Compare and Contrast; a Look at Some Common NDT Techniques

By Nick Schmoyer, Vice President and Technical Director at Inspectioneering. June 1, 2015

Nondestructive Testing (NDT) makes up the majority of testing performed in our industry. There many different types of nondestructive testing techniques. Each method has its own advantages and disadvantages; each can detect different types of flaws. Some are simple to perform, others are difficult. Some are used on several different types of equipment, others are more restricted.

In this blog post, we'll examine some of the more common NDT techniques in the industry. Even if you already know about all of these techniques, this may still serve as a nice refresher on the basics. If you don't already know about these techniques, you might learn something new.

Visual Testing (VT) is the oldest and most basic type of nondestructive inspection. It is simply looking at equipment to check for defects like cracks or corrosion. Despite this, it is still one of the most efficient and effective techniques. It can be used on nearly any kind of equipment. It is relatively inexpensive because it doesn't require specialized equipment. VT doesn't work in all situations though. It's not useful for finding defects in hard to reach places or those that are too small to see with the human eye.

Liquid Penetrant Examination (LPE) is a process that involves the use of fluorescent dye to highlight surface flaws. This makes otherwise unseen flaws more easy to see with the naked eye. It's a fairly simple technique. It involves applying a penetrant dye to the surface of a piece of equipment. This penetrant seeps into any defects on the surface of the equipment. Excess penetrant is removed and developer is applied. The penetrant remaining in the flaws will be drawn out, making them more visible to the naked eye. LPE is useful because it works quickly and is fairly inexpensive. It only works on surface flaws though, so it isn't applicable in all situations.

Magnetic Particle Testing (MPT) is a technique which is able to detect both surface and subsurface flaws in ferromagnetic materials. The process involves magnetizing the part then pouring metallic particles over it. Defects on or near the surface cause the magnetic field applied to the part to be distorted. This causes the particles to cluster around the distortions. It's useful because it's a highly portable technique and is good at detecting sub-surface cracks. On the other hand, if the defects are too deep in the material, MPT won't be able to detect them.

By the way, if you want to know even more about MPT, download our free Asset Intelligence Report on the topic.

Magnetic Flux Leakage (MFL) is a technique that involves the use of electromagnetism to inspect for defects. It is able to detect flaws like corrosion, pitting, and wall loss. The first step in the process involves magnetizing an object using electromagnets. The magnetic field around the object forms distortions using either coils or Hall effect sensors. These distortions are usually signs of defects in the material. While good at detecting flaws, this method is sensitive to distortions in the magnetic field.

Pulsed Eddy Current (PEC) is a technique used to detect flaws or corrosion in ferromagnetic materials. It can also measure the thickness of said materials. The technique works by applying a step function voltage to an eddy probe. The probe then induces a similar field when brought near an object. Like with MFL, this field is then measured for distortions, which signify defects. It's useful because it doesn't require surface preparation. The method can be used on insulated materials as well. It's also fairly cost-effective and is effective with a variety of different equipment.

Long Range Ultrasonic Testing (LRUT) is an ultrasonic testing technique that is used to inspect pipelines for corrosion. This technique is done by strapping a series of rings around a pipe. These rings release sound waves that move throughout the entire pipe. The speed of these waves can then be measured to detect wall thickness, which is an indicator of corrosion. This technique is one of the fastest tools for performing pipeline surveys. It is also particularly useful in inspecting pipelines that are in difficult to reach locations, many of which can't be examined using other methods.

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