CIT - Certified Irrigation Technician Hydraulics and Water Management Questions and Answers — Questions and Answers
Question 1: An irrigation system is experiencing loud banging noises in the mainline whenever a large, fast-closing electric zone valve shuts off. What is the MOST effective long-term solution to mitigate this issue?
- Install a pressure regulator before the valve.
- Install a water hammer arrestor near the valve. (Correct answer)
- Increase the static pressure of the system.
- Replace the mainline with a smaller diameter pipe.
Correct answer: Install a water hammer arrestor near the valve.
The banging noise is water hammer, a hydraulic shockwave caused by the abrupt stop of water flow. A water hammer arrestor is a device specifically designed with an air cushion to absorb this shockwave, protecting pipes and components.
Question 2: If the flow rate (GPM) in a given size of PVC pipe is doubled, what is the approximate effect on the friction loss per 100 feet of that pipe?
- It is cut in half.
- It doubles.
- It increases by approximately four times. (Correct answer)
- It remains the same.
Correct answer: It increases by approximately four times.
Friction loss is proportional to the square of the velocity. Since doubling the flow rate in a fixed pipe size doubles the velocity, the friction loss will increase by a factor of approximately two-squared, or four times. This is a fundamental principle in hydraulics.
Question 3: To prevent excessive friction loss and the damaging effects of water hammer, what is the generally accepted maximum design velocity for water in PVC irrigation mainlines?
- 2 feet per second (fps)
- 5 feet per second (fps) (Correct answer)
- 8 feet per second (fps)
- 10 feet per second (fps)
Correct answer: 5 feet per second (fps)
The irrigation industry standard, recommended by organizations like the Irrigation Association and manufacturers, is to keep water velocity at or below 5 feet per second (fps). Exceeding this velocity dramatically increases friction loss and the risk of surge pressure (water hammer).
Question 4: A technician is calculating the Total Dynamic Head (TDH) for a pump application. Which of the following factors is considered part of the dynamic head calculation, but NOT the static head?
- The vertical elevation change from the water source to the highest sprinkler head.
- The pressure required at the sprinkler head for proper operation.
- The friction loss through pipes, valves, and fittings. (Correct answer)
- The pressure of the water at the source (e.g., in a pressurized municipal line).
Correct answer: The friction loss through pipes, valves, and fittings.
Total Dynamic Head (TDH) is the sum of static head (elevation changes), operating pressure required, and all friction losses. Friction loss through pipes and fittings is a key component of the *dynamic* head because it only occurs when water is in motion. Static head exists whether the water is moving or not.
Question 5: A 25-year-old irrigation system with an unlined cast iron mainline is being evaluated. When calculating the current system's expected friction loss, what is the most important adjustment a technician should make compared to calculations for a new PVC system?
- Use a lower C-Factor value in the friction loss calculations. (Correct answer)
- Assume a higher static water pressure.
- Use a higher C-Factor value in the friction loss calculations.
- Disregard the pipe material and only use the pipe's diameter.
Correct answer: Use a lower C-Factor value in the friction loss calculations.
The Hazen-Williams C-Factor represents the smoothness of the pipe's interior. New PVC has a very high C-Factor (e.g., 150), indicating a smooth surface. Over time, materials like unlined cast iron corrode and build up scale, making the interior rougher. This increases friction and is represented by using a lower C-Factor (e.g., 100 or less).
Question 6: An irrigation system has a static pressure of 80 PSI at the backflow preventer. The total pressure loss from elevation change is 5 PSI. The friction loss through the mainline and zone valve is 11 PSI. The friction loss through the lateral lines for that zone is 9 PSI. What is the available dynamic pressure at the base of the sprinkler heads in that zone?
- 69 PSI
- 60 PSI
- 55 PSI (Correct answer)
- 80 PSI
Correct answer: 55 PSI
To find the available dynamic pressure at the head, all pressure losses must be subtracted from the initial static pressure. The calculation is: 80 PSI (Static) - 5 PSI (Elevation) - 11 PSI (Mainline/Valve Loss) - 9 PSI (Lateral Loss) = 55 PSI.
CIT - Certified Irrigation Technician Hydraulics and Water Management Questions and Answers
An irrigation system is experiencing loud banging noises in the mainline whenever a large, fast-closing electric zone valve shuts off.
What is the MOST effective long-term solution to mitigate this issue?