What defines a pressure vessel?

A pressure vessel is primarily defined as a container specifically designed to hold fluids, including gases and liquids, at a pressure significantly greater than the surrounding atmospheric pressure. This definition encompasses both the strength and construction of the vessel, which must be capable of withstanding the internal pressure without failing.

The correct choice emphasizes that a pressure vessel operates at pressures exceeding 103 kPa, which aligns with industry standards that categorize any vessel operating under such conditions as a pressure vessel. This threshold is important because the materials and design techniques used for pressure vessels differ significantly from those used for containers that operate at atmospheric pressure.

The other options are more restrictive or inaccurate. The first option limits the definition to liquids, negating the important role of gases in this context. The third option mentions an ordinary storage tank, which typically operates at or near atmospheric pressure and does not fulfill the design requirements for a pressure vessel. Lastly, the fourth option describes a device used for mixing gases, which is not the purpose of a pressure vessel, as its primary function is to store fluids under pressure rather than to mix them.