API 650 Upper plate thicker than lower plate

[IFRs]

API 650 11th Edition Section 7.2.3.2: The upper plate at a horizontal butt joint shall have a 4:1 taper when its thickness is more than 3 mm (1/8 in.) greater than the lower plate.


When would the upper shell plate be thicker than the lower plate?


Does API offer guidance when the lower plate is more than 1/4" thicker than the upper plate?

 

[Bonswa]

The only situation i can think of would be a thickened insert plate on the upper course. It's not very common considering your main nozzles are usually on the first course, but it is possible. Check out figure 5-7B. there's a small detail about thickened insert plates requiring the 4:1 taper.

That's the only situation that i can think of.

 

[JStephen]

See 5.6.1.3. This could happen where the design stress of the lower course is higher than the upper course (IE, very large tanks using high-strength lower courses). Without this provision, you'd be stuck using the high strength material most of the way up or be adding a lot of extra weight of plate just to keep the lower courses thicker.

It could also come up where a corrosion allowance is higher for the upper course than the lower course.

 

[IFRs]

Bonswa and JStephen -
I agree 100% with both of your comments. It is very common to taper the edge of a thickened insert when the insert plate is more than 1/8" thicker than the shell, they are almost always installed flush inside.

What do you think should be done when the lower plate is more than 1/4" thicker than the upper plate so when center stacked (the default condition) there is more than a 1/8" change in thickness at the joint? Is this covered anywhere in 650 or 653 (I could not find it).

 

[Bonswa]

I cant seem to find anything on it either. I would think that it would be too much of an offset. Just for reference, The 300ft diameter tanks that i've worked on have shell courses like this:

1st course - 1.105"
2nd course - 0.850"
3rd course - 0.637"
4th course - 0.433"
5th course - 0.375"

The greatest offset being exactly 1/4". This allows, like you said, 1/8" on each side.

 

[IFRs]

I'm working on a tank whose shell thicknesses are 1.50, 1.14, 0.77 and 0.41. I have more than 1/4" difference between courses.

 

[Bonswa]

I would think that as long as you have the 4:1 taper it would be acceptable. Now that i think of it, that tank i mentioned even has a thickened insert plate that's either 1.25" or 1.5". I know they have a massive taper on it too.

 

[rab1944]

Please refer to Section 5.6.1.3(a).

Section 7 is dealing with construction issues including plate alignment.

 

[IFRs]

My lower shell course is more than 1/4" thicker than the upper shell course. My question is: does API give any guidance on this situation, in regards to acceptability or the need to bevel the thicker plate. The answer seems to be "no".

Section 5.6.1.3 does not cover my situation.

Section 7 does not cover my situation either.