A Practical Comparison of ASME Sect VIII Div 1, 2 and 3 Designs
Imagine this conversation between a buyer looking for a pressure vessel and salesman trying to supply one……
Buyer – “I need a 130 gallon vessel rated at 7,500 psi”
Salesman – “Great. Now do you want it to be a Div 1, 2 or 3 stamped vessel?”
Buyer – “Don’t know and don’t care. Just quote me the lightest and least expensive thing you can.”
Salesman – Hmmm, OK…..
This conversation happens more frequently than people think. While the various divisions within the pressure vessel code have been around for a long time, there is a still a lot of uncertainty about what they each mean, especially for the end user.
The typical well-meaning engineer will be happy to talk at length about calculations and test requirements, or the fabricator will go on and on about welding and purchasing requirements. The primary thing that the buyers and end-users want to know is:
“What does this all mean to me?”
So from a practical end-user perspective what are the real differences?
The easiest way to flesh this out is to use an example. And for this exercise, we will use the vessel that the fictitious buyer above was looking for: A 130 gallon vessel rated at 7,500 psi.
As a simple basis for this discussion, let’s establish the design as a spherical welded vessel made from SA-516 70. The concepts will apply across just about any layout.
For the table above, the Div. 1 vessel is considered the benchmark as there is no reduction in weight or time. The other division options list relative reductions in wall thickness, associated weight, and fabrication time compared to the Div. 1 vessel. So, from a technical standpoint and general engineering concerns, what this will mean to the buyer/end-user?
Going beyond a Div. 1 vessel will get you a lighter and possibly less expensive vessel due to the labor reduction to produce. At the time of this writing, steel prices are going higher and higher, so the reduction of the wall thickness can equate to a significant savings. Accounting for the reduced labor as welding of the thinner wall takes less time, and it seems like an easy decision. Going up in the divisions will get you a lighter vessel that will take less time, and cost, to fabricate.
But then someone inevitably chimes in about additional testing and engineering costs associated with the higher ASME Divisions. And, don’t forget about needing to have licensed engineers involved in the process. So let’s talk about those items.
U-1 (d) in Div. 1 requires that once the design pressure goes over 3,000 psi, additional work must be done to ensure the integrity of the vessel. For a welded vessel, that can involve RT inspection and Post Weld Heat Treatment. You will also need to perform additional calculations which are often performed per Div. 2 using Div. 1 material strengths. Now this is not universal, but if the designers and manufacturers are doing their due diligence on the technical analysis, you are more than half-way to a Div. 2 vessel as far as engineering & testing expense goes but 1,200 lbs heavier and facing 5+ hours of additional labor cost for no benefit.
“But don’t you need a licensed engineer involved in the design? That has to be expensive?”
Actually, you need two, and that could get expensive, but often the cost is built into the design. If your fabricator does not have engineers on staff then this could be an additional cost. If you choose wisely, and safely, they will have the experienced and qualified engineers needed to handle this work. Typically the same engineers who make Div. 1 vessels can perform Div.-2 work, so you are just paying more for the material and fabrication while not fully using the engineers.
Div.-2 also requires additional material testing beyond the typical Div.-1 requirements to ensure proper integrity. However, the typical savings associated with removing 1,000+ pounds of steel and hours of additional welding usually more than makes up for the additional testing.
Now it’s important we add the caveat that this is not a universal proposition. There could be specifics about your vessel requirements that make going to a higher division cost the same or more than the lower division. Each design is unique and needs to be evaluated to ensure you are getting the best and most economically efficient solution.
The point to remember is that higher divisions can be a viable option to possibly save money and reduce weight in your final vessel design. Although such a change does require experience and specialized knowledge, the resources and additional requirements involved are available and applying them is relatively straightforward.
If you are working with an experienced team of engineers and fabricators, the process will be seamless and simple for the end-user.