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 4th 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.
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