Wet Film, DFT, and Holiday Detection on a Pipeline Coating
A field-applied fusion-bonded-epoxy-equivalent liquid pipeline coating (two-component epoxy, 12-18 mils DFT) is being applied to 24-inch pipe in a laydown yard. The applicator sprays, and the inspector checks wet film thickness (WFT) with a comb gauge, reading 14-16 mils on the top of the pipe and 8-10 mils on the bottom. The product data sheet gives a volume solids of 65% and a recommended WFT-to-DFT relationship. The applicator says the bottom will 'flow out' and even up, and wants to proceed to holiday detection with a high-voltage (jeep) tester set at 100 volts per mil. Ambient is 95°F, coating surface is 110°F, and the product's pot life at 95°F is 45 minutes; the batch was mixed 50 minutes ago. A holiday test on the first cooled section shows several sparks concentrated on the bottom of the pipe.
1. Compute the expected DFT from the WFT readings and determine whether the bottom-of-pipe readings are acceptable. Show the calculation.
Read the worked answer
DFT = WFT × volume solids (as a fraction).
Top of pipe: 14-16 mils WFT × 0.65 = 9.1-10.4 mils DFT. Bottom of pipe: 8-10 mils WFT × 0.65 = 5.2-6.5 mils DFT.
Specified DFT is 12-18 mils. Even the top-of-pipe readings are below the 12-mil minimum (9.1-10.4 mils), and the bottom is far below at 5.2-6.5 mils - roughly half the minimum. The applicator's 'flow out and even up' claim is wrong on two counts: (1) coatings do not level from a thin region to a thick one across a pipe circumference; gravity actually thins the bottom further and can cause sagging on the top, and (2) the WFT gauge reading is a snapshot, and the volume-solids calculation already accounts for shrinkage on cure.
The correct action is to stop, adjust the application (gun angle/distance, multiple passes, possibly different tip size or reduced atomizing pressure), and re-apply to achieve WFT that yields 12-18 mils DFT (i.e., approximately 18.5-27.7 mils WFT). The existing thin bottom must be corrected before cure or removed and re-applied per the product data sheet.
2. The batch is 50 minutes old with a 45-minute pot life at 95°F, and the holiday test sparked on the bottom. What do these two facts imply, and what should the inspector do?
Read the worked answer
Both facts point to a process out of control and require stopping work.
Pot life: the mixed batch has exceeded its 45-minute pot life at 95°F by 5 minutes. Material beyond pot life is thickening, losing solvent/ reactivity balance, and will not cure to specified properties; applying it risks poor adhesion, incomplete cure, and solvent entrapment. The batch must be discarded and a fresh mix made, and the mixing/applying cycle re-planned (smaller batches or chilled material) to stay within pot life at this temperature. The 110°F surface temperature is also above many products' maximum application temperature and will accelerate pot life and solvent flash-off, causing dry spray and poor flow.
Holiday sparks on the bottom: at 100 V/mil over a nominal 12-18 mil target, the test voltage is appropriate only if the actual DFT is in range. On the bottom, DFT is 5.2-6.5 mils, so a 100 V/mil setting is actually testing at roughly 15-19 V/mil of actual thickness - but more importantly, the sparks confirm the coating is too thin and/or has pinholes there, consistent with the low WFT. The inspector should: stop application, discard the over-age batch, re-verify DFT after correction, re-run holiday detection at a voltage appropriate to the verified DFT (per the test standard and product data sheet), and document the nonconformance and corrective action.