How reliable is a dye pen for leak testing on flange?

Question

how reliable is a dye pen for leak testing on flange?

Answer 1

Leak detection DYES are usually put inside the line to be tested and then the line is pressurized.  The dye makes it easier to see the leaking fluid.

Ultraviolet leak detection PENS on the other hand will make a wide variety of liquids and gases glow.  Very handy for finding leaks of synthetic oils, refrigerants,  and bio waste.  I've also seen PEN-style combustible gas and toxic gas detectors.

And, of course, there's DYE penetrant testing of welds.  But dye penetrant usually comes in a brush-can, not a pen.  If that's what you're talking about, all it really does is make small cracks that would have gone unnoticed stand out a bit more for visual inspection.

15 years working in power plants and chemical plants and I've never heard of a DYE PEN for the purpose of leak detection?  How would that work?  Would you scribble all over the welds and sealing faces of the flange, including the gasket or flex seal, and hope that it makes it easier to see the leak?  Or is it just another form of dye penetrant for inspecting welds?  If so, then the only type of lines where dye testing is ok are usually water lines or non-critical lines.  Anything important requires x-rays for the welds and pressurized leak testing before use.

Depending on the substance inside the pipe, there's many different ways to detect leaks.  Pixie sticks work great for anhydrous ammonia.  Tic sniffers are best for combustibles and hydrocarbons.  Soapy water works for just about any type of invisible gas.  If it's a suspected slow leak you can tape a Pig Pad, clean rag, or plastic sheet around the flange and reinspect it after an hour.  Heck, even spraying Talc powder all over the flange works for most liquids.