Metallurgical evaluations were performed on alloy 800H/HT pigtails removed from three different hydrogen reformers due to failure and/or excessive diametral growth. The pigtails were exposed to syn gas at a minimum temperature of 1565oF (852oC) on the ID and air on the OD. Pigtails from two reformers failed at the bend areas. The pigtails from the third reformer tolerated up to 14% diametral growth without rupture. Microstructures of the pigtails were identified via optical and scanning electron microscopy with energy dispersive x-ray spectroscopy. Important factors that might be responsible for causing pigtail rupture are discussed.
BACKGROUND
Hydrogen is routinely produced via the steam methane reforming process by reacting natural gas with steam in catalyst filled reformer tubes at 1500-1700oF (815-870oC). The hydrogen-rich gas (or syn gas) exits the reformer tube and flows to a common collection header via a pigtail. Pigtails are generally fabricated from alloy 800H or 800HT pipe. The pigtails contain numerous bends to accommodate thermal growth of the reformer tubes and collection header. The bends are typically formed by cold forming followed generally by solution annealing. The syn gas flowing through the pigtails has a typical composition of 44% H2, 7.4% CO, 5.7% CO2, 3.7% CH4, 0.01% N2, and 39% H2O. The pigtail is located outside of the furnace and its external surfaces are exposed to the ambient atmosphere.
Longitudinal, through-wall cracking developed in the side-wall of the bend section of pigtails at two different plants. At reformer plant A, through-wall cracks developed in the bend area of a nominal 1.5 (38 mm) diameter x 0.33 (8.4 mm) wall pigtail after 10 years of service. At reformer plant B, the through-wall cracks developed in the bend area of a nominal 1.25 (32 mm) diameter x 0.250 (6.4 mm) wall pigtail after 6 years of service. The typical through-wall crack is shown in Fig. 1. The pigtails from both plants were cold bent and annealed after bending. Lastly, large diametral growths were experienced in numerous pigtails at reformer plant C. The large diametral growth occurred in the straight pigtail sections welded to the reformer tubes. The straight pigtail sections were installed in the as-received condition from the mill and did not experience cold bending and subsequent annealing. The reformer C pigtails had been in service for 7.5 years and operated above the design temperature during the last 6 months of operation. The pigtails were removed from service due to excessive diametral growth. The operating conditions and design details for pigtails at reformers A, B, and C are summarized in Table 1. Metallurgical evaluation was performed on the samples removed from pigtails at three plants, and the results of the evaluation are presented herein.
