Flanges Joints Numbers on Welded Gas Transmission pipeline

Minimizing flanges is important for integrity and cost savings. High pressure flanges are EXPENSIVE and add a lot of weight-both directly and indirectly in structural steel.

However, ASME codes do not allow welding on a pressure vessel after it is pressure tested and stamped unless the weld procedure is that required by the vessel manufacturer, the welder is appropriately certified, the weld is inspected per procedure and/or vessel is re-hydrotested. This is why 'flanged connections' are most common.

If the equipment/vessel requires periodic inspection (what does not?) some means of isolating the equipment/vessel from the process is mandatory. either a pair of 'block and bleed' valves closed, a flange has to be opened and a suitable 'blind' installed, a drop-out pipe spool with a 90 degree bend is removed, or a section of the piping cut out and a positive diverter or cap placed over the end entering the vessel. Just an open gap does not prevent leakage from flowing into the vessel and rendering it hazardous to enter.

My dear Keith E Bowers

A. The thread is about the flanged joints on welded pipelines not vessels.

B. The ASME code allows welding for any vessel (U-stamped or not) after completion its fabrication and testing to proceed any repair, modification and/or alteration, but under the rules of ASME code such as:

1. Certified welder(s) and/or welding operator(s).

2. WPS's and PQR's in accordance with the code.

3. Carrying out all tests and inspection work.

4. Re-hydrostatically tested.

5. Issuing R-stamp for U-stamped vessels.

And I don't find any relation between the prescribed items with the number of flanged connection of a pipeline, unless you have another idea.