Inlet assembly weld failures
The standard adopted for the examination of Reformer Tubes in Steam Methane Reformers (SMR) has been to inspect the heated length of the tube within the furnace. The portion of the tube outside of the heated length is normally not inspected after being in service. However, recent trends have changed that perspective due to internal defects encountered in these portions of the top ends. These defects have been found in two completely different design arrangements which will be described separately as Case I and Case II.
Case I - Top Fired / Down Flow:
In these reformer designs, the burners are top fired with a down flow process. The inlet gas is injected through a pigtail into the tube above the furnace radiant box in the penthouse. The unheated tube top section is made of wrought material with metallurgies ranging from low-chrome alloy steel (1.25 Cr - 0.5 Mo) to stainless steel 304H, 347H, or similar materials while the heated length within the furnace is a centrifugally cast material mostly 25Cr/35Ni/Nb or with Micro-alloy additions. For further details see Figure 1.
During transient and cold weather conditions, the temperature of the tube top flange can drop below the process gas dew point resulting in condensation in the area shown in “black”. The process gas dew point is typically in a range of around 300-400 °F.
Case II - Bottom Fired / Up Flow:
In these reformer designs, the reformer is bottom fired with an up-flow process. The inlet gas is injected from the bottom of the tube through an inlet pigtail and the reformed gas comes out from the top of the tube through an outlet pigtail. The outlet pigtail is located on the portion of the tube outside of the heated length on an uncovered roof open to the atmosphere. The tube material is made of the same material as the heated length which is centrifugally cast length which is a centrifugally cast material mostly 25Cr/35Ni/Nb or with Micro-alloy additions. In this scenario, the process gas can also reach its dew point around 300-400 °F and cause “condensation”. See Figure 2.
The cyclic stresses brought on by these variations in temperature result in the damage mechanism as “thermal fatigue”. In both Case I and Case II, the defects manifest themselves in the form of circumferential cracks or pinhole cracks that initiate from the ID and propagate towards the OD resulting in a leak.
Top End Ultrasonic Inspection
Inlet Assembly Weld Failures
Several furnace designs utilize a dissimilar weld to connect the inlet assembly to the reformer tube. In some cases, this has led to cracking in the heat affected zone of the weld on the lower alloy side. MP has developed a customized testing device and procedure for testing these welds in some furnaces.
Procedures can be adapted for many furnace designs. This is an ultrasonic test incorporating special devices to assure complete coupling and accurate probe manipulation from the ID. Cracks are detected and sized allowing the continued safe operation of the furnace.