Start by separating the readings from the calculations.
A hydrant flow test report tells a story about a particular water system location under the conditions observed during a particular test. It should identify where the readings were taken, what was measured in the field, what was calculated from those measurements, and the limitations needed to interpret the result.
Five terms worth knowing
- Static pressure: The water system pressure measured before the test hydrant is flowing.
- Residual pressure: The pressure remaining in the system while water is being discharged from the flow hydrant.
- Pitot pressure: The velocity pressure measured at the flowing outlet and used with outlet size and coefficient to calculate discharge.
- Flowing GPM: The calculated volume of water discharged from the hydrant, expressed in gallons per minute.
- Available flow at 20 PSI: A calculated projection of the water supply available when the residual pressure reaches 20 PSI; it is not a directly measured field reading.
Measured readings and calculated results
Static, residual, and pitot pressures are field readings. Outlet size, discharge coefficient, and test configuration help turn the pitot reading into calculated flowing GPM. If more than one outlet is flowed, the reported total reflects the applicable outlet calculations. The available flow at 20 PSI is then projected from the observed pressure change and flow; it is not water directly discharged at that pressure.
A water supply curve can show the test data and projection visually when it is part of the scope. The report should identify the hydrant locations and relevant field conditions so readers can understand what the numbers represent.
The design professional and the reviewing authority decide how to use the data for a particular design. A flow test does not, by itself, certify that a proposed fire protection system will meet every project requirement.
Why two tests can differ
Water demand, system operations, valve positions, test locations, hydrant arrangement, and timing can affect readings. Compare the full test setup and conditions before comparing headline GPM values from two reports. If a result is unexpected, the water provider and project team may need additional context or investigation.
Four questions to ask when reviewing a report
- Where were the pressure and flow measurements taken?
- Which values were measured, and which were calculated or projected?
- What date, time, and system conditions were documented?
- Does the report meet the engineer's and local authority's requirements for this project?
Next step: Explore PFS flow test documentation or request a hydrant flow test.