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MTOC | het Materiaal & Technologisch Onderzoeks Centrum
technical analyses
dynamic behaviour
Sintra Engineers
Remaining life assessment
Structural Integrity and Reliability Analysis, abbreviated to SINTRA Engineers. We are specialists in all integrity issues that can arise in machinery and equipment. Working alongside other companies operating within MTOC,(Material & Technological and Research Centre) we can provide you with a total solution for all your integrityissues. Our clients include several multi-nationals and SMEs within the chemical, petro-chemical and offshoreindustries.
When damage occurs in process equipment such as vessels, piping, storage tanks, turbines and pumps, the obvious response is to shut down the unit and repair the damage. Such unscheduled shut-downs are inconvenient and the question quickly arises as to whether it would be possible to continue operation safely for a while before taking action. There is a � nancial advantage in being able to continue production in that delivery agreements can be full� lled. There is then also more time to prepare the repair work properly. A Fitness-For-Service assessment offers a solution in such cases. One of the steps in a FFS analysis is a remaining life assessment
Remaining life assessment
A remaining life assessments can only be performedon equipment for which the damage mechanism is predictable. The damage mechanisms creep, fatigue, ora combination of these two are typical predictable damage mechanisms.
FatigueFatigue occurs when a construction undergoes alternating stress. Temperature and/or pressure swings are often the cause of damage. Welds are particularly sensitive to this damage mechanism because of the local peak stresses and the high residual stresses in the weld. The following steps can be taken to carry out a fatigue analysis:
Damage investigationIf damage has occurred it is wise to perform a fractographic investigation. This is in order to conclusively determine whether fatigue is the cause of cracking. It can also be determined whether the medium has caused a rapid crack initiation and/or propagation (corrosion fatigue). This information can be taken into account in the fatigue analysis to ensure a more accurate result.
Determination and assessment of the local stressesIn order to determine the local peak stresses, Finite Element Analysis can be used. The assessment or determination of the remaining life can be performed according to several methods:
The cause of the alternating stress is often not known. In such cases we can offer support by performing strain gauge measurements. With the results of these measurements we can perform a remaining life assessment as well as determine the cause of the fatigue
integrity calculation
Design standards FFS standards Recommended Practice
EN13445 API579 FKM guideline AD2000
BS7910
ASME
BS7608
Figure right:striations on a cracked surface from which it canbe concluded that fatigue is the damage mechanism
remaining ligament fracture
crack initiation
materials science
loading. It may be caused by local thermal stress and/or strain or loads which cause the construction to resonate in its natural frequency. In most cases, these aspects are not considered in the design of the equipment.
Crack propagation phaseThe � rst fatigue cracks are visible after about 80-90% of the consumed life of the equipment. From this point forward, a fracture mechanics analysis is the only solution for a remaining life prediction. By using fracture mechanics it can be determined whether a crack is detectable before a construction breaks (leak before break). The results of the analysis also enable the optimum inspection frequency to be determined.
Creep
Creep is a relatively slow damage mechanism which is relatively easy to monitor. There are two available methods, metallurgical and calculative (calculating the remaining life fraction based on actual pressure and temperature). The down side of using only metallurgical investigation is that it is rather time-consuming and expensive. By using several calculation techniques, it is possible to determine the most critical locations of the installation. Performing metallurgical investigation only on these points allows the remaining life assessment to be carried out in a timely and cost-effective manner. The calculations can be performed in increasing degrees of complexity. The use of Finite Element Analysis can even determine the degree of stress at locations like corrosion spots, cracks or weld defects. By examining the creep strain rate, the effect of secondary stress (as a result of peak stress or thermal stress) can be included in the analysis.
Creep-fatigue
The location where creep damage can � rst occur is in most cases also the location with the highest peak stress. Often secondary stresses are the cause of the damage.
What is actually occurring at these locations isa combination of creep and fatigue. High secondary stresses are reduced due to creep and/or plasticity. The stress is reduced, which also causes a reduction in creep strain rate (relaxation). Because of the cyclic operation of the plant (start-up and shut-down), this relaxation pattern repeats, causing more damage to accumulate. Finite Element Analysis can be used to calculate this local cyclic behaviour and thus the remaining life. The accumulated damage can easily be made visible in simple diagrams. Equipment which is mainly cyclically operated should have a different design philosophy than equipment which is operated at high temperatures for long periods of time.
Figure right:stress in pigtail during several loading conditions
Graph right below:creep/fatigue assessment of equipment. Each point of the analysis line is equalto one year .
Figure and graph below:creep and relaxation damage in a T-piece
Sintra Engineers
MTOC buildingKampstraat 866163 HG GeleenThe Netherlands
+31 (0)46 475 7528
50.
45.
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30.0.0 2.0 4.0 6.0 8.0 10.0 (x1.E3)
Time [hour].
Stre
ss (M
Pa).
0.010
0.008
0.006
0.004
0.002
0.000
Stra
in (-
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Creep strain in crotch cornerCreep strain in saddle pointVon Mises stress in crotch cornerVon Mises stress in saddle point
A. Internal pressureB. Start-upC. After 100 hours at high temperature
A B C D E F
D. Shut-downE. Start-upF. After 1000 hours at high temperature
Creep
Fatig
ue
Failure Assessment DiagramAnalyse
