Fitness for Service & Residual Life assessments of the Laminated Shell of the Filter Separator Vessel as per API 579 FFS-I
Ideametrics Private Limited
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Problem Definition
Fitness for Service (FFS) is a standard practice used in the oil and gas industry to determine the fitness of working equipment for continued use. Process equipment used in the oil and gas, power generation and chemical process industry deals with high pressure and high temperature variations that can cause brittle and ductile fracture. FFS provides a quantitative measurement of the structural integrity of a component containing flaws. One of our clients detected 2 laminar defects in their main filter separator. This could cause an unplanned shutdown with a high level risk factor. Any unplanned shutdown of a manufacturing or process plant is expensive in terms of both loss of production and the manpower required to solve a particular mechanical integrity issue. FFS method can be used to support design, operation, fabrication, change of service and life extension programmes.
Below image shows the lamination area on the shell which was taken during the inspection (NDT)
Challenges
Increasing demands of Natural gas expect this gas transportation system to work on higher capacity. During scheduled inspection of the main filter separator lamination defects were observed on a shell. External pitting corrosion due to water stagnation on the top chamber near North side saddle closure was observed.
The critical region out of all the defects found was the lamination defect as you can see the below image which was taken by our NDT team on site.
Main challenge was to find out the residual life of the equipment for the defect which were observed on the NDT testing and is the equipment even suitable to consider the same in further woking due to the effect which was observed.
Solution
Our client has a Main Filter Separator in their plant which is a horizontal filter built in two chambers. Top Chamber has an inside diameter of - 914 mm and length 3454 mm and Bottom Chamber - 415mm and length of 3156mm with one side ellipsoidal dish head & other side closure. This filter was constructed in 1987.
The possible damage mechanisms of the Main Filter Separator were identified by considering various modes of deterioration, presence of corrosion elements in instrument air/surrounding environment & Visual Inspection findings. These include Brittle Fracture, General Metal Loss, Local Metal Loss, Pitting Corrosion, Blisters & Laminations, Crack & Crack Like Flaws and some Mechanical Damages.
Non-Destructive Testing (NDT) methods were used for identifying the defects in the main fitter separator. these mainly contains Visual & Closed Visual Inspection, Dye Penetrant Testing, Ultrasonic Thickness Gauging, Ultrasonic Flaw Detection & Wet Fluorescent Magnetic Particle Testing.
Non-Destructive Testing (NDT) observations
On Visual and closed observations during NDT, we observed that there were no defects on saddle supports, circumferential as well as longitudinal weld joints of shell, head & attachment nozzle welds. Pipe to flange nozzle welds were unaffected. No damage found on foundation of filter, on the shell and dish ends.
Two laminations were observed on the shell plate towards the north side below the inlet nozzle of horizontal weld while carrying out ultrasonic flaw detection. External pitting was observed on the top chamber external shell above the saddle support towards the north side near to the closure during visual inspection. The largest pit depth on the top chamber shell was measured to be 1 mm. Corrosion observed on the west side dish end weld to the shell joint due to stagnation of water underneath the paint.
As per API 579 FFS-I we decided to model the laminar defects in the model and study the same in Finite Element Analysis as per Level 3
API 579-1 FFS Assessment as per Level 3
As per API 579-1 FFS assessment there are 3 levels for assessment i.e. Level-1, Level-2, Level-3. We considered Level-1 Assessment for top Shell & Dish Head and Level-3 assessment for laminations defects. For ease of operations we named lamination defects as lamination1 and lamination2.
We compared the thickness of top & bottom shell as well as top & bottom shell dish head with designed thickness. We concluded that measured thickness is adequate for further operations in future considering allowance taken in the design phase. We haven't found metal loss in any of the parts of the separator.
Lamination1 to the nearest weld seam did not satisfy the equation of level 1 & 2 assessments as distance edge of lamination and nearest weld seam was lesser. Hence a detailed analysis i.e. Level 3 (FEA analysis) had been carried out for both the laminations.
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In Level-3 assessment we created a 3-D model of the main filter separator shell with lamination defect in it and checked stress levels in the lamination region. Lamination 1 is located at distance of 30 mm from longitudinal weld line and is of 55*25mm size, at a depth of 18.9 mm from outer face of shell. Lamination 2 is located at distance of 120 mm from longitudinal weld line and is of 35*24mm size, at a depth of 26 mm from outer face of shell. See below images for the defects modelled -
Loading Condition
Internal MAWP pressure is applied to the shell with the thrust induced due to internal pressure at one shell cutout as shown below in the figure
Boundary Condition
Displacement boundary condition is applied in such a way that it is free to expand in radial direction due to internal pressure and restricted in axial and tangential direction as shown in below figure.
Equivalent Von Mises Stress
Linearised Stress as per ASME Section VIII Div 2 Part 5
FEA Analysis of the model was carried out for the Internal MAWP Pressure load. Considering equivalent stress and deformation analysis for shell max stress location was found. Primary and secondary stress for max stress location, Lamination 1, Lamination 2 are ranging between 89.67 Mpa to 93.55 Mpa which is less than allowable stress.
No weld under-cuts, cracks or any dents, gouges & dent-gouges observed on the shell during visual inspection. Creep temperature range of carbon steel material is 371℃ and maximum design temperature is 65℃ so there is no chance of creep damage. No visual cracks were observed on the instrument.
Remaining life assessment
According to the remaining life assessment calculation remaining lifespan of equipment were calculated. Based on the visual inspection observations, operating parameters & thickness calculations, Main Filter Separator is still having remaining life more than the original design life of 20 years. It is considered that condition of vessel is not deteriorated to large extent with original corrosion allowance still available and therefore the expected remaining life is as calculated as 56.5 years.
Conclusion
Main filter separator successfully passed the Fitness For Service & Residual Life assessment test. We have given some recommendations. That includes painting of filters, greasing of nuts, bolts and closure connections. Pressure valve calibration & internal visual inspection was recommended too. Fitness for Service & Residual Life assessment ensured performance of the instrument with recommendations that helps to use investments precisely.
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Research Officer at DASIT, Universiti Teknologi PETRONAS
8 个月how did you calculate the remaining life? appreciate it if you can explain about it or provide the link to learn about it