Painting of pressure vessels before hydrotest 2

[1169]

ASME VIII UG 99 (k)indicates that vessels may be painted prior to pressure test. We want to do this to save time.
However same paragraph indicates that paint may mask leaks.
With this kind of wording nobody will allow painting before pressuring.
Is there any pressure limit above which leaks will not be masked?
If paint can mask leaks, why could this not happen with mill scale?

Has anybody had experience with this ambiguous code statement?

 

[unclesyd]

My copy of the code is an older version and UG-99 refers to "enameled" vessels. This is not paint but a vitrified coating like used on water heaters, bathtubs, and such a long time ago.

We were never able/allowed to paint a vessel prior to the initial hydrotest, only prime all areas except the areas of interest such as welds, nozzles, and such.

 

[lostsailor]

No ambibuity in this code paragraph whatsoever. This is a decision that should be made made with the involvement of the AI and the customer/end user. I've been involved in numerous hydrostatic tests where the vessels have been either completely or partially coated/painted. Factors I consider in whether or not masking of leaks will present a problem are the
configuration of the vessel, materials of construction, extent
of NDE, type and thickness of paint/coating, test pressure, test temperature. Check with the coating manufacturer as well, most of them have guidelines addressing pressure testing. Hope this helps.

 

[metengr]

I concur with the above posts. In reading paragraph k) of UG-99 the owner is simply cautioned about coatings that could mask pressure testing results. You must factor the type of coating, and hydrostatic test pressure, and most important, the recommendation of the AI. If the AI is not comfortable with the coating, they will have the final say because they are the ones that sign the Data Report after the completion of hydrostatic testing.

 

[VeryPicky]

IF you want to save time by coating the vessel prior to hydro, coat the vessel but put masking tape over the welds prior to applying the coat (make it weld width +1 inch each side of weld. Strip the tape before filling the vessel with fluid. After the hydro just coat the welds over. We have done it on a couple occasions. Nobody should challenge that but clear this procedure with AI prior. Hope this helps.

Putting Human Factor Back in Engineering

 

[robsalv]

Interesting. I asked a similar question elsewhere regarding Hydro of piping - coated versus uncoated.

B31.3 says you can coat prior to hydro except for category M services or piping that is to be sensitive leak tested.

I think the same should apply for PV's.

In Victoria Australia, I think the overwhelming experience is to hydro prior to coating. That is definitely my preference. In a way, it makes sense to do the hydro while the vessel is still in the shop and repairable should something "develop". Also generally speaking, paint shops are not really set up for hydro's.

BTW, the masked weld thing **can** result in substandard coatings over the welds. Metalurgically speaking, weld zones are preferential corrosion zones... so this is definitely not an ideal combination if the vessel is in an environment/situation that promotes corrosion, e.g., insulated and operating in the CUI range.

Cheers

Rob

 

[JStephen]

We run into the same issue with atmospheric storage tanks, in which the hydrotest consists only of filling the tank with water.

One of the issues is that some customers tend to view a hydrotest as ONLY a leak-detection test, which is not the point of it.

 

[VeryPicky]

robsalv

I see no reason why the coating over the weld should be inferior to the rest of the vessel body if the coating procedure applied is equally acceptable.

One more. That method was applied where the vessels' welds were ground flush and were invisible on the outside. We used a the same type of coating but different colour after the hydro to mark their location. Maintenance team was greatfull for that.

Putting Human Factor Back in Engineering

 

[robsalv]

VeryPicky

I agree that stripe coating of the welds post hydro can be done without impacting the integrity of the coating system... but there are some issues that could impact on the integrity - here are some that I can think of off the top of my head:

* The feathering of the vessel coat is not done well.
* the masking tape mastic leaves some adhesive behind,
* enough time elapses from removal of tape for a rust blush to begin,
* human fingerprints, sweat and/or oils are left behind in the tape removal process,
* the atmospheric conditions are different and unaccounted for in the application,
* the substrate is cold because of the hydro,
* the weld paint doesn't adhere to the previous coating for whatever reason

and there are also other reasons too...

Cheers

Rob