Digital Radiography on insulated pipes

[Gem1]

Hi All,

We are looking into using digital radiography to check wall thickness on lagged steam pipes at a power station. There is lots of literature and advertising out there but we would be interested to know what people think of the technology. What methods did you use and did you get good results?

Thanks.

 

[stanweld]

Actual wall thickness checks are somewhat dubious with RT. Identification of areas subject to corrosion (both internal qnd external)is commonly done with digital RT.

 

[metengr]

You can use digital RT to detect wall loss provided you have developed a standard to compare wall loss. In other words, locally remove insulation from an area of the pipe that can be exposed for UT. Obtain a digital RT and compare the results. This would be used for comparison with other locations for digital RT. Digital RT is expensive and can be cumbersome versus computed RT or conventional RT.

 

[darthsoilsguy2]

Film RT and Digital RT (DRT) are the same with regards to your question.

the good thing about DRT compared to conventional is the quick turnaround time on shot development, ability to share downgraded images easily, ability to incorporate downgraded images into reports, and hopefully reduced cost because you don't get nickled and dimed for film (that being said, some firms charge a much higher hourly rates for the DRT over RT and probably in attempt to recover the big-money equipment cost)

A major element to this will be pipe diameter. These "elevation" type shots require the film or the digital film to be placed straight and behind the pipe. The source has to be far enough away that the projection of the radiation is nearly parallel so that the sidewalls get picked up crisp and the image looks most like a cross-section of pipe. In addition to sidewalls, you can see variations in image density which could correspond to pitting/thinning. As the pipe diameter gets larger, the source must be farther away which greatly increases exposure times. If the pipe diameter is more than the film, then it really gets out of control and you are splitting the shots. i assume the pipe lagging is also steel, so the image density aspect to it is a little more nebulous since you have layers of steel not associated with the pipe that could also be corroding.

RT is good at picking up the stuff that a ultrasonic testing program (UT) would miss like thinning around small penetrations (<2") in the pipe, and it gives a better result for corrosion at bends than a bunch of numbers. One outcome with any form of RT is that you may find out weld problems that you were blissfully ignorant of. in your case, you might not find any problems since you're doing it on steam lines (if they were installed per B31.1), but i've done plenty of RT corrosion investigation on process and building service piping and you find problems with the welds left and right, mostly incomplete penetration.

and yes.... UT should also be done for fact-checking.