INTRODUCTION
When we talk about Reformer Tube the first thing that strikes our mind is how can a tube life be maximized and secondly what measures need to be taken if a tube fails. Needless to mention, that, the reformer tube is the most significant and exorbitant component of the plant. Its replacement is certainly very expensive.
Design of Reformer Tube
It is in accordance with known international standards, typically following the guidelines given in API530 for a designed creep life for the selected material. Laboratory short–term test is executed to derive the minimum required wall thickness for tube to combat the creep. Time to rupture is assessed for a range of temperature and stress.
Metallurgical development
Wrought alloy steels are burgeoning. Historically, a HK40 material was utilized for the last 40 years. However, the HP modified grade of steels which are micro-alloyed with the addition of zirconium, titanium tungsten and other rare earth elements is within easy reach that substantially increased the creep life. It is more expensive when compared to HK40. However thinner tubes should be opted for as it provides longer life, high heat transfer efficiency and with the same price yield.
Failure Mechanism
An increase in diameter is generally observed as a result of creep strain due to prolonged service at elevated temperature and internal stress. This eventually culminates in rupture. This is certainly a dominant damage Mechanism and it is life limiting at elevated temperature and internal stress. This eventually culminates in rupture. This is certainly a dominant damage Mechanism and it is life limiting.
Another factor that accelerates normal “end to life” is over-firing or Flame impingement. Besides thermal cycling is also responsible for “end of life”.
Life assessment methods
Routine skin temperature measurements that are ought to be taken. The data forms a guideline for life fraction consumption calculations.
Ultrasonic Attenuation- It is obviously an excellent principle for detection of mid-wall fissures. It is often found difficult to calibrate unless references with similar creep damaged tubes are made available. Ultrasound passing in through mode transmission mode will get attenuated due to creep voids and almost blocked when there are fissures (micro cracks) in the sound path. The ultrasound attenuation method is the best detection tool by NDT. Scaffolding is not required here if automation is used to crawl up and down on the tubes. The suspected tubes identified to have creep fissures by ultrasound method are often offered for a follow up NDT inspection method of Radiography.
Radiography- Radiology is usually done in suspected areas to image the creep fissures. It is primarily advantageous to confirm the presence of mid-wall cracks. It takes up much of the time and restricts the work area due to radioactivity during time of testing and hence it is often limited to sampling – generally based on ultrasound attenuation results.
Microstructural examination – Although, the creep voids and fissure formation starts from mid-wall, their incubation and initiation periods are dependent on metallurgical condition of the material. The microstructural examination is an excellent tool to judge the metallurgical condition. The coalesced carbides, grown up secondary carbides, absence of secondary carbides with blocky primary ones are all indicators of metallurgical aging of the tube at different stages. An active feedback of the metallurgical condition with creep strain data provides more accurate prediction of remaining life.
Hammering the final nail, it can be stated that tube life can be maximized by properly analyzing different measurements of tube condition - that typically involve – diameter measurements throughout the length of tube, wall thickness measurements, metallurgical ageing and detection of internal creep fissures.
Tube Metallurgy has reached a plateau as there is nothing new on the Horizon. Future enhancements are more likely to be in smart coatings to improve Heat transfer.