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Condition Monitoring - Process Plant Tube Inspection an Ongoing Commitment by Plant Owners and OperatorsCharles. Panos
International Tube Testing Pty Ltd Qld Australia
Equipment reliability forms an integral part of success in today's aggressive and competitive international market. Plant equipment such as boilers, pressure vessels, heat exchangers, condensers, reactors and other ancillary items require ongoing periodic monitoring and inspection to ensure that uninterrupted production is achieved.
Over the years plant owners and operators have adopted various approaches towards inspection in an attempt to ensure equipment runs at optimum production levels for as long as possible.
Typically inspections are either carried out in-house or contracted to third parties.
Today's market offers superior technology to that of the previous decade, which is being used to ensure accurate test data, is available for effective predictive maintenance of plant.
Options being adopted in plant predictive maintenance through the effective use of ongoing inspection test data, though varied within the international industry, are heading towards the use of professional focused third party services.
More often than not, third party specialised technical service providers offer superior technology and experienced personnel who are able to focus on the methodologies, technologies and techniques being used.
This presentation reviews a number of inspection processes, which are proving effective in successful plant maintenance through accurate test results.
Keywords: IRIS, Tube testing, Condition monitoring, Boilers, RFT
Industries rely on their plant operating efficiently and uninterrupted during production. Plant equipment includes boilers, pressure vessels, heat exchangers, AFC, heaters, condensers, shafts, rollers, turbines, pipe work etc. Purely by design plant suffers from a number of degradation mechanisms which when left un-monitored and without preventative - remedial measures being taken often lead to failure. The failure of plant not only results in loss of production, loss of income and costly unscheduled repairs but also all too often results in injury and sometimes fatalities.
Industries worldwide are now more than ever recognising condition monitoring as an important factor in the success of their operations. The effective monitoring and maintenance of plant can result in fewer unscheduled stoppages, equipment failures, loses in production, unplanned expenditure on repairs, injury and fatalities. Plant owners and operators are increasingly aware of effective methods available for plant monitoring and are adopting responsible, proactive approach towards predictive and preventative maintenance.
Specific technologies and industry experts are utilised by plant owners and operators to assist in the success of these programs. Industry specific technologies, procedures and ideologies have progressively evolved over the years, based on industry experience and demand and are successfully being utilised by proactive plant owners to assist in achieving uninterrupted plant operation. Technologies including specific test equipment, techniques and procedures are being applied by experienced operators to accurately test a range of plant. Specialised techniques which are now more commonplace include IRIS (Internal Rotary Inspection System) tube testing, RFT (Remote Field Testing) tube testing, Ultrasonic and Surface method techniques for crack detection and monitoring of rollers / shafts / drives, surface methods / EC (Eddy Current) techniques for crack detection and monitoring of gearing, Pulsed Eddy Current and Long Range UT (Ultrasonics) for corrosion under insulation and MFL (magnetic flux leakage) / B' Scan for Underfloor Corrosion.
The complexities of plant design and operating conditions gives rise to a variety of in-service defects which, in some cases and in particular when left unattended may give rise to plant / machinery failure. This paper therefore concentrates on a range of typical defects and the test methods being successfully utilised in combating plant failure and increasing plant productivity, unhindered operation and plant longevity.
Equipment such as boilers, heat exchangers, AFC, condensers, re-boilers, HP & LP heaters etc. form an integral part of the production system. Successful production processes are reliant on the smooth, uninterrupted operation of this plant.
Equipment failures, which occur during production, have an adverse effect on the performance of the plant resulting in production losses, expensive down time, repairs and associated expenses and can lead to injuries and fatalities.
Equipment often suffer from a number of degradation mechanisms some of which maybe operator dependant whilst others are process, design or manufacture related.
Many plant owners / operators are introducing structured monitoring programmes which incorporate periodic inspections, monitoring, analysis, as well as the introduction of external independent third party survey services.
Regardless of design or operating environment most tubes can be effectively monitored using the latest proven technologies and this approach is yielding surprising results for many operations.
Some failure modes associated with various plant equipment include, CO2 corrosion, metal dusting corrosion (after breakdown of the protective scale or oxide layer), erosion (general or localised), mechanical vibration (fretting), oxidation and wet corrosion including crevice corrosion.
Erosion may occur on the external or internal surfaces of tubes and can be caused by impurities in the gas stream or product.
One form of typical erosion, which may occur in heat exchanger tubes, is impingement erosion near inlet nozzles. (Figure A)
|Fig a: 3 Zone Feedwater Heater.|
|Fig b: Impingement Erosion.|
Erosion can also occur inside tubes at the tube inlet and can be caused by cavitation.
Boiler tubes in the process industry often suffer severe external erosion caused by high velocity flue gases containing ash impurities at baffle locations or at areas influenced by soot blowers (figures C & D).
|Fig c: Thermal representation of boiler.||Fig d: External tube erosion.|
Corrosion occurs due to a variety of reasons and at various locations along tubes and maybe general, localised or pitting in nature (figure E)
For example recovery boilers in the pulp & paper industry may suffer from near drum corrosion.
Corrosion may occur at tube surfaces just above the mud drum where deposits containing sulphur products react to moisture creating sulphuric acid. This mechanism causes wall loss at the tube surface and may result in tube failure over a period of time (figure F)
|Fig e: Tube internal & external pitting.||Fig f: Near drum corrosion.|
Water treatment if it is ineffective may lead to the formation of deposits on the internal surfaces of tubes (figure G)
Over a period of time these deposits may increase in thickness and may adversely effect flow or in some cases may also result in localised overheating of the tube material causing failure through bursting under pressure (figure H).
|Fig g: Internal Deposits.||Fig h: Burst tube.|
Tube Baffle cutting.
Tubes in certain exchanger designs may suffer from mechanical wear at tube support or baffle plate area (figure I). In principal, the mechanical vibrations at areas such as De-Superheating zones in Feedwater Heaters may lead to tube material wastage between the tube support plate and the tube outer surface leading to failure (figure J).
|Fig i: Mechanical wear.||Fig j: Tube bundle, support plates & tubes.|
The restriction posed by inadequate access to tubes is no longer a problem with the invent of tube testing systems designed for application from inside the tube. Technologies such as IRIS, EC and RFT are based on inserting a probe into each tube being tested. This is particularly beneficial where inspection is carried out from inside the steam drum, (figure K), from inspection openings in headers or from tube sheets in exchangers (figure L) as all tubes are readily accessible where proper preparation is carried out.
|Fig k: Shell & tube exchanger.||Fig l: Shell & tube exchanger.|
Testing from inside the tubes ensures that thousands of tubes in the generating / convection banks or other types of tube bundles are readily accessible and that the logistical access problems and potential safety hazards associated with external access are avoided.
Production rates are high and test results are instantaneous with the IRIS system and available after signal evaluation, review and confirmation using the EC/RFT systems.
Of these techniques, the IRIS system has proved to be the most effective with the ability to take full circumferential, continuous thickness readings, detectability of small area wall loss and real time wall thickness imagery and its application to all tube materials, though it is the slowest of the three systems.
The conventional EC (eddy current) technique has been effectively used for decades and is largely proven in its effectiveness for examination of non-ferrous tube materials, further it is fast and readily detects internal and external defects as well as cracking.
RFT (Remote Field Technique) has been used commercially for little over a decade and is considered to be the newest of the tube testing systems. Probe frequencies used are generally much lower than those used by conventional EC techniques resulting in inferior defect detectability compared to the conventional EC methods. The RFT technique is used as an effective detection and sorting tool for inspection of ferrous tubing. Production rates are high and unlike the IRIS & EC techniques, does not require the same extent of internal tube cleanliness.
There are benefits and limitations associated with each tube testing system and the effectiveness of test results relies heavily on operator experience, familiarity with the use of the test equipment as well as knowledge of the plant being tested.
Application of these systems includes inspection of a wide range of tube sizes, configurations and materials.
The IRIS tube testing system is one of several NDT techniques available for inspection of tubes. Access is required to the tube internal surface, which needs to be generally free of internal deposits or contaminants for optimum signal response and sizing accuracy.
IRIS operating principle
The IRIS system uses the ultrasonic pulse echo technique to generate high frequency sound waves into the tube wall measuring the thickness over all scanned surfaces. The probe (figure M) transmits an ultrasonic pulse, which is reflected at right angles by the rotating mirror (45º) housed in the probe turbine assembly (figure N). The system produces a B' Scan or end view/cross sectional image of the tube (figure O) and can be viewed in typical IRIS rectilinear format or circular view (representing a true end view of the tube, figure P). The image generated includes the full tube circumference (360º) and both internal and external defects can be readily distinguished.
|Fig m: IRIS probes.|
Figures O & P, shown below are typical of a digital IRIS system showing the two display types referred to above, both depict the wall loss defects shown in figure N above.
|Fig n: IRIS Technique Concept.||Fig o: IRIS Image C & B'.||Fig p: IRIS Image end view.|
Information such as configuration, design, history, failure mechanisms, operating conditions, material type and numbering convention etc is considered essential prior to the commencement of tube surveys.
Testing of tubes follows a pre inspection meeting with the plant engineer discussing operational history, a brief investigation (visual check) on the external condition of the tubes after which testing commences by inserting the test probe through each tube.
The probe is drawn through each tube at a steady rate and any recordable wall loss indications being immediately evaluated and recorded as required.
Records of the test are maintained in accordance with designated standards such as ASME V, AS2452.3 and reports are issued to the client upon completion of the survey.
A colour coded tube layout depicts each tube showing by colour the wall thickness range for each tube. This is often used for tube plugging purposes or re-tubing.
Plant subject to fatigue may over time crack at highly stressed or under designed areas. When unattended these cracks may lead to equipment failure. Surface cracking can generally be detected and monitored using surface test methods such as magnetic particle, penetrant, eddy current and visual testing.
Ultrasonics and radiography techniques may also be applied for detection of cracking which may occur at areas, which cannot be readily accessed.
Ultrasonic examination techniques are also used to gain appreciation of defect depth, though this can be restricted where crack orientation is not favourable for accurate sizing.
A combination of these methods assists in achieving accurate defect detection, sizing, orientation and location as well as ongoing monitoring.
For example process mills contain a range of mechanical components including low speed gearing used in mill crushing trains which may suffer from cracking caused through fatigue (figure Q).
High stresses encountered during the operating process may result in cracking in the gear tooth root / face areas and sometimes at the spokes (figure R).
Similar to other plant equipment, rollers, shafts and drives play an integral role in the plant process. These items also have a tendency to suffer from fatigue cracking which usually occurs in a radial direction at areas of high stress such as changes of section at drives or journals (figure S).
|Fig q: Mill second motion gear.||Fig r: Typical surface cracks.|
|Fig s: Roller, shafts.|
As is the case with most industries in today's competitive environment companies often do not have the luxury of having adequate spare parts as replacements for failed components. It is therefore important to ensure that each item of plant is used to its full capacity and that defects are identified and monitored and the item taken out of service or repaired prior to failure. This is more effectively achieved where historic data is available to the engineers. For example, knowing the propagation rate of cracking in a shaft may greatly assist in determining whether or not to take the shaft out of service, or when considering erosion rates of boiler tubes determined through periodic thickness surveys, planning can be extremely accurate ensuring that the optimum life is extracted from tubes prior to re-tubing.
Periodic testing along with accurate reporting and maintenance of records helps ensure that important data of this type is available to engineers for planning maintenance, repairs and plant asset replacement.
Test results should therefore be presented in concise and detailed formats ensuring all the relevant data is available for review and consideration by the engineer prior to assessment evaluation and rectification planning taking place.
The problems faced by plant owners and operators relating to the success or otherwise of their equipment is not unique. Rather these are common occurrences throughout many industries and many parts of the world.
Technologies exist today which ensure accurate data is available for engineers to calculate trending and plant life expectancy.
The tools available through current technologies are numerous and the calibre of technicians makes it easier for owners and operators to harness the benefits of these services.
It is encouraging that more and more industries have soundly chosen to adopt a proactive approach towards sustained, structured periodic condition monitoring, preventative maintenance and inspection policies.
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