13.2 Fire Hydrants: Types, Testing & Maintenance

Dry Barrel vs Wet Barrel

Dry barrel drainage is the critical operating detail. After the main valve is closed, water in the barrel drains out through weep holes (drain ports) at the base into a drain rock pocket. If the weep holes are plugged, or the hydrant sits in a high groundwater table, the barrel stays full and can freeze and split. Where drainage is impossible, the weep holes are plugged deliberately and the hydrant is pumped dry after every use and tagged accordingly.

[!IMPORTANT] Never leave a dry barrel hydrant partially open. With the main valve cracked open, the weep holes are still open too, so pressurized water is forced continuously out the drain ports into the surrounding soil. That erodes bedding, undermines the hydrant and adjacent main, and can wash out a roadway. A dry barrel hydrant is opened fully or closed fully.

---

Components

Thread compatibility matters. California has both National Standard Thread (NST) and legacy local threads in some jurisdictions. A mutual aid engine that cannot connect is a life-safety failure, so thread type must be documented and adapters carried.

---

Operating Technique

1. Confirm the hydrant is in service and note any out-of-service tag (typically a bagged or specially painted hydrant).
2. Remove a cap slowly with the operating nut closed - a cap that has trapped pressure behind it can launch.
3. Open fully and slowly - dry barrel hydrants especially. Count the turns and record them.
4. Watch the discharge. Discolored water means the operation is scouring deposits; direct the flow safely and be ready to flush until clear.
5. Close slowly, particularly the last few turns. A fast closure on a 4.5-inch outlet flowing 1,000+ gpm is a serious water hammer event.
6. Verify drainage on a dry barrel: hold a hand or a listening device at an open outlet; you should hear or feel the barrel draining.
7. Replace caps loosely enough to breathe but tight enough to exclude debris, and lubricate threads.

Discharge Control

Hydrant discharge is not free of obligations:

• Erosion and flooding - use a diffuser or splash pad; do not undermine a road or a neighbor's yard
• Traffic safety - cones, signage, and a spotter
• Dechlorination - discharge reaching a storm drain, creek, or the ocean generally requires dechlorination to protect aquatic life; systems commonly use tablets in a diffuser or a metered sodium bisulfite/ascorbic acid solution
• Water loss accounting - a metered diffuser or an estimate from the pitot flow and duration, so the volume is recorded rather than becoming apparent loss

---

Inspection and Maintenance

Many agencies color-code hydrant bonnets and caps to indicate available fire flow using the NFPA 291 classes, which lets an arriving engine company pick the right hydrant instantly.

---

Flow Testing

A hydrant flow test measures what the system can actually deliver:

1. Choose a residual hydrant (closest to the point of interest, where pressure is read) and one or more flow hydrants downstream.
2. Record static pressure at the residual hydrant with nothing flowing.
3. Open the flow hydrant(s); measure the outlet discharge with a pitot gauge held in the center of the stream about half a diameter out from the outlet; record residual pressure at the residual hydrant.
4. Compute discharge:

$Q = 29.83 \times c \times d^{2} \times \sqrt{p}$

Example: a 4.5-inch outlet with a coefficient of 0.90 and a pitot reading of 9 psi: Q = 29.83 x 0.90 x 20.25 x 3.0 = 1,631 gpm

1. Project the flow available at 20 psi from the static and residual readings using the standard hydrant flow test relationship, and report that to the fire authority.

[!WARNING] Stop the test before system pressure reaches 20 psi anywhere. Falling below 20 psi violates 22 CCR 64602 and creates an intrusion pathway. Notify the fire department before large tests, and notify customers in areas prone to discolored water. Test during low-demand hours where possible.