Ironworkers hold themselves to a standard of excellence to be the most skilled, productive and safest craftsmen in the building trades. To achieve that goal, the National Ironworkers and Employers Apprenticeship Training and Journeyman Upgrading Fund (NIEATJUF) conducts instructor training activities that include the annual Ironworker Instructor Training Program. Hosted by Washtenaw Community College, Ann Arbor, Mich., more than 800 apprentice coordinators, instructors and business managers participated this year.
Among the more than 40 courses presented during “train the trainers” week, 11 focused on welding and cutting, including a 20-hour block that covered oxyfuel cutting and heating. Highlights from the four days of classroom discussions and hands-on training sessions included a deeper dive into acetylene safety, the benefits of LP (liquified petroleum) fuel gases and how to manage pressure drop from the gas source to the point of use.
One-seventh for safety
The liveliest discussion focused on the science behind the utmost fundamental safety rules to obey, including the 1/7th safety rule, when using acetylene as a fuel gas:
- Never use acetylene above 15 psi.
- The withdraw rate of an acetylene cylinder must not exceed 1/7th the capacity of the cylinder per hour.
- Store and use acetylene cylinders in an upright position.
Without stabilization, pure acetylene (C₂H₂) can self-ignite and behave like an explosive even without oxygen present. When triggered by heat, pressure above 15 psi or shock, the molecule breaks down in a self-sustaining exothermic reaction: C₂H₂ → 2 C (solid) + H₂ (gas) + heat … lots of heat, violently released.
A Swedish engineer named Gustav Dalén solved the acetylene stability problem in 1905 when he invented what is now the modern acetone-filled, porous-mass acetylene cylinder. Acetone can hold about 25 times its own volume of acetylene at atmospheric pressure. The porous mass (today, calcium silicate) uniformly disperses the acetone and creates a large surface area to keep acetylene dissolved.
The reason for the 1/7th rule is that withdrawing too much acetylene can pull acetone out of the cylinder, and insufficient acetone can destabilize the acetylene. Putting a cylinder on its side also increases the chance of pulling out acetone. (Note: if a cylinder must be set on its side, restore it to an upright position and wait 24 hours before using). If the flame turns purple, it can indicate the presence of acetone. Because acetone can prematurely degrade rubber and plastics, manufacturers qualify hoses, seals and gaskets so that they are acetone-resistant. However, eliminating potential hazards is always the safest route.
To put the 1/7th rule into practice, one class calculated that the maximum withdrawal rate for a 322-cu.-ft. cylinder (a large size) was 46-cu.-ft. per hour (CFH; 322 ÷ 7 = 46). Next, they cross-referenced tip charts to determine that a size 6 multi-flame attachment, which has a maximum withdrawal rate of 40 cu. ft. per hour, was the largest attachment that could be safely used without an operator accidentally creating an unsafe condition by setting excessive flow rates.
Solving pressure problems
The class participants noted that larger attachments (size 10, 12 and 15) are more common for preheating and post-weld heat treating in structural steel applications. To achieve higher flow rates would require using a manifold to connect multiple cylinders together or, more commonly, switching to an LP fuel gas such as propane or propylene.
LP gases are stable and do not have the withdrawal rate, pressure, or handling and storage concerns associated with acetylene. While the hotter, more concentrated primary flame of acetylene works better for cutting, LP gases offer more benefits for heating:
- Their secondary flame has more energy (BTUs) and heats a larger area, reducing total heating time.
- LP gases can be used at flow rates of up to 125 cu. ft. per hour to run larger heating attachments.
- Stored as a liquid, LP gases deliver longer run time per cylinder.
- Per BTU, LP gases cost a fraction of the price of acetylene.
Because fuel gases have different flow, pressure and burning velocity requirements, regulators, heating attachments and cutting tips are fuel gas specific. While an LP regulator could technically be adapted for acetylene, it should never be done because it sets the stage for a catastrophe. First, LP regulators enable setting pressure in excess of 60 psi, a rate that would cause acetylene to disassociate. Second, they lack visual warnings at the 15-psi flow limit.
Pressure drop
Given the large scale of structural steel erection, bridge work and fabrication projects, setting gas pressure and flow volume at the regulator is not as simple as consulting a tip chart. When using long hoses, operators also need to consult a pressure drop reference chart. Long hoses require increasing set flow rates above the values specified on the tip chart.
Incorrect flow rates do not just hamper cutting or heating efficiency. Because cutting and heating tips are designed to function optimally within a specified range, if the gas velocity coming out of the tip drops below the flame’s burning speed, the flame can travel backward into the torch and create burnback or flashback (a “pop” from a flame burning back inside the torch or a whistling sound of a sustained flame deeper inside the torch, respectively, both of which are unsafe).
Small-diameter hoses, quick couplings, multiple connections, and additional or clogged flashback arrestors all reduce gas flow to the torch. Rather than guess at the amount of compensation required, the class learned about test gauges that thread to the back of the torch. By reading pressure at the gauge and adjusting pressure/flow at the regulator, they learned exactly how much additional pressure was required.
Always learning
Ironworkers hold their annual Ironworker Instructor Training Program so that their journeymen instructors can bring skills, knowledge and teaching techniques back to their local training halls. The ironworker commitment to training makes them better teachers and helps them create a safer and more productive workforce for contractors.
To make the training week possible, ironworkers collaborate with manufacturers that send teams of subject matter experts to the event. Much of this training is also available to other fabricators, including on-site oxyfuel safety audits and training, as well as extensive e-learning modules around which fabricators can build their own training.
Even the most experienced operators may learn something from these classes and materials, and everyone can use a refresher on safety fundamentals. Millions of people cut and heat safely every day without incident, and the continuing training helps ensure everyone goes home to their family at the end of the day.
Stay informed on safety best practices by visiting our dedicated archive on the topic.
