We have the necessary resources, the tools of the so-called sub-size technique, and finally the practical experience to determine the remaining service life, as we have already carried out several service life calculations based on direct material tests at Slovnaft’s Bratislava Refinery.

The sub-size technique for the modern and accurate determination of remaining service life is a modern test method, which allows the determination of the characteristic properties of the structural material on miniature (sub-size) specimens prepared from material samples taken from the apparatus/equipment using a special sampler. In this case, the sample taken is so small (roughly a 2 EUR coin) that it does not compromise the integrity of the structure, and under certain conditions the sampling can even be performed in-service. Mechanical tests, chemical composition determination, optical and scanning electron microscopy, metallographic, fractographic and fine structure tests are carried out on the extracted pieces to determine the actual mechanical behavior of the material. In the case of remaining service life determination, the need for sub-size tests is motivated by the fact that the actual condition of the material can only be reliably determined by direct mechanical and destructive material testing of the material of the container or apparatus.

More information on remaining service life determination and the closely related sub-size techniques is provided below.

Determination of remaining service life

Today, there is an increasing demand for the calculation of the remaining service life of structures/equipment already in service, which on the one hand helps preventive maintenance and on the other hand allows the technical basis for Lifetime Extension (LTE). This concerns the entire spectrum of the chemical and energy industries, in particular pressure vessels and pipelines and all equipment where mechanical wear and tear or other visible forms of deterioration do not limit the continued use of the equipment, structure or component.

Aging-degradation processes occur in the materials of the various structures due to the operating conditions (e.g.: degradation of pearlite grain structure, TiC precipitation), which cause degradation of the properties of the structural materials in question. This degradation of properties limits or may limit the service life and lifetime of the equipment. These unfavorable changes are various material-related processes that take place at microstructural or submicrostructural levels.

Pearlite degradation under operational heat stress

TiC precipitation in austenitic steel

The operation of an equipment/structure beyond its design lifetime requires adequate toughness and strength of the structural material to guarantee safe operation. However, these properties cannot be fully determined by non-destructive testing (NDT), as the changes occurring at the micro/sub-micro level (transformation, precipitation-degradation and transport processes) cannot be detected or only partially be detected by NDT.

The determination of the remaining service life therefore requires the use of a modern test procedure that provides quantitative information on the current state of the structural material of the equipment in question, which provides the basis and allows the necessary calculations to be made. The current material properties can only be determined by direct destructive testing (DT), which requires, by definition, the extraction of a specific material volume from the equipment, on which the necessary tests can be performed. Given that the primary objective is to determine the remaining service life, the removal of a material volume from the structure should not compromise the integrity of the structure and should not appreciably reduce the remaining service life itself. Taking these two conditions into account, a test procedure is required which requires only a minimum amount of material.

Sub-size sampling

For small-volume sampling, classical material removal processes are not or only partially helpful for the following reasons: (I) the thermal input of the material removal process and/or (II) the deformation caused by the process locally changes the structure and thus the properties of the material volume to be tested. To avoid these, we have aimed to use a sampling procedure that simultaneously ensures the preservation of the extracted volume in its current state, with negligible or minimal impact on structural integrity and in any case without compromising operational safety.

The sampling process in principle

Vacuum clamping

The remaining material shortage

The extracted material sample

The sampling device developed by Corweld Ltd. takes a small, shallow, spherical sample from the pressure vessel or pipeline, which – as proven by preliminary finite element simulations – is guaranteed not to affect the operational safety of the structure. In some cases, depending on the type of equipment, wall thickness, operating medium and, finally, operating temperature, sampling can be performed even during operation.

Thanks to the vacuum clamping of our developed sampler, it is suitable for sampling from flat, cylindrical, and double curved surfaces. If required, an inert gas (Ar) atmosphere can be provided in the cutting environment. In all cases, the size of the volume of material to be extracted, in the form of a spherical cap, is in accordance with the wall thickness and load of the equipment, which is defined prior to the design. The height (h) of the extracted volume can be varied between 0.6 and 3.3 mm, in accordance with which the depth (H) of the residual void is between 2.8 and 5.5 mm. The diameter of the extracted sample (Ød) thus varies between 10.6 and 25.2 mm, and the diameter of the residual void (ØD) between 24.3 and 33.4 mm, depending on the setting.

 

Sub-size tests

A tensile test specimen is machined from the extracted sample, using a method free from thermal and mechanical impact, and tested with a special tensile tool developed by Corweld Ltd. In addition, analytical, microstructural and fine structure tests (SEM, TEM) and, in the case of dynamic loading, micro-Charpy impact tests are carried out, on the basis of which a reliable statement can be made about the current condition of the structure in question. The great advantage of the whole sub-size methodology is that it provides quantifiable results, which allow a good engineering estimate of the remaining service life of the structure. The sub-size dimension also provides the possibility to target both the material, the weld metal and the heat affected zone (HAZ).

 

In all cases, the sub-size test results allow the determination of ageing-degradation trends, which is essentially the very reason and purpose of the test itself. The initial data can either be taken from databases or determined by conventional tests on material of the same quality but in new condition.

For the future, it is advisable to record the 0-state when installing a new equipment, which will make the determination of ageing-degradation trends based on sub-size studies, probably decades from now, even more accurate. From the above, it can be concluded that sub-size testing is currently the best and most reliable methodology for the assessment of the current condition and determination of the remaining service life of pressure vessels and pipelines, as well as of all structures and equipment subject to mechanical stress and thermal stress.

The sub-size specimens and the full-size specimen (below)

Extracted material samples and sub-size specimens

Sample of material (left) and the worked out specimen (right)

Weld joint environment and sub-size specimen machined from the heat affected zone (HAZ)

Metallographic image of a sub-size sample

Fractographic image of a sub-size tensile test specimen

TEM image of a sub-size sample

(1) In sub-size testing, small samples can be taken from materials, especially from pressure vessels and piping or other structures, in such a way that the continued operability of the equipment is not affected. At the same time, the material analysis of the extracted sample allows a well-founded, responsible opinion to be given on the current condition of the structure, the trend of the ageing-degradation process and a good estimate of its remaining service life, which is also the basis for an extension of the service life.

(2) The use of sub-size testing allows a reasonable reduction in the scope of non-destructive testing, thus allowing significant cost reductions in this area. It also reduces maintenance costs by generally increasing maintenance cycle times significantly when using sub-size testing, making maintenance outages (overhauls or overhauls) less frequent and shorter. This allows the maintenance organization to concentrate on its most important tasks and to work more efficiently.

(3) The definition of the remaining service life in this way is essentially a well-founded, moderately conservative good engineering prediction that allows responsible decisions to be made.

(4) Cost reductions, simultaneous reduction of downtime and service life extensions based on remaining service life will bring significant profit gains for both operator and owner.

(5) The above benefits also significantly increase operational safety, which is in line with modern chemical and energy safety philosophy.