Heat Treating

Furnace Atmosphere Conversion - Exothermic Gas to Hydrogen/Nitrogen

September 12, 2012
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Plymouth Tube operates two continuous heat-treating furnaces for annealing their products, and the facility is certified as ISO/TS 16949:2009. They had been producing an exother mic atmosphere for annealing furnaces from a combination generator/steam-boiler system.



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Fig. 1. An implementation team runs through the pre-start-up checklist to plan for and start up the furnaces during Plymouth’s maintenance shutdown.


The Challenge: Process Inefficiencies and Assessment of Aging Equipment

The exo-atmosphere/steam-generating equipment was identified by Plymouth as inefficient and in need of replacement. The exo-gas production satisfied the required flow for both of the annealing furnaces, but the boiler capacity was insufficient. The generator required the use of carbon monoxide (CO) alarms in the plant. These issues, coupled with the uncertainty over future legislation around CO levels, factored into the analysis.

Generator maintenance costs were rising, and the generator had limited gas-ratio flexibility. Plymouth Tube also considered the possibility of adding catalytic control for CO on this or any new generator system in the future. They were also interested in upgrading their atmosphere-control and data-collection capability. An updated atmosphere system would lower the dew point from the current 40-50°F, creating opportunities for higher productivity downstream and cosmetic improvements.

A decision was made to replace the combined boiler/exo generator with either a current exothermic gas generator plus CO control or to change the atmosphere to delivered hydrogen and nitrogen.



The Analysis

Economics
Plymouth Tube initially gathered the costs for both atmosphere alternatives, evaluating capital and operating costs for each option. Exothermic generator operating costs include natural gas, water, electric power and maintenance. Included with the exo-gas evaluation was a cost for catalytic clean up of CO. Capital costs for a delivered hydrogen/nitrogen atmosphere include concrete foundations, distribution pipelines for both gases and a gas mixing panel. H2/N2 operating costs include costs for gas used and monthly facility fees for the gas supply systems. The exothermic generator plus catalytic clean-up system was judged to have a higher capital cost. Operating costs with H2/N2 were higher than exothermic gas but acceptable. Plymouth Tube assessed that there was still a need for nitrogen supply to comply with NFPA 86 with an exothermic system. However, there were certain operating characteristics with H2/ N2 that were highly valued. Among those characteristics were the ability to change gas mix ratios and gas flows to meet certain conditions.  

Technology
Plymouth Tube, with 11 locations throughout the U.S., has experience with both generated atmospheres and hydrogen/nitrogen atmospheres. Corporate engineering is aware of the benefits offered by hydrogen and nitrogen gases mixed together in a blending panel. Engineers at Plymouth Tube concluded that the Streator product line – primarily 4130 steel tube – would benefit from adopting a nitrogen/hydrogen atmosphere. The precision and consistency of nitrogen and hydrogen would result in improved and more reproducible tubes. The lower dew point from cryogenic gases would produce better surface quality. Better surface quality would open up opportunities to reduce pickling. According to Plymouth Tube’s Judd Ferrin, mechanical engineer, “A gas mixing panel offering the ability to change gas ratios and gas flows meant that we could do things like lower the hydrogen ratio for intermediate annealed products, increase the hydrogen ratio to widen the range of steel tubing grades annealed or even manage the timing for furnace maintenance downtime.” Switching to straight nitrogen on weekends and during downtime will maintain the furnace dew point without using hydrogen.

The conclusion reached was that the technical advantages afforded by a hydrogen/nitrogen atmosphere outweighed the slight operating-cost advantage offered by an exothermic atmosphere.



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Fig. 2. Plymouth Tube’s Judd Ferrin (right), Praxair’s Keisha Owens (middle) and Plymouth Tube’s Mike Danko stand in front of the Praxair nitrogen and hydrogen supply systems. Praxair’s liquid-to-gas vaporizers reduced the overall footprint saving Plymouth Tube on the construction costs.

The Praxair Approach

“Praxair, along with its marketing partner, Atmosphere Engineering (AE), offered a combination of gas supply capability, gas control technology, atmosphere expertise and teamwork and was determined as the right combination for Plymouth Tube,” according to Mike Danko, Plymouth Tube’s quality and technical-services manager. “Among the benefits we saw from Praxair is a supply-tank-system foundation design that minimized our excavating and cement work.”

“Atmosphere Engineering offered a very well-integrated mixing panel with the flexibility to build to order for our operation,” Ferrin said.

Preparations began for the furnace atmosphere changeover to take place during a maintenance turnaround. Praxair coordinated the resources for the design of the hydrogen and nitrogen system foundations and assisted with the engineering of the hydrogen and nitrogen distribution lines from the supply systems to the furnaces. Plymouth Tube, AE and Praxair worked collaboratively in the atmosphere-control strategy designed into the AE gas mixing panel. The goal was to use this opportunity to improve the furnace atmosphere control and atmosphere measuring capability.

“We wanted to come away from this maintenance turnaround with a system that gave us substantially more insight into what was happening with the products in our furnaces,” Danko explained.

The fully automated mixing panels were designed with the latest in precision differential pressure-flow measurements and touch-screen controller capability to control H2/N2 flow and ratio, natural gas flow and to measure dew point and conveyer-belt speed. The gas control panel calculates and automatically sets the hydrogen and nitrogen flow rates based on desired hydrogen percent. The gas control panel also automatically begins a nitrogen purge during power outages.

Concurrent with the infrastructure design, plans were developed for the furnaces’ start-up process. A team from Plymouth Tube, AE and Praxair laid out the steps for a successful and safe introduction of a hydrogen-nitrogen atmosphere during the restart of the annealing furnaces. A checklist was developed to guide the team though the construction phase, pre-start-up phase and the start-up phase of the atmosphere project. Frequent team meetings were held to cover each new challenge that arose prior to the start-up.



The Results

An on-site team worked on the introduction of hydrogen and nitrogen into both annealing furnaces. As the furnaces were reheating, Praxair purged the hydrogen lines with nitrogen to remove any residual oxygen to below 1%, and AE worked through a final check of the gas panels. A panel systems check was accomplished during the nitrogen purge. When the furnaces were up to the normal operating temperature, H2/N2 mixed gas was introduced. Following some atmosphere-flow balancing work and within 24 hours of the H2/N2 atmosphere being in the furnace, the dew-point measurements for furnaces #2 and #3 were substantially lower than those achieved with the previous exothermic generator. Product was running through an annealing cycle on furnace #2 and then furnace #3. The new atmosphere start-up process was successful and on time, within the bigger Streator maintenance turnaround project.

Plymouth Tube reports both furnaces are operating at a much lower dew point, tube surface quality is improved and pickling times have been reduced. “Finished product is significantly better. We are looking at additional markets for our products because of what we have been able to do with improved surface appearance,” Ferrin said. IH  

For more information:  Contact Bob Esper, marketing manager, Praxair, Inc., 39 Old Ridgebury Road, Danbury, CT 06810; tel: 206-741-1841; e-mail: bob_esper@praxair.com; web: www.praxair.com

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