Piping hydro, to coat or not to coat query?

[robsalv]

Hello folks.

This might be a more suitable query in another forum... apologies if so... I was hoping I could draw on your experience in regards to piping hydros.

We're about to launch into a significant amount of piping work in a plant revamp [Australian site]. It's been our practice to date to leave all piping welds [shop or field] uncoated until the piping successfully passes a hydro test. This is on the understanding that a pinhole/pore failure in a weld may not necessarily be picked up by RT and is also undetectable in a hydro situation if the weld is coated due to being plugged by the coating system.

We're considering a proposal to modify this practice by way of prime coating all shop spooling prior to hydro. This means that the final hydro will be carried out once all closure welds have been done and the piping is fully installed in the field. The field welds will be left unpainted until after the hydro, at which time the piping will be fully coated to the appropriate protective coating specification.

B31.3 stipulates uncoated welds for Cat M fluids or sensitive leak tests - neither which apply to us. This clause actually struck me as odd given the pin hole plugging that can occur with a coating system. Wonder why the committee has let that one go past?

Anyway, I'd appreciate it greatly if you would share the gist of the piping hydro practice on your sites?

Thanks in advance.

Rob

 

[ApC2Kp]

Robsalv,
If your fluid services are non-hazardous or low concern for leakage, then the consequences of a pin hole that opens up and leaks later is simply a maintenance task. The reality of pinhole plugging was investigated by Union Carbide after leaks of ethylelene oxide (carcinogen) in their hydrotested piping. It was demonstrated with a pipe spool drilled with small holes, painted with an industrial epoxy coating, and it would pass hydrotest. In service the ethylene oxide acted as a solvent to dissolve the epoxy paint plug. So if the fluid service is severe pressure / temperature, or toxic / hazardous (Category M) then piping of those services need to be tested rigorously.
There are many jobsites that fabricate and prime coat the piping. You could have the shop welds masked before painting. You might consider a shop leak test with low pressure air and soap bubbles. The urgency of short turn around schedules has driven more shop hydrotesting of the fabricated piping. There seems to be less time to do leak repairs during the shutdowns.

 

[robsalv]

Thanks ApC2KP for your contribution.

I'm familiar with that paper, but couldn't remember the specifics. It's the prime reason I'm reluctant to move away from our current practice of masking welds when prefabbed shop spools are painted without a shop hydro'd. Also, even though we aren't talking category M fluids, the "simple" maintenance job you speak of could be far from simple when the plant has to be shut down to effect the repair.

I prefer the shop hydro then paint scenario, but the economics become significant when looking at the volume of work we have coming up.

I take it then your practice is to coat prior to hydro for non category M fluids?

Thanks
Rob

 

[ApC2Kp]

Robsalv,
The paint prior to hydro would apply to non Category M piping. Prime coat in shop (and finish coat in field) saves some surface preparation and painting time as well as protecting the pipe during temporary storage. A zinc rich primer coat might not seal tight any larger leaks, and the leaks then show up during hydrotest.