Overview of Outlet Manifolds Integrity in Ammonia Plant Primary Reformers

Introduction

The primary reformer in an ammonia plant is vital for producing synthesis gas, which is essential for ammonia production using the Haber-Bosch process. In this reformer, hydrocarbons react with steam at high temperatures of 1292°F (700°C) to 1832°F (1000°C) within catalyst-filled tubes to yield hydrogen and carbon monoxide. The resulting gas mixture, known as process gas, then travels through the outlet manifolds and transfer line to enter the secondary reformer for additional reforming processes.

Maintaining the reformer's efficiency and safety is paramount, particularly regarding the integrity of outlet manifolds due to extreme temperatures and stresses. This involves understanding materials and design factors to mitigate stress concentrations. Materials must withstand high temperatures and resist creep and thermal fatigue, with high alloy steels like 20Cr-32Ni-Nb often chosen for their oxidation and creep resistance. Precise mechanical design is crucial, considering the material’s thermal expansion properties and ensuring manifold adaptability to expansions without inducing excessive stress.

Cracking in outlet manifolds may result from thermal fatigue, creep, stress corrosion cracking (SCC), or stress relaxation cracking (SRC). Thermal fatigue occurs due to cyclic heating and cooling, leading to crack initiation and propagation. Creep happens with prolonged exposure to high temperatures, causing permanent material deformation and creep cracking. SCC develops from exposure to corrosive environments along with tensile stress. Nickel-based alloys such as Alloy 800H, 800HT, and 22Cr-35Ni-Nb, commonly used in the construction of outlet manifold assemblies, are also vulnerable to SRC, also known as reheat cracking. This type of cracking occurs in metals due to stress relaxation through grain boundary deformation within the creep temperature range, either during post-weld heat treatment (PWHT) or while in service at elevated temperatures. The onset temperature of SRC varies depending on the specific alloy used and its grain size, and it is particularly prevalent in thick-walled components. Careful control of heat treatment parameters and selection of alloys with appropriate grain sizes can mitigate the risk of SRC, particularly in thick-walled components.