Liquid Penetrant Testing: How Has DPT Evolved in the Industry?

Liquid Penetrant Testing (DPT), or Liquid Penetrant Inspection (DPI) as it is often known, is referred to as PT (Liquid Penetrant Testing) or LT (Liquid Testing). It is a non-destructive testing method, used on non-porous materials, such as metals, plastics, and ceramics, to detect surface defects, which may not necessarily be visible to the naked eye.

Liquid penetrant testing dates back to the 1900s, when the “oil and whiting” method was used in the railroad industries to detect cracks in parts such as wheels, rod shafts, and crankshafts. The piece was first cleaned with boiling caustic soda, before being dried, dipped in oil, and dried again, before the final coat of whitish or chalk was applied. Where there were cracks in the structure, oil seeped out and mixed with the whiting. This made it much easier to locate hairpin cracks.

Later, a dye was added, and in the 1940s, a fluorescent or visible dye was added to make the distinctive cracks more obvious at first glance. Ultraviolet light is used to detect cracks with fluorescent dye and white light in the case of visible dye.

So how does it work?

Dye Penetration Inspection uses capillary action to draw liquids into tight spaces against external forces such as gravity, in the same way that a paintbrush draws water. The cohesion within the liquid and the intermolecular forces between the liquid and the solid act to pull the liquid upward without external force and despite opposing forces.

Liquid penetrant testing is a widely used method due to its low cost and flexibility. It can be applied to a wide range of industries and applications, including;

– Storage tanks

– Pipelines

– Structural membranes

-Welds

– valves

– And much more

It is used in various industries due to its ease and low cost, but also due to legal requirements and security concerns. Mandatory testing of equipment at various time periods for safety is required in various industries. And not doing so could have catastrophic consequences. Additional testing is often performed between required uses, to help maintain personnel safety and prevent costly failures.

Some industries that use NDT include; Chemical, power, oil, bulk and offshore storage.

In today’s world, newer technologies have devised more efficient methods to scan certain structures in some situations, such as ultrasonic and radiographic. They can provide more information or be faster. Although for complex forms, penetration testing can still be a simpler and more efficient method. Instead, technologies such as ultrasonic and radiographic can complement liquid penetrant testing to provide an efficient toolset for all situations, rather than replace it. For a while, it seemed like this newer technology might have arrived to replace DPI, but for now and for the foreseeable future it seems like it still has its place in today’s world of NDT.