Why are boiler tubes thinner-walled than boiler drums if both withstand the same pressure?

Why are boiler tubes thinner-walled than boiler drums if both withstand the same pressure?

• A. They operate at higher temperature
• B. The tubes are much smaller in diameter
• C. They are made of a different alloy
• D. They experience less corrosion

Answer: (B)

The key idea is how pressure creates stress in different-sized parts. When a vessel holds internal pressure, the outward pull on the wall increases with the radius of that wall. For a thin-walled cylinder, the hoop stress is roughly proportional to pressure times diameter divided by wall thickness. So, for the same internal pressure, a smaller-diameter tube needs less wall thickness to keep the stress within what the material can safely handle, while a large-diameter drum must have thicker walls to achieve the same safety margin. Since boiler tubes have a much smaller diameter than the boiler drum, they can be thinner-walled and still withstand the same pressure. Higher temperature, different alloy, or corrosion rate can affect design in other ways, but the primary reason the tubes are thinner is the much smaller diameter reducing the stress they experience.

The key idea is how pressure creates stress in different-sized parts. When a vessel holds internal pressure, the outward pull on the wall increases with the radius of that wall. For a thin-walled cylinder, the hoop stress is roughly proportional to pressure times diameter divided by wall thickness. So, for the same internal pressure, a smaller-diameter tube needs less wall thickness to keep the stress within what the material can safely handle, while a large-diameter drum must have thicker walls to achieve the same safety margin. Since boiler tubes have a much smaller diameter than the boiler drum, they can be thinner-walled and still withstand the same pressure. Higher temperature, different alloy, or corrosion rate can affect design in other ways, but the primary reason the tubes are thinner is the much smaller diameter reducing the stress they experience.