Which of the following is NOT considered a pressure vessel?

A vertical cone-roof tank is designed to store liquids at ambient pressures and is not typically classified as a pressure vessel. Pressure vessels are containers designed to hold gases or liquids at pressures significantly different from ambient pressure, which often involves a design that accommodates high pressure conditions.

In contrast, a horizontal tank, a spherical tank, and a steel cylinder can all qualify as pressure vessels, depending on their design pressures and intended use. Spherical tanks, for instance, are often used for storing gases under pressure due to their ability to withstand high internal pressure. Similarly, steel cylinders can be designed to handle various pressure levels, making them suitable for different materials under pressure.

Therefore, the vertical cone-roof tank stands out as it is not used to contain materials at pressurized conditions, reinforcing its classification outside of pressure vessels.