Welding is a necessary chore when it comes to pipes. Curved steel plates can be welded together to form a pipe, and various lengths of pipe can be girth welded--a weld that goes in a circle to connect the pipes at their two ends--so that a pipe has an extended length. There are a number of defects and errors that can occur throughout this welding process though, and they can have disastrous effects on the pipe's integrity.
A pinhole is one of the easiest flaws in a pipe weld to miss even during a thorough inspection. A pinhole defect is a very small area that simply doesn't get welded, and a hole is left from the inside of the pipe to the outside of the pipe. At the very least, this causes the pipe to leak. If the pipe is under high pressure, that hole may create a weakness in the pipe that could lead to a violent rupture or explosion. Even if the pipe isn't under high pressure, that hole might grow over time and lead to a burst pipe.
Another type of welding defect that is particularly bad where pipe welding is concerned is called an off-seam weld defect. This defect occurs when someone is using the double submerged arc welding (DSAW) process. This means the interior and exterior of the pipe's seam are welded at the same time, theoretically providing a stronger weld seam. An off-seam weld occurs when the DSAW process isn't properly aligned, which means that rather than making a seam stronger, it makes the seam weaker.
One of the more dangerous types of welding defects is called an undercutting, and when pipes are going to be under pressure, this can be extremely bad. Undercutting is the thinning of the pipe wall during a seam weld. This means that when a pipe is put under pressure, there is an area of weakness around the pipe wall near the weld seam. While it might hold for a time, if the pressure increases in the pipe because of a backup or an increased material flow, then the weakened pipe wall could rupture and burst.