Dig into Underground Fire Water Piping and Appurtenances


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    • Commercial Kitchen Fires
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    • Scope of Damage/Cost of Repair
    • Fires & Explosions Analysis: Origin & Cause
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Many sites that are protected by fire sprinklers will have at least some amount of private underground fire water piping. Its purpose is to make water available for fire protection or suppression at a needed flow and pressure. Its presence is usually quietly evidenced by the connected objects that occasionally surface along its course, like valves, fire department connections and private hydrants, termed appurtenances. Underground water piping commands attention, though, when there’s a leak or blowout. You may have seen news reports of burst water mains accompanied by fantastic fountains of blasting water carving huge holes in streets and dirt, flooding buildings or cars, or undermining building foundations unfortunately close to the break. Problems with private underground fire water piping can be similarly fantastic but can be quiet and slow to develop, also.

Appurtenances: fire main accessories

Accessories on the fire water main give control of water flow (valves and check valves) or access to get water from it (hydrants) or put water into it (fire department connections). Among the first things that may be found along private fire water mains, beginning with an assumed connection to the public water main, include a backflow preventer and valves , and an FDC (fire department connection).

A valve vault and an FDC

On a larger private fire system, there may be fire hydrants, divisional valves, riser control valves, and fire pumps.


Common materials of construction of existing, older private fire mains include cast iron, unlined ductile iron, and cement asbestos. Newer piping could be ductile iron, cement-lined ductile iron, with an increasing trend toward plastic piping, including HDPE and PVC. Factors considered for selection include cost, corrosion resistance, and the number of connections.

Potential Problems with Underground Fire Water Piping

If a private fire water main doesn’t work correctly to flow water when needed, the number one reason is that a valve was shut that shouldn’t have been.  Other problems that can occur include leaks, corrosion, tuberculation or calcification, obstructive biologics like zebra mussels, appurtenances getting mechanically struck by vehicles or lawn equipment (after which the device may not be operable), improper installation (improper backfill is an example), improper or inadequate thrust restraint.

A mechanically damaged fire hydrant

Water pipe clogged with Zebra mussels.  Image credit: worldurbanparks.org

Zebra and quagga mussels, both invasive species, are major nuisances along waterways in the US infested with them. Microscopic baby mussels can enter, attach to, and grow on the inside of piping, including fire protection systems, that draw up infested raw water from affected rivers and lakes. (There are readily available online maps that show affected waterways). Breathe a sigh of relief if your fire protection systems are supplied by treated water, because the treatment process would eliminate them.


Older cast iron and unlined ductile iron pipes, especially in areas with hard water, are prone to develop tuberculation. As tuberculation builds over time,  the effective diameter of the pipe gets smaller and the friction for water flow increases.

Tuberculation closed up the effective diameter of this removed pipe; right side – the interior of the same removed pipe

After new pipes are installed or when work is done on existing piping, the piping  is flushed. This is done to remove any debris that could have made its way into the pipe from laydown or installation. All sorts of objects could be in there, from dirt, mud, and rocks to work gloves, hand tools, or clothing. Flushing is done at a certain flow rate according to the pipe size; flowing a garden hose would be totally inadequate for any size of pipe used for fire protection water.

Inspections and Testing

For the piping itself, little is required for inspections and testing. Where a pipe is aboveground, it should be visually inspected. Flow testing is needed every 5 years. While some problems are unseen, applying the information gained from this required periodic flow testing will ensure that adequate flow and pressure remain available to supply fire protection systems. Flow test results can be compared to fire protection demand flows and pressures, to previous tests and evaluated for trends.

The appurtenances – the devices that are on the pipe, like valves and hydrants –  require more attention than the pipe. Control valves (whether you want to call it a control valve, a sprinkler control valve, a divisional valve, sectional valve, or an abbreviation like SCV or PIV, whatever) are visually inspected weekly, monthly, or quarterly depending on their locked or supervised status. If there are locked PIVs but unlocked, unsealed, unsupervised roadway valves, the PIVs would be inspected monthly, but the roadway valves would be inspected weekly.  All valves are fully traveled annually; lubricant is applied to OS&Y (outside screw and yoke) valves before moving them so that the lubricant is distributed along the stem.

Did you know? For post indicator valves (PIVs), the installation convention is to orient the open/shut windows so that they are in line with the pipe that the valve is installed in. This provides indications as to how the pipes below are laid out.

PIVs situated on adjacent perpendicular pipes, showing the installation convention of the indicator windows.

This article illustrates that there can be more going on with private fire water mains than what easily meets the eye. We have seen how items visible aboveground hint at what may be below, glimpsed underground and inside of piping. We have briefly discussed some of the problems that can occur with underground fire water mains. Equipped with more information on private fire water mains and their appurtenances, have you become aware of any inspections or tests that need to be done?

Amy Anderson, PE, CFEI, has a Bachelor of Science in Chemical Engineering from Clemson University and is a licensed Professional Engineer in Fire Protection, as well as a Certified Fire and Explosion Investigator. Amy has over 20 years of engineering experience including property loss prevention engineering specializing in fire protection, chemical and pharmaceutical facilities. She has partnered with clients to identify, assess, avoid, and reduce risk at their commercial and industrial properties. Additionally, she has assisted with the development of building and fire protection specifications, reviewed plans and performed site visits. She has reviewed project documents for compliance with applicable standards – construction, fire protection, process, and combustible dust hazards. Amy is a member of the National Association of Fire Investigators, the Society of Fire Protection Engineers, the National Fire Protection Association and the American Institute of Chemical Engineers.

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