A hydrant is only as good as the water main behind it, and the only way to know what a main can deliver is to measure it. NFPA 291, Recommended Practice for Water Flow Testing and Marking of Hydrants, defines how that measurement is made and how the result is painted onto the hydrant so a pump operator can read it at a glance. This guide explains the three pressures involved, the flow formula, the colour classes, how to project a test result to the standard 20 psi residual, and the quicker fireground methods operators use to estimate available water with the hydrant they are already on.
Static, residual, and flow pressure
Three pressures describe a hydrant, and confusing them is the root of most mistakes.
Static pressure is the pressure in the main when no water is flowing from the hydrant. It is what the intake gauge reads when you connect to a hydrant, open it, and have not yet opened a discharge. Static pressure comes from the elevation of the reservoir or the setting of the pumps that feed the system.
Residual pressure is the pressure remaining in the main while water is flowing. It is always lower than static, because the water moving through the main loses pressure to friction on the way to you. The bigger the flow, the bigger the drop. Residual pressure is what your intake gauge reads once lines are charged and flowing.
Flow pressure, or pitot pressure, is the velocity pressure of the stream leaving an open outlet, measured with a pitot gauge held in the stream. It is used to calculate how much water is flowing. You read it at the flowing hydrant during a test; you do not see it on the apparatus.
The drop from static to residual is the key piece of information. It tells you how hard the main is working to deliver the current flow, and from that you can estimate how much more it can give.
NFPA 291 hydrant classes and colours
NFPA 291 rates hydrants by the flow available at 20 psi (140 kPa) residual pressure and recommends painting the bonnet and nozzle caps to show the class.
| Class | Available flow at 20 psi residual | Colour |
|---|---|---|
| AA | 1,500 gpm (5,680 L/min) or more | Light blue |
| A | 1,000 to 1,499 gpm (3,785 to 5,675 L/min) | Green |
| B | 500 to 999 gpm (1,900 to 3,780 L/min) | Orange |
| C | Under 500 gpm (under 1,900 L/min) | Red |
Public hydrants are normally painted chrome yellow on the body, with the bonnet and caps in the class colour. Private hydrants are often painted red overall to distinguish them, and hydrants on non-potable systems should be violet. Many jurisdictions use their own scheme, so learn yours. The colours are only as current as the last flow test, and a hydrant that was green ten years ago may be orange today if the main has tuberculated or the system has grown.
Measuring flow with a pitot gauge
A flow test uses two hydrants on the same main: a test hydrant, where a cap gauge reads static and residual pressure, and one or more flow hydrants downstream, where water is discharged through an open outlet and a pitot gauge reads the velocity pressure in the stream. The flow from each open outlet is calculated with the discharge formula:
Q is flow in gpm, d is the outlet diameter in inches, P is the pitot reading in psi, and c is a coefficient that depends on the shape of the outlet where it meets the hydrant barrel:
- 0.9 for an outlet with a smooth, rounded entrance (most modern hydrants)
- 0.8 for an outlet with a square, sharp-edged entrance
- 0.7 for an outlet that projects into the barrel
You find the coefficient by looking into the outlet with the cap off and feeling the entrance with your fingers. When in doubt, 0.9 is the common default for current hydrants.
Worked example. A 2½-inch outlet with a smooth entrance reads 25 psi on the pitot gauge.
Q = 29.83 × 0.9 × 2.5² × √25 = 29.83 × 0.9 × 6.25 × 5 = 839 gpm (3,175 L/min).
If two outlets are flowing, calculate each and add them. In metric, the same relationship is usually written with d in millimetres and P in kPa, Q = 0.0666 × c × d² × √P in L/min, and gives the same answer within rounding.
The test hydrant gauge reads static pressure before the flow hydrants are opened and residual pressure while they are flowing. You now have the three numbers you need: static, residual, and total measured flow.
Projecting the result to 20 psi residual
A test rarely pulls the main down to exactly 20 psi, so NFPA 291 provides a formula to project the measured flow to what the main would deliver at that residual. NFPA 291 asks for enough test flow to drop the residual by at least 25 percent, so the projection rests on a meaningful drop; the example below drops it 26 percent. The formula comes from the Hazen-Williams relationship between flow and friction loss in pipe:
Q_F is the total measured flow, S is static pressure, R is the residual pressure during the test, and Q_R is the projected flow at 20 psi residual. The exponent 0.54 is the Hazen-Williams exponent, and it reflects the fact that flow in a pipe grows more slowly than pressure drop does.
Worked example. The test above measured 839 gpm. Static pressure was 65 psi and residual during the test was 48 psi.
The pressure drop available down to 20 psi is 65 − 20 = 45 psi. The drop during the test was 65 − 48 = 17 psi. The ratio is 45 ÷ 17 = 2.65. Raised to the 0.54 power, that is about 1.69. So Q_R = 839 × 1.69 = 1,420 gpm (5,375 L/min).
This hydrant is Class A and gets a green bonnet. If the utility wanted to confirm it, it could open a second flow hydrant, pull the residual closer to 20 psi, and measure directly. The projection gets you there without stressing the main.
The same equation, rearranged, is what the Fire Pump Simulator uses to predict residual pressure as you open lines on a simulated hydrant. Pull more flow and the intake gauge falls along the same curve a real main would follow. The seeded "Standard Residential" hydrant, for instance, has a static pressure of about 80 psi (550 kPa) and a residual of about 60 psi (415 kPa) at its test flow, which the simulator rates as Class AA.
Estimating available water on the fireground
A pump operator at a working fire does not have a pitot gauge and a second hydrant. What they have is the intake gauge: a static reading when the hydrant is first opened, and a residual reading after the first line is charged. Three field methods turn those two numbers into a decision about whether the hydrant can supply more lines.
The percentage method
Calculate the drop from static to residual as a percentage of the static pressure. Then:
| Pressure drop | Additional lines of the same flow available |
|---|---|
| 0 to 10 percent | 3 |
| 11 to 15 percent | 2 |
| 16 to 25 percent | 1 |
| Over 25 percent | Additional water is doubtful |
Example. Static 80 psi. You charge a 250 gpm attack line and residual settles at 72 psi. The drop is 8 psi, which is 10 percent of 80. Three more 250 gpm lines are available, about 750 gpm more, for a total around 1,000 gpm.
Example. Static 50 psi. You charge a 250 gpm line and residual drops to 36 psi. The drop is 14 psi, which is 28 percent of 50. This hydrant is nearly tapped out at 250 gpm. Do not add a second line without finding a second source.
This method is the one most texts teach and the one most likely to appear on a certification exam.
The first-digit method
Compare the pressure drop in psi to the first digit of the static pressure. If the drop is less than or equal to the first digit, three more lines are available. If it is less than or equal to twice the first digit, two more. If it is less than or equal to three times the first digit, one more.
Example. Static 70 psi, so the first digit is 7. A drop to 63 psi (7 psi) means three more lines. A drop to 56 psi (14 psi) means two. A drop to 49 psi (21 psi) means one.
The first-digit method is a mental shortcut for the percentage method and gives the same answers for static pressures in the common range.
Squaring the lines
Because friction loss in the main rises with the square of the flow, the pressure drop for two identical lines is roughly four times the drop for one, and for three lines roughly nine times. If one line dropped the residual 5 psi, two lines will drop it about 20 psi and three lines about 45 psi. Square the number of lines, multiply by the first drop, and check whether the residual stays above 20 psi.
Example. Static 70 psi, one line drops it to 65 psi, a 5 psi drop. Two lines: 4 × 5 = 20 psi drop, residual 50 psi. Three lines: 9 × 5 = 45 psi drop, residual 25 psi. Four lines: 16 × 5 = 80 psi drop, which is below zero. Three lines is the limit, and the percentage method agrees: 5 psi is 7 percent of 70.
Mains, grids, and dead ends
Two hydrants a block apart can behave completely differently, because what matters is the pipe behind them. A hydrant on a looped grid is fed from two or more directions, and the residual pressure holds up well under flow. A hydrant on a dead-end main is fed from one direction only, and the residual drops fast. Hydrants near the end of a long dead-end 6-inch main in an older subdivision can be Class C regardless of the static pressure they show when you first open them.
Static pressure, on its own, tells you very little about available flow. A hydrant at 90 psi static on a dead-end 4-inch main may deliver less water than a hydrant at 55 psi on a looped 12-inch grid. The residual drop after the first line is the number that reveals the main.
Two engines on the same main will each see the other's flow in their residual pressure. If the second engine connects to the next hydrant down the street and starts flowing, the first operator's residual drops, and the available water calculation for both must now account for the total flow on the main. Pre-incident planning that maps which hydrants share a main is worth more than any formula at a large fire.
Managing intake pressure at the pump
NFPA 291 rates hydrants at 20 psi residual for a reason that goes beyond hydraulics. Below about 20 psi (140 kPa), a pump drawing hard on a hydrant can pull the pressure in the main toward zero or negative. That risks collapsing old cast iron or thin-walled mains, and it risks pulling groundwater or contaminants into the drinking water system through leaks and cross connections. Water utilities treat 20 psi as the floor, and so should pump operators.
Practically, watch the intake gauge and treat 20 psi residual as the point where you stop adding flow. Some departments allow a 10 psi absolute minimum for a short period at a working fire; know your department's standard. If you need more water than the hydrant can give at 20 psi, the answers are a second hydrant on a different main, a relay from a stronger hydrant, or a supplemental supply from draft or a tanker shuttle. They are not a deeper pull on the hydrant you have.
Intake pressure also feeds directly into your pump discharge pressure. Hydrant pressure arriving at the intake is pressure the pump does not have to generate, which is why the same handline needs less throttle on a strong hydrant than on tank water. The pump discharge pressure guide covers the discharge side of that equation.
Practice this
- Work the percentage method on the next five hydrants you connect to at drills. Note the static, charge a line, note the residual, and say the available lines out loud before you look anything up. It becomes automatic quickly.
- In the simulator, the seeded hydrants range from a strong urban main to a low-pressure rural one. Connect to each, charge a line, read the residual drop, and decide how many more lines you would add. Then add them and see whether the intake gauge agrees with you.
- The hydrant supply job performance requirement in the simulator scores the full evolution: connecting the supply line, bleeding the air, opening the intake, and transitioning from tank to hydrant while refilling the tank and keeping water on the line.
- Ask your water utility for the most recent flow test results in your first-due area. Many will share the data, and comparing it to the bonnet colours you see on the street is often eye-opening.
- The NFPA 1002 study guide covers how available-water questions are framed on the written exam, and the practice test has a bank of them.
The hydrant is the most common water supply in the fire service and the one operators think about least, because it usually works. Knowing how to read what the main is telling you, and knowing where the floor is, is what keeps it working when the fire is big.