Minimum weld spacing requirement

[Syahar1975]

Good afternoon all,

API 650 figure 5-6 states the fillet weld spacing requirement for shell nozzles.

Column C in this table state 75 mm minimum distance from the edge of pad plate (toe weld) to the corner weld (toe weld).

Is there any technical problem if we are not following this requirement ? Let say the actual distance is only 30 mm.

Many thanks for your respond.

 

[IFRs]

Well, you are not in compliance with the code, you should have used a tombstone shaped repads. There are good technical reasons for weld spacing.

 

[JStephen]

The problem you run into is that a rule-of-thumb gets converted into a law. So no, the tank probably won't fall down because of that. And yes, it should have been a tombstone repad. Whether it is actually better to leave well enough alone at this point or to start cutting things apart and re-welding in the proper configuration is a different issue.

Note that in Appendix A, some of these weld clearances are reduced dramatically, and in API-12D, they aren't required in the first place, although it may be the same materials, same weld, and at the same stress level.

 

[Syahar1975]

Good morning Gents,

Thanks for the comments.

Yes we did use low type RF pad. Tombstone = Low type ?

The problem was there was fabrication error. So there is about 30 mm gap from the bottom of this low type RF pad to the corner weld.

The tank had been hydrotested. I do not know why this issue was not caught during fit up inspection nor visual inspection nor release for hydrotesting.

So far we do not see any un-usual appearance on this area.

 

[IFRs]

You have two square corners on the bottom of the repad, and it is 30mm above the bottom plate? If this is a new tank, I would not accept it and I would look for other non-code items. the hydrotest is god but was it at the low design metal temperature? Time for more information: Shell thickness and grade, nozzle size, repad thickness, nozzle neck material, tank location or lowest one-day mean temperature, product density, corrosion allowance. Is the nozzle-to-shell weld full penetration? It might be time for a lower half-pad modification.

 

[Syahar1975]

Good morning IFRS,

It is impossible to modify this RF pad at this stage.

Yes, it is 30 mm above the annular plate.

Hydrotest at ambient temperature.

Shell and RF pad A516-60 9 mm thickness. Neck is A53B pipe. Full penetration weld.

CA for shell 3 mm.

Visually, there is no something strange in this area.

 

[IFRs]

Sigh. I suppose worse things have been done to steel tanks, and they remain standing! Still, it is a new tank and it would be nice for it to be to code. Can you verify that there is adequate reinforcement below the nozzle? Could you grind or gouge the sharp corners to have at least some radius? A516-60 is a nice tough steel, the thickness is quite thin and the risk of brittle fracture is probably low. Based on the thickness, this is not a large tank so that is good. What is the max and min temperature the tank will see environmentally and operationally?

 

[Syahar1975]

Good morning IFRs,

Thanks for your comments.

At present nothing we can do. No one will allow me to do such modification.

The maximum operating temperature is 60 deg C.

In this area the minimum ambient temperature during winter is about 2 to 5 C. deg.

The tank diameter is 18 M. Dome roof tank.