Dye Penetrant Testing - How Has DPT Evolved In the Industry?

dye Penetrant Inspection

Dye Penetrant Testing (DPT), or Dye Penetrant Inspection (DPI) as it's often known is often called PT (Penetrant Testing) or LT (Liquid Testing). It's 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.

Dye Penetrant Testing goes back to the 1900's when the 'Oil and Whiting' method was used in the railroad industries to detect cracks in parts such as wheels, rods axels and crankshafts. The part was first cleaned using boiling caustic soda, before being dried, submerged in oil, and dried again, before applying the final coating of whiting or chalk. Where cracks were present in the structure oil would seep out and mix with the whiting. This made locating hairpin cracks much easier.

Later a dye was added and by the 1940s a fluorescent or visible dye was added to make distinguishing cracks more obvious at first glance. UV 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 utilises capillary action to draw liquids up into narrow spaces against external forces such as gravity, in much the same way a paint brush draws up water. Cohesion within the liquid and inter-molecular forces between the liquid and solid, act to draw the liquid upwards without external force and in spite of opposing forces.

Dye penetrant testing is an oft used method because of it's cheapness and flexibility. It can be applied to wide range of industries and applications, including;

- Storage Tanks 
- Pipelines 
- Structural Membranes 
- Welds 
- Valves 
- And much more

It's used in several industries because of its ease and cheapness, but also due to legal requirements and safety concerns. In several industries mandatory testing of equipment is required at various time periods for safety. And failure to do so, could have catastrophic consequences. Additional testing is often performed in between mandatory uses, to help keep staff safe, and expensive failures from occurring.

Some industries that utilize NDT include; Chemical, Power, Petroleum, Bulk Storage, and Off Shore.

In today's world, newer technologies have come up with more efficient methods for scanning certain structures in some situations, such as ultrasonic and radiographic. They can provide more information or be faster. Though for complex shapes penetrant testing can still be a simpler more efficient method. Instead, technologies like ultrasonic and radiographic can complement dye penetrant testing, to provide an efficient toolkit for all situations, rather than to replace it. For a while it looked like these newer technology may have come along to replace DPI, but for now and the foreseeable future it looks like it still has its place in today's NDT world.

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