Threaded fittings are almost found in any process plant, mostly in auxiliary lines. Even if the main process lines are welded (based on service fluid, service air/water (utility),lines are usually threaded), instrument connection, impulse lines are usually threaded. In many instances unions are provided for ease of removal of equipment.
It is upto the designer and the Owner to specify the fluid service category and do the correct material selection and fabrication within the rules and prohibitions under the code.
Many Owners specify their own specification built on the code. It is the experience and incidents that matures them to specify additional rules. Code rules are minimum.
B31.3 also brings the concept of Safeguarding (Appendix G).
Whether we talk of a process plant or home heating lines with natural gas, threaded joints are inevitable. A good design will be to minimize the joints, select appropriate joint type and provide safeguarding as much as possible. On the incident above, the question to ask would be: from where the heat source came? Was the joint was next to a heat source that can potentially ignite the gas. Many times, small lines that are threaded are abused by personnel working in the plant by push and pull. If the line is toxic, hazardous, flammable, the concept of safeguarding need to incorporate in the design.
GDD
Canada