Many metal fabrication shops and welders perform repair or modification work on old equipment, both in a shop setting and in the field. Performing repair work on hollow structures can pose a significant hazard other than a large internal pressure buildup from heating which we discussed in the last welding safety article.
As an easy introduction, consider the need to perform cutting or welding on a tank that used to contain a flammable solvent. Empty of liquid means it is safe, right? Not necessarily. It is the vapors that burn, not the liquid. An “empty” hollow structure such as a tank could be the worst case scenario, full of flammable vapors. Given the right ratio of vapor to oxygen, that hollow structure is like a bomb, ready to explode as soon as enough energy is transferred from the hotwork process.
Pittsburgh, PA. October 8, 2012 - Most of us are familiar with the concept that if a gas is heated it expands. This is described by Charles Law. If you inflate a balloon at room temperature and then place it in a freezer it decreases in volume. The balloon then returns to its original volume if warmed to room temperature again.
But, what happens when the gas is heated inside a rigid container such as a hollow metal structure which can’t expand? As the temperature in the hollow space increases the pressure increases. This is one of the reasons we have to be careful when doing hotwork, such as welding, on hollow structures.