Purpose Statement

This article provides an overview of the calculations used during hydrant flow testing within First Due. Understanding these calculations helps users accurately record hydrant test data and interpret the resulting rated capacity of a hydrant.

Background Information

Hydrant flow testing is used to measure the available water supply within a distribution system. These tests help fire departments and water authorities determine the water flow capacity available for firefighting operations and planning.

First Due uses the Hazen–Williams formula to calculate hydrant flow test results.

The Hazen–Williams equation is an empirical relationship that relates the flow of water in a pipe to the physical properties of the pipe and the pressure drop caused by friction. This formula is commonly used in the design and analysis of water pipe systems such as:

• Fire sprinkler systems

• Municipal water supply networks

• Irrigation systems

(Source: Wikipedia)

During a hydrant flow test, several key data points must be collected to perform the calculation.
 

Calculations  

1. When flow testing hydrants you will need the following information:
   1. Outlet Size
      1. Outlet size is the diameter of the hydrant opening (in inches) that will be used during testing.
   2. Static Pressure
      1. Static Pressure is the pressure that exists at a given point under normal distribution system conditions measured at the residual hydrant with no hydrants flowing. (Source: NFPA 291 3.3.4, 2022 ed.)
   3. Residual Pressure
      1. Residual Pressure is pressure that exists in the distribution system measured at the residual hydrant at the time the flow readings are being measured at flow hydrant(s). (Source: NFPA 291 3.3.3, 2022 ed.)
   4. Coefficient
      1. For hydrant testing there are three coefficients of discharge used: 0.7, 0.8, 0.9.  
         1. The design of the hydrant will determine which of the three coefficients will be used. (Source: NFPA 291 4.9.1, 2022 ed.)
            
            
             
      2. If a stream straightener is used during testing NFPA suggests using 0.95 as the coefficient unless the coefficient of the tube is known. (Source: NFPA 291 4.9.2, 2022 ed.)
          
2. Once you have entered the static pressure, residual pressure and coefficient the program will use the Hazen Williams formula to calculate the Rated Capacity of the hydrant.
   1. Rated Capacity is the flow available from a hydrant at the designated residual pressure (rated pressure), either measured or calculated. (Source: NFPA 291 3.3.2, 2002 ed.)