1.
Introduction:
The steam reformer is one of the main critical equipment in Methanol and Ammonia production plants. The cost of the reformer is a substantial part of the investment of the complete plant. This paper covers in brief the service, design, and construction of the steam reformer with concentration on the construction materials, damage mechanisms and their mitigation for the radiant section (catalyst tube – inlet and outlet pigtail – inlet and outlet manifold). This is mainly to help in maintaining high levels of safety, reliability, and structure integrity of the reformer
2.
Service
The steam reformer is used in the production of synthesis gases from the natural gas. In the steam reformer steam is mixed with the natural gas and the combined stream is further heated and routed through tubes in a reforming furnace containing nickel oxide catalyst. Here a reforming reaction occurs in which methane in the natural gas is partially converted into hydrogen, carbon dioxide and carbon monoxide.
CH4 + H2O CO + 3H2 (Steam Methane) CO + H2O CO2 + H2 (Water Gas Shift)
3.
Description
The steam reformer is a rectangular insulated structure containing vertically supported tubes filled with nickel oxide catalyst in which the steam reforming takes place at elevated temperatures. The endothermic heat of reaction is supplied from downward firing burners situated in the roof of the Reformer. These are fired on a mixture of combined process waste gas streams supplemented with natural gas. The burners are arranged in rows between the reforming tubes and are positioned such that no
flame impingement occurs on the tubes or on the furnace walls.
The natural gas and steam reactants are evenly distributed by a system of headers on the top of the reforming furnaces and the connections to each tube are made by solid drawn alloy flexible connectors.
4.
Main components
4.1. Burners :Main and auxiliary burners
4.2. Radiant coils:
Catalyst tubes
Inlet and outlet manifolds Inlet and outlet pigtails
4.3. Convection section coil 4.4. Ducting
Air ducting with supports and guides Turning Flue gas ducting with supports Main burners duct
Transition duct Air intake
Flue gas duct to stack
4.5. Refractory radiant and auxiliary firing lining 4.6. Steel works
4.7. Piping included in reformer package
Interconnection piping (convection to radiant) Steam and mixed feed gas
Inlet manifolds
Inlet and outlet pigtails
Main and auxiliary burners piping complete Instrument, drain and vent connection
4.8. Reformer gas main 4.9. Flue gas stack 4.10. Equipment:
Steam superheater no. 1 Mixed feed heater Steam superheater no. 2 Mixed feed heater no.2 Combustion air preheater Flue gas stack
Flue gas fan with steam turbine Combustion air fan with steam turbine
5.
Mechanical design and material selection:
5.1. Reformer tube assembly
Because of the severity of the operating conditions, reformer tubes assembly are fabricated from centrifugal cast materials. This material provides high potential to withstand the operating conditions that it has superior stress-to-rupture strength at high temperature. The latest trend is to utilize micro-alloys, which have a higher creep resistance. The most successful alloy is the micro-alloys, which is obviously the stronger alloy. The reformer tubes were fabricated from CA 4852-Micro centrifugal cast austenitic stainless steel tubes. This tube material is based on the standard heat resistant HP-type casting alloy containing 0.35-0.75 weight percent (wt%) C, 2 % (max) Si, 25% Cr, 35% Ni, and 2% (max) Mn. These materials have high stability of carbide, increased creep strength, higher durability and oxidation resistance compared to the conventional materials.
Fig. 1: Key principle conducted in reforming Fig.2: Tube assembly
The advantages of using these micro-alloys are:
Possibility of operation of the reformer at higher temperature & pressure Reduced reformer wall thickness
Increased quantity of catalyst packing in the same space – this aspect has been utilized advantageously, for increasing the capacity and reducing the energy consumption of existing Reformers.
These materials provide high resistant to metal dusting that corrosion mechanisms like oxidation and carburization govern metal dusting. Uniform oxidation of the alloys definitely inhibits carburization and metal dusting temporarily through formation of protective scales. Scale formation is supported by high chromium contents, medium silicon contents, and small grain sizes and induced surface deformation. All these factors make chromium and other stable oxide formers (Mn, Si, AI) diffuse faster to the surface and therefore increase the re-healing capacity of the dense
protective chromia layer. Controlling these most favored oxidation mechanisms therefore bears the potential to control metal dusting.
The HP-40 modified Nb tube showed a severely aged structure within its short service period. Chromium carbides (M23C6) and niobium carbides,
precipitated at elevated temperatures, did not adversely affect the mechanical strength of the tube. Creep voids observed between the cracks did not contribute the tube failure. The tube failed as a result of thermal shock caused by the temperature gradient difference during the furnace operation.
5.2. Outlet manifold assembly
Outlet components such as manifold tube, Cone sections and T-pieces are made from centrifugal casting. Fig.3 shows the assembly of the outlet manifold. The outlet assemblies were fabricated from G4859 (20%Cr–32%Ni-Nb). These components need to cope with the expansion stresses; therefore, ductility is of primary importance. Creep strength is of secondary importance.
Fig 3: outlet manifold assembly
5.3. Outlet Pigtail (Hairpin)
The outlet pigtail is fabricated from pipe 1-1/2 in under the rules of ASME B31.3. Fig. 4 shows the arrangement of the outlet pigtails. The Outlet pigtails are made from ASTM B 407 alloy 800H (30%Ni, 20%Cr). Alloys 800H and 800HT are the standard materials for intermediate temperatures (620 to 925 °C) 1. These alloys combine corrosion resistance with strength and metallurgical stability (Fig. 5).
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data for vari was up to 1 rom pipe 1 ASTM A3 at relatively is 440o C a ngement o let pigtail a ious nickel-b 10,000 hr. 1 1-1/4 in und 12 Grade y lower te at the inle of the inlet rrangemen base and st
der the rul TP304H a emperature et pigtail a pigtails. nt tainless alloy es of ASM austenitic s e than the and 770oC ys. Test E B31.3. stainless e outlet at the
6.
Damage mechanisms:
6.1. Reformer tubes
6.1.1 Description of damage
The main damage mechanism for reformer tubes is the combination of thermal stresses across the tube wall and internal pressure stresses. This combination causes that creep damage typically develops at the inner diameter or just below the ID surface. Also, the creep damage occurs over the complete circumference (or at least a large part of the circumference) and over a longer (axial) part of the tube. The damage process results in diameter increase and creep damage (cavitation) at the inner diameter. Final rupture occurs in a longitudinal direction. The tube materials HP40 micro shows relatively high creep resistance. Fig.7 compares the average100,000 hours creep resistance of several reformer alloys.
Fig.7: Comparison of creep resistance of several reformer alloys3
Another main damage mechanism can be overheating by catalyst degeneration or by operating upsets. Typically, catalyst degeneration results in creep damage over a small part of the circumference and over a short (axial) part of the tube. This means bulging and the final rupture occur also in axial direction.
Fig.7: Creep growth in reformer tube
6.1.2 Prevention /Mitigation
There is a little to be done to prevent this damage, the best way to avoid creep and stress rupture is the selection of the proper material, which is applied in our reformer as described above.
The catalyst tubes were inspected and base line readings of the diameter and thickness were recorded using the advanced tool of Quest Reliability at shop. This will help in the future monitoring and inspection of any minor changes takes place in the tubes and detection of deformation before damage.
6.2. Outlet manifold assemblies
6.2.1 Description of damage
The damage mechanisms of outlet components (manifolds, cones, T-pieces) are generally much simpler than that of the reformer tubes, because the outlet components are not subject to firing conditions. Thermal gradients across the tube wall are not significant and do not cause thermal stresses.
The main damage mechanism for outlet component is hindered thermal expansion. The outlet system cannot expand or shrink freely during shutdown and start up. Very often, there is an interaction with creep, because of the long hold-times involved. The damage starts at the outer diameter and concentrates near the welds. The final rupture occurs in circumferential direction.
Another damage mechanism is creep under internal pressure resulting is creep under internal pressure resulting in diameter increase and creep damage at the outer diameter. Final rupture occurs in longitudinal direction.
6.2.2 Prevention and mitigation
One of the important parameter to prevent/mitigate the damage –in addition to the proper material selection which is applied- is to control the heating and cooling rates during startup and shutdown.
6.3. Outlet pigtail (hairpin)
6.3.1 Description of damage
The commonly reported failure mechanism in the outlet pigtails is intergranular oxidation cracking, due to the combination of creep, oxidation. This mechanism involves interaction of mechanical fatigue, high temperature creep, diffusion of carbon and chromium, and high temperature oxidation. Figure 8 shows a recorded failure location and the orientation of the crack in a outlet pigtail pipe.
Mechanical fatigue is due to the repeated expansion and contraction of both tubes and headers that occur during both on-stream and transient periods. During operation these pigtails see process temperatures at about (750°C) where creep can occur and can accelerate formation of creep fissures.
Fig.8: crack on pigtail in a longitudinal direction
6.3.2 Prevention and mitigation
Since fatigue is one of the major factors causing damage, so damage can be efficiently mitigated/prevented by controlling the rates of heating and cooling during startup and shutdown.
7.
References:
1. ASM handbook Volume 13 "Corrosion".
2. API 571 “ Damage mechanisms affecting fixed equipment in the refinery industry
3. Kirchieiner. R. and Woelopert, P. Niobium in certified cast tubes for petrochemical applications.
4. NACE paper no. 01374; correlation of oxidation, carburization and metal dusting; "controlling corrosion by corrosion".
5. NACE paper no. 03657; Failure mechanism of alloy 800H in steam reformer furnace pigtails.
6. Paper of the 6th Schmidt + Clemens Group Symposium; "Life assessment and
inspection techniques in reformer furnace".
7. Paper of Johnson Matthey Catalysts about Re-tubing your primary reformer The Katalcojm performance concept.
8. Three-dimensional analysis of creep voids formation in steam-methane reformer tubes; Azmi Abdul Wahab; university of Canterbury.
