Some years ago, I was talking with a group of people at an engineering conference when the subject turned to measuring how far molten metal penetrated up a very narrow tube before freezing. “That’s easy,” said one guy, “you just take a microsection of the tube.” “Okay,” said another, “but what do you do with the second one?”
To put this into perspective, we were talking about verifying the quality of products coming off an assembly line, meaning hundreds or thousands of products, each containing dozens of these tubes. Slicing half of these, or even one in ten, into pieces and examining the remains under a microscope would have taken untold hours, never mind costing more in lost materials than the manufacturer would ever have earned from the production run.
As technology became more advanced, performance requirements higher and profit margins tighter, industries began to feel the need for ways of testing their products in ways that wouldn’t damage them. This often meant “seeing” inside solid metal without cutting through it.
Railroads and pipelines are some of the biggest beneficiaries of these modern techniques. In both cases, they have to tolerate major strains, whether pressures of 1,500 psi or rapidly moving trains weighing over 100 tons, and do this for years on end. If either should fail, the probable consequences range from the expensive to the tragic.
If you watch a movie set in medieval times, you’ll probably hear the hero’s sword making a ringing sound after every impact. This isn’t entirely a case of Hollywood taking liberties with metallurgy, but was actually a primitive way for smiths to gauge the quality of a weapon or other piece of metal.
One of the earliest ways in which this was applied to NDT (non-destructive testing) was for a person to tap one end of a rail with a hammer while a colleague listened at the other, sometimes by pressing their ear to the rail but also with the help of an amplifier. A solid, well-tempered piece of metal sounds louder and more clear than a possibly defective one. This is because, in the latter case, voids (air bubbles), inclusions (solid impurities) and fissures all absorb or reflect sound as it travels through the material.
Modern acoustic testing is computerized and uses frequencies too high for a human to hear, as this allows even tiny defects to be detected.
A Pig in a Pipe
The next most common NDT technique involves applying a magnetic field to metal and measuring how much of it “leaks” out. These and other semi-automated techniques are fast and highly accurate, but what are engineers to do when it comes to a pipe 1,000 miles long…and mostly several feet underground?
Pipeline ruptures can be caused not only by material flaws, but also accidents or sabotage. To make the latter less likely, pipes are usually buried, but they still need to be inspected regularly for signs of corrosion or strain.
The best solution currently available doesn’t even require the flow to be shut down: the “pig” is a device that fits the diameter of the pipe and is carried along by the flow of oil or gas, measuring the integrity of the pipe walls and joints as it goes. As long as this is done regularly and prompt action is taken when a flaw is discovered, the risk of petroleum seeping into groundwater is minimized.