A C-27 contractor is installing a new irrigation system for a sports field. The design specifies that the mainline should be sized to maintain a maximum velocity of 5 feet per second (fps) to minimize water hammer and friction loss. If the required flow rate for the entire system is 150 gallons per minute (GPM), approximately what minimum nominal pipe size (IPS) should be used for the PVC mainline?
- A2-inch
- B3-inch
- C6-inch
- D4-inch
Show answer & explanationAnswer & explanation
Correct answer: D. 4-inch
To find the minimum pipe size, we use the formula: Area (sq ft) = GPM / (Velocity (fps) * 448.8). First, Area = 150 GPM / (5 fps * 448.8) = 150 / 2244 = 0.0668 sq ft. Then, use Area = π * (Diameter/2)^2 to find diameter. Diameter = 2 * sqrt(Area/π) = 2 * sqrt(0.0668/3.14159) = 2 * sqrt(0.02126) = 2 * 0.1458 = 0.2916 feet. Convert to inches: 0.2916 ft * 12 in/ft = 3.499 inches. Therefore, a 4-inch pipe (with an actual internal diameter typically around 3.8-4.0 inches) is the closest and smallest standard size that satisfies this requirement.
Why the other options are wrong
- A. 2-inch pipe is too small, leading to velocities well over 5 fps and increased friction loss.
- B. 3-inch pipe is likely too small, resulting in velocities slightly above 5 fps.
- C. 6-inch pipe is oversized for this flow and velocity, leading to unnecessary material cost.
Pipe Sizing for Velocity
Selecting the appropriate pipe diameter for an irrigation system to maintain water velocity below a specified maximum, preventing issues like water hammer and excessive friction loss.
- Max velocity for mains is often 5 fps to prevent water hammer.
- Velocity is inversely proportional to pipe cross-sectional area.
- Larger pipes reduce velocity for a given flow rate.
Memory trick: Flow, velocity, area, diameter – that's the pipe size chapter.