Before revision: Taking the general form and exponentiating each side by gives (rounding exponents to 3–4 decimals) Rearranging gives The flow rate , so The hydraulic radius (which is different from the geometric radius ) for a full pipe of geometric diameter is ; the pipe's cross sectional area is , so U.S. customary units (Imperial) When used to calculate the pressure drop using the US customary units system, the equation is:2007 version of NFPA 13: Standard for the Installation of Sprinkler Systems, page 13-213, eqn 22.4.2.1 where: Spsi per foot = frictional resistance (pressure drop per foot of pipe) in psig/ft (pounds per square inch gauge pressure per foot) Pd = pressure drop over the length of pipe in psig (pounds per square inch gauge pressure)L = length of pipe in feetQ = flow, gpm (gallons per minute)C = pipe roughness coefficientd = inside pipe diameter, in (inches) Note: Caution with U S Customary Units is advised.
After revision: Sfoot of water per foot of pipeTaking the general form and exponentiating each side by gives (rounding exponents to 3–4 decimals) Rearranging gives The flow rate , so The hydraulic radius (which is different from the geometric radius ) for a full pipe of geometric diameter is ; the pipe's cross sectional area is , so U.S. customary units (Imperial) When used to calculate the pressure drop using the US customary units system, the equation is:2007 version of NFPA 13: Standard for the Installation of Sprinkler Systems, page 13-213, eqn 22.4.2.1 where: Spsi per foot = frictional resistance (pressure drop per foot of pipe) in psig/ft (pounds per square inch gauge pressure per foot) Pd = pressure drop over the length of pipe in psig (pounds per square inch gauge pressure)L = length of pipe in feetQ = flow, gpm (gallons per minute)C = pipe roughness coefficientd = inside pipe diameter, in (inches) Note: Caution with U S Customary Units is advised.