In-Service Welding on Ethylene Piping: Defining and Demonstrating Safe Conditions

In-service welding is a common practice used to perform repairs and install hot taps to both piping and pipelines. It involves arc welding on a systems containing fluids that are under pressure in either flowing or static conditions. Safety considerations for in-service welding are well defined for many hydrocarbon fluid streams, but there have been reservations to perform in-service welding on high purity ethylene systems due to concerns of exothermic decomposition reactions leading to failure of the pipe. This work examined existing literature, including past industry ethylene incidents, and experimental data to develop an analytical model that may be used to define safe in-service welding conditions on high purity ethylene systems.

The test vessel consisted of commercially pure ethylene (<5ppm O₂) contained in a thin walled pipe section (<0.203”) at high pressure (1,200 and 1,500 psi) under static conditions. Nine in-service welds were completed using various welding heat inputs with the intent of producing the highest possible pipe wall temperatures to facilitate conditions that may lead to failure due to self-propagating ethylene decomposition. Piping failure was not observed; however, all welds did result in localized ethylene decomposition that produced a carbon deposit on the inner diameter pipe surface, local diffusion of carbon into the pipe wall resulting in the development of carburized layer on the inner diameter pipe surface of all welds, and eutectic iron forming adjacent to the carburized layers on the ID surface on several welds. The experimental results were used to validate the models. The experiments also provided evidence that limiting inner diameter surface temperature to ≤1,800°F, an already common practice for mitigating the risk of burning through the pipe wall during in-service welding, is also suitable when performing in-service welding on ethylene piping and pipelines to prevent ethylene decomposition related failures and eutectic iron formation.

This paper provides an overview of ethylene decomposition, a summary of the work and an approach that maybe used to define the safe in-service welding conditions on high purity ethylene systems.