2.2 Hydrants

Hydrants

Quick Answer: Fire hydrants provide critical access points for fire suppression and distribution system flushing. Operators must understand the differences between dry and wet barrel designs, the importance of breakaway safety features, and the procedures for flow testing and color-coding to ensure adequate fire protection.

Fire hydrants are the most visible components of a water distribution system. While their primary purpose is to provide water for firefighting, they are also essential tools for system operators, used for flushing mains, testing pressure and flow, and temporary water supply.

Dry-Barrel vs. Wet-Barrel Hydrants

The climate dictates the type of hydrant installed in a distribution system.

Dry-Barrel Hydrants

Used in areas subject to freezing temperatures. The main valve that controls the water flow is located underground, below the frost line.

• Operation: When the hydrant is closed, the entire barrel above the main valve is empty (dry).
• Drain Valve: A critical feature is the automatic drain valve at the base. When the main valve is fully opened, the drain valve closes. When the main valve is fully closed, the drain valve opens, allowing the water remaining in the barrel to drain into the surrounding soil, preventing the hydrant from freezing and cracking.
• Crucial Rule: Dry-barrel hydrants must be operated either fully open or fully closed. If partially opened, the drain valve remains partially open, washing away the soil around the base and potentially undermining the hydrant and the water main.

Wet-Barrel Hydrants

Used exclusively in warm climates where freezing is never a concern (e.g., parts of California and the Sun Belt).

• Operation: The barrel is constantly filled with water under pressure. Each nozzle has an independent, operating valve right at the connection point.
• Advantage: They provide instant water access and allow multiple hoses to be connected and controlled independently without shutting down the entire hydrant.

Installation Requirements

Proper installation is critical for hydrant stability and operation.

• Connection: Hydrants are typically connected to the main via a 6-inch branch line, which should include an isolation gate valve (the auxiliary valve) so the hydrant can be maintained without shutting down the main.
• Thrust Blocking: The force of water pushing against the base of the hydrant can be immense. Concrete thrust blocks or mechanical joint restraints must be installed to prevent the hydrant from blowing off the pipe.
• Drainage (Dry-Barrel): A substantial volume of coarse gravel or crushed stone must be placed around the base of a dry-barrel hydrant to ensure the drain valve functions properly and the barrel empties quickly.

Safety Flanges (Breakaway Design)

Hydrants are unfortunately common targets for vehicular accidents. Modern dry-barrel hydrants feature a "traffic model" or breakaway design.

• Mechanism: The barrel has a safety flange (a weak point) near the ground line, and the operating stem has a breakaway coupling.
• Benefit: If struck by a car, the upper section snaps off cleanly. Because the main valve remains closed below ground, water does not geyser into the street, and repairs can often be made quickly with a repair kit without digging up the entire hydrant.

Hydrant Flow Testing

Flow testing is performed to determine the water availability in the distribution system, primarily for fire protection. A standard flow test requires at least two hydrants: a residual hydrant (where pressures are read) and one or more flow hydrants (where water is discharged).

The test involves taking three specific pressure readings:

1. Static Pressure: The pressure at the residual hydrant before any water is flowing. This represents the normal system pressure.
2. Flow Pressure (Pitot Pressure): The pressure of the water stream discharging from the flow hydrant, measured using a pitot gauge. This is used to calculate the flow rate in gallons per minute (GPM).
3. Residual Pressure: The pressure at the residual hydrant while water is discharging from the flow hydrant. This shows how much the system pressure drops under demand. (Safety standard: Residual pressure should never drop below 20 psi during a test to prevent backflow and cavitation).

Color-Coding Flow Classes (NFPA 291)

The National Fire Protection Association (NFPA) provides a standard color-coding system (NFPA 291) for hydrant bonnets (tops) and nozzle caps to instantly communicate a hydrant's flow capacity (at 20 psi residual pressure) to arriving firefighters.

(Note: The barrel of the hydrant is typically painted chrome yellow for visibility, though local jurisdictions may use different colors.)