WTO Water Distribution & Storage Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a pressure-reducing valve (PRV) in a water distribution system?
- Increase pressure to elevated storage tanks
- Reduce high upstream pressure to a safe downstream level (Correct answer)
- Prevent backflow from customer meters
- Regulate chlorine residual levels
Correct answer: Reduce high upstream pressure to a safe downstream level
A PRV automatically reduces high upstream pressure to a lower, controlled downstream pressure to protect pipes and meters.
Question 2: Which type of water storage tank is typically used to equalize pressure in a distribution system while providing fire flow reserve?
- Ground-level clearwell
- Hydropneumatic tank
- Elevated storage tank (Correct answer)
- Surge tank
Correct answer: Elevated storage tank
Elevated storage tanks use gravity to maintain system pressure and provide reserve volume for peak demand and fire protection.
Question 3: A distribution main showing a consistently low chlorine residual at the end of the line most likely indicates:
- The pipe diameter is too large for the flow
- Excessive detention time causing chlorine decay (Correct answer)
- A malfunctioning pressure gauge
- The PRV is set too low
Correct answer: Excessive detention time causing chlorine decay
Long detention times in dead-end or low-flow mains allow chlorine to dissipate, resulting in low or zero residuals at the extremities.
Question 4: The Hazen-Williams formula is used in water distribution to calculate:
- Chlorine demand of a water sample
- Head loss due to friction in a pipeline (Correct answer)
- Velocity of flow through a meter
- Volume of a storage tank
Correct answer: Head loss due to friction in a pipeline
The Hazen-Williams formula relates flow rate, pipe diameter, pipe roughness coefficient, and hydraulic gradient to estimate friction head loss.
Question 5: Cross-connection control programs are primarily designed to protect against:
- Excess fluoride entering the distribution system
- Backflow of contaminated water into the potable supply (Correct answer)
- Corrosion of cast iron mains
- Pressure surges from pump startup
Correct answer: Backflow of contaminated water into the potable supply
Cross-connection control prevents non-potable substances from entering the drinking water system through unprotected connections.
Question 6: When flushing a water main to restore chlorine residual, the operator should flush at a velocity of at least:
- 0.5 ft/s
- 1.5 ft/s
- 2.5 ft/s (Correct answer)
- 5.0 ft/s
Correct answer: 2.5 ft/s
AWWA recommends a minimum flushing velocity of 2.5 ft/s (0.76 m/s) to effectively scour sediment and biofilm from pipe walls.
Question 7: In a looped distribution system, what is the main advantage over a dead-end (branching) system?
- Lower construction cost
- Water can reach any point from two directions, improving flow and reliability (Correct answer)
- Simpler pressure management
- No need for storage tanks
Correct answer: Water can reach any point from two directions, improving flow and reliability
Looped systems allow flow from multiple directions, reducing pressure loss and ensuring supply continuity if one segment is taken out of service.
WTO Water Distribution & Storage Systems 2
What is the primary purpose of a pressure-reducing valve (PRV) in a water distribution system?