Question 70 3AE01 - Third Assistant Engineer

The Correct Answer is C ### Explanation for Option C (2.5 inches) The minimum nominal pipe size of 2.5 inches is typically mandated by mechanical and fire codes (such as NFPA 31, Standard for the Installation of Oil-Burning Equipment, or jurisdictional amendments to the International Mechanical Code (IMC)) for fuel oil service tank vent lines. This size is required primarily for safety during fuel transfer operations. Fuel oil service tanks (often referred to as day tanks) are frequently filled rapidly using transfer pumps. A 2.5-inch vent pipe ensures: 1. **Adequate Venting Capacity:** It provides sufficient relief area to prevent the tank from becoming over-pressurized due to the incoming volume of fuel and displaced vapors during the fastest possible filling operation. 2. **Surge Protection:** It accommodates pressure surges that occur when high-flow pumps are suddenly shut off, which could otherwise damage the tank if the vent were too small. 3. **Code Mandate:** Many codes specifically require that the vent pipe be equal to or larger than the fill pipe, but set a prescriptive minimum of 2.5 inches for service tanks regardless of the fill line size, especially in commercial and industrial applications (like those supporting emergency generators). ### Explanation for Other Options **A) 1.5 inches:** This size is insufficient for commercial fuel oil service tanks and would create a high risk of over-pressurization and potential tank failure or rupture during typical transfer pump operations. **B) 2 inches:** While 2 inches is often the minimum vent size required for some older or smaller fuel oil *storage* tanks, it is often considered the *minimum fill pipe size* for standard commercial operations. To ensure the vent capacity exceeds the fill capacity (a common requirement), the vent size must be increased to 2.5 inches or greater, making 2 inches insufficient for the mandatory minimum vent size in many modern installations. **D) 3 inches:** Three inches is a safe and acceptable size, but it is larger than the minimum requirement set by regulatory codes. It is typically used for very large tanks or systems with extremely high throughput, but 2.5 inches satisfies the minimum nominal size requirement.