Transmission pipeline welding often requires preheating of the weld zone to ensure that the joint achieves its required strength and hardness. Proper preheating also helps minimize the risk of delayed hydrogen-induced cracking, which is a significant concern that impacts weld quality and integrity in transmission pipelines.
There are several heating methods available when preheating and stress relieving of parts is necessary in pipeline welding. Open flame has been one commonly used method. However, it presents some challenges that can negatively impact weld quality and integrity.
Another option to consider is induction heating, a method that offers numerous benefits for weld quality, efficiency and safety not found with other heating methods.
Induction provides greater consistency in heating and eliminates a potential hydrogen source that is a byproduct of open-flame heating. These advantages make induction a good solution to help transmission pipeline contractors meet code and quality requirements — both for new pipeline construction and the repair and maintenance of existing in-service transmission lines.
Induction on Transmission Pipelines
Induction heating has been successfully used in transmission pipeline applications with high-strength steels for many years.
Induction heating systems quickly heat conductive metals by inducing current into the part. Induction does not rely on a heating element or flame to transfer heat. Instead, an alternating current passes through the heating device, creating a magnetic field around it. As the magnetic field passes through the conductive workpiece, it creates localized eddy currents within the part. The resistance of the metal fights against the flow of the eddy currents, generating heat in the part. The part becomes its own heating element, heating from within, which makes induction very efficient since little heat is lost in the process.
Applications that typically require hours to heat can be completed in minutes by using induction heating and the different liquid-cooled and air-cooled options. Induction heating systems can be paired with various coil configurations to induce heat, depending on the part size and geometry.
When using open-flame heating, temperatures are typically monitored manually using temperature crayons, which do not provide the accuracy of induction. In comparison, induction systems use feedback from thermocouple probes for automatic and uniform temperature control.
Eliminating a Hydrogen Risk
One of the main quality challenges when using open flame for preheating pipe in the field is that a byproduct of the process introduces a hydrogen risk.
A byproduct of burning any fuel in flame heating is water vapor. The moisture in water vapor can be a source of hydrogen in the weld that could result in
hydrogen-induced cracking. Reducing the risk of trapping hydrogen in the weld is critical for achieving high-quality welds in pipeline applications.
Using induction heating instead of open-flame heating eliminates that hydrogen risk by keeping moisture out of the process — and therefore helps improve
weld quality and integrity to meet necessary code
Quick and Consistent Heating
Transmission pipeline applications typically have minimum and maximum temperature requirements for preheating that are determined by the welding procedures for that specific alloy of pipe. Staying within the temperature window is important for weld quality and to achieve the desired properties in the finished weld.
A typical minimum preheat temperature in pipeline welding is 250 degrees Fahrenheit. Maintaining a minimum temperature helps to eliminate any moisture that can develop, since pipes are generally stored outside on pipeline jobsites, where it can be cold and damp.
Open flame often results in inconsistent heating throughout the part, and it’s also harder for the operator to hold a specific temperature or ensure the temperature remains within the specified window. Falling below or going over the required temperature window can adversely impact weld quality.
By contrast, induction heating provides consistent and uniform heating throughout the part. Induction systems also make it easier to hold the temperature at a specific level and to constantly monitor heat — to ensure the pipe remains within the proper temperature window throughout the weld.
Induction heating also provides a much faster time to temperature, which is important when constructing new transmission pipelines. Speed is critical in these applications because operators may be trying to weld as many joints as possible per day. These jobs typically involve numerous weld stations along the right of way. The aim is to heat pipe along the right of way, then move on quickly to heat the next weld joint ahead of the weld station.
Preheating for In-Service Pipelines
While speed is not as critical in repair or maintenance projects on in-service transmission pipelines, induction heating systems offer numerous benefits for these applications as well.
In these applications, it’s difficult to properly heat the steel with an open flame because anything flowing through the pipe has a heat sink effect, which pulls the heat from the steel. Once the operator heats the area and pulls the torch away, the steel can cool down within seconds.
In these applications, it is often not practical or feasible for the pipeline to be shut down while the repair or maintenance work is completed.
Induction heating allows the operator to maintain the necessary level of preheat in order to slow down the cooling of the weld puddle and minimize the risk of cold cracking in the weld. This can be done without stopping the flow of oil or natural gas through the pipe.
Liquid-cooled induction coils work well for jobs on in-service transmission pipelines because coils can be arranged to accommodate geometry transitions common with the split tees used for hot taps and pipe-to-valve welds. Results from testing funded by the Pipeline Research Council International (PRCI) supports the use of induction heating for welding projects on in-service pipelines to reduce risks of hydrogen cracking.
Additional Benefits for Weld Quality
Induction heating also offers benefits when considering several other factors involved in transmission pipeline welding.
• Coated pipes: When completing transmission pipeline repair projects that require cutouts and tie-ins, there is often a need to reapply coating to protect the pipe from corrosion. Many materials used in the industry will not cure properly if the steel is not warmed up to a certain temperature level in these cases. Air-cooled induction blankets can be used to heat up zones of the pipe, to create a wide area of heated steel so the epoxy coating can be applied with the pipe at the temperature necessary for proper curing.
• Pipe segments: Some applications require welding pipe segments to valves. In these situations, induction heating can minimize the risk of damaging seals and valves, which is a common challenge that can occur with flame heating. Valve ends are typically much thicker steel compared to the pipe, which increases the risk of causing damage to the valve seals when using flame. Induction provides a much more localized heat that can be directed to a specific area of the part, which helps prevent damage. Because of these benefits, several major pipeline companies specify the use of induction heating for preheating in applications that involve a pipe segment or valve.
When Quality Is Critical
More transmission pipeline companies are transitioning to induction, successfully using the process for preheating in new construction projects and repair and maintenance work.
Among its numerous benefits, induction provides great consistency in heating and faster time to temperature of the part — while also eliminating a potential hydrogen source that is a byproduct of heating with open flame. In addition, induction offers benefits for safety on the jobsite.
The result is a preheating process that contributes to improved weld quality and integrity — a critical factor in helping pipeline contractors achieve success.
Steve Latvis is an induction heating specialist for Miller Electric Mfg. Co., a welding equipment manufacturer based in Appleton, Wisconsin.