Relaxation of In-Service Welding Procedure Flow Restrictions

Abstract

Edison Welding Institute (EWI) and Enterprise Products Operating LP (Enterprise) worked together to develop an in-service welding program. The objective of this project was to relax flow restrictions on current in-service welding procedures to allow for welding onto liquid pipelines with flow rates outside of current flow limits. Enterprise’s current products include liquid propane, liquid ethane, and propane and ethane mixes in addition to other refined products. The current Enterprise in-service welding procedures restrict welding onto liquid pipelines with a flow rate between 1.3 and 4.0 ft/s (0.4 and 1.2 m/s). The minimum flow rate of 1.3 ft/s (0.4 m/s) was used because it was Enterprise’s minimal operating flow rate. The maximum flow rate of 4 ft/s (1.2 m/s) was grandfathered into the procedures. When welding onto an in-service pipeline to repair a damaged section of pipe or to install a branch connection (i.e., hot-tapping) there are two main concerns (burnthrough and hydrogen cracking) and both concerns needed to be evaluated for both flow conditions. The results from the project allow welding onto no-flow liquid pipelines with wall thicknesses between 0.25 and 0.5 in. (6.4 to 12.7 mm). Even though welding onto a no-flow thin-walled liquid pipeline [i.e., less than 0.25 in. (6.4 mm)] would not increase cracking susceptibility, the risk of burnthrough and eutectic iron formation would make the procedure unacceptable. The results of this project also indicated that acceptable welds can be made onto a high flow liquid pipeline [up to 12 ft/s (3.7 m/s)]. It was recommended, however, that Enterprise only use the temper bead welding procedures for such applications. Proper use of the temper bead welding procedures (i.e., proper heat input, weld toe spacing and stringent low hydrogen welding practice) has been shown to produce acceptable, crack-free welds. It is important to note that none of the welds showed signs of cracking, but the hardness levels of the heat input control procedures all exceeded the critical hardness level for their intended carbon equivalent materials. Increasing the flow rate from 4 to 12 ft/s (1.2 to 3.7 m/s) does appear to increase the cooling effect but it is not possible to determine the magnitude of the effect from the results of this work.

Study on the Dynamic Evolution Behavior and Failure Mechanism of Burn-Through Instability during In-Service Welding by Combining In-Situ Observation and Failure Analysis

“…Boring et al [ 26 ] evaluated the burn-through risk based on their test device. They concluded that excessive inner diameter (ID) surface bulging, the carburized and eutectic iron layer on the ID surface, were the precursor of burn-throug instability.…”

In situ observation of deformation and failure behavior in the burn-through instability zone of in-service welding

“…Based on the consideration of engineering value, previous studies mainly focus on the evaluation criteria of in-service welding. The previous criteria mainly include the minimum wall thickness criterion [3], inner wall maximum temperature criterion [4,5], residual strength criterion and plastic failure criterion [6]. It can be found that all the failure models are based on the phenomenon and mechanism of in-service welding failure instability, but there is still a lack of research on the plastic deformation and failure mechanism of burn-through instability.…”