Boilers are safer than they’ve ever been, but the dangers of working with water and steam should never be taken for granted. One of the most critical, and basic, things to remember about boiler operation is that water flashes to steam at 212 ° F, and when this happens it will instantaneously try to increase to 1,700 times its original volume. The following true story demonstrates how this fundamental fact of boiler operation, if overlooked, can lead to serious injury or worse.
The job started like many others: A repair crew in a large powerhouse was attempting to replace the piping between the main steam stop and the non-return valves on a high pressure boiler. The main steam valve next to the boiler was leaking steam, but upon initial inspection, the crew didn’t consider it to be a problem.
The first circumferential root pass, while only a fifth of the total thickness, created a pressure containment vessel. The crew left for the weekend, and for the next two days the leaky valve continued to leak steam into the new spool piece, which had a short drain valve that was closed. The steam condensed, and the hot water accumulated in the spool pieces and the valve line.
When the workers returned the following Monday, they neglected to open the short drain valve to ensure the area was free of water before they started to weld — a huge oversight that would come back to haunt them.
The welder finished the circumferential weld next to the non-return valve, with four more passes. All the while, the condensed water inside the spool piece was being reheated until it flashed into steam. The welder and crew didn’t realize this, and the steam was trapped within the spool piece under immense pressure as it tried to expand to 1,700 times its liquid volume.
The welders continued working and began the second pass at the other end of the spool piece. At this point, the welding arc was hotter than 10,000° F, the molten weld puddle was nearly 2,800° F, and the heat-affected zone ranged from approximately 1,200° F to just below the temperature of molten steel. This extreme heat, combined with the steam inside the spool piece would lead to tragedy.
When the welder was about two-thirds of the way around the pipe on the second pass, the weld and adjoining metal blew out. The weld root pass and the section of the still-molten second pass exploded, sending moton metal mixed with superheated steam into the welder’s neck and chest. The welder’s assistant was hit directly in the face and eyes, blinding him.
A lack of diligence and respect for the power of steam ended with two people being seriously injured during what should have been a routine repair job. These types of tragedies can be avoided by keeping these guidelines in mind:
- Never weld on any pressure-retaining surface which contains unvented liquids that can be pressurized.
- When welding on a vessel containing either a liquid or gas vapor, always remember that stress values of the metal in the weld and heat-affected zones are low due to high temperatures. Therefore, the weld strength will contain virtually no internal pressure.
- Before beginning any welding, always check to see if anything might be contained by or on the opposite surfaces. Welding on containers holding unknown substances can be extremely dangerous.
- Always use proper welding procedures qualified in accordance with ASME Code Section IX.
Information for this blog was taken from The National Board of Boiler and Pressure Vessel Inspectors. Learn more at NationalBoard.org.