By Georgios Vassiliadis, Sales Engineer EMEA, and Henrik Häll, Product Specialist, EMEA, Sandvik
Steam methane reforming (SMR) is a critical reaction in several refining processes, including the production of oils, ammonia, methanol, and syngas. At the heart of the SMR process lies the reformer, which is a complex, energy intensive and expensive piece of equipment. However, within the reformer unit, one of the most frequently replaced components are the pigtails.
As the name suggests, pigtails are curved pieces of pipe that carry reformed gas from catalyst tubes to the collection manifold. Their function is to discharge the mechanical stress and displacement experienced by the reformer unit system because of the thermal expansion resulting from the reforming process. Located in the inlet and the outlet of the reformer, pigtails are susceptible to a range of mechanical and thermal stresses and potential failures.
Inlet pigtails are less critical than outlet pigtails as they are required to perform at relatively lower temperatures; but it is still important to take great care in their material selection. Outlet pigtails, in contrast, must be able to withstand temperatures up to 900°C, which creates harsher hot corrosion challenges. This means the pigtails must be made from high alloy steel grades that are able to withstand such high temperatures and corrosion. Steels working at temperatures above 500°C experience high temperature corrosion and diffusion of gases into the protective oxide layer, which can affect the material’s properties.
The failure of outlet pigtails is a common cause of plant downtime and creates a potential risk to personnel. That increases the total cost of ownership of the asset. Choosing the right material can avoid unplanned shutdowns and lost production while extending the lifecycle of the pigtails and improving the efficiency of the steam reformer.
In the high temperature corrosive environment of steam methane reforming, there are several factors that influence the lifespan of pigtails, including their design, the material used for their manufacture, metallurgy and methods deployed to inspect their integrity. Using heat-resistant materials is obviously essential, and in this respect the two most important aspects to consider are:
- Creep strength/resistance
- Grain size
Creep strength is defined as the maximum stress in a material that will result in a specified amount of creep in a given time at a constant temperature. It is essentially used to measure a material’s ability to withstand sustained loading without significant continuous deformation. In steels, creep strength is only a significant factor at significantly elevated temperatures. Creep strength is also known as creep limit or creep resistance.
Mechanical fatigue and the bent shape of pigtails make creep resistance critical. To prevent cracking and potentially catastrophic ruptures it is essential to use creep-resistant materials.
Grain size has a huge influence on the mechanical behavior of the materials used for pigtails. It is important to understand the precise character of the grains as well as grain size control. A coarser grain size improves the creep strength (assuming all other conditions remain the same).
The industry requirement for outlet pigtails can be found in standard specification ASTM B407, which calls for an average grain size of ASTM No. 5 or coarser. Measurement is done in compliance with the standard ASTM E112 and set one average grain size value to the entire batch with no information around distribution.
However, the upper and lower values of the statistical grain size distribution have an influence on the performance of the outlet pigtails.
Manufacturing pigtails with a specific grain size interval and a uniform grain size structure throughout both thickness and length of the tube will naturally improve mechanical and creep resistance performance to optimal levels so you’re never faced with the unexpected.
Extend your possibilities
Pigtails are one of the most important components in a steam methane reformer (SMR). To extend service life, raise personnel safety and naturally avoid high-cost unplanned shutdowns, it is important to make sure each component is made of materials that can withstand high temperatures while also having good creep properties. Additionally, by increasing the material requirement with a uniform grain structure across both the wall thickness and the full length with a narrow grain size interval, can raise the security and stability in your process to the next level. The benefits of specifying high quality pigtails ensure an increased trust in your equipment, extends the service time, and results in less unplanned failures of your SMR.