Actual vessel thickness greater than nameplate 2

[daily123214]

Throughout my career I have worked with a ton of older vessels, 30s, 40s, and i notice the actual readings on these vessels were higher than what the nameplate and Appendix G forms said.

The nameplate and Appendix G forms don't normally say "nominal" but just "thickness".

So why was it common back then for the actual thickness to be greater than the specified thickness? i'm talking 15, 20% higher.

I don't see that with newer vessels but more metal would be nice wouldn't it?

 

[TGS4]

This might be a corrosion allowance?

 

[r6155]

There may be a difference between the design drawing and the as built drawing.
Do you have them?

Regards

 

[LittleInch]

Difference between min design and what was produced plus nominal thicknesses being different ( higher)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Christine74]

Are you talking about the UT readings of the heads only or the cylinders as well?

If it's just the heads it's possible that the manufacturer listed the required thickness on the Appendix G Data Report instead of the nominal thickness. This could account for the large discrepancy.


-Christine

 

[mk3223]

It seems that the Code of 30s/40s might be different from the current design requirement, and the vessel manufacturing process is improved a lot through the years.

 

[dig1]

LittleInch says what I have seen on older vessels as well (1960s vintage) on the shells and heads. Heads I can understand more, especially in the "spherical" portion of 2:1 elliptical. What I was told by a long retired and now deceased CB&I engineer is that first the plate thickness always ran heavier compared when we had talked about this (late 1990s) and it was not uncommon to put the minimum on the Data Report but the plate was sometimes (I'm not saying often) order to nominal; so if the required thickness was 1.412" then sometimes 1-1/2" was used in the shell course then add a plus on the slight mill excess on the plate. I've never seen 15%-20% higher but more like 8%-10%. I have seen shop drawings from the 60s w/ tmin, tnom, and an MAWP on the shell drawing. These are moderate sized vessels (14'-15' diameter an 150' tall).

 

[daily123214]

@TGS4: Using corrosion allowance is included in the nameplate thickness at least in my experience.

@r6155: I don't sadly

@LittleInch: When you say min design you mean T-min correct?

@Christine74: Both the shell and heads.

@mk3223: perhaps. also i notice in the older code books the phrase "nominal thickness" is not used.

@dig1: that makes sense. I also notice for many of these vessels the T-min that i calculate happens to be equal to the thickness listed on the nameplate or at least very close.

 

[GD2]

Did the manufacturer get the plates at T[sub]min[sub][/sub][/sub] or at the thickness commercially available in 30s and 40s? And what was the thickness they were required to mark or report in Appendix G -, would be the question?

GDD
Canada

 

[r6155]

The head manufacturer uses a greater than required thickness to compensate for the reduction in thickness during the forming method.

Regards

 

[Geoff13]

When I started (early 80's) the mills in Canada had a base price for fractional thicknesses (3/8", 1 1/4", etc), and charged extra for a decimal thicknesses (0.481", etc). I don't recall the precise thicknesses that were base price. I think it started at 1/16" increments (i.e. 1/4", 5/16", 3/8"), then 1/8" increments (i.e. 3/4", 7/8", 1") and finally 1/4" increments.

Thus we would sometimes round up the thickness to save costs. The nameplate would still list the Code thickness for future inspection comparison with the required thickness.

 

[dig1]

Geoff13,

Thank you; this makes sense to what I have seen on older drawings and ties into with what I have been told before.

 

[daily123214]

@geoff13

That makes a lot of sense thank you!

 

[Christine74]

If we're talking about cylinders being significantly thicker than nominal, my best guess is that plate manufacturing tolerances weren't as tight in the 1930s/40s so they produced plate that was thicker on average so that the thin spots still met T[sub]min[/sub]. But I don't have any documentation of what tolerances were required for plate back then.


-Christine

 

[r6155]

Some manufacturers sometimes use a higher thickness based on stock and save additional time to make an as-built drawing. It depends on the customer's requirements.

Regards