What minimum pipe size is needed to carry 5 GPM at a velocity of 5 FPS or below for a 100-foot run?

What minimum pipe size is needed to carry 5 GPM at a velocity of 5 FPS or below for a 100-foot run?

• A. 1-1/4 inch
• B. 1/2 inch
• C. 1 inch
• D. 3/4 inch

Answer: (B)

To carry a given flow at or below a target velocity, you must ensure the pipe’s cross‑sectional area is large enough for that flow at the desired velocity. Start by converting the flow to consistent units: 5 gallons per minute equals about 0.01114 cubic feet per second. With a velocity limit of 5 feet per second, the required area is A ≥ Q/V = 0.01114 / 5 ≈ 0.00223 square feet. For a circular pipe, Area = πd^2/4, so the minimum diameter is d = sqrt(4A/π) ≈ sqrt(4×0.00223/π) ≈ 0.0533 feet ≈ 0.64 inches. That means the pipe must be at least about 0.64 inches in diameter to meet the velocity requirement. The next standard size up is 3/4 inch (0.75 in), which would satisfy the condition. A 1/2 inch pipe would yield a velocity around 8 ft/s with the same flow, exceeding the 5 ft/s limit. The 100-foot run doesn’t change this velocity requirement, though head loss could matter in a full design.

To carry a given flow at or below a target velocity, you must ensure the pipe’s cross‑sectional area is large enough for that flow at the desired velocity. Start by converting the flow to consistent units: 5 gallons per minute equals about 0.01114 cubic feet per second.

With a velocity limit of 5 feet per second, the required area is A ≥ Q/V = 0.01114 / 5 ≈ 0.00223 square feet.

For a circular pipe, Area = πd^2/4, so the minimum diameter is d = sqrt(4A/π) ≈ sqrt(4×0.00223/π) ≈ 0.0533 feet ≈ 0.64 inches.

That means the pipe must be at least about 0.64 inches in diameter to meet the velocity requirement. The next standard size up is 3/4 inch (0.75 in), which would satisfy the condition. A 1/2 inch pipe would yield a velocity around 8 ft/s with the same flow, exceeding the 5 ft/s limit. The 100-foot run doesn’t change this velocity requirement, though head loss could matter in a full design.