Coating of pipe welds prior to hydro?

[robsalv]

Hello folks.

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

 

[Garly]

Gday Robsalv,

i am in australia but am a bit confused - you mention an australian site but i assume B31.3 is an american standard? The aust standards AS 4037 is for Examination & testing of Pressure piping. also, AS4041 is the general pressure piping standard. If you welders are prequalified, and the welding method prequalified, i would assume you would have faith in the end result, and expect that the finish product will pass the hydro.

 

[robsalv]

G'day Garly.

Thanks for your response.

Our owners are American, so we have American management practices. Inside the plant boundaries we apply the more stringent of American and Australian standards. It's an interesting tight rope.

I couldn't determine from your post whether you coat or don't coat your welds prior to hydrotest. If you mean take it on faith that there's no need to hydro because the welder will do good work... then I have to part company with you there.

Never take anything on faith. Minimum testing requirements are there for a reason. AS4041 and 4037 don't say anything about hydro's with or without coated welds. Granted its less likely these days than in days gone by that a shop weld will have a gas pore pinhole in it... particularly with a properly managed QA system and a controlled environment... but if there's an undetected hole and the coating will hold back the pressure test... then your piping system integrity is open to question. Not something we wish to contemplate in the hydrocarbon industry.

Cheers
Rob

 

[bvi]

It has been shown that coating systems can effectively plug leaks. In fact, there was work done where very small holes were drilled in a pipe, the pipe coated, and it could be taken up to burst pressure without leaking. There was a paper published by Batey on this; I saw it but don't recall where.

You have a trade off in construction issues versus leak tightness issues, and I think it wise to consider the consequences of pin hole leaks should you coat the welds prior to hydrotest. Coating prior to hydro is permitted by ASME B31.3 and many folks do this.

Another way to deal with the dual issues is to shop hydro the fabricated spools prior to coating them and field hydro the complete system and field welds, prior to coating them.

 

[robsalv]

Thanks for adding to the thread BVI.

I've seen that paper (but can't recall the specifics either!) and that is the prime reason wny I'm wary of moving away from our current practice.

We often do what you suggested. To me it's the best middle road that meets most of the competing obligations. There's an economic driver to test this though given the volume of work coming up.

What's your practice?

Thanks.

Rob