7.7 Fire Hydrants, Dry Barrel Operation, Fire Flow Testing & Unidirectional Flushing

The most visible asset the utility owns

WPI lists "mains and related equipment (hydrants and valves, pressure regulating valves, air relief valves)" as distribution components and "maintain and repair distribution components (mains, services, meters, valves, hydrants, pumps, storage)" as a job task. Hydrants deserve their own treatment because they are simultaneously a fire protection asset, a flushing tool, a flow-testing instrument, and, in Colorado, a freeze risk.

Dry barrel versus wet barrel

Every hydrant in Colorado is a dry barrel design. The main valve sits at the hydrant base below the frost line, operated by a stem running the length of the barrel. When the hydrant is closed, small drain (weep) holes at the base open automatically and let the barrel drain into the surrounding gravel drainage pocket. When it is open, those same drain holes are sealed by the valve.

Two operating consequences follow directly:

1. Open a hydrant fully. Partially opening a dry barrel hydrant leaves the drain holes partly open while the barrel is pressurized, eroding the drain area and washing out the surrounding soil, and it also risks contaminating the barrel. Turn the operating nut fully open, then back off a fraction of a turn to relieve stem tension.
2. Verify the barrel drains. After closing, listen or check with a plumber's rod or a vacuum check. A hydrant that does not drain will freeze and be unavailable at the fire. Barrels fail to drain when the drainage pocket is silted, the water table is high, or the drain holes are plugged. Where the barrel cannot drain, it must be pumped out after every use and flagged for winter attention.

Other Colorado-specific hydrant items: breakaway flanges at grade so a vehicle strike shears the hydrant cleanly without pulling the buried piping; a gate valve on the hydrant lateral so the hydrant can be isolated without shutting the main; hydrant markers for snow visibility; and antifreeze is never put in a hydrant barrel, because the barrel is directly connected to the potable main when the hydrant is opened.

Hydrant maintenance

An annual or semi-annual program, documented per hydrant asset ID:

1. Inspect for damage, leaning, obstruction, and clearance.
2. Remove caps, inspect threads and gaskets, lubricate cap threads.
3. Open fully, flow briefly to flush the barrel and confirm the main valve seals, and check for a full-open stream.
4. Close slowly to avoid water hammer; confirm the barrel drains.
5. Check operating nut and stem lubrication.
6. Verify the isolation valve on the lateral operates.
7. Paint per the color code the fire authority uses for available flow.
8. Record findings and generate corrective work orders.

Closing slowly is not optional. A hydrant flowing 1,000 gpm slammed shut creates a pressure surge that can break mains and services elsewhere on the system.

Fire flow testing

Fire flow testing determines how much water is actually available at a location and at what residual pressure. It uses two hydrants:

• The residual hydrant is closest to the point of interest; a pressure gauge is installed on it and it is not flowed.
• The flow hydrant is downstream; it is opened and the discharge measured with a pitot gauge or a flow diffuser.

Procedure:

1. Record static pressure at the residual hydrant with nothing flowing.
2. Open the flow hydrant and let the flow stabilize.
3. Record the residual pressure at the residual hydrant and the pitot pressure at the flow hydrant outlet.
4. Compute the flow: Q = 29.83 x C x d² x √P, where d is the outlet diameter in inches, P is pitot pressure in psi, and C is the outlet coefficient — 0.90 for a rounded outlet, 0.80 for a square-edged outlet inside the barrel, and 0.70 for a projecting outlet.
5. Compute the flow available at a target residual, conventionally 20 psi, using the hydrant flow test equation.

Worked example. Static pressure is 74 psi. With one 2.5-inch square-edged outlet flowing, pitot pressure is 16 psi and residual pressure is 58 psi.

• Q = 29.83 x 0.80 x (2.5)² x √16 = 29.83 x 0.80 x 6.25 x 4 = 597 gpm
• Pressure drop is 74 − 58 = 16 psi.

Because available flow rises roughly with the 0.54 power of the pressure drop, dropping to a 20 psi residual (a 54 psi drop) yields substantially more than 597 gpm — the calculation gives roughly 597 x (54/16)^0.54, or about 1,170 gpm at 20 psi residual. That number is what the fire authority needs.

Distribution system flushing

WPI lists "conduct distribution system flushing" as a job task. Flushing removes accumulated sediment, biofilm, corrosion products, and stale water, and restores disinfectant residual. There are two fundamentally different approaches, and the difference is worth knowing precisely.

Conventional flushing opens hydrants and blows water out until it runs clear. It is simple and it works, but water arrives at the flushed hydrant from every direction, velocities are modest, and sediment can be drawn from clean areas into dirty ones.

Unidirectional flushing (UDF) is a systematically planned program in which valves are closed to create a single defined flow path to each flushed hydrant. Because flow is forced through one route, velocity is much higher — the design target is at least 5 feet per second, and preferably 6 or more — which actually scours the pipe wall rather than merely diluting. UDF proceeds outward from the source so that clean water always flushes into dirtier pipe, never the reverse. It uses substantially less water for a better result, and it verifies valve operability as a by-product because every valve in the sequence must be exercised.

Planning a UDF program requires an accurate map with valve and hydrant locations, a sequence of flush segments, calculated velocities, and estimated flush volumes. Operationally:

• Notify customers in advance; discolored water complaints follow flushing.
• Monitor chlorine residual and turbidity at the flushed hydrant and flush until both meet targets.
• Dechlorinate the discharge where it reaches a storm drain or a stream, because chlorinated water is acutely toxic to aquatic life. Tablets, diffuser baskets, or liquid feed on the discharge are standard.
• Control erosion and traffic, and manage discharge in freezing conditions so streets do not ice.
• Record flush date, location, duration, volume, starting and ending residual and turbidity.

Velocity: the number to remember

The reason UDF works and conventional flushing often does not is velocity. Normal distribution velocities are 2 to 5 ft/s; a main sized for fire flow may sit at a fraction of 1 ft/s under domestic demand, which is precisely why sediment settles there. Reaching 5 ft/s or more is what re-suspends and carries out the accumulated material.

The flow required to reach a target velocity comes from the continuity equation:

Q (gpm) = 2.448 x d² x V, where d is inside diameter in inches and V is velocity in ft/s.

Worked example. To scour an 8-inch main at 5 ft/s:

Q = 2.448 x (8)² x 5 = 2.448 x 64 x 5 = 783 gpm

That is comfortably within a single hydrant's capacity. To do the same in a 16-inch main:

Q = 2.448 x (16)² x 5 = 2.448 x 256 x 5 = 3,132 gpm

which exceeds what one hydrant will pass — which is why large transmission mains are usually cleaned mechanically by pigging or swabbing rather than by flushing.