Summary: The following article is a part of the National Board Technical Series. This article was originally published in the October 1984 National Board BULLETIN. (3 printed pages)
Certain laws of physics are taken for granted at times. One of the most basic laws we learn, and one that should never be forgotten, is that water will flash to steam when it reaches 212°F.
This was the case recently when 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, which was in battery with three other boilers. The workers noted the main steam valve next to the boiler was leaking steam, however, they did not consider it a problem at the time. It was believed that because the members of the company-employed repair crew were experienced maintenance workers, it wasn't necessary that they be performance qualified-tested in accordance with the ASME Boiler and Pressure Vessel Code , Section IX, or use welding procedures similarly qualified. They, therefore, began the job of replacing the spool piece.
The first circumferential root pass was made on both ends of the spool on a Friday afternoon. The root pass, while only welding approximately one fifth of the total through thickness, effectively created a pressure containment vessel, and at this point the repair crew quit for the weekend. The leaky steam valve continued to leak steam into the new spool piece. The spool piece had a short drain valve which was closed. Over the long weekend the steam condensed and the hot water accumulated in the spool piece and the drain valve line.
On Monday, it didn't occur to the workers to open the short drain valve to make certain the area inside, where they were about to weld, was free of water. The welder completed the circumferential weld adjacent to the non-return valve with four additional passes. During the welding of these four passes, the condensed water was reheated and flashed into steam. Since the steam was confined within the spool piece, it could not expand to 1700 times its original volume and, therefore, compressed and built up great pressure. This is in conformance with another basic law of physics of importance to this trade; when water flashes to steam, it will always attempt to increase to 1700 times its original volume.
The workers, unaware of this tremendous pressure build-up, began to weld the second pass at the other end of the spool piece.
To explain how this flashing of steam and pressure build-up occurred; the welding arc was in excess of 10,000o F, the molten weld puddle was nearly 2800°F, and the heat affected zone ranged from approximately 1200°F to just below the temperature of the molten steel. Since the root pass was approximately 3/32 inch- thick, the second pass had molten metal within 1/16 inch of the inside of the pipe. Experience tells us, therefore, that the inside of the pipe was cherry red to hot white. Steels are very weak at these elevated temperatures and this contributed to the failure.
The welder was about two-thirds of the way around the pipe with this second pass when tragedy struck. As it always does, the weld and adjacent metal blew outwardly with frightening speed and surprise. The weld root pass and the section of the molten second pass exploded, striking the welder in the neck and chest with molten metal mixed with steam, causing serious injuries. The welder's helper was hit directly in the face and eyes. At this writing, he is blinded and it is doubtful that he will regain his sight. These injuries are a high price to pay for the errors made during a seemingly minor repair job.
Tragedies such as these can be averted. Awareness of the danger is the best defense. Keep 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 the high temperatures, therefore, the weld strength will contain virtually no internal pressures.
- 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.
- Always use welders that have demonstrated their proficiency and have passed a performance qualification test in accordance with ASME Code Section IX.
It appears that most of these guidelines were ignored in the incident described above. The results of this carelessness too often encompasses death, injury, and destruction of property.
Safety is no accident. We have to work at it at all times!
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.