Welding onto in-service pipelines and piping systems allows cost-effective installation of branchconnections (hot taps) and repair sleeves. Beyond the normal in-service welding concerns ofburnthrough and hydrogen cracking, there are often additional concerns when welding ontopipelines and piping systems in chemical service. For example, some products can undergochemical changes when exposed to the heat from an in-service weld. In spite of these concerns,some companies routinely weld on lines in chemical and other potentially hazardous service,although limitations and special precautions are often...
Welding onto in-service pipelines and piping systems allows cost-effective installation of branchconnections (hot taps) and repair sleeves. Beyond the normal in-service welding concerns ofburnthrough and hydrogen cracking, there are often additional concerns when welding ontopipelines and piping systems in chemical service. For example, some products can undergochemical changes when exposed to the heat from an in-service weld. In spite of these concerns,some companies routinely weld on lines in chemical and other potentially hazardous service,although limitations and special precautions are often specified. Examples include wallthickness limitations below which in-service welding is not permitted and the use of weldingprocedures that limit heat input. The objective of this project was to review the concerns forwelding on in-service pipelines in chemical service, the techniques that have been used in thepast to address these concerns, and to develop criteria that will allow this type of welding to becarried out safely. Criteria for in-service welding were developed for a wide range of chemicals that were identified as being of interest. The format for these criteria is similar to material safety data sheets (MSDSs). Many of the potential concerns that were identified can be controlled by limiting the inside surface temperature. Factors that affect inside surface temperature include: (1) pipe wall thickness, (2) the introduction of heat into the pipe wall by the welding process, and (3) the removal of heat by the contents. For many of the concerns that were identified, if protection from burnthrough using the inside surface temperature approach is practiced, protection from these other concerns is also accomplished. The ability to accurately control heat input levels is an important aspect of being able to weld safely onto in-service pipelines, particularly when inside surface temperature needs to be limited. Methods for controlling heat input levels in the field were reviewed and detailed guidance was developed. The benefits of using small-diameter electrodes (lower current levels), which allows the attachment of a hot tap fittings at relatively low heat input levels, were identified. Techniques to address other concerns that are not inside surface temperature related were also identified.