sections of the linings shall be removed for shell inspection. This will also permit a cross sectional examination of the lining. In cases where bare metal has been exposed because of lining failure, visual inspection shall be made of the exposed metal. Thickness of the shell shall be measured from outside.

Concrets, Gunite and Refractory Linings inside a pressure vessel shall be visually checked for mechanical damage such as spalling and cracks.

Particular attention should be given at locations where hot spots have been noticed during operation. Minor cracks and areas of porosity are more difficult to find. Light scrapping will sometimes reveal such conditions. Bulging which can be located visually is usually accompanied by cracking in most cases. If corrosion occurs behind a concrete lining, the lining will lose its bond with steel. The sound and feel of light hammer tapping will usually make such looseness evident. If corrosion behind a lining is suspected small section of the linings shall be removed for shell inspection. This will also permit a cross sectional examination of the lining. In cases where bare metal has been exposed because of lining failure, visual inspection shall be made of the exposed metal. Thickness of the sell shall be measured from outside.

ii) RUBBER LINED PRESSURE VESSEL

Some pressure vessels are rubber lined from inside for protection against

corrosion. The rubber lining shall be inspected for mechanical damage, holes, cracking, blistering, bonding etc. Holes in the lining is evidenced by building. A holiday detector should be used to thoroughly check the lining for leaks and holidays. Care must be taken so that the test voltage does not approach a value that might puncture the lining. Standards are available for values of test voltages as per thickness of rubber lining. For inspecting rubber lined vessels, IS-4682-Part-I shall be referred. Bonding of the rubber lining should be checked ultrasonically from the outside.

ii) RUBBER LINED PRESSURE VESSEL

Some pressure vessels are rubber lined form inside for protection against corrosion. The rubber lining shall be inspected for mechanical damage, holes, cracking, blistering, bonding etc. Holes in the lining is evidenced by bulging. A holiday detector should be used to thoroughly check the lining for leaks and holiday. Care must be taken so that the test voltage does not approach a value that might puncture the lining. Standards are available for values of test voltages as per thickeness of rubber lining. For inspecting rubber lined vessels, IS-4682-Part-I shall be referred. Bonding of the rubber lining should be checked ultrasonically from the outside.

10.2.3 Riveted Vessels

Besides the internal and external inspection as given earlier, Riveted vessels shall be examined for tightness of rivets, soundness of caulking and seal welds and other conditions. For the insulated riveted vessel, insulation should be removed from all joints for checking at 18 months’ intervals.

10.3 CORROSION COUPONS/PROBES

Corrosion coupons are installed in the pressure vessels to evaluate accurately the corrosion rate or to evaluate a new material in the existing environment. While doing the internal inspection the corrosion coupons if installed should be taken out. Nature of corrosion attack on the corrosion coupons shall be studied. The coupons are then thoroughly cleaned and weight loss in a specified length of

11

time shall be calculated. This gives the corrosion rate and cleaned coupons are again installed for future evaluation. Corrosion probes may be installed at vulnerable locations on the pressure vessels for onstream monitoring of corrosion rates. Coupons and probes can be either fixed or retractable type.

10.4 SAFETY RELIEF DEVICES

The safety valves and safety relief valves on the pressure vessels should be revisioned and tested. For details on inspection of pressure relieving devices OISD-Std-132 shall be referred.

11.0 RETIRING THICKNESS

Before determining the limiting or retiring thickness of any pressure vessel, it should be determined under which code it has been manufactured. Retiring thickness shall be calculated as per applicable code as most vessels are built with some excess thickness in vessel walls and heads, over that required to withstand the internal operating pressure. The excess thickness may result from any one or more of the following factors:

i) Excess thickness as a result of using a nominal thickness of plate rather than the exact (smaller) value calculated.

ii) Excess thickness available as a result of setting minimum thickness of the plates for construction purposes.

iii) Excess thickness available as a result of change in vessel service, by reducing safety valve setting or maximum metal temperature or both.

Retiring thickness for many accessories of pressure vessels are not covered in the ASME code; neither are the methods of calculating such thickness. Some of these parts are trays, internal tray supports, valves, grid, baffles, ladders and platforms. For some of the equipment, there are generally accepted methods of setting retiring thickness. Minimum thickness should be developed for all such equipment. The results of possible failure of such equipment. The results of possible failure of such equipment should be considered for setting these limits. Safety and continuous efficient operation are the prime factors affecting retiring thickness for these components. Repair or replacement should be carried out when they have lost one half their original thickness. The retiring thickness for nozzles and internal pipings shall be calculated

by applicable codes or ANSI standards. Widely scattered pits may be ignored provided:

a) no pit depth is more than one half the vessel wall thickness exclusive of the corrosion allowance.

b) the total area of the pits does not exceed 45 square centimeters within any 20 centimeter diameter circle, and

c) the sum of their dimensions along any straight line within the circle does not exceed 5 centimeters.

As an alternative to the procedures described above, any thinning of components below minimum required wall thickness due to corrosion or other wastage may be evaluated to determine the adequacy for continued service by employing the design by analysis methods of the ASME code. Section VIII Divn. 2 Appendix- 4.

12. INSPECTION DURING MAINTENANCE

12.1 WELD BUILD UP

In pressure vessels where some of the small areas within rejection limit as specified in 11.0 sub clause b, have thinned down and entire corrosion allowance has been eaten away, repair by local weld filling may be required to build up the thickness . The area to be repaired should be marked at site and should be cleaned thoroughly. The area is filled with weld deposits, in a staggared manner to avoid warping, with suitable electrodes matching with the base part. After weld build up, the area should be visually/dye penetrant inspected for cracks and defects. Thickness spots are made at a few locations at the built up area by grinding. Thickness shall then be measured ultrasonically to check whether requisite thickness has been obtained. Preheating and post weld heat treatment whenever required should be carried out as per the code.

12.2 NOZZLE REPLACEMENT

Thinned and deteriorated nozzles shall be replaced. Rejected nozzle shall be removed by gouging the welding. New nozzles fabricated out of piping having thickness equivalent to original nozzles are installed. Welding shall be carried out from inside as well as outside with suitable electrodes matching with base metal and nozzle material. Preheating and post weld heat treatment of the

12

welding shall be carried out as per the requirement of relevant code. The weld joints shall be checked visually and also by dye penetrant test. Defects, if found are repaired. The weld joints shall be checked for leaks by pressurising with air at a pressure 1.03 kg/cm2 through the tell-tale hole provided in the reinforcing pad. Pressurising the entire column to check the nozzle weld joints should be avoided. In some cases where the area is accessible from inside, a box may be provided around the nozzle. The box is pressurised with water to the test pressure of the column/vessel calculated by applicable code. The weld joints and HAZ are checked for possible leaks. If any defect is found in the weld joints, these are gouged, rewelded and retested.

12.3 PARTIAL REPLACEMENT OF SHELL PLATES AND DOMES

Some portion of shell plates and domes of pressure vessels may get thinned due to corrosion or erosion. The thickness of the affected area may reach the retiring thickness. In such cases, partial replacement of shell plate or dome is carried out as weld repair of the big area is not practically possible. The affected portion is cut and removed. The new plates matching with the metallurgy and thickness of the original plate is made available.

The edge preparation shall be done as per the code requirement by grinding. The prepared edges shall be checked for cracks, flaws and defects by magnifying glass and by using dye penetrant kit. The welding procedure is developed for welding the old and new piece as per the relevant code. The welding may be performed either from inside or outside. The root run shall be thoroughly inspected for cracks and flaws. After completing the welding from one side, the other side is chipped and grounded. Before welding again, the groove is checked for cracks and defects. Welding is then completed from the other side. Complete welding shall be visually checked and radiographed as per the applicable code. Detailed inspection of welding shall be done as outlined in Annexure II.

Preheat and post weld heat treatment shall be carried out as per the requirement of relevant code. In order to check whether post weld treatment has been carried out properly, hardness readings on the weldment and HAZ shall be taken after PWHT. The hardness readings should be minimum as indentation marks required during hardness measurement

act as stress riser and this leads to stress concentration. If post weld heat treatment is required it is recommended to carry out radiography before post weld heat treatment also. The defects in the welding are repaired by gouging and rewelding. In lieu of radiography, ultrasonic inspection of weld joints may be carried out.

12.3.1 Hydrostatic Test

After satisfactory inspection and radiography, the column/vessel is hydrostatically tested at a pressure calculated by applicable code. The pressure should be held for a minimum of 30 minutes. During hydraulic testing the pressure gauge should be installed at the highest point. It is recommended that two pressure gauges be used. The range of the pressure gauge should be 30% more than test pressure and calibrated pressure gauges shall be used. The area which has been repaired should be thoroughly checked for leaks and signs of deformation. The pressure drop shall also be noted. Before subjecting the column/ vessels to hydrostatic test, the foundation/supporting structures of the pressure vessels should be checked for water load. Austenitic SS pressure vessels shall be pressure tested using DM water or passivating solution. Hydrostatic testing of vessel operated under vacuum conditions shall be done as per the relevant code.

12.3.2 Pneumatic Testing

When testing pneumatically, a soap solution should be used as an aid to visual inspection. This soap solution is brushed over the seams and joints on the vessel. The vessel is then examined for evidence of bubbles as an indication of leaks. Often a vessel which operates at a vaccum may be pressure tested. This is the preferable testing method when feasible, because it permits the location of any leaks. When pressure testing is not feasible, a vacuum vessel can be tested for leaks by creating a vacuum by means of evacuators or vacuum pumps installed in the units. If the vacuum can be held for a specified time after closing of the evacuators or vacuum pumps, it is an indication that the vessel is free of leaks.

If the vacuum cannot be held, leaks are present but this method gives no indication of their location. A search, which may be difficult, must then be made to trace the leaks. It is

13

suggested that pneumatic testing should be avoided as far possible and if at all is to be carried out it should be done in accordance with relevant code.

12.4 REPAIR OF CLADDING AND STRIPLINING

The bulged, cracked or heavily pitted cladding inside the pressure vessels shall be repaired. The deteriorated cladding is removed by cutting. The edges of the remaining cladding is sealed by welding, using proper electrodes as per cladding and shell metallurgy. If the area of the damaged cladding is small, the area is weld overlaved using suitable electrodes. The area is then ground smooth. The repaired portion shall be checked visually and by using D.P. for defects and cracks etc. The welding should be done in a staggerd manner to avoid distrotion of the shell. When the damaged area is big, after sealing the remaining cladding, striplining of the area can be done. (Details of striplining and welding procedure is given in Annexure - II). Bulged striplining is replaced by puncturing the lining to remove entrapped air. The bulged portion of the lining is flattened by light hammering and then welded. If the striplining has cracked or heavily pitted the damaged lining should be removed and fresh lining put. The weld joints are checked for flaws and cracks by DP and visual examination. While removing or puncturing the cladding/striplining, necessary precautions should be taken as hydrocarbon may be entrapped in between the lining and shell.

12.5 REPAIR OF PAINTED AND RUBBERLINED AREAS

If the painting in a small area of a vessel has peeled off or has been damaged patch painting repair can be done. The damaged areas shall be painted with original painting system with proper curing time etc. If the area of damage is large, the area is shot blasted to Swedish, standard Sa 2-1-/2 to clean the surface and original painting system is applied with proper curing time. DFT is measured with paint thickness gauge. If internal rubber lining of vessels has bulged or cracked in a small area, the deteriorated lining shall be removed and fresh rubber lining is put in that small area. New lining shall be checked for holes and flaws. Local curing should be done to achieve hardness of 65+50A (shore hardness-A). When a large area of the rubber lining has cracked and bulged, the damaged lining is taken out. Bare metal is cleaned by

shot blasting. New rubber lining is provided. Curing shall be done to achieve 65 + 50A (shore hardness A). The lining shall be visually checked for cracks, holiday and bulging. The holidays shall be checked by using holiday detector. For inspecting the rubber lining IS-4682-Part I shall be referred.

13.0 DOCUMENTATION

Observations of each inspection shall be properly recorded. After determining the corrosion rate and remaining corrosion allowance, repair and replacement of a pressure vessel can be planned. The following cards shall be used for proper documentation of the Inspection findings:

i) Data card (Ref. Form No. 1)ii) Index card (Ref. Form No. 2)iii) History card (Ref. Form No.3)iv) Data record card (Ref. Form No. 4)v) Development sketch (Ref. Figure 5 & 6)

14.0 REFERENCES

The following codes standards and publication have either been referred or used in the preparation of this standard, and the same shall be read in conjunction with this standard. i) API Guide for Inspection of Refinery

Equipment - Chapter VI - Unfired Pressure Vessels.

14

ii) API Guide for Inspection of Refinery Equipment - Chapter V - Preparation of Equipment for Safe Entry and Work.

iii) ASME - Pressure Vessel Code. Section VIII Divn. I & II.

iv) Indian Standard for Unfired Pressure Vessels - IS-2825.

v) BS-5500-Specification for Unfired Fusion Welded Pressure Vessels.

vi) API-510-Pressure Vessels, Inspection Code-Maintenance, Inspection, Rating, Repair & Alteration.

vii) IS-9964 Part-I, Preparation of Tank for Safe Entry and Work.

viii) IS-4682 part I, Code of practices for lining of vessels and equipment for chemical processes-Rubber Lining.

ix) Pressure Vessel Inspection Safety Code-Part 12 Institute of Petroleum.

15

FORM 1

VESSEL DATA CARD

INFORMATION WEIGHTS

DESIGN CODE_________________________ SHELL______________________________

MANUFACTURER_______________________ INTERNALS_________________________

MANUFACTURER’S INSULATION_________________________ ORDER NO.____________________________

DRG.NO.______________________________ EMPTY VESSEL______________________

JOB NO._______________________________ FULL OF WATER

DIMENSIONS OPERATING CAPACITY________________

TOTAL HEIGHT________________________ MATERIALS

HEIGHT BETWEEN TANGENTS__________ SHELL_________________________________

DIAMETER___________________________ HEADS_____________________________

WALL THICKNESS____________________ SKIRT_______________________________

TYPE OF HEADS______________________ BASE PLATE_________________________

CORR. BENCH MARKS_________________ MANWAY NOZZLE______________________

CONDITIONS

DESIGN TEMPOC_____________________ OPERATION TEMPO C_______________________

DESIGN PRESSURE KG/SQ.CM__________ OPERATING PRESSURE KG/SQ.CM____________

HYDROTEST PRESSURE KG/SQ.CM_____ CORROSION ALLOWANCE mm________________

STRESS RELIEVED____________________ RADIOGRAPHED____________________________

JOINT EFFICIENCY LONG SEAM________ HEAD_____________________________________

16

17

18

FORM 4

DATA RECORD CARD

UNIT_________

INSP.POINT

DESCRIPTION SIZE SCHDL ORG.THICKN

RETTHICKN

THICKNESS

1986 1987 1988 1989 1990 1991

19

20

UJ+

21

ANNEXURE – IINSPECTION CHECK LIST FOR COLUMNS IN SERVICE

UNIT_______________ EQUIPMENT No_____________ DATE_______________

1. SERVICE

2. REASON FOR INSPECTION

I) Shutdownii) On-StreamII) Breakdown

3. INTERNAL INSPECTION

A. TOP ZONE

a) Scaling Natureb) Domec) Shelld) Weldinge) Nozzle Weldingf) Internalsg) Spouts and Counter Spouts

B. MIDDLE ZONE

a) Scaling Natureb) Shellc) Weldingd) Nozzle Weldinge) Internalf) Spouts and Counter Spouts

C. BOTTOM ZONE

a) Scaling Natureb) Shellc) Domed) Weldinge) Nozzle Weldingf) Steam Coils

22

4. EXTERNAL INSPECTION

a) Foundation & Foundation Boltsb) Insulationc) External Corrosiond) Ladder and Stair Casee) Nozzle Flangesf) Bosses and Nipplesg) Grounding Connectionsh) Testing Nipple of Liners on Nozzle

5. THICKNESS SURVEY OF COLUMN INCLUDING ALL NOZZLES YES/NO

6. CONDITION OF INTERNAL LINING, IF ANY

7. REPAIR, IF ANY

8. CORROSION COUPONS: YES / NO

9. REMARKS

INSPECTION ENGINEER

23

ANNEXURE-II

INSPECTION OF WELDING

1. DUTIES OF WELDING INSPECTOR

The duties of a welding inspector usually involve the performance of a number of operations, including :

i) Interpretation of drawings and specifications.

ii) Verification of the metal being welded.iii) Verification of procedure and welder

qualification.iv) Checking application of approved welding

procedures.v) Verification of proper heat treatment.vi) Assuring acceptable quality of welds.vii) Preparation of records and reports.

2. INSPECTION PRIOR TO WELDING

i) The faces and edges of material should be examined for laminations, blisters, scabs and seams.

ii) Heavy scale, oxide films, grease, paint, oil and moisture should be removed.

iii) The pieces to be welded should be checked for size and shape. Warped, bent or otherwise damaged material should be detected in the early stages of fabrication.

iv) Edge preparations, bevel angle, alignment of parts and fit up should be checked. The groove surface should be smooth (equivalent to machined/filled/ground surface). The root gap should be uniform.

v) Tacks to hold alignment of joint must be checked for soundness. Tacks which are to be included in weld must be done by qualified welders in accordance with the welding procedure and must be of the same quality as root pass.

3. INSPECTION DURING WELDING

Visual inspection is employed to check details of the work while welding is in progress. The details to be considered are:

i) Welding processii) Cleaning.iii) Preheat and interpass temperatures.iv) Joint preparation.v) Distortion control.vi) Filler Metal.vii) Interpass chipping, grinding or gouging.

The inspector should be thoroughly familiar with the items involved in the qualified welding procedures. Compliance with all details of the procedure should be verified. The root pass is most important from the point of view of weld soundness. The root pass may be checked by dye-penetrant testing. The inspection of root pass offers another opportunity to inspect for plate laminations.

In the case of double-groove welds, slag form the root pass on the side of the plate may from slag deposits on the other side. Such deposits should be chipped, ground or gouged out prior to welding the opposite side. Where slag removal is incomplete, it will remain in the root of the finished welds. Emphasis should be placed on the adequacy of the tack welds and clamps or braces used to maintain the root opening to assure penetration and alignment.

4. INSPECTION AFTER WELDING

Visual examination is the first stage in the inspection of a finished weld. The following quality factors should be checked:

i) Dimensional accuracy of the weldment (including distortion).

ii) Comformity to specification requirements regarding extent, distribution, size, contour and continuity of the welds.

iii) Weld appearance, surface roughness, weld spatter etc.

iv) Surface flaws, such as cracks, porosity, unfilled craters and crater cracks particularly at the end of welds, undercutting, overlap, excessive weld reinforcement, excessive grinding etc.

24

v) The areas where fitup lugs were attached or where handling lugs, machining blocks or other temporary attachments were welded on, must be checked carefully after the attachment is removed. The area must be ground smooth and any pits or tears shall be filled in with weld metal ground smooth Air hardening materials should be preheated before any thermal cutting.

vi) Postweld heat treatment time, temperature and heating/cooling rates. For groove welds, the width of finished welds will fluctuate in accordance with the groove angle, root face, root opening and permissible tolerances. The height of reinforcement should be consistent with the specified requirements. Where not specified the inspector may have to rely on his judgement, guided by what he considers a good welding practice.

The finished weld, should be thoroughly cleaned of oxides and slag for its final inspection.

After visual inspection the finished weld may be examined by one or more than one of the following techniques.

5. NON DESTRUCTIVE TESTS

i) DYE PENETRANT TESTING: Unless otherwise specified, the extent of this test will be 100% for all root runs for alloy steel welds. Adequate precautions as specified in applicable code should be taken while interrupting the welding cycle.

ii) MAGNETIC PARTICLE TESTING

iii) RADIOGRAPHY: Unless otherwise specified the extent of radiographic examination will be as follows:

a) carbon and carbon molybdenum steels-10% of the welds.

b) alloy steel - 100% of the welds. The weld joint for radiography will be marked by the inspector.

Radiographic examination of weld joints of two dissimilar materials shall be considered as per the higher metallurgy stipulations.

iv) ULTRASONIC TESTING.v) EDDY CURRENT TESTING.vi) FERRITE DETERMINATION.vii) ULTRASONIC HARDNESS TESTING

Hardness testing by portable hardness testers may be considered as NDT method. Hydraulic testing may be done to check for leaks through welds, cracks etc.

6. DESTRUCTIVE TESTS

i) Mechanical tests like tensile, bend, impact, hardness, drift, flattening tests etc.

ii) Chemical analysis, microscopic examination, grain size determination etc.

The method and extent of examinations will be governed by applicable code requirements.

7. REPAIR OF WELDS

i) No repair should be carried out without prior permission of the inspector.

ii) Weld discontiniuties which are beyond acceptable limits shall be removed from the joint completely by the process of chipping and grinding.

iii) Where random radiography is specified, the first weld of each welder shall be completely radiographed. In case of pipe size 150 mm dia and below, the first two welds shall be completely radiographed.

iv) For each weld found unacceptable due to a welders fault two additional checks should be carried out on welds performed by the same welder.

25

ANNEXURE-III

HYDROGEN BLISTERS-INSPECTION, EVALUATION AND REAPIR OF PRESSURE VESSELS

1. INSPECTION

A. Visually inspect exterior and interior of vessel to determine location of all hydrogen blisters.

B. Determine blister thickness by ultrasonic survey or by drilling.

C. Conduct magnetic particle inspection at the edge and crown of any blister 2 inches and greater in diameter to locate cracks which originate at, or have progressed near to the surface.

D. In order to detect plate cracks (fissures), conduct magnetic particle inspection of the plate for a distance of 6 inches beyond the limits of blisters 2 inches and greater in diameter appearing on the inside of the vessel. See Figure 1.

2. EVALUATION

A. If the blisters are clustered and originate at varying depths, replace the plate.

B. If plate fissures are detected under Paragraph II. F, replace the affected plate.

C. Hydrogen blisters vissible in the cylindrical section of the shell, the crown of flanged and dished or eliptical heads, or in hemispherical heads and those that are away from seams of localised loading such

as support pads should be considered acceptable.

D. Hydrogen blisters visible in and very near highly stressed areas such as head knuckles, openings, seam and seam junctures, and support pads should be considered unacceptable in pressure vessels.

1. Affected components should be repaired or replaced. Repair or replacement should be approved by an appropriate inspector aided by material and design specialists as required.

E. If the diameter of any blisters listed in paragraph III.C exceeds the thickness of the plate, and the vessel is operated below the metal transition temperature, the material should be replaced.

USE ASME Code, Section VIII Division 2, impact Test Exemption Curves for Carbon Steels.' as a measure of transition temperature.

3. REPAIRS

A. Cracked blisters on the outside surface of vessels in hydrofluoric acid service and cracked blisters on either inside or outside surfaces of vessels in other services shall be repaired as follows:

1. Drill 1/8" diameter holes at ends of each crown crack or edge crack to a depth equal to blister depth as determined by thickness measurement.

B. Relieve hydrogen pressure in uncracked blisters 2 inches and larger in diameter by drilling a 1/16 inch diameter hole in the center of the crown as follows:

1. Blisters showing on outside of vessels. a. Drill from outside.

2. Blisters showing on inside of vessel. a. Drill from inside.

26

b. For vessels in hydrofluoric acid service, drill from outside.

3. Blisters showing on both inside and outside of vessel.

a. Drill from inside. b.For vessels in hydrofluoric acid service, drill

from outside surface only.

D. Vessels in hydrofluoric acid service.

1. If blisters 2 inches and larger in diameter on the inside surface have crown or edge cracks, gouge out complete blister, fill with weld metal and grind smooth with plate surface.

2. If blisters on the outside surface are cracked, treat as in Paragraph III A.

D. Preheat, welding procedures, stress-relief, etc. should be in accordance with current acceptable practice for the specific vessel material.

E. Spheres and other vessels with tubular legs.

1. Hydrogen may diffuse through the vessel wall and become trapped inside tubular legs that are welded to the vessel. This can form an explosive mixture with air in the legs.

2. Prior to any welding or cutting on or near the legs of a blistered vessel, the legs should be purged of any explosive gases as follows:

a) Drill 1/4" diameter holes at top and bottom of legs, with a non-sparking drill.

b) Flush with inert gas or with air from the bottom hole.

3. Other dead spaces of significant volume should be treated in a similar manner.

27