pin holes in boundary welds on tanks

By welder_guy2001 (***)

Date 01-14-2003 03:12

post this question in the "Technical" section of this BBS...you might get some responses from some weld inspectors there.

By jon20013 (*****)

Date 01-14-2003 14:05

Don; I'm not familiar with the MIL-STD you are referring to but I've got over 30 years welding and weld inspection experience. The porosity sizes do not sound abnormal for visual acceptance under most of the standards I am familiar with however, I am curious, are there no NDE requirements for these welds other than visual? Also, in reading your description of the weld areas, it would seem that there would be hydrostatic testing or some other form of pressure testing to assure leak tightness. Again, I'm not familiar with the standard you're discussing so my comments are only based on my experiences in other arenas.

By DONK

Date 01-18-2003 14:05

JON,THE TANKS ARE TESTED AT 2 PSI HOLDING TEST FOR 10 MINUTES. THE BOUNDARY WELDS ARE NOT CHECKED USING A SOAP AND AIR TEST.THE CONTRACTOR IS SUPPOSED TO DO THIS BEFORE WE ARE CALLED TO WITNESS THE 2 PSI HOLDING TEST.HOWEVER THESE TANKS ARE ON A DESTROYER AND THEY ARE BIG TANKS.SO WHEN YOU TEST THEM AT 2 PSI SMALL PIN HOLE LEAKS COULD GO UNDETECTED.I KNOW THAT ON CONTRACTS INVOKING ABS STDS. USUALLY A TANK IS TESTED USING SOAP AND AIR AT ALL BOUNDARY WELDS OR THE TANK IS FILLED WITH WATER[HYDRO TEST].THERE HAVE BEEN AT LEAST ONE INSTANCE WHERE A SHIP THAT LEFT HERE HAD LEAKS ON FUEL TANKS AND THEY HAD TO BE CORRECTED. DON KUMPUNEN

By dasimonds (**)

Date 01-18-2003 20:08

I used to work at a shipyard where all our work was for the U.S. Navy, destroyers and cruisers. As I remember, ALL air, water, and oil tight welds were tested for leaks using a vacuum pump, soapy water, and a plexiglass box. This ensured the welds were tight, and didn't require pressurizing the whole tank.
As I recall, most problems occured at starts and stops, and in corners, in which people started or stopped in the corner.

Hope this helps.
Dale Simonds

By - Date 01-20-2003 04:41

I have worked the last ten years on construction of floating offshore platforms (TLPs, semi-subs, FPSOs, etc). These are constructed in accordance with a myriad of codes and US Federal regulations, including various ABS rules which require "tank testing" (looking for leaks at tank boundaries including welds and bolted connections such as manways). The test method you are referring to is commonly called vacuum box, and I have found that all things considered it is the most cost effective way to test welds before assemblies go to paint on their way to being erected and becoming tanks (ballast, void spaces, equipment spaces, fluid storage tanks). For testing of bolted connections at completed tank boundaries, we pressurize the entire tank, as mentioned in the previous posting, to 2-5 psi and then soap the joints. Also very effective.
I disagree with the statement, however, that when pressurizing the entire tank to 2 psi leaks at boundary welds would go undetected. 2 psi in such a large volume is a substantial amount of stored energy. If there is a through wall leak you will find it, provided of course that the surface is properly prepared (i.e. clean and unpainted) and examined with a bubble forming solution. As I inferred above, however, this is not a cost effective way to test boundary welds as the welds must remain unpainted. Normal practice at most large shipyards is to paint assemblies before they are taken to erection, therefore it is most convenient to test welds before paint.
Mankenberg

By DONK

Date 01-21-2003 14:11

KIPMAK,WHAT I AM SAYING IS THE TANK IS PRESSED UP TO 2 PSI AND HOLD FOR 10 MINUTES.THE TANK BOUNDARY WELDS ARE NOT CHECKED WITH SOAP AND AIR.DO YOU AGREE WITH THIS? DON KUMPUNEN

By - Date 01-29-2003 08:22

In this case, yes. If the boundary welds are never examined with a bubble forming solution, then I agree that small leaks could exist. Most likely with that volume of air you would also not notice a drop in pressure during the duration of the test.
As I mentioned in my previous post, according to the ABS Rules, which apply to the structures that I have been involved with as well as many types of commercial ships, boundary welds must be tested as well. It has been many years since I have worked with NAVSEA or NAVSHIPS specs (and I wasn't involved in tank testing when I did work with them), but I've got to presume that the extent of testing is spelled out in there somewhere. So if the boundary welds are not required to be tested according to the applicable spec, maybe that's okay.
Mankenberg