Div-2 VS Div-1--- Which one is more reasonable? 1

[Paulettea]

Dear All

I have a basic question regarding design as per ASME BPVC VIII.

I want to know that if VIII-2 allows higher allowable values for stresses in design then why don't we just fabricate all pressure equipment as per Div-2 and forget about Div-1?

Suppose that I am in the position of a user who is supposed to pay all the money to construct a plant. I have heard that using Div-2 makes the thickness lower and therefore can save lots of money. Why should anybody advise me not to go for Div-2.

Is it all about safety?

I think there must be some good points about using Div-2 and some bad points so that there will be some justification to use Div-1.

And from a manufacturer point of view what is the reason that the manufacturers do not offer Div-2 as the design code. Is it harder for the manufacturer to follow Div-2 rules?

Warm Regards

 

[XL83NL]

Im sure others may provide a more valuable comment from their point of view, based on more experience with the codes, but; consider the (possible) material savings on material due to thinner walls, versus increased engineering cost since the engineering required
a) is more time consuming
b) requires more experienced engineers with a profound understanding of Div 2, hence a more expensive engineering assignment to pay.

I can do a div 1 calc on the back of a napkin, find out an 8" 40S pipe suffices for my shell, as a standard stock size has sufficient over capacity. For such situations, I wouldn't want to pay for div 2 engineering assignment (which is more expensive) which would point me out I need 8" 40S for my shell. There's no advantage for taking div 2 over div 1 in such cases. Of course, this will apply only for a certain scope, e.g. small sizes vessels made from reeadily available standard (stock) sizes. That may be the reading for the continuous update cycle of div 1.

Another thing is, which I see as a benefit for ASME in general, that applying design codes for pressure equipment on the EU market, ASME has the best of both worlds. Sure, you can go for a thinner design using AD2000, but that code isnt (sufficiently) updated anymore. EN13445 also offers some advantages (e.g. thinner walls required), and also covers DBA (sort of Div 2 approach), but it currently has too much flaws, imo. The code doesnt (yet) have sufficient practice to be readily applied. Other (EU) codes arent either updated anymore, are too difficult (e.g. CODAP), or out of my league PD5500).
This means that for most instances I direct all our suppliers into using ASME, even when the equipment needs CE-marking. This is the most easy.

Having said that, I believe that (either here on eng-tips in one of TGS4's comment, or via some ASME document), Ive heard the 2 will eventually merge as FEA design are becoming more n more standard these days. I wouldnt be surprised that at some point we no longer have div 1 DBR, and only have div 2 DBR & DBA.

 

[Paulettea]

XL83NL, Thank you very much for your reply. What I cannot fully understand is that how engineering cost in Div-1 and Div-2 are different. If I am supposed to do DBR, I will go for software like PV Elite or Compress no matter what Division I am supposed to use. These software cover the code. If by any chance I am forced to perform some calculations that is not covered by Div-1,it does not mean that I am not asked to perform those detailed calculations. In fact, in many cases if the detail of design is not provided by Div-1 we have to follow another acceptable code or Div-2. So I think designing as per Div-1 does not reduce engineering task.

 

[marty007]

I am also not overly familiar with Div.2, but here are a few items that I do know about...

Div 2 requires a few additional steps that are otherwise not needed for Div.1. Two documents are required, the "User's Design Specification" and the "Manufacturer's Design Report". These must be certified by engineers with specific training and experience.

For a Div.1 vessel, the purchaser could sketch up a criteria drawing on the back of a napkin if they're in a rush...

A lot of these types of differences however have the potential to change in the coming years. Div.2 for 2017 has introduced a new Class system to the code, that could eliminate some of this additional work. Class 2 vessels would be like the current editions of Div.2, whereas the new Class 1 eliminates much of these additional documentation requirements (except when fatigue or Part 5 are used). The material savings of a Div.2 vessel however are reduced, as the allowable stress values for a Class 1 vessel will fall somewhere between the current Div.1 and current Div.2 values.

I'm quite interested to see if user's will start asking for ASME VIII-2 Class 1 as the default, and ASME VIII-2 Class 2 when the pressure increases enough to justify the additional documentation work.

 

[TGS4]

Excellent question Paulettea. And some excellent responses so far by both XL83NL and marty007.

What I would like to add would be a follow-on to what marty007 started. The Class 1 and Class 2 designations in the 2017 Edition of Division 2 are intended to close the gap so to speak on the administrative costs associated with Division 2.

Most Division 2 vessel that I have worked with or certified the UDS/MDR for have been strictly DBR, with no DBA. The choice of selecting Division 2 was purely economic, and the administrative burden was kept to a minimum. But requiring a PE/P.Eng./other to certify a UDS and MDR is additional cost and effort, for starters. For many smaller vessels (as XL83NL indicated) that additional cost is not made up for in the material and welding savings for the thinner vessel - but it can be made up multiple times over for larger vessels or multiple vessels.

I am going to be very strongly advocating that owners start requesting Division 2, Class 1 as their standard Code of Construction, with Division 2 Class 2 being used in special circumstances. That can only be enhanced when manufacturers start offering this as a (preferred) option when quoting supply.

The biggest variable in such a brave new world is the regulatory agencies. Right now, performing modifications (repairs, re-rates, etc) on Division 1 vessels is very straight-forward as far as regulatory paperwork and oversight goes. But there is a substantially higher burden for Division 2 vessels. For many of the owners that are my clients, that regulatory burden actually is their primary factor in deciding what the cut-over point is for Division 1 vs Division 2. That's where I am personally focusing my effort for the time being, in order to affect the change that I would like to see.

 

[SnTMan]

I'm going to agree with much of what XL83NL & marty007 have to say. Not familiar enough with Div 2 to know really, but:

My class of work is exclusively Div. 1. Good luck getting a UDS, half the time our clients can't even do the " back of a napkin" decently. Minimum thicknesses are generally governed by TEMA rules, no cost savings there. Because of this we hardly realized any savings from the design margin going to 3.5 from 4.

I really like a DBR approach. We can all do the same problem and get the same result, unlike DBA, of which thread794-432237 is but one example of many. Hard enough to get Div 1 work through a client review as it is. Never mind cyclic loading. Add to that, it has gotten to where the time performing the Div 1 part of the design is almost negligible. The nozzle loading, wind & seismic, Zick analysis, unit lift lugs, etc, etc. seem to dominate.

Whatever happens, something like Div 1 must be available, IMO.

Starting to think retirement is looking good

Regards,

Mike




The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[XL83NL]

Class 1 & 2, thats exactly what I was referring to. Is there any foresight when div 1 will be annuled?

 

[JStephen]

"I have heard that using Div-2 makes the thickness lower and therefore can save lots of money. Why should anybody advise me not to go for Div-2."

Two thoughts here- first off "can save" is not the same as "will save". Secondly, if you don't have a strong preference to go with one or the other, it would seem logical to request vendors to quote whichever is cheaper, rather than trying to force them to do one or the other in hopes that it is cheaper.