Purpose

1. To provide an overview on the calculations used for hydrant flow testing.
   
   

Formula

1. First Due uses the Hazen-Williams formula to calculate the results of a hydrant flow test.
   

   
   

   The Hazen–Williams equation is an empirical relationship which relates the flow of water in a pipe with the physical properties of the pipe and the pressure drop caused by friction.
   It is used in the design of water pipe systems such as fire sprinkler systems, water supply networks, and irrigation systems. (Source: Wikipedia)

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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.)