NFPA 13 Section 11 addresses the hydraulic design ensuring adequate water flow from sprinklers. Design density is the required flow rate per square foot of area to suppress fires. Ordinary hazard design density is 0.15 gallons per minute per square foot over 1,500 square foot design area. This means a 1,500 square foot area requires 225 gallons per minute total system flow. Extra hazard areas require design densities of 0.30-0.40 gallons per minute per square foot, requiring much larger system capacity. Pressure at the sprinkler head must be sufficient to achieve proper spray pattern, typically 7-40 pounds per square inch depending on head type. Pressure is calculated using friction loss formulas based on pipe diameter and flow rate. Larger diameter pipe reduces friction loss, while smaller diameter increases loss. System supply must deliver adequate pressure at the highest elevation of the system.