Lap Welded for Bottom and Annular Plates

[hamidun]

Dear All,

As per API 650 ed 12 2016, para 5.1.3.5 Lap-welded joints, as tack-welded, shall be lapped as follows: - Single-welded joints shall be lapped at least five times the nominal thickness of the thinner plate joined, or 25 mm (1 in.), whichever is smaller.

From the statement above, is it right if we lap:
1. bottom plate to bottom plate by minimum 25 mm?
2. bottom plate to annular bottom plate by minimum 25 mm?

Help me with this, as I could not find a specific statement that rules the lap between bottom to bottom plate or bottom to annular plate.

Thanks for the help!!!

 

[IFRs]

There is no specific statement for bottom-to-bottom and bottom-to-annular laps, they are covered in 5.1.3.5. Yes you can have a 1" minimum lap on bottom-to-bottom and bottom-to-annular joints.

 

[hamidun]

Thanks IFRs!!!

However, in many storage tanks drawing I have seen, the lap between bottom to annular is normally higher than 25 mm... I understand the minimum lap is 25 mm, therefore lap higher than 25 mm is not a problem.

But what becomes my concern is why the designer would lap the bottom to annular with higher lap in which it will make the use of material is more than if we use 25 mm lap welded joint.

Is there any calculation for to determine the lap between bottom to annular plate?

 

[IFRs]

There is no calculation for lap width other than what you have found. The designer may have used 2" as the design lap, and the field crew used 1" on the inside laps to hedge against running our of plate. Sometimes plate comes into the shop on the plus side of the length tolerance. It is so much worse to run out of plate! The field crew could also have gone on the light side of the extension outside the tank shell. I always design for 2" to accommodate camber in the bottom plates, tanks being out of round, field crews being slightly sloppy, etc. I would never design for the minimum lap.

 

[JStephen]

In my experience- laps exceeding the minimum required dimension are nearly always to allow for field variation in plate placement or for shop tolerance in cutting (or both).

 

[hamidun]

IFRs, JStephen,

What about the code/standard? Is there any reference covers about this (AWS, ASME)?

I think I need the code/standard to argue to the client as I have limited experience about this.

 

[IFRs]

If these are API 650 tanks, you have the applicable code - API 650. I see no value in dragging in another standard and trying to apply it to this situation. In API 650, the maximum bottom lap is not defined - only a minimum lap is specified. You may be making to much of this - API 650 does not care if the lap is more than the minimum 1" and nor should you, the owner or their inspectors. In my opinion, there is nothing more to add and no more time is needed. Put it to bed and move on!

 

[sajk14]

Typically i allow more than the minimum lap required by the codes and standards to take into account weld shrinkage and cutting/burning tolerances of the bottom plates on site.

The European tank standard 'EN 14015:2004' does actually specify a minimum lap of 60mm for the bottom plate to annular. Laps between 2 bottom plates remian the same as API 650 i.e. 5 x the plate thickness.

However as IFRs stated if your tank is in accordance with API 650 then a lap of 1" ( 25mm ) should suffice as it complies with the minimum requirement of that standard.

 

[hamidun]

IFRs,

Well, I guess I should move on, thanks IFRs!

sajk14,

Thanks for the reference, that really helps!!!