What is a pressure vessel?

A pressure vessel is specifically designed to hold gases or liquids at a pressure significantly different from the ambient pressure. The definition includes those vessels that are built to withstand internal or external pressures of 15 pounds per square inch gauge (psig) or higher. This capability is critical because the structural integrity and safety of pressure vessels are paramount, particularly in industrial settings where hazardous materials or high-energy processes might be involved.

The other options do not capture the essence of what defines a pressure vessel. For instance, the description of a vessel used for chemical reactions at atmospheric pressure does not meet the criteria of being a pressure vessel since it operates without the need for withstanding significant pressure. Similarly, a device for controlling pressure in pipelines focuses on pressure management rather than storing or containing substances. Lastly, a large tank used for storing liquid at ambient pressure clearly falls outside of the pressure vessel category, as it does not involve structural requirements for high-pressure conditions.