Final answer:
Hot tapping is indeed permitted on pressure vessels under specific conditions, with safety being the primary concern. The process should follow rigorous risk assessments and industry-specific standards to ensure the structural integrity and safety of the operation.
Step-by-step explanation:
The question of whether hot tapping is permitted on pressure vessels primarily pertains to safety and industry standards. In general, hot tapping, which is the process of making a connection to existing piping or pressure vessels without emptying them first, is permitted under specific conditions. This is because the process involves a significant risk, especially when dealing with substances that are under pressure. To ensure safety, particular standards and best practices must be followed, which usually include ensuring the structural integrity of the vessel, strict control of the process, and rigorous risk assessments.
Similar principles of expansion and contraction due to temperature changes, as illustrated in daily experiences, apply to this industrial process. For example, when handling a metal peg and block, the peg should be cooler than the block for easier insertion because metal expands when heated. Similarly, running hot water over a tight metal lid causes the lid to expand slightly, making it easier to open. These examples rely on the principle of thermal expansion, which is also a critical consideration in hot tapping, as the materials involved will expand or contract based on their temperatures during the process.
Lastly, an understanding of thermal expansion underpins why a cold alcohol thermometer initially dips before rising in hot liquid, due to the momentary contraction of alcohol before it begins to expand with rising temperatures.