Re-rating of Pressure vessels

Peers,

I am not a Pressure Vessel designer. We've engaged a designer to check design calculation of 5 scrubber vessels which needs to be re-rated from DP 2056psig to 2082psig for the purpose of production optimisation. All 5 vessels are identical design apart from following difference. 5th vessel has all the same design conditions apart from 3 nozzles of 4" dia (connected to shell) have different material schedule (and hence different nozzle pipe wall thickness) compared to other 4 vessels. All the other design conditions are identical. Now, the design calculations carried out on 5th vessel results that design pressure limit 2074psig. All other 4 vessels have met 2082psig.

I am not sure how such a minor difference in nozzle pipe sched can cause this difference in DP. Could repad thickness be a factor? What could be a possible solution to get all the 5 vessels for same rating in this case?

Thanks in advance for your comments/suggestions.

You should check what is the limiting factor in your design.
PVElite shows this in (I'm not 100% sure about this so don't shoot me if I'm wrong) the vessel design summary.
It can be the nozzle wall thickness itself, or the nozzle reinforcement.
If it is the nozzle wall thickness, you should use a thicker pipe for the nozzle. If it is the reinforcement, you can add extra reinforcement. You should check the applicable code however to see how you may add that extra reinforcement.

Yes, the nozzle schedule could make the difference, but that is easy to determine in PV Elite;- change the nozzle schedule and run an analysis with the higher DP. Then try to change back the nozzle schedule , add a reo pad and run again the analysis. However, is another story with adding a reo pad for up-rating a vessel...that's the vessel modification!
Is there any corrosion allowance included, have you had a shell thickness monitoring program in place? There are many other factors you have to consider for a re-rate.