UG-44 (b) 2

Pressure vessel designers still have to design the nozzle to shell junction for external piping loads.

mechengineer,

I'm coming to this (and your previous thread) late as I don't swing through this particular sub forum very often. However, you have some amazingly misguided views about pressure vessel and piping system design.

As a piping engineer, we have our vessel engineers include "standard load tables" in the purchase orders for pressure vessel vendors to consider in their quotations and design work. These purchase orders are generally started long before I as a piping engineer am even on a project, much less have any idea what the attached piping system will look like. Years of experience have taught EPC firms that not including such load tables in the PO means that the quotations will be based on zero applied loads and even submitting loads later will result in additional charges by the vendor for the analysis and change orders if they upgrade the flange class beyond what was selected for a pressure only design.

The notion that the vessel vendor doesn't need to consider those loads for a flange evaluation because they only apply to the nozzle/shell junction is deeply wrong. A typical nozzle projection is on the order of 12" or so. The only difference between the loads applied at the junction and the flange face will be some changes to the moments do to the slightly different length of the moment arm. That flanged joint has to be capable of transmitting the piping loads to the shell interface.

The notion with regard to piping loads that "...it is rare case that exert an axial tensile force and moment to flange joint (make the flange or piping as a structure/equipment support) in addition to the piping system with internal pressure, thermal load and boundary point constraints." is so laughably untrue that I find it difficult to take anything you have said seriously. Equipment flanges, especially for pressure vessels, are often a primary support/anchor point for a connected piping system. In many cases, there is no feasible adjacent structure to serve this role or it would be a major cost adder to design, fabricate, and erect a whole separate structure to relieve/reduce loads on the vessel connection.

Engineering a project is a schedule critical activity, one that has only become ever more aggressive since I started in this business back in the 90's. I don't have time (or budget) to evaluate every single vessel nozzle on the job and then coordinate with a vessel engineer on a piece of equipment that is deep into its own design to try and get selected flanges upgraded to a higher class with all the paperwork tracking, cost adders, and delivery delays that can result. We give those vendors a standard table so they know the rules when they start and I know those same rules (just as I do when evaluating piping connected to all kinds of other equipment that has nozzle allowable loads built into it's specification (i.e. API-610, 617, 618, 661, etc) and can evaluate and modify the piping system design to meet them.

Then, in the rare case when I can't get a nozzle load on a critical piece of equipment to pass because the combination of temperature/expansion and layout constraints making getting all six forces and moments under the allowables impossible, I can then devote the extra scrutiny to those and work out a solution with the equipment vendor. Often times, we can avoid changes because one component may be 120% of the allowables while the others are all 80% or less and the combination is acceptable.

In the grand scheme, typical pressure vessels are just chunks of metal and wide spots in the road that a fluid travels through in a refinery or chemical processing plant. They don't have the critical tolerances of high speed rotating equipment that are heavily sensitive to piping loads and to which stress engineers like myself have to devote most of our time on a project.

Your thinking about this topic is ultimately penny wise, pound foolish. Yes, the cost of a pressure vessel is going to be a little higher if it gets built with CL300 flanges instead of CL150 that might have been passable. However, compared to the cost savings in schedule, both for the critical path design/approval/fabrication/shipping/installation of those vessels, as well as the EPC able to focus precious hours and budget on details for more critical equipment and piping systems, a few extra tons of metal in flanges is just noise. The ultimately goal is to get the project online as quickly as it can safely be put together so our client can convert it from a money pit and into a money printer.

That said, I do think the UG-44 approach is still excessively conservative, even if it is a big improvement over the ridiculous "Equivalent Pressure" method. But, to suggest that UG-44 should be ignored by vessel companies in favor of just having piping engineers evaluate each flange connection separately is simply a non-solution.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.