Chapter 2: In-Service Inspection Methodology

2.0          In-Service Inspection Methodology

2.1        Methods of Inspection

2.1.1     Introduction

The following types of inspection can be performed on pressure vessels, boilers and heat exchangers.

2.1.1a   External Inspection

General visual inspection to detect damages, leakage, wet insulation, condition of paint, corrosion etc.

Close visual inspection to detect cracks in welds.  Special emphasis shall be put on nozzle (especially nozzles which may be subjected to external loading or vibration from piping).  Brackets and supports shall also be treated. NDT (normally MPI) to detect cracks in welds.

2.1.1b   Internal Inspection

Internal Inspection shall be performed on larger vessel every 3 or 4 year.  The Inspection shall be coordinated with a shut-down and shall cover: General visual inspection of the shell and internal part (to detect corrosion, loose parts, damage to coating, cracks in weld and fouling).NDT (normally MPI) of weld to detect any crack. Procedure for preparation before inspection shall be existed. If cleaning is necessary, this shall be specified in the programmed.  It shall be assessed if samples of sediments shall be taken for analyses. The visual inspection shall also assess if all internal corrosion has been monitored by Wall Thickness Measurement (WTM) if any.  Is e.g. number of WTM points sufficiently and are the WTM points correctly located? In addition to looking for cracks in the pressurized shell, one shall be aware of cracks in the shell. Vessels with internal coating (e.g. polyester for corrosion resistance) shall be treated in a particular way.  As long as the coating may cause severe pitting corrosion.  This kind of corrosion is very difficult to detect by ordinary corrosion surveillance.   The inspection shall thus be concentrated on a very close inspection of the coating to detect any defects in it.

2.1.1c   Non-destructive Testing

i.    Liquid Penetrant Testing

In liquid penetrant testing, the liquid penetrate is applied to the surface of the specimen, and sufficient time is allowed for penetration into surface discontinuity.  If the discontinuity is small or narrow, as in a crack or pinhole, capillary assists the penetration.  When the opening is gross in nature, such as a tear, the liquid may be trapped when poured over the specimen.

+ Process selection

Selection of the suitable penetrate type and process for a particular liquid penetrate test depends upon the sensitivity required; the number of articles to be tested; surface condition of the material under test; configuration of the test specimen; and the availability of water, electricity, compressed air, suitable testing area, etc.

+ Capabilities of test

Liquid penetrate testing is capable of locating discontinuities open to the surface in articles made of nonporous material.  With penetrant tests, detectable discontinuities such as surface cracks, porosity, and “through” leaks can be found.

Process Selection Guide

Testing Problem	Preferred Process	Remarks
High Production of Many Small Articles Required High Production of Large Individual Articles Highest Sensitivity to Fine Discontinuities Shallow Discontinuities Scratches, etc, Must be Detected Articles Having a Rough Surface Articles Having Threads And Keyways Articles Having Medium Rough Surface Spot Testing of Local Areas Desired Portable Equipment Necessary Water and Electricity Not Available Anodized Articles, Cracked after Anodizing, to be Tested Repeated Application of Process is Desired Leak Detection	Water Washable Post Emulsified Post Emulsified Post Emulsified Water Washable Water Washable Water Washable Or Post Emulsified Solvent Removed Solvent Removed Solvent Removed 1.   Solvent Removed 2.     Post Emulsified 3.     Water Washable Solvent Removed Water Washable or Post Emulsified	Small Articles Handled in Baskets Large Forgings, Extrusions, Etc. Brightest Indication Most Sensitive Depth of Emulsification Can be Controlled Post Emulsified Penetrant Might Lodge in Corners Choice Depends Upon Production and Sensitivity Requirements Order of Preference Indicated Five or Six repeats Shall be the limit

ii.   Radiography

Radiography can be used for the detection of localized corrosion weld defects and with aid of standard “image quality indicators” wall thickness measurement can be made.The technique depends on the opacity of materials to either gamma or x-ray radiation passing through the component reacting with a photographic film or a fluorescent screen.  The density of the image produced on the film is related to the thickness and density of the material under examination. An x-ray source requires and electrical power supply and water-cooling, where as gamma radiation is obtained from a small amount of appropriate radioactive material.  Consequently gamma radiography is more suitable for in-plant applications.  Gamma radiography also has the advantage of having penetrating power, but the resolving power is lower than for x-rays, that can be focused. The use of radiography involves trained and licensed personnel.  The interpretation of results also requires experience since radiography is essentially sensitive to volume losses due to corrosion.  Therefore, pitting is fairly easily recognized but cracking, especially transverse to the radiation, is difficult to detect. In order to precisely define localized internal corrosion in a shell radiographs must be taken in several different directions, and step wedges used to calibrate optical density to estimate thickness.  It is often best to use ultrasonic to confirm actual thickness and depth of pitting. Recent developments utilize enhancement of very weak images, therefore allowing use of much lower (and safer) radiation levels.  Enhanced images can be displayed on a cathode – tube screen, from a moving source/screen, allowing real – time monitoring of accessible lengths of pipe. This technique makes radiography more flexible and comprehensive in coverage, freeing it from the limitations of photography and hazardous radiation levels.  The permanent “hard” image of the photograph is lost, but continuous images can be stored on videotape or computer disc.

iii.  Ultrasonic:

This method depends on the measurement of transmission and reflection time of a high – frequency sound generated by a piezoelectric crystal at the surface of the material under test. The sound pulses are reflected by the front and back surface of the material and by any defect in between, and detected by the same or another piezoelectric crystal on a cathode ray oscilloscope, or other output device. There have been considerable improvements in instrument technology over the past few years, and hand-held instruments are now available demonstrating excellent accuracy and sensitivity. Ultrasonic are useful not only for measuring metal thickness from one side, but for determining the extent and configuration of pitting and other localized corrosion, and for finding inclusions, laminations, hydrogen blisters and stress cracks within the examined structure. If ultrasonic thickness measurements are to be used to estimate corrosion rates over time it is important that the probe be accurately located in the same place for consecutive measurements, and that baseline measurements are made before the equipment is commissioned so that conclusions are based on actual initial thickness rather than the nominal value.

Calibration standards are used to adjust instrument calibration at the beginning and end of a series of measurements. In order to transmit the sound waves into the material under test it is necessary to have clean, close contact between the probe and the surface that may be assisted by various acoustic coupling fluids or gels. In order to be able to slide the probe across the surface (for example to precisely locate corrosion pits on the blind side) the surface may require grinding or sanding to remove mill scale or other imperfections and achieve the proper degree of smoothness. When the nature of corrosion is scattered pitting, even coupon data must approached statistically.  Several coupons of a cumulative area equivalent to actual pit spacing in the system must be used to reliably obtain a “sample” of a corrosion pit on the coupon surfaces exposed. In sampling, as in corrosion monitoring methods, “one is seldom enough”.  Reliability is increased by increasing data population. The number of samples to achieve a required degree of accuracy can be determined by statistical analysis methods available on hand-held calculators.  Ultimately, cost will be a controlling factor in determining the number of samples required in a given situation.

iv.  Chemical Analysis

All of the available laboratory and field techniques of chemical analysis can be used to monitor corrosion indirectly in a variety of ways.  This section shall include: -

·         Analysis of fluid stream content

·         Analysis of changes in fluid stream composition

·         Analysis for corrosion products

·         Analysis for insoluble corrosion products

·         Microbiological analysis

2.2  Choice of Areas for Inspection

The primary objective of inspection is to detect damages, loose parts, cracks, corrosion or fouling.  The following shall be closely considered when choosing inspection areas, methods and intervals:

-     Probability of cracking

-     Probability of corrosion

-     Probability of fouling

-     Possible consequences of a failure

-     Design corrosion allowance

Vessels subjected to low and static pressure have low probabilities of cracks, whereas vessels subjected to high pressure, and especially varying pressure (fatigue loading) have high probabilities of cracking. It shall also be considered on which side (external or internal) a crack would most probably start. There is always a probability of cracking in welds of items subject to vibrations. These can be nozzles (vibrations from piping), internal items (flow induced vibrations), brackets, etc. Transition zones between stiff and flexible parts are also high-risk areas w.r.t. cracking.  This can be weld between large nozzles and shell between certain internal items and shell (bad design), etc. One shall be aware of cracks due to hydrogen brittleness, stress corrosion due to chlorides when assessing probability of cracking. Repaired areas shall be paid special attention since there is always a certain possibility for inclusion of failures in. The consequence of failure may in some areas be negligible, whereas it in other areas can be catastrophically (brittle fracture).  It is therefore important to assess if a failure will be detected (eg. by leakage) before it can lead to brittle fracture. On large vessels 100% NDT of all welds can be specified during a ¾ year period.  Access for the inspection/measurement equipment must also be considered. On smaller vessels the inspection can be concentrated on critical areas, eg. in the interconnection between circumferential and longitudinal welds. One shall also be aware that specification of 100% MPI can lead to superficiality.  An alternative can be emphasized thoroughly examination in the critical areas, as recommended for smaller vessels. The program shall be coordinated with shutdowns.  These may decide if eg. a 100% test shall be performed every 4^th year, or if a 20% test shall be performed every year.

Example 1

Choice of Inspection And Wall Thickness Measuring Points on a Pressure Vessel

Fig. 1 shows location of wall thickness measuring points on a fuel gas knock-out drum.

A total of 8 points have been chosen in areas where debris and/or water is expected to collect.  Four points (1-4) are located in the lower part of the vessel whereas the remaining four (5-8) is located just above the support for the mist catcher. The specification of ultrasonic equipment and procedures for scanning around each measuring point is the same for this vessel as for the piping mentioned in example 1. The interval specified for wall thickness measurements is initially 12 months, but will be subject to revision as practical experience is built up.  As the purpose of the vessel is to remove liquid from the gas, the presence of liquid is less stochastic than for pipes, and modification of the interval may be advisable.

In addition to wall thickness measurements, an inspection program covering both external and internal visual inspection, as well as detailed inspection (MPT) of welds has been specified. Internal visual inspection is specified to be performed every 4 year.  This is among others attended to be used to assess if the wall thickness measurement program shall be updated.  Eg. are the measurements points located correctly, and has eventual pitting corrosion been picked up?

Detailed inspection (MPI) is specified on 100% of the following welds during a 4 year period (ref. Fig 2):

External round weld, RW 1

Internal round weld, RW 2

External round weld, RW 3

And for the welds in connection with the nozzles mostly exposed to corrosion and external loadings.  These are:

-           External weld between reinforcement plate and shell in connection with nozzle A

-           External weld between nozzle A and reinforcement plate

-           External weld between nozzle C and shell

-           External weld between nozzle M and shell

Furthermore, all nozzles are subjected to an external visual inspection every 3-month.  The inspection program also specifies that the lower part of the support shall be inspected closely every 12 month for corrosion (see fig 3) since water and debris may collect in that area.

Example 2       

Inspection of Drain Separator

This vessel has a low operating pressure and the probability of crack growth is relatively small. NDT requirements are therefore rather small. The vessel has an internal coating of Polyester to resist corrosion.  As long as the coating is intact, there is no risk for corrosion.  However, even the smallest defect of the coating may cause corrosion in the vessel shell with large corrosion rates. Such kind of corrosion is nearly impossible to detect by external corrosion monitoring (WTM) so, the only applicable alternative must be a close visual internal inspection after eg. two years of service.

Chapter 1: In-Service Inspection

1. Introduction
This manual has been formulated to serve as a minimum reference base of ONE-M GROUP’s Inspection personnel to carry out plant In-Service inspection including turnaround as required by statutory body i.e Department of Occupational Safety and Health (DOSH)  Malaysia and plant owner.

The applicable Malaysian Laws and Regulations are;

a.     Factories and Machinery Act 1967 (Subsequent Amendments)

b.    Petroleum (Safety Measure) Act. 1984

1.1 Objective of In-Service Inspection

The purpose of In-Service inspection on items such as pressure vessels, heat exchangers, etc, are to intercept failure while it still in its initial stage, and to be able to plan and perform necessary remedial works at proper interval. Also, by carrying out in-service inspection, unnecessary events may be avoided from occurring. Thus, a well planned in-service inspection could:

a.     Prevent accidents that could cause undue risk to the personnel working at or near the plant.

b.    Prevent accidents that could cause severe material damage and consequently substantial economic losses.

c.     Prevent accidents that could cause undue risk of pollution to the environment.

d.    Prevent undue stop pages in the production of the plant.

If during in-serve inspection is carried out, some abnormalities are found on certain equipment; further investigation shall be called for, such as, the use of non-destructive testing methods.

1.2 Inspection Personnel (Inspector)

Inspector for the purpose of this inspection shall have tertiary education and have at least 4 years working experience in performing plant inspection and testing and/or repair on pressure vessels or other DOSH registered equipment. Inspector shall undergo mock up test as and when required by Client to proof its ability.  Inspector shall also confirm fit physically by medical practitioner and have to undergo annual check up to ensure fitness. Other requirements such as vision, color blindness, hearing etc shall confirm to standard health requirements set certification body such as API, PCN/CSWIP with API 510.

1.3 Inspection tools

For the purpose of the manual the following inspection tools are to be furnished to the Inspectors to ensure effective and accurate inspection result to be submitted to Client or DOSH;

Essential;

1.     Torch light

To provide adequate lighting for general visual inspection. To check the longitudinal straightness of shell, to detect buckle and to detect shell surface unevenness, dents, corrosion, pit, gauge, groove … etc.

2.     Scrapper

To remove scale/rust if any to have better view of the examined area.  Rag And Steel brush must be available for the same purpose.

3.     Straight Edge Ruler (6”)

To estimate thickness of plates /pipe size/nozzle size and flatness of the surface, straightness of nozzle.

4.     Welding Gauge

To measure lost in weld cap or lost in thickness at HAZ, groove depth hi-low in weld.      

5.     Magnification Hand Len

To have close-up view of surface defects especially fissure crack or to identify surface pits/marks.

For Further Measurement or Investigation

Optional;

6.     Pit Gauge

To measure accurately the pit depth.

7.     Camera

To record damages/failure or any non-compliances and as evidence for illustration, discussion, reporting, etc.

8.     Mirror/Boroscope

To inspect internal area of bends in nozzle inlet/outlet from vessel.

9.     Non-Destructive Testing: DPT/MPI

To detect surface crack if there is any sign of damage/impact on nozzle or vessel shell that may cause surface crack.

10.  Ultrasonic Testing

To check remaining wall thickness at specific location (eg. pit-hole, groove, etc) from opposite side. To detect and scan for defect at area which is suspected of having defect/internal crack.

1.4  Basic Principles of In-service Inspection

Described below are few basic principles that are to be implemented in an in-service inspection program

Types of Inspection

a.     General visual inspection

b.    Detail inspection

a.     General visual inspection:

Usually covers items such as header, shell, manhole, nozzle, coating, flange, gauge/meter, bolt-nut, etc.

Requiring only portable inspection instrument and no interruption on the part of the production.

b.    Detail inspection

Is a more thorough inspection by carrying out extra examination and testing on selected critical areas. Additional examination and testing such as NDT methods may be required in the inspection. Frequency and extent of inspection shall depend on the criticality of the items to be inspected and in case DOSH registered equipments its depend on DOSH regulation set force. A more critical parts or items require higher frequency of inspection. Therefore, the items or equipment to be inspected shall be categorized according to their criticality. For example, general visual inspection is to be carried out at an interval of 3 to 36 months depending on the criticality of their equipment. As for detail inspection, it is usually carried out annually or at a lower frequency than general visual inspection.

1.5  Inspection Reviews

An inspection review is carried out to suggest that an Inspection Category given at the detail inspection shall be changed. It involves carrying out such on-stream inspection as may be possible or necessary and examining other relevant data which may be available, eg from corrosion coupon, wastage rates, metal contents of effluent streams, etc. The pressure vessels, heat exchangers and boiler shall be subjected to an inspection review when:

i)      Significant changes occur in the places and services conditions of the equipment that may affect its deterioration rate in whole or in part.

ii)     An abnormal incident occurs, which has affected the safety operation of the equipment.

iii)    Any changes occur since the detailed inspection that may lead to a possible shortening of the interval allowed elapsing before the next detail inspection.

iv)    There are changes in the inspection results, where reduction of inspection interval may be necessary.

v)     There are changes in the inspection results, where reduction of interval between inspections may be necessary.

1.6  Selection of Inspection Category

The selection of the Inspection Category for the pressure vessels boilers and heat-exchangers can be considered with the following factors:-

(i)     The age of the item and length of time of the commissioning.

(ii)    Severity of the service duty and the consequences of the CO2, wet H2S environment.

(iii)   The standard of design, material and construction of item.

(iv)  Previous history of the type of equipment or items. (Reviews all the necessary technical papers related to the equipment).

(v)   The optimum utilization of the equipment.

1.7  Typical In-Service Instruction of a Petrochemical Plant on the Unfired Pressure Vessel (UPV)

1.7.1     Introduction

This Instruction outline the statutory requirements in the inspection of unfired pressure vessels in accordance to DOSH regulation currently set force. These inspection requirements are applicable to new and operating unfired pressure vessels.

1.7.2.    New Pressure Vessels

Detailed drawings, calculations, details of instrumentation and welding and testing procedures are to be submitted to DOSH for approval. DOSH will review these documents, stamp their approval and return it for record. DOSH has to be notified of all welder qualification tests, procedure qualification test, NDT and hydrostatic pressure test. DOSH Inspectors will witness these tests and indicate their acceptance. An application for permission to operate the pressure vessels at certain safe working pressure has to be submitted to DOSH after satisfactory completion of all required test. DOSH will issue a “Certificate of Fitness” valid for fifteen months from the date of issue. This certificate will contain the DOSH registration number (PMT…………) and this registration number has to be stamped on the nameplate of pressure vessels. Pressure vessels fabricated outside Malaysia have to be re-hydrotested at the plant for witnessing by DOSH. Certification by Inspecting Authority (as in the Fourth Schedule, F & M Act.) is necessary.

1.7.3     Operating Pressure Vessels

Internal visual examination is required to extend the certificate of Fitness. However , DOSH will consider extension of the Certificate of Fitness for a period of thirty six months from the data of original issue on external visual inspection, thickness monitoring and previous inspection findings. Hence all unfired pressure vessels are subjected to periodical inspection. All modification to be performed on the operating pressure vessels shall follow the same procedures as the new pressure vessels. To ensure that the operating pressure vessels are fit-for-service at all times, it is prudent to inspect them periodically according to technical  requirements and procedures as described in In-service Inspection Manual. Normally on External Visual Inspection of at test once a year is sufficient, frequency may increase on problem-vessels. A report is to be made on each inspection.

1.7.4     Inspection Manual

It is necessary to develop on Inspection Manual on Unfired Pressure Vessels with detailed instruction and Procedures for inspection, testing, reporting, recording and repairs.The area of coverage shall include and not limit to:-

i.    Internal Visual Inspection

ii.     External Visual Inspection

iii.    Overhaul, Inspection and Testing of PSV

iv.    Thickness Gauging Monitoring Program

v.     Turnaround and shutdown Inspection

The Inspection Manual shall include guidelines for the Evaluation and Analysis of all inspection findings and test results.

1.7.5     Internal Inspection

Internal visual inspection of all columns, drums reactors, heat exchangers, fired heaters, filters etc is to be performed at least once in three years and as instructed by DOSH. Internal of columns, drums, reactors and filters are to be checked for corrosion, discoloration, scales, integrity of attachment welds and condition of screens, wire mesh or filter cartridge. In addition, shell and tube exchanger visual inspections are to include checked of tube sheet welds and cleanliness of tubes. Ultrasonic thickness measurement of tube of fired heaters is to be performed if visual inspection reveals suspect areas. A few plugs are to be opened on air-cooled heat exchanger to inspect the condition of the inside of the headers. Flange faces of all connections that have been opened are to be installed reassembly. Individual reports are to be generated for each major equipment inspected. Further details are to be included in the Inspection Manual of Unfired Pressure Vessels.

1.7.6     Thickness Gauging Monitoring Program

Thickness gauging is part of the overall plant erosion and corrosion monitoring program. The thickness of pressure vessels are periodically checked to determined the predicted design life of the vessels as well as to identify the areas which may have deteriorated due to erosion and accelerated local corrosion. Normally twice thickness gauging per year is sufficient, frequency may increase at highly corroded & eroded area. The details of procedures, equipment, tools, reporting, frequency and recording are to be stipulated in the Inspection Manual of Pressure Vessels.

1.7.7     Inspection of Pressure Safety Valves (PSV)

All the PSV’s are to be visually inspected at least once a year are also they need to be dismantled, cleaned, overhauled, repaired and tested every three-year. The detailed procedures and requirements are described in the In-Service Inspection Manual of Pressure Vessels. However if there is any doubt on the integrity of any of the PSV, twice per year of visual inspection may be necessary.

1.7.8     Evaluation and Analysis

The In-Service Inspection Manual shall include the guidelines on the Evaluation and analysis of all the inspection findings and test results. An appropriate Evaluation and Analysis will provide a better picture on the actual condition and integrity of the Pressure Vessels. Trend analysis and Statistical Analysis may be included.

1.7.9     Other Statutory Requirement

Beside the statutory requirements on the inspection of unfired pressure vessels other relevant statutory requirements include:

i.    Reporting

ii.     Power of DOSH Inspector to carry out Investigation and Enquiries.

iii.    Notice to inform DOSH on sale, hire, transfer, alteration of the pressure vessels.

Inspection Section personnel shall observe these statutory requirements.

1.7.10   Records and Documentation

The records and inspection reports are to be kept properly according to the requirements of inspection Manual of Pressure Vessels. It is necessary to keep a record of all inspection results of each unfired pressure vessels, from the initial inspection to the latest inspection, so that comparison and analysis can be made from loss or damage, however they shall remain easily accessible or retrievable whenever necessary.

1.7.11   Corrective Action and Internal Audit

All unfired pressure vessels are to comply with strict statutory requirements, especially in relation to any hot-work repair on permanent parts of the vessels that need the approval of DOSH Inspector. Hence, Inspection Section personnel need to follow-up all the corrective Action on hot-work repair with specific attention on the statutory requirements. However, in the case of non-hot work matters such as painting, coating, insulation, cleaning, replacement of gauges etc which are to be executed by Maintenance or Operation personnel, then Corrective Action of such nature may not be the responsibility of Inspection Section. All Corrective Actions will remain outstanding until they are completed satisfactory. Outstanding Corrective Actions may lead to the situation of the vessels being classified as not fit for operation; hence all outstanding corrective actions after the due date shall be submitted to the Section Manager and Plant Manager immediately for further consideration. The head of the Section may carry out on Internal Audit on the inspection work and documentation related to the unfired pressure vessels. The purpose of the Internal Audit is to verify that the work done and documentation are in compliance with statutory and specified requirements; the findings and observations are useful for future improvement.

1.8  A Typical Inspection & Testing Instruction on Pressure Safety Valves

1.8.1     Introduction

This Instruction stipulates the inspection and test requirements of Pressure Safety Valves. Pressure Safety Valves (PSV) are vital to the plant safeguards system and are to be periodically tested to ensure proper function. Testing of PSV’s is performed once a year every three years as a minimum. In addition, if the Operations Department experiences problems/malfunction of any PSV, they may arrange with Maintenance Department for repair/readjustment of the scheduled test.

1.8.2     Procedure

Maintenance Department is responsible for planning and scheduling the testing of PSV’s. Test schedule will be forwarded to the Inspection Section. The Inspection Section will provide the set pressure requirement for each of the PSV’s from the Test Certificates on record. The test procedure shall follow in “PSV’ Inspection, overhaul and Testing Procedure”. Visual Inspection is to be performed once/year (See PSV Visual Inspection Form as enclosed). Inspection Section will witness the testing of PSV’s to procedure, the setting is within acceptable limits and to perform other checks as outlined in the procedure. Setting to be monitored and controlled by Inspection Section. PSV-seals are maintained and controlled by the Inspection Section. The inspector witnessing the tests will affix the seals to PSV’s after satisfactory completion of all required tests. Sealing of the PSV’s is to be witnessed by operation and maintenance department representative.

1.8.3     Documentation

Test Certificates are to be completed and certified by the Inspectors in conjunction with the tester and maintenance supervisor. The Inspection Section for record retains the completed test certificates. All certificates shall be kept in file with proper indexing for trace ability and prevention from damage or loss.

1.8.4     Internal Audit

The PSV test procedure, test reports and certificates are too subject to Internal Audit. The purpose of Internal Audit is to verify whether documentation and procedure are in compliance with the written requirements.