Fire Pump Suction Classification of Positive and Negative Suction One of the most critical and often misunderstood aspects of fire pump design is suction condition. As per NFPA 20, the performance, reliability, and even pump selection directly depend on whether the system has positive suction or negative suction. 1. Positive Suction (Preferred by NFPA) Positive suction exists when the water level is above the fire pump impeller centerline. This means the pump is gravity-fed, ensuring stable and