Holiday detection in FBE Pipe Coating
A discussion started in 2001 & continuing through 2017(2001)
FBE = Fusion Bonded Epoxy
Fusion bonded epoxy coating
Q. The general recommendation to check the discontinuity in a FBE pipe coating is 125 volts/mil, but I consider this voltage not enough to detect all holidays because the coating surface is not flat (but waves) and sometimes the thickness is higher than the average. What's more, the holiday wire doesn't touch the coating surface 100%. How much higher do you consider could be the voltage to check, if you agree with my point of view?. How much higher could the voltage be without burning the coating ?Gabriel Giraldo Alfonso
- Barranquilla, Colombia
A. The only point of reference I have, being in the electroplating industry, is the spark testing of PVC liners for steel tanks which will hold liquids.
I found your question interesting, since I never thought about the test requirements we use for the spark tester. I wonder if we have some similar volts/mil requirement.
Certainly, the limit of the practical use of the test would occur if the higher voltage being used to test the coating could burn a hole right through the coating.
Falls Township, Pennsylvania
A. A simple way to test what voltage a coating can withstand is to apply the voltage with a brass brush probe applied, then slowly wind up the voltage, note; use a DC voltage detector not pulse. When it breaks you will know the dielectric strength of the coating at that test point. It may surprise you, as how much a well applied cured coating, will withstand, generally a lot more voltage than most could imagine. For general testing purposes the breakdown voltage could not be applied as a test voltage.
I would think that you would be tied to a specification, that has a preset test voltage which would not allow for this, but maybe in the future. Pipelines need the maximum test voltage applied as you can do little after they are buried.
PCWI P/L - Newcastle NSW Australia
September 12, 2012
A. The 125 volts/mil is old school. Todays FBE's have a dielectric strength of 1100 to 1300 volts per mil. Coatings are usually applied in the 14-16 mil thicknesses providing a minimum of 15,000 volts of protection. At 125 volts per mil at 14 mils is 1750 volts. This will usually find holidays that are through the coating with exposed steel, assuming the coating is not cratered. This means that the coating is deformed from the impact and is thicker at the edges leaving an air gap of maybe 21 mils. These often will go undetected at the 125 volts per mil. Now assume the coating is only damaged leaving 1 mil of coating to remain on the steel. We need 1100 volts to burn through it plus 75 volts per mil to jump the air gap of 13 mils. Damage goes undetected. Why do we get detection at 125/mil? Its because of contaminants on the surface of the defects assisting the electrons to flow to the steel. So, yes, 125 is not enough.Darren Lemmerman
- St. Paul Minnesota, USA
August 23, 2016
A. It's extremely concerning to read that someone might actually think that dielectric strength has anything to do with holiday detection.
Dielectric strength/breakdown and discontinuity testing (holiday testing) are two completely different subjects. The two test methods (one destructive, the other non-destructive) are used to determine completely different characteristics of materials.
You cannot holiday test with 1,000v/mil because a product data sheet lists a 1,000v/mil dielectric strength value.
125v/mil is not old school. If anyone thinks they can conduct discontinuity testing with 1,000v/mil, then they should look for a carrier that doesn't involve coating inspection.
Anyone reading this who has any doubt that dielectric strength testing and holiday testing are not related. Please read ASTM D520 [link by ed. to spec at TechStreet], NACE SP0188 [link by ed. to spec at TechStreet] & NACE SP0490 [link by ed. to spec at TechStreet]. Please.
NACE III CIP QC MGR - Panama City Florida, USA
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