11.4 NFPA 291 Hydrant Flow Tests, Pitot Math & Marking Colors

Three pressures, two hydrant jobs

NFPA 291 (2022) is a recommended practice for water-flow testing and hydrant marking, not a sprinkler-install chapter. Independent OpenExamPrep material covers it because NICET Water-Based I still tests assisting with a flow test (task 1.5.3) and reading the resulting numbers.

Memorize three defined readings:

• Static pressure (3.3.4): pressure at the residual hydrant under normal distribution conditions with no hydrants flowing. In the field sequence you open that hydrant, vent air, then read static with the other hydrants still shut.
• Residual pressure (3.3.3): pressure at the same residual hydrant while the flow hydrants are discharging. Residual is not the pitot number.
• Pitot / velocity head: the pitot-tube reading in the flowing stream, used only to compute discharge from that outlet.

Rated capacity (3.3.2) is the flow available at a designated residual—20 psi (1.4 bar) for uniform marking (4.2.1). Many health departments forbid dropping mains below 20 psi (4.2.4). Fire flow itself is defined at 20 psi residual (3.3.1).

Layout and the Level I job

For available water in the main (4.4), pick a group of hydrants. The residual hydrant (R) sits between the flowing hydrants and the large supply mains. Flow hydrants (F) discharge. Weak systems may need only one or two flowing hydrants; strong systems may need several (4.4.8–4.4.9). Give thought to traffic, flooding, landscaping, and pedestrians (4.4.2). Inspect the hydrant and the area for safety before flowing (4.3.3). Tests should be during peak demand when practical (4.3.1).

To support theoretical rated-capacity calculations, 4.4.6 wants a residual-hydrant pressure drop of at least 10 percent. (Older classroom notes that insist on 25 percent are not the 2022 sentence.) If municipal pumps rise as you open more hydrants, the recommended practice even discusses declaring an artificial 10 percent static drop to draw a usable curve.

Level I assists. The qualified person sets the test plan—which hydrant is R, which are F, how many butts to open, when to shut down, and how to notify the water utility. Your hands hold the pitot in the correct position, your eyes read gauges on cue, and your body keeps bystanders and vehicles out of the stream. You do not freestyle a second flowing hydrant because the pitot looks low.

Equipment in 4.6 includes a tapped hydrant cap with a cock, a 100 psi or 200 psi Bourdon gauge (200 psi if static exceeds 100), a pitot and gauge per simultaneous flowing hydrant, wrenches enough to operate together, and preferably playpipes or stream straighteners with known coefficients. Calibrate gauges at least every 12 months (4.6.3).

Procedure in 4.7, condensed: attach the gauge to a 2½ in outlet of R; open R and vent air; close the cock; read static; on signal open flow hydrants one at a time; let debris clear; on signal read pitot(s) and residual together; shut down slowly, one hydrant at a time, to limit surge.

Pitot math — Equation 4.9.3a

NFPA 291 (2022) Equation 4.9.3a:

Q = 29.84 × c × d² × √p

Q is gpm, c is the outlet coefficient, d is outlet diameter in inches, p is pitot pressure in psi. Fire-service pocket cards often round the constant to 29.83. The difference on a 671 gpm example is about 0.2 gpm. Use whichever constant the stem prints; this section uses 29.84 because that is the 2022 equation.

Figure 4.9.1 coefficients (do not swap the middle and projecting rows):

Prefer 2½ in (65 mm) butts; they usually flow full. Pumper outlets often have a void at the bottom (4.8.1–4.8.2). Hold the pitot orifice about one-half the outlet diameter downstream, in the center of the stream, at right angles to the face, air chamber up (4.8.3–4.8.5). Avoid pitot readings under 10 psi when you can (4.8.6). Dry-barrel hydrant valves should be wide open so the drain is not cheating the stream (4.8.8).

Worked pitot (clean squares): 2½ in rounded outlet, c = 0.90, pitot 16 psi.

√16 = 4

d² = 2.5 × 2.5 = 6.25

29.84 × 0.90 = 26.856

26.856 × 6.25 = 167.85

167.85 × 4 = 671.4 gpm (report 671 gpm)

Same outlet, projecting c = 0.70: 29.84 × 0.70 × 6.25 × 4 = 522 gpm. Same outlet with a 0.95 tube: 709 gpm. Using c = 0.80 on a rounded butt understates flow; using 0.90 on a projecting butt overstates it.

Second clean pitot: same 2½ in rounded butt, pitot 36 psi, √36 = 6; 167.85 × 6 = 1,007 gpm. That flowing-butt number is still not the class color until you convert to 20 psi residual.

Convert to 20 psi residual — Equation 4.12.1.2

QR = QF × (hr / hf)^0.54

QR is predicted flow at the desired residual, QF is measured test flow, hr is the pressure drop from static to the desired residual (usually static − 20 psi), and hf is the drop actually measured (static − residual). Same units on both drops.

Worked prediction: static 70 psi, residual 50 psi, QF = 671.4 gpm, desired residual 20 psi.

hr = 70 − 20 = 50 psi
hf = 70 − 50 = 20 psi
hr / hf = 2.50
2.50^0.54 ≈ 1.640
QR = 671.4 × 1.640 ≈ 1,101 gpm at 20 psi residual.

Marking colors at 20 psi residual (Chapter 5 / 5.1)

The 1,101 gpm result is Class A, green. The 671 gpm pitot flow, if someone painted from it without the 0.54 conversion, would have been orange—and wrong. Barrels are commonly chrome yellow unless the jurisdiction has adopted another barrel color. Hydrants whose rating is below 20 psi should have that pressure stenciled in black on the top. Public hydrants should be flow tested at least every 5 years and flushed at least annually (4.15).

A single-hydrant test that uses the same hydrant for static, residual, and flow (4.5) measures that hydrant, not the main the same way a two-hydrant test does. Do not color an entire block from a 4.5 hydrant-only test unless the qualified person says that is the rating method.

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