The coefficient of discharge of a fire hydrant when the outlet is smooth and well-rounded outlet is?

The coefficient of discharge (Cd) is a dimensionless number used in the formula for calculating flow from a fire hydrant outlet. It accounts for the energy losses due to friction and the contraction of the water stream as it exits the orifice. For a hydrant outlet that is smooth and well-rounded, the flow is more efficient with minimal stream contraction, resulting in a higher coefficient. The standard value used in hydraulic calculations for this type of outlet is 0.90. This value is empirically derived and widely accepted in fire protection engineering for flow testing.

B. 0.87: This is not a standard coefficient for any of the common fire hydrant outlet classifications.

C. 0.82: This is not a standard coefficient for any of the common fire hydrant outlet classifications.

D. 0.70: This coefficient is used for a hydrant outlet that is square and projects into the barrel, which represents a significantly less efficient flow path.

1. NFPA Fire Protection Handbook, 21st Edition. Volume 2, Chapter 18, Section 18.5.2.2, "Calculating Hydrant Flow Rate." Table 18.5.2 provides the coefficients of discharge for hydrant outlets, listing 0.90 for "Outlet smooth and rounded."

2. NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, 2022 Edition. Chapter 4, Section 4.9.2. This section provides the formula for computing discharge and references the standard coefficients, stating, "The coefficients for outlets of the following shapes are as follows: (1) Outlet smooth and rounded: 0.90..."

3. Oklahoma State University, Fire Protection Publications. Hydraulic Calculations for Water-Based Fire Protection Systems Workbook (FPST 3333). Chapter 3, "Water Supply Testing," typically outlines the hydrant flow test procedure and lists the standard coefficients of discharge, including 0.90 for smooth and rounded outlets, consistent with NFPA standards.