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What is a fire
hydrant flow test? A fire hydrant flow test is a process of flowing water
out of at least one fire hydrant while obtaining the required pressures
(static, residual, and pitot) needed to calculate the amount of water in GPM
(gallons per minutes) that the water system is capable of providing at a
certain location.


Who needs fire
hydrant flow test data? The following entities utilize fire hydrant flow
test data: architects, fire safety
engineers, plumbing engineers, city governments, fire departments, water
companies, corporate and private contractors, City State and federal
municipalities, land developers, etc.

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How is the static
pressure obtained? The static pressure is obtained by attaching a pressure
gauge typically to a 2.5-inch outlet using a special hydrant cap. Open the hydrant
valve, be sure all excess air is released, and wait for the needle on the
pressure gauge to settle. This is your static pressure.


How is the residual
pressure obtained? When fire hydrants are fully opened, and their
associated pitot readings are obtained, note where the static pressure has
dropped down to on the pressure hydrant. This is your residual pressure.


How is a pitot
pressure obtained? The pitot pressure is obtained by opening a hydrant
valve, letting water flow directly out of an outlet, and placing a pitot tube
in the center of the water steam so that a reading on the pressure gauge is
obtained. This is your pitot pressure. Hand held pitot gauges, diffusers, and
hose monster equipment are all ways that you can obtain the pitot pressure


What is the
coefficient of discharge? This is a number that is used when computing the
flowing gpm. There are three general types of hydrant outlets and their
associated coefficient of discharge. These are as follows:

(NFPA-291, 2013 edition)

How is the flowing
GPM (gallons per minute) calculated? The flowing GPM is calculated using the
following formula:

(NFPA-291, 2013 edition)



Q = GPM (gallons per minute)

c = coefficient of discharge

d = outlet diameter in inches

p = pitot pressure in psi




What is fire-flow and
how is it determined? Fire-Flow is the available GPM at 20 PSI. This is the
amount of water (GPM) that there is if the residual pressure is at 20 PSI. It
can be calculated by using the following equation:

(NFPA-291, 2013 edition)


Qr = flow predicted at desired residual pressure

Qf = total flow measured during test

hr = pressure drop to desired residual pressure

hf = pressure drop measured during test


What information,
required and recommended by the IFC, NFPA, and Authorities Having Jurisdictions,
is to be obtained from a fire hydrant flow test?

(1) Location and elevation of static and residual test gauge

(2) Flow location

(3) Static pressure, psi (bar)

(4) Residual pressure, psi (bar)

(5) Flow, gpm (L/min)

(6) Date

(7) Time

(8) Name of person who conducted the test or supplied the

(9) Other sources of water supply, with pressure or

(10) Results of the flow test displayed on a hydraulic graph


What time should fire
hydrant flow tests be conducted? It is generally preferred that fire
hydrant flow tests are conducted during a period of high demand. This is
typically between 6am and 8am. Check with local AHJ and/or Water Department for
test area to determine period of high demand.


What is the recommended
drop in pressure that should be obtained during a flow test? AWWA (American
Water Works Association) recommends that a 10 PSI drop in pressure is obtained
during a flow test. NFPA-291 recommends that at least a 25% drop in pressure is
obtained and/or to flow the demand necessary for fire-fighting purposes.


How many fire
hydrants should I utilize during a flow test? Generally, two fire hydrants
are utilized (one for the static-residual pressures and one for the flowing
pitot pressure(s)). To achieve a larger drop in pressure and a larger flowing
gpm, more outlets need to be flowed, and this can be done by flowing multiple
fire hydrants or hydrant outlets during a test.


If I’m flowing two
fire hydrants during a test, where should the pressure hydrant be located? Per
NFPA-291 the pressure hydrant (where the static-residual pressures are
recorded) should be located in between the two hydrants that are being flowed.


Should the flow
hydrant or pressure hydrant be closest to the project site? The pressure
hydrant should be located closest to the project site. The results obtained
during a flow test evaluate the available water supply at the pressure hydrant.


How should hydrants
be tested on a dead end main? The dead end main should always be flowed.
The static-residual hydrant is placed so that it is located between the “large
mains that constitute the immediate sources of water supply in the area” and
the flow hydrant.


How often should
private fire hydrants be flow tested? The International Fire Code states
that private fire hydrants shall be flow tested annually.


How often should public
fire hydrants be flow tested? NFPA-291 states that public fire hydrants
should be flow tested every five years.


How long is water
supply flow test data considered current and reliable for? NFPA-13 states
that when flow test data is to be used for system design, then the flow test
data should be no more than one year old. The Authority Having Jurisdiction can
supersede this requirement.


Why is important to
know the elevation of a fire hydrant? Elevations affect pressures. 0.433
PSI per foot of elevation is either gained or lost.


If given the
opportunity to flow either the 2.5″ outlet(s) or the pumper outlet, which one
is preferred? The 2.5″ outlet(s) will give you a more accurate reading in
most cases as the entire cross-section of the outlet is filled by the discharge
of water whereas there is frequently a void space near the bottom of the stream
on the pumper outlet.


Where can I obtain
more information about fire hydrant flow testing? Two published guidelines
about fire hydrant flow testing are: NFPA-291 (Recommended Practice for Fire
Flow Testing and Marking of Hydrants) and AWWA M17 (Installation, Field
Testing, and Maintenance of Fire Hydrants).



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