Induction Heating for Cold Weather Preheating and Post-Curing Of Liquid Epoxy Coatings On Gas Pipeline Girth Welds

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In 2013, Pacific Gas & Electric Company (PG&E) did not have a specific standard or guidance for selection, application and inspection of cold weather, field-applied coatings on transmission piping when the pipe surface temperatures went below 50 F (10 C). In November of that year, unusually cold weather hit northern California. Night temperatures were below freezing, which caused the pipe surface to be below the 50 F minimum necessary to cure the approved epoxy coatings.  

As seen in the January 2016 Edition of the Journal of Protective Coatings and Linings (JPCL) and on Paintsquare.com

At this time, the application procedure consisted of enclosing the work area and using space heaters to raise the ambient temperature sufficient to increase the pipe temperature. This approach was costly and cumbersome, and resulted in cure times of 6-to-10 hours or more. With these long cure times, construction crews could not quickly backfill the pipelines, and corresponding construction production rates were slow and costs high.

COLD WEATHER COATING CHALLENGES

A fundamental challenge of applying traditional two-component, epoxy amine pipeline coatings at low temperatures is the slowing of curing reaction as the pipe temperature decreases. If the temperature drops below a minimum threshold, the reaction will cease completely. The curing process may restart when the temperature increases, but other issues associated with the interrupted cure may prevent the coating from achieving the optimum coating properties. Epoxies can be effectively applied in cold weather by preheating the pipe, and force-curing the applied epoxy.

At low temperatures, the applicators will need to deal with changes in coating characteristics. As the temperature decreases, the coating will become more viscous (thicken). The chemical composition of a coating will determine its freezing point, but typically a coating containing solvent will freeze below the freezing point of water.  

Two-component materials will be harder to mix when they have a higher viscosity. Some coating manufacturers require that the coating be stored and mixed at higher (“normal”) temperatures. A higher-viscosity coating will become inherently harder to apply and application techniques may become limited to trowels. A higher-viscosity coating will not sag as quickly as it might normally; if attention isn’t paid to achieving the proper wet-film thickness, sags may develop after the applicator has completed the work. In extremely cold temperatures, attire will be necessary to reduce risks associated with low-temperature environments such as hypothermia and frostbite, and this can interfere with the applicator’s productivity.

LOW-TEMPERATURE COATING

Aside from enclosing and heating the environment with space heaters, two general solutions are available for coating pipe at low temperatures — heating the pipe and applying a traditional epoxy or applying an alternative, non-epoxy coating material at lower temperatures. Subsequent to extensive research and testing, PG&E concluded that heating the pipe and applying a traditional epoxy amine coating was the best solution in this case. 

INDUCTION HEATING 

Following extensive research and testing, PG&E selected an induction heating system that uses an induction blanket wrapped around the pipeline. The system is an adaptation of an induction heating system that is commonly used in the pipeline industry for welding preheat and/or stress relief. 

Induction heating involves heating a conductive metal using coils of electromagnets and alternating current electricity. The alternating current produces eddy currents within the substrate and the internal resistance produces heat. As electronic power sources using inverter technology have become much smaller and more efficient, induction heating has become better suited for field use. In addition, flexible coils have been developed which make induction heating equipment easily adaptable to a variety of shapes and configurations.

Air-cooled induction heating systems have been specifically designed for preheating applications up to 400 F (204 C). In the case of induction heating blankets for girth weld coating on existing fusion-bonded epoxy (FBE) coated pipe, the maximum temperature is controlled to 180 F (82 C) in order to prevent damage to the existing FBE. These systems can be operated in manual modes where power output is applied to a part for a specified length of time or in the temperature-controlled mode where the power output is automatically adjusted to maintain a specified temperature at a sensor location on the pipe.

Air-cooled blankets are available for pipe diameters from 8-to-60 inches (20-to-152 cm). A disadvantage of these systems is that the blankets may get the surface dirty where they contact the pipe. To avoid contaminating the cleaned surface and curing coating, the operator should develop a heating strategy and exercise care when handling the blankets. Induction heating provides the optimum QA/QC for preheating and coating as the entire pipe wall can be heated and precise temperature of the pipe surface can be controlled throughout the entire process. In addition, the induction heating system also allows the pipe heating to be continued during the post-cure process, permitting the coatings to be cured in approximately one hour. Short cure times are desirable since they decrease the time to backfill. Precise temperature control or post-cure heating cannot be attained using propane preheat systems.  

Laboratory Demonstrations

The system was demonstrated on girth weld coatings at the PG&E Applied Technology Services Laboratory (ATS). Figure 1 shows the equipment in the ATS facility for the demonstration, during which a range of PG&E personnel were brought in to observe the process, learn how to operate the equipment, collect data on heating efficiency, and (because this lab allows for operation of equipment on 20-foot sections of pipe), provide feedback on field use of the system. Several sections of FBE-coated pipe were brought to ATS facilities to allow the induction heating equipment manufacturer to demonstrate the system. Figure 2 shows observers during one of these demonstrations. PG&E engineering and ATS personnel collected data on the heating and cooling rates under different combinations of pipe diameter, blanket spacing and system control settings. Based on the data collected and input from the observers, a draft procedure and plan was prepared for field demonstrations.

Fig. 1: Induction heating equipment set up for demonstration at PG&E Applied Technology 
Services facility.
Fig. 2: PG&E personnel participating in the demonstration at PG&E Applied Technology Services facility.

Field Demonstrations

After the demonstration at ATS, the first of several field pilot projects was performed during a 24-inch line installation near Petaluma, California. The induction heating equipment can be mounted on a vehicle for easy access along the pipeline right-of-way (Figure 3). Before surface preparation, the induction heating blankets were placed directly on, or immediately adjacent to, the girth weld area (Figure 4). Thermocouple (temperature) probes underneath the middle of each blanket were used to ensure the pipe temperature remained below the temperature set point, with the coating applicator able to adjust the blanket spacing and induction heating control temperature (below 180 F) based on site conditions. Ambient temperature, wind, pipeline conditions and other variables will affect heat transfer from the pipe, and thus the optimum adjustment of the induction heating system.

Fig. 3: Induction heating equipment set up for demonstration at PG&E Applied Technology 
Services facility.
Fig. 4: Induction heating blankets installed adjacent to girth weld.

Once the area underneath the blankets has been warmed to the desired temperature and recorded by the temperature probes (typically taking several minutes), the heating blankets may be moved so they do not get damaged during surface preparation (Figure 5). The blankets may be energized during surface preparation to ensure continuous heating of the pipe during the blasting process.

Fig. 5: Abrasive blasting of the heated girth weld. Note that blankets have been moved to avoid damage.Fig. 6: Heating blankets are reinstalled for coating application and cure.

Once the surface was prepared and inspected, the induction blankets were placed as close as possible to the edge of the coating application area without interfering with the work. With induction heaters in place, the epoxy coating was applied in accordance with standard practices (Figure 6). 

Fig. 7: Time to backfill versus temperature for approved liquid epoxy coatings.

The induction heaters may remain in place as required for adequate cure. Figure 7 shows the time to backfill as a function of temperature for two PG&E-approved liquid epoxy coatings for buried pipeline. Note that both manufacturers indicate a minimum cure temperature of 40 F (10 C). By maintaining the pipe surface temperature above 100 F (254 C), the time to backfill can be reduced from over 10 hours to less than one hour. This expedited cure results in shortened construction time, less impact on the public and reduced project costs.

During the laboratory and field demonstrations, data was collected to understand the effect of induction heating on the pipe. Figures 8 and 9 show some of the data from the field demonstration. Induction coils quickly raise the temperature of steel in their immediate vicinity. Figure 8 shows temperature data with a 3-inch blanket spacing. Note that it takes five minutes to heat the center section of that pipe to 160 F (71 C) with the blankets controlled at 180 F. Once the blankets were removed, the pipe temperature remained above 140 F (60 C) for an additional five minutes. 

Fig. 8: Pipe temperature profile with 3 inches between induction heating blankets.
Fig. 9: Pipe temperature profile with 11 inches between induction heating blankets.

Steel surfaces several inches away from the induction coils can take considerably longer to heat and will not reach the same temperature as the steel under the coils. Figure 9 shows that with an 11-inch spacing between the blankets, the weld only reached 140 F. Obviously these temperature profiles will be affected by the ambient temperature and other factors such as airflow and pipe product temperature.  

The data demonstrates how pipe temperature can be adjusted through spacing of the blankets and adjustment of the temperature control under the blankets. Adjusting these parameters allows the coating applicator to control heating in various pipe configurations and ambient conditions. During the PG&E testing and pilot project, the optimum conditions were determined to be 11-inch spacing between blankets and 180 F temperature control set point under the blankets.

AN ALTERNATIVE PIPE-HEATING PRACTICE

PG&E’s new standard practice for coating pipelines when the pipe surface temperature is below 50 F involves preheating the pipeline prior to surface preparation and coating application. The company also tested and allows use of propane torches as a preheating alternative in situations where induction heating is not available or not practical. Open propane flame heating is also a common preheating method used in pipeline construction. Propane torches are used in welding to preheat the pipe to between 250 F (121 C) and 400 F. Lightweight, rugged portable heating torches are available with capacities in excess of 200,000 BTU/hour at 2,000 F. Open-flame heating is generally inefficient because the heat energy in an open flame is lost heating air surrounding the targeted area. Inconsistent heating and hot and cold spots can also occur when there is inefficient heat transfer within the part. This can result in inconsistent coating quality.

At extremely low temperatures, propane torches become difficult to use and storage tanks/cylinders may need to be warmed for proper operation. There are also safety issues anytime anyone works with an open flame. Workers must be careful not to ignite flammable sources that may also be on the jobsite. In addition, propane heating has a limitation in that it cannot be used to post-cure epoxy coatings. Therefore, this method is considered a second alternative to the induction heating system.

SUMMARY AND CONCLUSIONS

PG&E has determined that the optimum approach for cold weather coating application on girth welds is to use the existing approved epoxy coatings with induction preheat of the pipe to above 50 F during surface preparation, coating application and cure. As a second choice, preheating can alternately be achieved with propane torch heating if the induction preheat system is not available at jobsites.  

PG&E is now successfully using this pilot standard for cold weather coatings. The first of several sets of induction heating equipment has been purchased. Personnel have been trained in induction heating procedures and propane heating is also available for use. 

The system was demonstrated on girth weld coatings at the PG&E Applied Technology Services Laboratory and also during several 24-inch line installations near Petaluma, Calif. as a pilot program. Based on the data gathered and success of the demonstration, PG&E has an accepted process for heating with induction blankets when coating girth welds in cold weather.

ACKNOWLEDGMENT

The authors would like to acknowledge Judy Cheng, Mike Hernandez, Keldon Cox and Aziza Tarin of PG&E whose participation in this program helped immensely.

ABOUT THE AUTHORS

J. Peter Ault has been actively involved in various aspects of corrosion control and materials engineering for over 25 years. Since 2006, he has been a principal of Elzly Technology Corporation. Ault is an active member of several technical societies including SSPC, ASTM, NACE International, ASNE, SNAME and NPSE. He is a registered professional engineer in New York and New Jersey and holds coatings specialist certifications from both SSPC and NACE. Ault has a B.S. degree in mechanical engineering and an MBA from Drexel University.

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Bruce J. Wiskel has been a practicing corrosion engineer for over 30 years. He was a partner and owned a major corrosion protection company for over 14 years, held the positions of managing director, V.P. and president for several divisions over a 17-year employment for a major corrosion protection company, and for the past two years has been working for a major U.S. gas transmission and distribution company. Wiskel has called three countries home and his career has allowed him to work on corrosion protection projects in over 20 countries around the world. Wiskel holds a Bachelor of Science degree in metallurgical engineering from the University of Alberta and a business management certificate from the University of Calgary. His is a registered professional corrosion engineer (P.E.) in California, a registered professional corrosion engineer (P.Eng.) in Canada (Alberta), a NACE cathodic protection specialist and a NACE CIP coating inspector. Wiskel has published and lectured extensively around the world. He was a lecturer for the University of California and is presently a NACE CP instructor.  

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