What is the difference between a water flow test & design basis water supply & why do we require a safety margin after adjusting a water flow test?

A water flow test when performed in accordance with NFPA 291 “Recommended Practice for Fire Flow Testing & Marking of Hydrants” is applicable at the time, on the day, & at the effective point where it was performed (it is a snapshot in time).

A design basis water supply BEGINS with a water flow test & conversations with the water purveyor to determine system design & capacity over time. The design basis water supply is extrapolated from the water flow test by adjusting for known daily & seasonal fluctuations & may include a safety margin.

A safety margin is a reasonable allowance made to account for the future degradation of the water supply due to growth in the surrounding area that may reduce the ability of the water purveyor to supply the design basis water supply to the site. A safety margin is not provided to account for installation differences from the shop drawings & hydraulic calculations.

Some municipalities have mandatory safety margins. Georgia for example, requires a minimum safety margin of 10 psi. Many, however, have no requirements. Georgia also requires water tests to be performed at the period of highest system demand. This has led some local municipalities to require a reduction of the water flow test by the difference between the observed static pressure & the lowest sustained pressure observed on a 24-hr pressure recording at the Static/Residual hydrant. Is this enough? If the water test is performed in the high-water use summer months, maybe. If the test is performed in the lower water use winter months, maybe not. When a water flow test is adjusted with an evaluation of the water purveyor’s capacity over time, the anticipated daily & seasonal fluctuations, maximum daily demand, & annual low water conditions, one can determine an appropriate design basis water supply that will be available anytime a fire may occur.

HGI has required a min. safety margin of 10 psi (of the design basis water supply static pressure or 10% whichever is less) between the highest sprinkler demand & the design basis water supply at the effective point. Recently, we have relaxed our safety margin requirement for sprinkler systems supplied by a full duration water tank to 5 psi.

Why not drop the safety margin entirely? How will the water supply contained within a full duration water tank degrade over time? An annual fire pump test may be considered to have passed if the fire pump performance is at least 95% of the nameplate condition per NFPA 25 “Standard for Inspection, Testing, & Maintenance of Water-Based Fire Protection Systems” (2023 edition) § If the pump performance degrades further, an investigation & remediation is necessary to return the system to the necessary capability. Providing a 5 psi safety margin will account for most of the allowed degradation of the fire pump performance over time.