Compressed gas cylinders are vessels that contain high pressure gases such as propane and natural gas. The ubiquitous propane cylinder can be found throughout the country in applications from temporary heating to industrial vehicle fuel reservoirs. As the propane cylinders are used and refilled, inspections are periodically performed to assess pressure vessel integrity as required by the U.S. Department of Transportation regulations.
Among the several inspection requirements, is the visual inspection of the outside surfaces of the cylinder. Impact marks, scrapes, corrosion, erosion and wear are noted during these inspections. The underlying premise of these requirements is to remove from service any cylinder that exhibits deficiencies on the outside surface that could cause a failure. Inspection of the extent of corrosion of the exterior metal is an important examination that is performed in order to requalify a propane cylinder for service. Inspection is visually made and areas of suspicious corrosion damage are noted. In the instance of severe surface corrosion, a small tool is used to measure the depth of the corrosion. Pitting and crevice corrosion cause thinning of the pressure vessel wall, resulting in higher stresses and stress concentration. When excessive corrosion depth is found, the propane tank is deemed to be unacceptable and is retired.
FIGURE 1 FIGURE 2
When these inspections are not performed and the propane cylinder suffers from corrosion damage, the condition could prove disastrous. Figure 1 shows a building that sustained considerable explosion damage after the failure of a propane cylinder. This building was undergoing a renovation with a 100 lb. propane cylinder on the first floor, connected to a temporary construction heater. One evening when the building was unoccupied, an explosion occurred on the first floor. The 100 lb. propane cylinder rocketed upward through the first 3 floors and became lodged in the fourth floor. Figure 2 shows the top of the propane cylinder with resulting impact damage and part of a wood beam that was in the path of trajectory. Figure 3 is an overall view of the 100 lb. propane cylinder.
FIGURE 3 FIGURE 4 FIGURE 5
The bottom of the cylinder separated and was found on the first floor (Figure 4). As would be expected, as the propane at approximately 100 psi escaped from the bottom of the cylinder, over 7 tons of force pushed the cylinder upward. The accident investigation focused on the cylinder end cap and why it had fractured. The arrow in Figure 5 points to the fractured end of the cylinder at the foot ring. Severe crevice corrosion was evident (see “Corrosion Failures”, Claims Magazine, Sept. 1993). Crevice corrosion occurs at a corner between two pieces of steel where trapped debris, dirt and moisture accumulate in the crevice. Figure 6 depicts the corroded area in the crevice between the tank envelope and the foot ring, with the arrow pointing to severe crevice corrosion. Figure 7 is a cross-sectional cut through the foot ring showing the failure origin. The left arrow points to the original cylinder wall. The right arrow points to the foot ring support which does not carry pressure but merely the weight of the cylinder. The center arrow points to the base of the bottom cap on the cylinder. The Y like crevice traps moisture and debris, resulting in the corrosion that significantly reduced the end cap steel thickness (center arrow). In this case, approximately 25% of the original wall thickness remained at time of failure. This condition was obviously hazardous and lead to a large property loss. Apparently this propane cylinder had not been properly inspected.
When a loss occurs that mandates a failure analysis of a propane cylinder, the following recommendations are offered. Preserve the fracture surfaces from corrosion by applying a protective film such as mineral oil. Attempt to recover as many parts of the cylinder as possible. This may require extensive excavation at the scene to find lost parts such as the one described above. Perform a failure analysis as soon as possible since the process of corrosion is ongoing and can lead to loss of evidence.
FIGURE 6 FIGURE 7