When placing a shell-and-tube heat exchanger with a removable tube bundle such as a bayonet-tube heavy fuel oil service heater into service, to avoid thermal shock what statement represents the correct operating procedure?

Correct Answer: Option D — Introduce the heavy fuel oil first, then gradually introduce the steam until full flow is established to avoid thermal shock.

Introducing fuel first ensures the tube bundle is submerged in the process fluid before steam is applied. Gradually admitting steam prevents localized overheating and excessive thermal stress on the tube sheet and joints. This procedure is a standard operational safety practice to prevent mechanical failure of heat exchangers.