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Flat Heads-ASME Boiler Code Question

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I was looking at a drawing for an existing  pressure vessel a Client had the 
other day.  I was looking at it in regards to the support frame and 
foundation.  I noticed the drawing said it was a 10 psig design pressure with 
a 1 psig operating pressure.  I am not an expert in the boiler code, but I 
did see that the vessel is 5' in diameter with a 3/16" thick flat top.  The 
top does not have any stiffener and was made as a circular flat head that had 
the knuckle turned down (1-piece head) .  It is my understanding that a flat 
head with a turned knuckle (bend top with a radius) is only allowed on tanks 
24" and narrower.  I checked the formula for the thickness and arrived at a 
5' diameter would have to be over 3/4" thick.  The tank was supposed to meet 
the ASME boiler code Division VIII.

I looked in the Pressure Vessel Handbook by Megyesy saw the same rules I had 
learned thru osmosis.  The head shown in the book as "A" has limits that this 
existing tank far exceeds.  The t/d is .003 which is much less than the .05 
limit placed on this type of head.

Does anyone know if you can have a flat head (one-piece) greater than 24" in 
diameter?   If not, what are reasonable methods of retrofit?.  The tank is 5' 
OD and about 5' tall with a cone bottom.  The Client has already confirmed 
that the 10 psig shown on the drawing is incorrect.  

Ron Martin
Martin Structural Services, Inc.