Standard drop tubes should allow for at least what inches of clearance from the tank bottom?

The correct choice is based on industry standards that dictate the minimum clearance required for drop tubes installed in underground storage tanks (USTs). Standard drop tubes are designed to ensure that the intake for fuel is placed above the tank bottom to prevent the pickup of sludge, sediment, and water that can accumulate over time. This is critical for maintaining the quality of the stored fuel and ensuring the proper operation of dispensing equipment.

The requirement for at least 3 inches of clearance specifically allows enough space for fluid movement without disturbing the bottom sediments while minimizing the risk of contaminating the fuel supply. Furthermore, having a clearance of up to 9 inches provides an additional safety margin in cases where tanks may experience slumping or settling. Thus, the stated clearance range in the correct answer is aligned with good practices in UST management to prevent corrosion, contamination, and other issues that can arise from sediment disturbance. This standard is essential for compliance with regulations and promoting environmental protection, as any contamination can lead to significant risks and liability.