A sewage ejector will discharge through a 2-inch pipe. Take these values as given: the discharge piping must maintain a velocity of at least 2 feet per second, the inside cross-sectional area of the 2-inch pipe is 0.0233 square foot, and 1 cubic foot per second equals 448.8 gallons per minute. What is the smallest pump capacity that satisfies the velocity requirement?
- A10.5 gallons per minute
That result comes from converting the area straight to gallons per minute without multiplying by velocity. Area alone is not a flow.
- 20.9 gallons per minute
- C31.4 gallons per minute
That result uses 3 feet per second. Some references quote higher scouring velocities for other applications, but the value to use is the one the problem supplies.
- D41.8 gallons per minute
That result uses 4 feet per second, doubling the required velocity. Oversizing this way produces a pump that short cycles and hammers the check valve.
Why B is correct
Velocity is what keeps solids suspended in a pressurized discharge line. Multiplying the pipe's area by the required velocity gives the flow in cubic feet per second, and the conversion factor turns that into the gallons per minute a pump is rated in.
What this question is testing
Whether the candidate can convert between velocity, pipe area and pump capacity, keeping cubic feet per second and gallons per minute straight.
On the job
Ejector selection usually starts with the head the pump must overcome and stops there, which is how undersized discharge flows happen. Running the velocity check tells you whether the pipe and pump are a matched pair, and it explains the classic complaint of a two-year-old ejector line that plugs solid: a big pipe with a small pump moves water slowly enough for everything else to settle out. Bigger discharge pipe is not automatically better.
Memory technique
Area times velocity, then four hundred forty-nine to get gallons per minute.
Exam tip
Flow equals area times velocity. Get to cubic feet per second first, then convert once at the end.
Where to look it up
IPC Chapter 7, the sumps and ejectors provisions covering discharge piping. The UPC states its own minimum velocity and pump capacity requirements for sewage ejectors, so verify in your adopted code.