Which equation should be used to determine the frictional resistance in a standpipe system?

• A. p = 3.14Q/D
• B. p = 4.87Q/Cd
• C. p = 2.71Q/Cd
• D. p = 6.28Q/Cd

Answer: (B)

Frictional resistance in a standpipe is assessed with a simplified friction-loss relation that links pressure loss directly to the discharge and the system’s discharge coefficient. The equation p = 4.87 Q / Cd is used, where p is the friction loss (psi), Q is the discharge (gpm), and Cd is the coefficient of discharge that accounts for losses due to piping, fittings, and hose. The constant 4.87 comes from unit conversions so the result is in psi when using gpm and a dimensionless Cd. As flow (Q) increases, friction loss rises; a higher Cd (better discharge efficiency) lowers the pressure loss. The other constants (3.14, 2.71, 6.28) don’t match the standard NFPA 14 friction-loss formula, so they aren’t used for this purpose.