CIT - Certified Irrigation Technician Hydraulics and Water Management 2 — Questions and Answers

Question 1: A sprinkler system mainline has a static pressure of 65 PSI. After opening a zone with four spray heads, the pressure drops to 48 PSI. What is the pressure loss in the system?

48 PSI
65 PSI
17 PSI (Correct answer)
113 PSI

Correct answer: 17 PSI

Pressure loss is calculated as the difference between static pressure (no flow) and dynamic pressure (during flow): 65 PSI - 48 PSI = 17 PSI. This loss occurs due to friction in pipes, fittings, valves, and the elevation change.

Static pressure is measured when no water is flowing, representing the full available pressure from the supply. Dynamic (working) pressure is measured while sprinklers are operating. The 17 PSI difference represents the cumulative friction loss through the water meter, backflow preventer, mainline, lateral pipe, fittings, and the sprinkler heads themselves. This loss must be accounted for during system design to ensure adequate pressure at the most remote sprinkler. If the sprinkler heads require 30 PSI to operate properly, the 48 PSI dynamic pressure provides adequate margin.

Question 2: According to the Hazen-Williams equation, as pipe diameter increases, friction loss per unit length:

Increases proportionally
Remains the same
Decreases significantly (Correct answer)
Fluctuates unpredictably

Correct answer: Decreases significantly

As pipe diameter increases, friction loss decreases significantly because the larger cross-sectional area reduces water velocity for the same flow rate. Friction loss is proportional to approximately the 4.87th power of the pipe diameter (inverse relationship).

The Hazen-Williams equation shows that friction loss is inversely proportional to the pipe diameter raised to approximately the 4.87 power: hf = (4.73 x L x Q^1.85) / (C^1.85 x D^4.87). This means doubling the pipe diameter reduces friction loss by a factor of about 29 (2^4.87 = 29.3). For example, changing from 3/4-inch to 1-inch pipe at the same flow rate reduces friction loss by approximately 70%. This is why proper pipe sizing is critical in irrigation design — undersized pipes create excessive pressure loss and poor sprinkler performance.

Question 3: What is the recommended maximum velocity of water flow in irrigation mainline pipes to minimize water hammer and friction loss?

2 feet per second
5 feet per second (Correct answer)
10 feet per second
15 feet per second

Correct answer: 5 feet per second

The recommended maximum water velocity in irrigation mainlines is 5 feet per second (fps). Exceeding this velocity significantly increases friction loss and the risk of water hammer, which can damage pipes, fittings, and valves.

The 5 fps guideline is an industry standard established by organizations like the Irrigation Association and ASABE. Above 5 fps, friction loss increases rapidly (friction loss is proportional to velocity squared), and the potential energy for water hammer increases dramatically. Water hammer occurs when fast-moving water is suddenly stopped (by a valve closing), creating a pressure wave that can reach pressures several times the normal operating pressure. At 5 fps, water hammer pressure surge is approximately 60 PSI; at 10 fps, it would be approximately 120 PSI — potentially bursting pipes. Lateral lines to sprinklers can have slightly higher velocities (7 fps) because they are shorter.

Question 4: A pressure gauge at the base of a hill reads 60 PSI. The irrigation zone at the top of the hill is 46 feet higher in elevation. What is the approximate pressure available at the top of the hill (ignoring friction loss)?

40 PSI (Correct answer)
60 PSI
80 PSI
20 PSI

Correct answer: 40 PSI

Pressure loss due to elevation change is approximately 0.433 PSI per foot of elevation gain. For 46 feet: 46 x 0.433 = approximately 20 PSI loss. Available pressure at the top: 60 - 20 = 40 PSI.

The relationship between pressure and elevation is based on the weight of water: one cubic foot of water weighs 62.4 pounds, which translates to 0.433 PSI per foot of elevation (or 1 PSI per 2.31 feet). This is a fixed physical relationship that cannot be changed by pipe size or flow rate. At 46 feet elevation change: 46 / 2.31 = 19.9 PSI loss, leaving approximately 40 PSI at the hilltop. Conversely, zones downhill from the pressure source gain 0.433 PSI per foot of drop. This elevation pressure change must be calculated during design in addition to friction losses to ensure adequate pressure at all sprinkler heads.

Question 5: What is the purpose of a pressure regulator in an irrigation system?

To increase pressure for zones at higher elevations
To reduce and maintain constant downstream pressure regardless of supply pressure fluctuations (Correct answer)
To eliminate water hammer
To filter debris from the water supply

Correct answer: To reduce and maintain constant downstream pressure regardless of supply pressure fluctuations

A pressure regulator reduces high supply pressure to a consistent downstream pressure, protecting system components and ensuring proper sprinkler performance. Most residential regulators reduce 80-120 PSI supply pressure to 40-50 PSI for the irrigation system.

High water pressure (above 80 PSI) can cause sprinkler misting, premature component wear, water hammer, and pipe failure. A pressure regulator uses a spring-loaded diaphragm to maintain a constant outlet pressure. For spray heads (typically 30 PSI), rotor heads (45 PSI), and drip systems (25 PSI), regulators ensure optimal operating pressure. There are two types: in-line regulators (installed on the mainline) and individual head regulators (built into each sprinkler). When supply pressure varies significantly (common in municipal systems during peak and off-peak hours), regulators maintain consistent precipitation rates and spray patterns throughout the day.

Question 6: The 'flow rate' of an irrigation system is typically measured in which unit?

Pounds per square inch (PSI)
Gallons per minute (GPM) (Correct answer)
Feet per second (FPS)
Cubic feet per hour (CFH)

Correct answer: Gallons per minute (GPM)

Flow rate in irrigation is measured in gallons per minute (GPM). This measurement indicates the volume of water moving through the system per unit of time and is used for pipe sizing, valve selection, and determining how many sprinklers can operate simultaneously.

GPM is the standard flow rate unit in North American irrigation. Available GPM determines how many sprinklers can run on one zone — for example, if the supply provides 15 GPM and each spray head uses 2 GPM, a maximum of 7 heads can run per zone (leaving a safety margin). GPM is measured using a flow meter, a pressure gauge and flow chart, or by timing how long it takes to fill a known container. In commercial irrigation, flow may also be expressed in cubic meters per hour (m3/hr). The relationship to velocity is: GPM = velocity (fps) x cross-sectional area of pipe (sq ft) x 448.8 (conversion factor).

CIT - Certified Irrigation Technician Hydraulics and Water Management 2

A sprinkler system mainline has a static pressure of 65 PSI.
After opening a zone with four spray heads, the pressure drops to 48 PSI.
What is the pressure loss in the system?