In which situation is copper tubing not suitable for steam service?

Copper tubing is not suitable for steam service when the temperature exceeds 406°F, as at this high temperature, copper can lose its strength and structural integrity. Under extreme thermal conditions, the mechanical properties of copper may degrade, leading to potential failures such as deformation or bursting. The pressure rating at which copper tubing can safely operate is closely tied to temperature; although it can handle certain pressures, the combination of excessive temperature and pressure can surpass the material's limits.

In steam applications, maintaining the integrity of the tubing is crucial to ensure safe and reliable operation. The maximum allowable temperature for copper tubing in steam applications is typically around 406°F, beyond which the risk of failure significantly increases due to the material's limitations. This understanding is vital for proper boiler operation and safety.

For the other situations presented, while they might represent conditions that could warrant caution, they do not inherently disqualify copper tubing for steam service as definitively as the temperature criterion does.