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for Extended Service beyond Design Service Life, there by Recycling Nickel
Satish B . Kunte
Chief Engineer, Plant Health Service Department
Rashtriya Chemicals and Fertilizers Limited,
Mahul Road, Chembur,
Mumbai- 400 074
ABSTRACT
The case study deals with conservation of centrifugally cast reformer tubes in fertilizer industry for an extended period beyond their design life. The study describes two such exercises, one dealing with HK-40 tubes and the other with HP- micro alloyed tubes. In both cases reworking done on used tubes has helped in exceeding their life beyond design life. The recovery/reinstallation procedures were successful, giving good life extension. Such attempts not only result in huge savings on the reformer tube inventories but also in conservation of the expensive raw material, namely nickel, that can be used for other strategic applications.
INTRODUCTION
There is a fear that we may not be able to meet the rising demand of nickel by the industry unless we stop the extravagant use of nickel. Hence it becomes a need of the time to conserve nickel and recycle nickel to the extent possible. Reformer tubes used in fertilizer and petrochemical industry is one such item which uses a large quantity of nickel. The author who has extensive experience in condition monitoring of reformer tubes, details in this paper the advantages gained in recycling the used reformer tubes to conserve nickel terming it as Nickel recycle.
CHANGES IN MATERIALS OF CONSTRUCTION
The industry has seen the material of reformer tubes change from HK40 to IN519 and now to HP- micro alloyed. The wall thickness of the reformer tubes reduced from about 17 mm in the past to the present about 10 mm without losing strength and other high temperature properties. This is a major advantage with respect to heat transfer, increase in volume and also with respect to the total weight of the tubes which result in series of benefits.
But, in the process, the requirement of Nickel has increased from 20% in the old HK40 tubes to 35% in the present HP- micro alloyed tubes. The attached table shows the nominal chemical composition of HK40, IN519 and HP micro-alloyed tubes, with the corresponding sound wall thickness used. One can see the substantial increase in the nickel content.
Nominal Composition of reformer tube alloys
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DESIGN LIFE OF REFORMER TUBES
The reformer tubes are normally designed for a service life of 100,000 running hours under ideal conditions. The life is generally limited by creep failures. In practice there are several factors contributing to the loss of creep properties thereby making it difficult to run the tubes up to the design life of 100,000 hours. These factors are
Since accidental failure of a reformer tube is a very expensive affair in terms of fire hazard and production loss, the reformer tubes are normally replaced after extracting 90% of the design life, depending on the maintenance practices being followed. This again means hike in Nickel demand beyond projection.
MANUFACTURE AND SERVICE CONDITIONS
As per the existing norms all over the world, the reformer tubes are supposed to be made from fresh raw materials and melting scrap material is not allowed in the recipe. The author, being a metallurgical engineer, had initial hands on experience in quality control and assurance steps in the manufacture of centrifugally cast reformer tubes and other heat resistant castings in a major foundry. The work involved planning and executing quality control steps followed by quality assurance steps for each tube. This includes all the testing right from spectrographic analysis of molten charge ready for pouring upto the hydraulic pressure test of the finished reformer tubes assembly to the complete satisfaction of the third party inspection agency as per specified concerned international standards.
This initial experience of the intricacies of the quality aspects of centrifugal castings has been of great help in his present responsibility, spanning a period of about twenty nine years of monitoring the Plant Health in RCF which is a huge fertilizer manufacturing complex, where hundreds of reformer tubes are in service. The responsibility of Plant Health Service includes, among others, monitoring the behavior/performance of reformer tubes and analyzing their routine and unusual challenging failures over the entire service life of the tubes. He also got an opportunity to carry out entire third party inspection activity for ninety numbers of HP micro alloyed reformer tubes being manufactured for RCF in a well reputed foundry. He has thus been a witness to the gradual transition of the reformer tubes metallurgy from HK40 through IN 519 to the present HP micro alloyed, which took place gradually over the last three decades or so.
The reaction in the reformer tubes is endothermic. The reformer furnaces in RCF are side fired. The reaction is at the peak in the top portion and it slows down towards the bottom side of the tubes. The tubes therefore tend to remain cooler at the top and become hotter towards the bottom. If the tubes are designed to operate at TST (Tube Skin Temperature) of 870OC and if the bottom portion of any given tube is at 890OC, the top portion of the same tube could be at 820OC or 830OC or even less. Under ideal conditions the thermal gradient from top to bottom of the tubes should be at minimum level. But in practice reformers are operated sometimes with this gradient as 100OC or even more. For all practical purposes, a gradient of 20 to 30OC can be considered as very good.
COMMON FAILURE PATTERN AND ITS ASSESSMENT
Due to continuous high TST, the reformer tubes are more prone to failure in the bottom one third portion. In the last 30 years at RCF, more than 90% of the reformer tube failures were recorded in the bottom one third portion only. Very rarely tube failures were recorded above bottom one third region due to localized overheating. Not a single failure is recorded in the top one third portion. As the tubes get older, they start loosing their high temperature strength, the rate of loosing depending mainly upon their skin temperature. Only the outer surface of the tubes is available for carrying out all Non destructive testing like microstructure, ultrasonic flaw detection, dye penetrant, magnetic permeability, OD measurement, Radiography, soap bubble test etc.
Out of these, the tests which tell the inside story are mainly carried out on the weld seams. The general feeling is that weld seam is the weakest portion and the cracks in the tubes originate from welds. However, many times it has been observed that the tubes crack in the parent metal away from the weld. So the weld seam need not be treated as a part of the tube susceptible for cracking. Besides reformer tubes with only one joint and that too without using filler wire are already in use in the industry.
It is not a very common practice to carry out Radiography of the reformer tubes to assess the health condition and residual life. Even if radiography is carried out it is done only for the weld seams. Ultrasonic flaw detection and magnetic permeability tests are very commonly carried out for the entire length of the tubes. It is very tricky and difficult to co-relate the results of these tests with the residual life of the tubes. The microstructure is studied only on the outer surface of the tubes and it cannot tell the story on the inside surface of the tubes. It may not be advisable to correlate the deterioration observed in the micro structure on outer surface of the tubes with the residual life of the tubes as it may not be leading to sudden failure. The dye penetrant test is done only on the weld seams and it tells about the damage that has already occurred. The soap bubble test can be done for the entire tube but it also tells about the defects that already exist.
GRADUAL AGEING OF THE TUBES AND ITS MONITORING
The inside surface of the tubes has a machined finish but the outer surface of the as cast tubes has a rough shiny granular finish. As the tubes grow older, the outer surface of the tubes starts becoming smooth and gradually looses the shine. Anyone closely associated with the reformer tubes for a long time can tell their approximate age of the tubes just by examining their surface finish.
The present generation reformer tubes have a good resistance to creep, but with ageing, as they start yielding and as they gradually bulge, the surface finish starts changing. The tubes can be taken as young for the first five years of their life as they exhibit very less bulging during this period. This largely depends on the operating conditions.
The TST of the tubes has the maximum impact on their ageing process. The design of the present reformer furnaces is generally spacious. Therefore the possibility of the tubes receiving undesired heat flux from the neighboring tubes and from the furnace walls is minimized. Some of the tubes which run below the design TST, exhibit very less increase in the outer diameter even at the fag end of their life, whereas some tubes running at higher TST show excessive sudden bulging even in their young age. For a particular design operating pressure and temperature the manufacturer of the reformer tubes specifies the maximum allowable bulging and advises not to use the tubes beyond this bulging which is normally 2% for the HP- micro alloyed tubes.
So the warning on the tubes could be best realized before the 100,000 running hours period or 2% bulging whichever is earlier.
As the reformer tubes run hottest in the bottom one third portion, the bulging of the tubes is maximum in this region and so are the failures. The bulging of each individual tube is monitored during every possible opportunity. It is the experience that the bulging of the tubes in top one third portions is insignificant and for assessing the residual life, the bulging of the tubes in the bottom one third portions needs to be monitored very closely. Following is the summary of the record of the measurement of the OD in bottom one third portion of all the ninety tubes in methanol reformer at RCF.
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REPLACEMENT AND REHABITATION STRATEGIES
The tubes showing sudden excessive bulging are more likely to fail than the tubes bulged to the same value steadily over a long period. The tubes bulged steadily to the maximum allowable limit over a prolonged period which is substantially less than the design life, can work further as the rate of bulging is not drastic. Similarly the tubes which have bulged less than 1% but have almost lived their designed life can also give some more service. In the case of methanol reformer at RCF a life of 1,10,000 running hours has been extracted from few HK 40 tubes as their bulging was less than 1% throughout their life.
In reality no one wants to take any chances on reformer tubes and they are replaced in time. Slightly unhealthy and failed tubes are replaced immediately, and when life of majority of the tubes comes to an end, all the tubes are replaced together, in toto, as no one prefers to run the reformer with a combination of fresh and old tubes. Everything ultimately leads to hike in the demand for Nickel.
CASE STUDY NO.1
RECOVERY AND REHABITATION OF USED HK 40 TUBES
More than 15 years back the reformer in the Methanol plant at RCF had 90 Nos. of HK 40 tubes. The reformer is of an old design and all the outlet pigtails and headers are inside the furnace. During a shutdown about 35 aged tubes were replaced with new HK 40 tubes as a planned activity. Due to such unavoidable partial replacements, the reformer always ran with a combination of some very old tubes and some fresh tubes. The removed 35 tubes had only scrap value and procurement of new tubes was not planned immediately. The failed/aged/scraped 35 tubes were inspected thoroughly. Bulging measurements and micro structure analysis was carried out on each tube. The bottom half portion of all these tubes was discarded, regardless of its condition. The top one third portion and about a meter extra (depending on the position of the first weld seam) of the discarded tubes was found useful from metallurgical point of view. The good portions of the scraped tubes were salvaged and 10 new tubes were fabricated from these portions, in-house. The bottom reducers and grid plates were salvaged from the discarded tubes only. These tubes were put into service and they worked satisfactorily for almost 30,000 hours (4 years) till all the 90 HK 40 tubes were replaced with HP- micro alloyed tubes. This has resulted in considerable cost saving and better utilization of the reformer tube inventory kept in Stores.
SIGNIFICANCE OF THE CASE STUDY ABOVE
This activity, never attempted before in the reformer tube application industry, was initially felt only as a salvaging operation for emergency use. The importance associated with the crisis for Nickel and all other expensive natural resources was not seriously taken those days as they are focused today.
CASE STUDY NO.2
RECOVERY AND REHABITATION OF USED HP- micro alloyed TUBES
Today in the same reformer many of the same HP- micro alloyed tubes have almost reached the end of their design life. There had been accidental failures of the reformer tubes and the reformer had to be operated with a combination of old and new tubes. During April 2007 turnaround, 25 damaged tubes were replaced with new ones. The reformer is still running with the undesired combination of some old tubes and some fresh tubes. It is decided to replace the entire reformer and the project is likely to be completed by October 2009. Till then the reformer is to be run with many old tubes which were installed in 1997, because it is very difficult and expensive to buy reformer tubes in smaller quantity.
All the scrapped 25 tubes were therefore inspected thoroughly. Eight tubes which had developed crack and which were run in blanked condition for some time were discarded. The bottom half of the remaining 17 tubes along with pigtail was discarded without any testing. It was planned to fabricate at least four good tubes from the 17 top halves of the scraped tubes. Eight good reducers and grid plates were salvaged from the scraped tubes. The 17 top halves were subjected to following tests:
Assembly of the reformer tube
The excessively bent portions and most of the portions of the tubes below the first weld seam were discarded. The macro structure did not reveal the original proportion of equiaxed and dendritic columnar grains. Some failed tubes exhibited good amount of equiaxed grains but the older tubes mostly showed columnar grains. No macro fissures were observed. The microstructure analysis and the tensile strength values did not show any abnormal deterioration and accordingly the bulging of these portions was also to the extent of around 0.6%.
Desired lengths were measured and marked for making four tubes. The inspection and testing norms were set just like making new tubes. The machined surface was checked with dye penetrant testing before welding and no porosity was tolerated. Welding of the selected pieces was carried out with the same WPS used for making fresh tubes. Root dye penetrant test, final pass dye penetrant test and 100% radiography was carried out. There was not much rejection in carrying out welding of the old used tubes and the weldability of the material was found well within acceptable limits. Brand new pigtails were welded to the reducers and four new tubes were made ready for hydro test. Hydro test was carried out at the same pressure like brand new tubes. The pressure was held for a much longer time than the time specified for new tubes. There was no leakage observed
All the fabrication, inspection, testing and witnessing was done in-house. Making of the tubes this way does not mean recycling Nickel. All the four tubes reborn from scrap were put into service. They were subjected to real acid test as they are installed in the corners where the tubes receive maximum heat from side walls. The tubes are working satisfactorily for the last six months at 920OC TST.
CONCLUSIVE COMMENTS
All the Nickel does not die in the reformer. Some more life is still left in the cold end of the reformer tubes. The cold end i.e. the top portion of the tube, does not get exposed to temperatures as high as the bottom portion and hence retains its original properties to a considerable extent. The cost for making one good tube from 3 used tubes is less than 10% of the cost of new tube and the life expected from this tube could be 30,000 to 40,000 running hours or 4 to 5 years.
The extravagant use of Nickel (or for that matter all resources) by us may deprive our future generations of fresh Nickel very early. Salvaging reformer tubes this way may postpone fresh Nickel stock out situation and we may escape abuses and curses from our own grand children and great grand children for our extravagancy.
ACKNOWLEDGEMENTS
The author is highly thankful to the RCF management for giving freedom to the author for carrying out the unique exercise of salvaging used Reformer tubes and putting them back into service which was never attempted before in the reformer tube industry. The author is also thankful to Dr. Ellaya Perumal, Director,Corrosion and Metallurgical consultancy centre, Bangalore.
Responsible Care (RC) is a Chemical Industry Initiative, which calls on Companies to demonstrate their commitment to improve all aspects of performance, which relate to protection of health, safety and environment.
