When you record a flow test, the system calculates several values automatically. This article explains what those values mean and how they are calculated.

Flowing GPM

The flowing GPM (gallons per minute) is how much water is actually coming out of the hydrant during the test. The formula is:

Q = c x d2 x square root of P

Where:

  • Q is the flow rate in GPM
  • c is the discharge coefficient (0.9 for smooth, 0.8 for square edge, 0.7 for projecting)
  • d is the outlet diameter in inches
  • P is the pitot reading in PSI

Worked example

You test a hydrant with a smooth 2.5-inch outlet and get a pitot reading of 25 PSI.

Q = 0.9 x 2.52 x square root of 25

Q = 0.9 x 6.25 x 5

Q = 28.13 GPM

That is the actual flow from the 2.5-inch port. For a steamer port, the numbers are much higher because the diameter is larger.

Another example with a steamer port

Same hydrant, but you open the 4.5-inch steamer instead, and the pitot reads 10 PSI.

Q = 0.9 x 4.52 x square root of 10

Q = 0.9 x 20.25 x 3.16

Q = 57.59 GPM

Rated capacity at 20 PSI

The rated capacity estimates how much water the hydrant could deliver if the residual pressure dropped all the way to 20 PSI. This is the standard used to compare hydrants across different water systems. The formula is:

Rated = Q x ((Static - 20) / (Static - Residual))0.54

Where:

  • Q is the flowing GPM from the formula above
  • Static is the static pressure in PSI (no flow)
  • Residual is the residual pressure in PSI (during flow)

Worked example

Your test gave you a flowing GPM of 800. Static pressure was 65 PSI and residual was 50 PSI.

Rated = 800 x ((65 - 20) / (65 - 50))0.54

Rated = 800 x (45 / 15)0.54

Rated = 800 x 30.54

Rated = 800 x 1.78

Rated = 1,424 GPM

This hydrant has a rated capacity of about 1,424 GPM at 20 PSI residual.

NFPA 291 classification

The National Fire Protection Association standard 291 assigns a class and color to each hydrant based on its rated capacity. This is the same color system many departments use when painting hydrant caps.

  • Class AA (Blue) -- 1,500 GPM or more. Excellent supply. Suitable for large commercial or industrial buildings.
  • Class A (Green) -- 1,000 to 1,499 GPM. Good supply. Suitable for most residential and light commercial areas.
  • Class B (Orange) -- 500 to 999 GPM. Fair supply. May require a second supply line for larger fires.
  • Class C (Red) -- Under 500 GPM. Poor supply. A tanker or second hydrant will likely be needed.

In the example above, the hydrant with a rated capacity of 1,424 GPM would be classified as Class A (Green).

What if the rated capacity does not calculate

The rated capacity cannot be calculated if the static pressure is 20 PSI or below, or if the static pressure is the same as (or less than) the residual pressure. In those cases, the system shows a dash instead of a number. This usually means the gauge was not read correctly or the water system was under unusual stress during the test.

Tip

Pay close attention to the residual pressure. If it drops below 20 PSI during a test from a single port, the water system at that location is weak. Note it in the test and flag the hydrant for your officers to review during pre-incident planning.