Fire Hydrant Testing

NFPA Guidance on Fire Hydrant Testing

During a fire emergency, reliable sources of water can mean the difference between life and death. This is why the National Fire Protection Association (NFPA) outlines fire hydrant testing that measures real-world pressure and flow in a city’s water distribution system.

Regular fire hydrant testing ensures the ability to provide water at an acceptable pressure and flow rate for public health and firefighting operations. Most jurisdictions also require hydrant flow tests to design fire sprinkler systems for commercial or residential structures.

NFPA 291 recommends testing fire hydrants every 5 years to ensure adequate pressure and flow, with annual inspections. Tests must be conducted by qualified personnel, often resulting in color-coded caps based on flow capacity

The 2022 edition of NFPA 291: Recommended Practice for Fire Flow Testing and Marking of Hydrants (4.2.2) recommends that fire hydrants should maintain a residual pressure of 20 psi (pounds per square inch), or 1.4 bar, for effective firefighting, as well as to prevent backflow that could contaminate the public water supply.

NFPA 291 stipulates hydrant flow tests every five years to ensure that changing conditions in the piping and system demands won’t impede hydrants’ ability to deliver water. From the 2022 edition of NFPA 291 (4.15.1) Public fire hydrants should be flow tested every 5 years to verify capacity and marking of the hydrant.

In the explanation that accompanies the section (A.4.15.1), NFPA clarifies the section’s intent. It states that it does not mean to mandate routine five-year testing of every hydrant—especially if there is no pressing need to test a specific hydrant or if test data less than five years old is available from an adjacent hydrant on the same grid.

Performed by city officials or professional contractors, fire hydrant testing verifies the performance of a city’s water distribution system, determining the pressure and rate of flow available at various locations. It measures static (non-flowing) and residual (flowing) pressure, as well as the rate of discharge in gallons per minute (GPM) of each fire hydrant.

The data that’s collected is used for two important purposes:

·        Uncovering closed valves, heavy pipe-wall deposits, or other problems in a water distribution system. Reduced rates of flow often stem from blockages or other infrastructure problems.

·        Properly designing fire sprinkler systems for commercial and residential structures. If water supply pressure and flow readings are off, it can lead to an underdeveloped system that requires additional fire pumps or an expensive overhaul of pipe fitting.

Besides delivering peace of mind that hydrants will work in an emergency, hydrant flow tests enable municipalities to color-code their fire hydrants according to their strength of output. The colors categorize hydrants by the GPM of their flow. For instance, the color-coding scheme recommended by NFPA 291 (5.1) and The American Water Works Association says light blue hydrants have a capacity of 1,500 GPM or more (“very good flow”) and red hydrants have a capacity below 500 GPM (“inadequate”). Below picture suggested Test Layout for Hydrants.

Two Hydrant Flow Tests with a Calibrated Flow Device Main Capacity Flow Test:

A Main Capacity Test evaluates the water supply of the fire main at the location of the test hydrant. The information derived from this test is used by city planners and contractors to consider the water supply for general use and fire sprinkler systems.

Setup at the Test Hydrant (pressure hydrant, static/residual hydrant):

1.   Attach gauge cap to the test hydrant. Tighten all other caps.

2.   Open test hydrant, vent air from hydrant body through the valve on the gauge cap assembly. Close it when air is vented.

At the Flow Hydrant

1.   Set the Little Hose Monster™ with gauge to the Pitotless Nozzle™ in an appropriate location for flowing water.

2.   Attach Remote Reader and gauge to the Pitotless Nozzle.

3.   Attach hydrant gate valve to the hydrant.

4.   Tighten other caps.

5.   Attach the hose to the Pitotless Nozzle and Little Hose Monster assembly.

Conduct the Test

1.   Record static pressure reading from gauge cap. (Test Hydrant)

2.   Open flow hydrant fully.

3.   When the flow rate stabilizes,

1.   Record nozzle pressure from the flow reader. (Flow Hydrant)

2.   Record the residual pressure reading from the gauge cap. (Test Hydrant)

At this point, the test is complete

1.   Slowly close Flow Hydrant. Remove test equipment from hydrant. Replace and tighten cap. If the hydrant is a dry barrel type, note that water drains properly from the hydrant.

2.   Record the number of minutes that water was flowing. This can be used to account for the amount of water used during the flow test.

At the Test Hydrant

1.   Close the hydrant. Remove gauge cap and replace hydrant cap. If the hydrant is a dry barrel type, note that the water drains properly from the hydrant.

SECTION 4.3.6

• To obtain satisfactory test results of theoretical calculation of expected flows or rated capacities, sufficient discharge should be achieved to cause a drop in pressure at the residual hydrant of at least 25 percent, or to flow the total demand necessary for firefighting purposes.

Specific instructions for conducting fire hydrant testing can be found in NFPA 291’s Chapter 4, “Flow Testing.” NFPA 291 guidelines require identifying a residual (test) hydrant to measure static and residual pressure, as well as one or more flow hydrants.

Static pressure represents the pressure at a given point under normal distribution system conditions. It is measured at the residual hydrant with no hydrants flowing. Residual pressure is the pressure that exists in the water distribution system while water is flowing. It is measured at the residual hydrant at the same time flow readings are taken at the flow hydrants.

To determine how many flow hydrants are needed, keep in mind that NFPA 291 recommends flowing enough water to provide at least a 10% drop in residual pressure compared to the static pressure (4.4.6). Further, it states that testers may need to “declare an artificial drop in the static pressure of 10 percent” in “water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened.” NFPA has updated this guidance from the 2019 edition, which recommended flowing the total demand necessary for firefighting purposes during the test, or enough water to provide at least a 25% drop in residual pressure compared to the static pressure. (In 2018, Sprinkler Age noted that the 25% drop was not necessary for a hydrant flow test used to design a fire suppression system.)

Both editions say that if the mains are small and the system is weak, only one or two hydrants need to flow (2022: 4.4.8/2019: 4.3.7). If the mains are large and the system is strong, as many as eight flow hydrants may be required (2022: 4.4.9/2019: 4.3.8).

The 2022 edition of NFPA 291 (4.3.1) now suggests conducting hydrant flow tests during periods of “periods of peak demand, based on knowledge of the water supply and engineering judgment,” which is an update from the 2019 edition’s guidance to do it during periods of “ordinary demand” (NFPA 2019: 4.2.1)  That’s likely because many fire protection professionals recommend performing flow tests during peak morning hours to reflect the worst-possible scenario during an emergency. Street pressures can fluctuate as much as 10 psi in the morning, compared to later in the day when demand is typically less.

Be prepared to record the following information during the test:

·        Date of hydrant flow test

·        Location of hydrants being tested (name of the street)

·        Time of day testing was performed

·        Static reading at the residual hydrant (pressure in the system with no flow)

·        Residual reading at the residual hydrant (pressure in the system during flow)

·        Flow reading at the flow hydrant, using a pitot gauge

·        Water main diameter in inches

·        Hydrant outlet size and type (determining the coefficient of discharge)

·        Hydrant elevation

Testers should always consult their local authority having jurisdiction (AHJ) and fire department guidelines for the most accurate information.

1.   Determine the location of the test by selecting a group of hydrants in the same vicinity. Remember, as many as eight hydrants may be required for robust systems with large water mains.

2.   Mark the hydrant measuring pressure as the residual hydrant. Both static pressure (when flow hydrants are closed) and residual pressure (when flow hydrants are open) are assessed from this hydrant. The residual hydrant should be between the hydrant(s) to be flowed and the large mains that supply water to the area.

3.   Flush the residual hydrant to remove any sediment and attach a nozzle cap with a gauge to the hydrant’s outlet.

4.   Slowly release the main valve until air is vented. Take a static pressure reading.

5.   Measure the inside diameter of the outlet nozzle or hydrant outlet where flow occurs. A hydrant’s inside diameter is usually 4”.

6.   Field personnel should slowly open each flowing fire hydrant, one at a time, to avoid pressure surges.

7.   After the residual pressure read from the outlet cap stabilizes, take readings at each flow hydrant using a pitot gauge. Residual pressure and pitot gauge readings must be taken simultaneously. For accurate pitot gauge readings, the pitot tube should be held downstream and in the center of the nozzle.

8.   Record both the residual pressure at the residual hydrant and pitot gauge readings at the flow hydrant(s).

9.   Slowly close each fire hydrant.

10.Use the PSI readings from the residual hydrant’s static and residual pressure, the coefficient determined by measuring the inside diameter of the hydrant’s outlet nozzle, and other factors to determine two sequential numbers using two sequential formulas: the discharge, aka the gallons flowing during the test (gpm), followed by the flow predicted at the desired residual pressure, aka the available fire flow.

As mentioned above, the tester needs to gather key information to run two equations in sequence.

The first equation determines the flow (gpm) from the tested fire hydrants based on the pitot gauge pressure readings. A version of it is found in section 4.93 of NFPA 291: 

Q = 29.83 * c * d² * √P

Where:

Q = discharge; the gallons flowing during the test (gpm)

c = coefficient of discharge, which represents friction loss. It’s determined by assessing the shape of the transition between the vertical barrel of the hydrant and the horizontal outlet. Most hydrants have a smooth and rounded transition resulting in a .90 coefficient of discharge but not all of them (as shown below):

d = diameter of the outlet

P = the pressure reading at the pitot gauge during the test (PSI)

The second formula estimates the “flow predicted at desired residual pressure,” which is sometimes called the “available fire flow” (AFF). This essential equation is found in section 4.12.1.2 of NFPA 291, but here is a version with more steps broken out:

Q_R = Q * (((S – 20)^0.54) ÷ ((S – R)^0.54)))

Where:

Q_R = flow predicted at desired residual pressure/available fire flow

Q = the discharge (gpm) measured during the test (the result of the first equation)

S = the static pressure measured during the test

20 = the amount of minimum pressure (in psi) required for most municipal water supplies to prevent backflow and achieve fire protection objectives.

(NFPA 291 calls the “S – 20” calculation above “h_r,” which equals “pressure drop to desired residual pressure”)

R = the residual pressure measured during the test

(NFPA 291 calls the “S – R” calculation above “H_f,” which equals the “pressure drop measured during test”)

0.54 = a constant within the Hazen-Williams equation

After conducting a hydrant test, testers plug in their measurements to the two formulas above, completing one after the other.

Section 4.5 Test Procedure

4.5.1 In a typical test, the 200 psi (14 bar) gauge is attached to one of the 2 1/2 in. (65mm) outlets of the residual hydrant using the special cap.

4.5.2 The cock on the gauge piping is opened and the hydrant valve is opened full.

4.5.3 As soon as the air is exhausted from the barrel, the cock is closed.

4.5.4 A reading (static pressure) is taken when the needle comes to rest.

4.5.5 At a given signal, each of the other hydrants is opened in succession with discharge taking place directly from the open hydrant butts.

4.5.6 Hydrants should be opened one at a time.

4.5.7 With all hydrants flowing, water should be allowed to flow for a sufficient amount of time to clear all debris and foreign substances from the stream(s).

4.5.8 At that time, a signal is given to the people at the hydrants to read the pitot pressure of the streams simultaneously while the residual pressure is being read.

4.5.9 The final magnitude of the pressure drop can be controlled by the number of hydrants used and the number of outlets opened on each.

4.5.10 After the readings have been taken, hydrants should be shut down slowly, one at a time, to prevent undue surges.

Section 4.6 Pitot Readings

 

Figure 4.6.9 – Pitot Tube Position

4.6.5 The air chamber on the pitot tube should be kept elevated.

4.6.6 Pitot readings of less than 10 psi (0.7 bar) and more than 30 psi (2.1 bar) should be avoided, if possible.

4.6.7 Opening additional hydrant outlets will aid in controlling the pitot reading.

4.6.8 With dry barrel hydrants, the hydrant valve should be wide open to minimize problems with underground drain valves.

4.6.9 With wet barrel hydrants, the valve for the flowing outlet should be wide open to give a more streamlined flow and more accurate pitot reading. (see figure 4.6.9)

Section 4.7 Determination of Discharge

4.7.1 At the hydrants used for the flow during the test, the discharges from the open butts are determined from measurements of the diameter of the outlets flowed, the pitot pressure, (velocity head) of the streams as indicated by the pitot readings and the coefficient of the outlet being flowed as determined from figure 4.7.1

4.7.2 If flow tubes (stream straighteners) are being utilized, a coefficient of 0.95 is suggested unless the coefficient of the tube is known.

Figure 4.7.1 – Three General Types of Hydrant Outlets and Their Coefficients of Discharge.

The standard gives the following coefficients for different outlet types: 

o   Smooth and rounded: 0.9

o   Square and sharp: 0.8

o   Square and projecting into the barrel: 0.7

Table 4.10.1(a) Fire Flow Testing and Marking of Hydrants

Table 4.10.1(a) – Theoretical Discharge Through Circular Orifices (U.S. Gallons of Water per Minute)

What the table is used for

·        Calculating hydrant flow: 

The table is used in conjunction with a pitot gauge reading to calculate the gallons per minute (GPM) being discharged from the hydrant nozzle.

·        Hydrant flow testing: 

It is a standard reference for fire departments and water utilities to conduct flow tests and determine the actual water supply available for firefighting.

·        Calculating theoretical discharge: 

By using the table's values and applying appropriate coefficients (which can be adjusted based on the specific hydrant and setup), the theoretical discharge of water can be calculated. 

How it is used in practice

·        A fire flow test is conducted by opening a hydrant and measuring the pressure with a pitot gauge. 

·        The reading from the pitot gauge is then used with Table 4.10.1(a) to determine the theoretical discharge. 

·        If multiple outlets are used, the discharges from all of them are added together to get the total discharge. 

Hose Friction Loss

Friction is the force resisting the relative motion of solid surfaces, fluid layers and material elements sliding against each other. Friction loss is the pressure loss due to the friction. Distance, diameter, and the GPM / volume, all affect friction loss.

In firefighting, friction between the water and the inside surface of the pump, connected appliances and fire hose all cause turbulence. This turbulence reduces the energy produced by the pump (PSI). The higher the GPM/LPM passing through a hose or pipe, the more turbulence and friction loss will result.

There is friction loss when flowing through a hose, but in a hydrant flow test, it doesn’t mater. The purpose of a hydrant flow test is to evaluate the water supply, or the flow-rate that will be available when the system is brough down to 20 psi residual.

A hydrant flow test requires three measurements: static pressure, residual pressure and test flow-rate. The reading from the gauge cap in the residual hydrant gives you static pressure and residual pressure.

The fiction loss created in the hose results in a lower test flow-rate and a greater residual pressure. This will not affect the predicted flow at 20psi as long as there is a sufficient drop in static-to-residual pressure. NFPA 291, 4,3,6, 2016 recommends a drop of at least 25% from static to residual pressure. AWWA M17 recommends a drop of at least 10 psi from static to residual pressure.

To illustrate that friction loss does note have an effect on the predicted flow-rate:

1. Test #1 measures the test flow through an open hydrant nozzle with a hand held pitot.

• Static — 85psi
• Residual — 60psi
• Pitot — 36psi
• Test flow — 1007GPM

Predicted flow at 20psi = 1687GMP

2. Test #2 measures the test flow through 1 1/2″ x 10′ hose and the 2 1/2″ Hose Monster.

• Static — 85psi
• Residual — 70psi
• Pitot — 20psi
• Test flow — 764GPM

Predicted flow at 20psi = 1687GMP

·        Static pressure is equal in both tests. Flow test equipment does not affect static pressure.

·        Test flow in Test #2 is less than in Test #1 because of the friction loss in 10′ of hose.

·        Residual pressure in Test #2 is greater than in Test #1. The friction loss in the hose causes a backpressure which increases residual pressure. The higher the residual pressure compensates for the lower test flow-rate.

·        Both flow tests result in a predicted flow at 20psi that is equal. Test points from both tests fall on the same line of the graph.

Hydrant Testing Pitfalls

·        Water gauges not calibrated.

·        Insufficient pressure dop on residual hydrant.

·        Elevation differential between test and residual hydrant not recorded.

·        Hydrant butt type not determined.

·        Hydrant test worksheet incomplete.