again. The static system riser pressure observed
prior to flowing water (i.e. static pressure) and the
residual pressure are used as a benchmark and are
compared to previous main drain flow test results.
Written records should be kept by the building
owner for future reference and troubleshooting.
2 Conduct an initial water distribution system
analysis
A key fundamental to using the main drain test
data to monitor changes in the water supply
assumes that the water supply was adequate at
the time of original installation of the system.
Performing a water distribution system flow test
(i.e. hydrant flow test) at the initial acceptance
testing for comparison to the sprinkler system
design is often overlooked and not validated
during the building construction inspection
process. Original data obtained well in advance of
construction may not accurately describe the
water supply strength when construction is
complete, and more importantly, at the time of
building occupancy.
Although a main drain test may repeatedly
produce similar residual pressures at regular
inspections, the system supply may have been
inadequate from inception. Without calibration of
the actual water distribution supply curve to the
sprinkler system design criteria, an inadequate
water supply can remain undetected despite
performing the required main drain tests.
NFPA 291, Recommended Practice for Fire Flow
Testing and Marking of Hydrants, is a recom-
mended practice that addresses hydrant flow tests.
For new building construction, a hydrant flow
test should be performed at the time of initial
acceptance testing for the sprinkler system, just
prior to building occupancy. For an existing system
in service, a water supply flow test
should be performed at the next
inspection interval, if the baseline
test was not performed and
recorded at the completion of
construction. The water supply
curve can be used to validate
the design flow data used by the
design professional and/or instal-
ling fire sprinkler contractor and
for calibration of the main drain
test residual pressures. If the
as-built data does not agree with
the design flow test figures and/or
the supply is inadequate to satisfy
the sprinkler system demand,
investigation of the water supply
should occur immediately.
After the water supply curve is
characterized and determined to
satisfy the sprinkler system
demand, future main drain tests
results can be checked against the
baseline value to confirm water
supply adequacy. With this foun-
dation established, the results of
the main drain test can be used
with confidence.
3 Investigate reductions in
main drain residual pressures
The 2008 edition of NFPA 25
includes a new provision that
states where a 10 percent reduction in the full
flow pressure is observed from previous results,
the cause of the reduction shall be identified and
corrected. However, good engineering practices
dictate that any reduction in the residual pressure
observed during a sprinkler system flow test
should be investigated. A decrease in residual
pressure is a possible indicator of a deteriorated
water supply, obstruction, closed valve, or other
system impairment. At a minimum, the investi-
gation of the reduced residual pressure should
consist of an impact analysis to determine the
effect on the sprinkler system hydraulic demand.
The pressure buffer of a sprinkler system can
vary depending on the system designer's anticipa-
tion of nominal consumption, future demand,
infrastructure improvements, seasonal pressure
changes, etc. Minor changes in the observed main
drain test residual pressure can swing the
demand/supply relationship into a deficit, resulting
in an inadequate water supply. When conducting
a main drain test, observation of a drop in residual
pressure from a previous result is a qualitative
indicator of diminished water supply strength.
Investigation of any drop in main drain test
residual pressure should occur prior to the 10 per-
cent threshold, since this prescriptive requirement
does not account for systems that have smaller
design cushions which can be overcome by
slight decreases in residual pressure. The main
drain test can be used to detect a weakening
water supply, but the impact on the sprinkler
system is best determined by conducting a hydrant
flow test.
If the hydrant flow test is conducted as recom-
mended in Item 2, and results show that the water
supply curve differs from the original designer's
data, the first step is to determine the impact on
72 INTERNATIONAL FIRE PROTECTION
Figure 3: Water supply
flow curves for multiple
tests versus sprinkler
demand
SPRINKLER TECHNOLOGY
Water Supply Flow Test
120
110
100
90
80
70
60
50
40
30
20
10
0
100 200 300 400 500 600 700 800 900 1000
Pressure(PSI)
Sprinkler Demand 600 GPM at 40 PSI
Initial Water Supply 800 GPM at 40 PSI
Supply Curve I 750 GPM at 35 PSI
Supply Curve II 750 GPM at 25 PSI
Q1.85 Flow (GPM)

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