What is the required slope for piping returning vapor to underground storage tanks?

• A. 1/4 inch per foot
• B. 1/6 inch per foot
• C. 1/8 inch per foot
• D. 1/2 inch per foot

Answer: (C)

The required slope for piping returning vapor to underground storage tanks must facilitate efficient drainage and prevent liquid buildup in the vapor return line. A slope of 1/8 inch per foot is specified because this angle is sufficient to encourage the flow of any condensate back to the tank, while also ensuring that vapor can travel freely without being obstructed by trapped liquids.

If the slope were to be steeper, such as 1/2 inch per foot, it might create excessive velocity, potentially leading to issues like pressure fluctuations or even damage to the system. Conversely, a shallower slope, such as 1/4 inch per foot or 1/6 inch per foot, might not provide adequate drainage, increasing the risk of liquid accumulation in the vapor return line. Thus, the 1/8 inch per foot slope is optimal for balancing effective vapor return and minimizing the risk of liquid retention.