Demand for transportation fuels and other crude oil-based products has been increasing. To meet growing demand for cleaner fuels, refiners are using more severe processing methods such as hydrocracking. Demand for refined products has increased to the extent that refiners desire larger hydrocracking reactors that can operate at higher pressures with design conditions that are even more severe. New feedstock diets for refineries utilize more difficult to crack crudes; demand for reactors that can withstand higher temperatures (over 450°C) and higher hydrogen partial pressures (12 MPa to 15 MPa) likewise is increasing. Under such severe processing conditions, reactor vessels are constructed from low alloy chromium (Cr)-molybdenum (Mo) steel of various grades.
Severe processing environment.
Hydrocracking or cracking, in the presence of hydrogen or dehydrogenating, is a catalytic process; heavy oils are converted into lighter fractions. The upgrading is done by several chemical reactions and involves the saturation of aromatics, cracking (breaking the bonds of chains of Carbon NDR) and isomerization in the presence of hydrogen. Hydrocracking is, therefore, one of the two major conversion processes used by the modern refining industry. The other important process is the fluid catalytic cracking (FCC). However, this processing operation is mainly used to produce gasoline.
Cracking operations play a more versatile role in refining hydrocarbons. This process can be adapted to produce middle distillates; thus, it is widely adopted due to its ability to provide a wider range and higher yield of quality products. Typical products from hydrocracking include: liquefied petroleum gas (LPG), naphtha, jet fuel (kerosine), diesel, ethylene, lubricating oils and gasoline.
The chemical reactions of the hydrocracking processes are grouped into two broad classes. The first group includes hydrotreating reaction, during which impuritiessuch as nitrogen, sulfur, oxygen and metalsare removed from the hydrocarbon mixture. The second group of reactions involves hydrocracking, in which the carbon-carbon bonds are broken with the help of hydrogen, using bifunctional catalysts. Typical variability of hydrogen partial pressure in representative applications is:
Mild hydrocracking of vacuum gasoil (VGO), Arab or Urals: 800 psia to 1,200 psia (5.5 Mpa to 8.2 Mpa)depending on the desired operating period
Arab hydrocracking of VGO with 70% conversion mode single-stage once through ( SOT)1,800 psia2,000 psia (12.4 Mpa to 13.8 Mpa)
Arab hydrocracking of VGO with 100% conversion mode TSREC1,800 psia2,000 (12.4 Mpa to 13.8 Mpa).
Typical variability of starting operation temperature of the catalyst in various treatment schedules are:
SSOT, 390°C to 430°C
Second phase of optimized partial conversion (OPC) or TSREC, noble metal catalyst/zeolite, 290°C350°C, catalyst and base metal/zeolite, 320°C400°C.
As shown here, the severe operating conditionshigh temperature, high pressure and high partial pressure of hydrogenincrease the activity of the catalysts. And only under these harsh conditions can the best performance be expected from catalyst materials; and therefore, the refining operations can be more effective.
Designing for severe service.
Under these extreme conditions, reactors need to be constructed from high-performance materials that are both resistant to high pressure at high temperature and are resilient to corrosive attack from the inside. In fact, the majority of hydrocracking reactors in operation today, are built from a low alloy Cr-Mo type 2.25Cr-1Mo steel. But the trend in recent years is to build hydrocracking reactors with materials with even better performance. A new generation of steels such as low alloy Cr-Mo with enhanced vanadium (V) 2.25Cr-1Mo-0.25V [plate steel SA542 D4a and forgings SA336 F22V (P-No.5C-ASME IX)].
Usage of better-performance materials has increased the service life of high-pressure vessels and can exceed the service life length as compared to those manufactured with more conventional materials, even in cases where hydrogen partial pressures are higher than those comparable to conventional 2.25Cr-1Mo reactors.
The industry has seen that 2.25Cr-1Mo-0.25V can provide better mechanical properties at room temperature, hot and creep as compared to conventional 2.25Cr-1Mo. In short, the 2.25Cr-1Mo-0.25V, when compared to conventional 2.25Cr-1Mo, is considered to be:
Stronger in tensile strength at elevated temperatures
Less vulnerable to temper embrittlement
Less vulnerable to hydrogen embrittlement
Less vulnerable to hydrogen attack
More resistant to weld overlay disbonding.
The American Society of Mechanical Engineers (ASME) in the Boiler and Pressure Vessel code has also recognized these advantages. In the ASME VIII Division 2 Ed. 2007, these additions are listed:
Design stress intensity changed for this material, ASME Code Allowable Stress Intensity Changed
2007 ASME Section VIII Division 2 Pressure Vessel Code permits significantly higher design-stress intensities for 2.25Cr-1Mo-0.25V steel than the previous edition:
2004 Edition: 169.1 MPa @ 454°C, 163.0 MPa @ 482°C
2007 Edition: 199.8 MPa @ 454°C, 164.6 MPa @ 482°C.
The conventional 2.25Cr-1Mo material properties are:
2004 Edition 127.8 MPa @ 454°C, 110.0 MPa @ 482°C
2007 Edition 149.8 MPa @ 454°C, 112.0 MPa @ 482°C.
The significant revision of the allowable stress intensity from the 2004 Edition to the 2007 Edition of the Code, shows, at 454°C an increase of 18.2%, and, in the 2007 Edition, shows an overall increase of the V-modified steel over conventional material of 33.3%. This increase above conventional material means that the V-modified steel will have even greater application in the future.
Thanks to all these benefits, reactors can be built lighter and, therefore, cheaper. For the reactor manufacturer, this translates into fewer and lighter movements in the factory, easier transportation, lighter loads on the roads and lighter lifting, which opens up crane availability and using a lighter crane while loading on a ship or during erection, which means less cost. The foundations where the reactor will sit can now afford to be lighter and shallower. Each of these activities provides cost benefits with a lighter weight reactor. The industry cannot deny these benefits as they give considerable economic advantage.
Manufacturers are becoming more confident in the construction of V-steel vessels and are able to assist the engineering, procurement and construction (EPC) companies in the evaluation of possible alternatives, even hybrid solutions between plate and forgings to assess the best results in terms of operational safety, quality and cost.
Another aspect to consider is the inside surface protection. Classically, hydrocracking reactors need to protect their inner surface from direct contact from process fluids. This protection is provided by an internal liner that is capable of protecting the base metal from high-temperature corrosion. This cladding is typically carried out by overlaying a weld metal over the base metal. This process of weld overlay uses austenitic stainless steel, usually the type SS 347, niobium-stabilized to resist the phenomenon of precipitation of carbides at the grain boundary, in particular, during construction and, especially, during post-weld heat treatment (PWHT).
However, the real purpose of the cladding is for process service of the reactornamely preventing hydrogen (H2) and other corrosive media attacks on the base metal wall of the reactor. A major problem is that an H2 attack can provoke:
Decarburization of the surface as carbon migrates to the surface of the material exposed to the process fluids
Carbon at the surface combines with the free hydrogen to form methane (CH4) and causes blistering on the undersurface (see Fig. 1).
| Fig. 1. Macro of 2¼Cro-1Mo-¼V |
decarburization and fissuring in high-
temperature hydrogen service.
Tough fabrication process.
Following so many positive characteristics, there must be another side to this coin. And there is, in fact, the only weak link in this design-materials-construction-in service chain is limited to fabrication. All the potential risks are borne by the manufacturer, so it is necessary to assign these projects to reliable manufacturersexperts with credentials. Some of the risks in using 2.25Cr-1Mo-0.25V are:
Greater sensitivity to weld cracking during fabrication
Susceptible to re-heat cracking
Intermediate stress relief (ISR) mandatory for highly stressed-pressure retaining welds and catalyst-bed supports zone
Greater control required on preheat and inter-pass temperatures
Higher weld-metal hardness compared to the conventional 2¼ Cr-1Mo steel
Difficult to guarantee toughness for the V-modified steel with 54 joule impact energy level at 29°C.
Welding consumables suppliers are limited globally
Very low toughness of as welded weld deposit prior to PWHT, can cause:
o Cracking from not carrying out ISR for sufficient time for nozzle welds
o Cracking resulting from weld flaw in nozzle welds
o Cracking resulting from cutting nozzle opening through a bed support weld build up after DHT
Field-weld repairs are much more difficult to carry out, due to heating steps necessary in the welding process.
Critical quality issues.
Material quality from the mill is critical; consumable-material quality and management are also critical. V-modified steel is difficult to work with and it needs to be managed well. The manufacturer needs to properly plan the construction of the reactor or vessel. From initial material handling, through to cutting, rolling, beveling, welding, heat treating and non-destructive testing (NDT) inspection, all need to be tackled by skilled trained personnel.
Moisture problems. The real price to pay for its advantages in mechanical properties is that V-modified steel is extremely difficult to weld. To make the welding easier, an increased overall material management system of the welding process and welding consumables are necessary. In particular, electrodes and flux are subject to intense drying, between 350°C and 400°C, and maintained at temperatures well above 130°C, to remove any sign of moisture. Moisture is extremely harmful inside the welding process; moisture contains hydrogenthe primary element for cracking. It is imperative that even the welding material held in the welding equipment during the feeding process of the weld should be kept at elevated temperatures. The elevated temperatures help avoid forming condensation and ensure that when weld consumable material reaches the weld zone, it is dry and fully cleared of moisture.
Managing the welding and controlling the heat treatments helps to obtain the desired mechanical properties, especially the required toughness. From historical evidence, typically the heat-affected zone is the weakest area in most welded metals. In V-modified steel, regarding toughness at low temperatures, the critical zone is the area meltedthe weld deposit. Today, despite all the technological efforts, the filler material still faces some difficulties keeping up with the requirements of industry.
The welding consumable materials are characterized by very low storage of hydrogen, specifically designed for welding steels with 2.25% Mo, 1% Cr, 0.25% V, resistant to creep and hydrogen attack. The weld metal is resistant to embrittlement caused by the high-temperature service, and is verified during step cooling tests. The valuesX factor and J factorare very low, on average below 15 and 100, respectively.
Another important factor in fabricating reactors in 2.25Cr-1Mo-0.25V is the PWHT. In fact, compared to the conventional 2.25Cr-1Mo, V-modified steel requires a higher temperature PWHT with longer holding times, typically 710° +/5°C for 89 hours.
Critical temperature parameters. Specialists in this field recognize that the weld metal on these types of materials has a critical PWHT temperature of 705°C and a holding time at that temperature for at least 8 hours. These two parameters of temperature and holding time are higher than the standard required by ASME where:
ASME VIII Div. 2 Ed. 2009b Table 6.11 for P-No.5C states a minimum 675°C
ASME VIII Div. 2 Ed. 2009b Table 6.11 P-No.5C states a Holding time minimum for tn < 50 mm 1h
Many specialists consider these temperatures and holding times to be insufficient.
Therefore, to have good mechanical properties of materials in welding, PWHT is carried out at higher temperatures and over longer periods of time in special furnaces capable of treating whole or sections of reactors from 800 tons to 900 tons.
In addition to these two parameters, the temperature profile is critical. It is essential that the temperature is the same all the way through the reactor body, and that during the temperature rise and fall, the differences in metal temperature is minimal.
Problems for reactors where temperatures of the PWHT are not homogeneous can include:
Potentially leave residual welding stresses and generate new stresses due to the different temperatures in various parts of the reactor
Reduce toughness (and risk H2 attack) in the zones under temperature
Increase hardness in the zones under temperature
PWHT at over temperature with over soak (e.g., 720°C/12 h) along with higher X factor and J factor, can compromise step-cooling test results (an accelerated thermal aging test)
PWHT and other cumulative heat treatments influence the properties of materials (base metal and weld metal).
It is clear that the target must be to create a homogeneous temperature profile over the whole reactor, where the temperature gradient must be steady enough to ensure that temperature differentials do not occur through the thickness of the metal. Also, it can be demonstrated that the PWHT during construction plays a vital role in determining the service life of the reactor, and that, critically, any one activity can jeopardize the success of a project, but none more so than PWHT. A well-executed PWHT can be proven to extend the service life of the reactor.
Another very important aspect in the construction of the reactors is nondestructive testing (NDT). For reactors in 2.25Cr-1Mo-0.25V, the acceptance criteria are necessarily higher and more stringent than conventional steels. Even small indications may give rise to problems later in fabrication, where they can be a trigger for defects with greater importance, such as cracks. Table 1 lists examples of typical examination procedures used on certain weld types in the manufacturing cycle.
Fitness for service.
A final consideration should be made to the minimum pressurization temperature (MPT). Process equipment fitness-for-service assessments using API RP 579 is a sophisticated prediction tool to assess the metallurgical condition of a section of process equipment. The analyses of stresses and strains of pressure equipment can assist in predicting whether operating equipment is fit for its intended service. The studies predict how the material will behave according to certain operating conditions and is used to establish an MPT curve. This curve provides an accurate limit for operating characteristics. In this manner, startup and shutdown procedures can be set closer to these limits, making the plant more flexible. If the MPT is under the curve, then we are in optimal conditions. Other critical information necessary to calculate the MPT include actual data from the material used. There is a direct correlation between the X and J factors and MPT. The lower the X and J factors, the lower the MPT. And to achieve a low X and J factor, then cleaner materials with fewer impurities are necessary. Fig. 2 shows a typical MPT curve.
| Fig. 2. Minimum pressure temperature cure. |
This field of research regarding the use of materials and process standards for fabrication of heavy-wall vessels of 2¼ Cr-1Mo-¼ V alloy for service with hydrogen at high pressures and temperatures is under continuous review. American Petroleum Institutes publication of API 934-F is under development exclusively for this topic.
In summary, there are only a small number of key factors that greatly influence safer reactor fabrication. Intensive training of all personnel involved in the fabrication and inspection of the reactor is paramount. It is important that each individual takes responsibility and care for himself, his (her) fellow workers, as well as the entire team. The importance of special care required in the management of welding consumables is also illustrated here. We have highlighted the importance of good reliable automated process control during each of the welding phasesfrom pre-heating, to welding, to post-weld heat treating. Control of the complete manufacturing process should be guaranteed by developing and following specific welding procedures, and by fixing welding parameters in production with automatic continuous recording and control methods. To minimize the risk of premature brittle fracture, it is advisable to have an ISR furnace in the shop. It is indispensable to be critical in NDT, as small indications can propagate into larger failures. Consequently, specific training and qualification are required for all technicians and operators. Finally, and arguably most importantly, it is important to have reliable execution of PWHT procedures, with well controlled furnaces and skilled personnel to guarantee precise temperature curves with a temperature profile no greater than +/- 5°C. HP
| Fig. 3. Circumferential welding of 2 x 2¼|
Cro-1Mo-¼V shell cans with preheat burners.
| Fig. 4. Submerged arc welding (SAW) of |
the 2¼Cro-1Mo-¼V shell cans.
API 934-A & -B
|The authors |
||Davide Quintiliani is an international welding technologist and international welding inspector, and II Level of several NDE techniques. He joined Walter Tosto in 1996 as a quality control Inspector; in 2004, he became head of the quality control department with roles of NDE and welding coordinator. From 2008 to present, he is the head of the welding department, chief welding coordinator and material selection specialist. Mr. Davide has a degree from the University of Chieti G. DAnnunzio, in health and safety at work and a second degree in techniques of loss prevention at work and the environment. He has authored 24 technical articles regarding PED, quality, NDE and welding. |
||Giacomo Fossataro is the technical and operation manager at Walter Tosto with global responsibility for design, manufacturing and quality control activities. He started his professional career in Walter Tostos technical department and has held many positions within Walter Tosto including head of technical department and manager of site activities. Mr. Fossataro holds a degree in engineering (industrial technologies) from the Politecnico di Milano. |
Michael De Colellis is a project manager at Walter Tosto SpA in Chieti, Italy. He has a BE degree in manufacturing systems engineering from the University of Hertfordshire, UK, and an MSc degree in advanced manufacturing systems and technology, from the University of Liverpool. Mr. De Colellis began in the automotive and earthmoving equipment industry, working from quality engineer to quality manager, at General Motors and Case New Holand, before transferring into the oil and gas pressure equipment construction business.