IA Pumping Systems & Pump Selection 2 — Questions and Answers
Question 1: A pump operating at a higher flow rate than its BEP is said to be running:
- In shutoff condition
- On the right side of the curve (toward runout) (Correct answer)
- At maximum efficiency
- In the cavitation zone only
Correct answer: On the right side of the curve (toward runout)
Running to the right of the BEP (runout condition) results in reduced efficiency, increased NPSH requirements, and potential motor overload.
Question 2: What is the purpose of a pressure relief valve on an irrigation pump discharge?
- To increase system pressure
- To protect the system from over-pressure caused by water hammer or valve closure (Correct answer)
- To regulate flow to each zone
- To allow air to escape from the system
Correct answer: To protect the system from over-pressure caused by water hammer or valve closure
A pressure relief valve opens automatically when system pressure exceeds a set point, protecting pipes, fittings, and emitters from damage.
Question 3: In a submersible well pump system, what determines the maximum practical depth for a single-stage centrifugal pump?
- Motor winding resistance
- The maximum NPSHa available
- The pump's maximum shutoff head (Correct answer)
- Pipe material friction factor
Correct answer: The pump's maximum shutoff head
A single-stage centrifugal pump can only lift water up to its shutoff head; for deeper wells, multi-stage submersible pumps are required.
Question 4: The affinity laws for centrifugal pumps state that when pump speed doubles, flow rate:
- Stays the same
- Doubles (Correct answer)
- Quadruples
- Is halved
Correct answer: Doubles
By the affinity laws, flow is directly proportional to speed: doubling RPM doubles GPM.
Question 5: A variable frequency drive (VFD) on an irrigation pump motor is used primarily to:
- Increase motor starting torque
- Adjust pump speed to match system demand and save energy (Correct answer)
- Step up voltage to the motor
- Filter electrical noise from the control system
Correct answer: Adjust pump speed to match system demand and save energy
A VFD modulates pump speed to maintain a target pressure or flow, reducing energy consumption significantly compared to throttling or cycling.
Question 6: Water hammer in irrigation pipelines is best mitigated by:
- Increasing pipe diameter at the pump
- Using slow-closing valves and/or pressure relief valves (Correct answer)
- Reducing operating pressure at the controller
- Installing check valves on each head
Correct answer: Using slow-closing valves and/or pressure relief valves
Slow-closing valves reduce the rate of velocity change that causes pressure transients; pressure relief or surge relief valves absorb residual pressure waves.
IA Pumping Systems & Pump Selection 2
A pump operating at a higher flow rate than its BEP is said to be running: