Painting or Coating Prior to Hydrostatic testing 2

[metengr]

FYI:
From time to time, questions are asked regarding painting or coating of a pressure vessel or even a pressure retaining item prior to hydrotesting.

I was forwarded this interesting article that was published in Materials Evaluation, September, 1993. The article is titled " Don't Paint Before Hydrostatic Testing" and is authored by Jon Batey.

To summarize, the author explains how the various construction codes and in-service repair codes like the NBIC are either silent or briefly mention caution on the issue of painting or coating before hydrostatic testing.

Several tests were actually conducted on a 4” Schedule 40 carbon steel pipe that was internally pressurized with specific hold times at 1100 (30 min), 2000 (10 min) and 2400 psig (5 min). The pressure was increased in 100 psig increments. The various coating systems that were evaluated consisted of;

- vinyl finish over inorganic zinc primer
- vinyl finish over vinyl primer
- epoxy finish or epoxy primer

The pipe section was pre-dilled with twelve, small diameter thru holes (0.0135" and 0.020") to simulate weld or fabrication defects.

The results of the pressure tests revealed some interesting results;

- in a test pressure range of 325 psi to 1125 psi, the vinyl coating systems failed. At the onset of failure, the vinyl coating locally disbonded into a blister that ruptured.

- the epoxy system adhered and did not disbond even at 2425 psig pressure.

So, you may want to think twice during fabrication of pressure retaining items to coat after hydrostatic testing to make it easier for your AI.

 

[weldtek]

Good point. Most customer specifications that I've worked with preclude the option of paint before hydrotest, but this is a good reminder for any of us who may from time to time be tempted to take a shortcut on a job which doesn't have the usual plethora of owner issued additional specs.

 

[svanels]

Interesting but I would conclude that vynyl coating is unsuitable for service between 325 - 1125 psi, what about above the 1125 psi range?

If the coating is necessary, describe the correct testing procedure in the IOM.

In the fabrication process this would be no problem, but in plants, hydrotesting is the last step done after repairs. It is un-logical to open the vessel again after all connections are made, and induce the risk of someone leaving tools behind.

Steven van Els
SAvanEls@cq-link.sr

 

[metengr]

The vinyl coating also failed at 1125 psig. Any pressure above 1125 psig is also a failure.