Pump Systems

A pump adds mechanical energy to a liquid, raising its pressure and/or elevation as it moves through a piping system. The energy added per unit weight of fluid is the pump head H, expressed in metres of the pumped liquid. The total dynamic head (TDH) that the pump must supply is the sum of the static head, the pressure head difference, and the friction and minor losses in the system: TDH = (P2 - P1)/(ρg) + (V2² - V1²)/(2g) + (z2 - z1) + h_L. The system curve plots the required head against flow rate; since friction losses vary roughly as h_f ∝ Q², the system curve is approximately parabolic. The pump characteristic curve, supplied by the manufacturer, shows the head the pump can deliver at each flow rate for a given impeller diameter and speed. The operating point is the intersection of the pump curve and the system curve. Pump power is P_hyd = ρ g Q H, and the shaft power is P_shaft = P_hyd / η_pump, where η_pump is the pump efficiency at the operating point. Pumps in series add head at the same flow; pumps in parallel add flow at the same head. Cavitation occurs when the absolute pressure at the pump suction drops below the vapor pressure of the liquid, causing vapor bubbles to form and collapse violently, which erodes the impeller and causes noise and vibration. Net Positive Suction Head (NPSH) quantifies the margin against cavitation. NPSH_available = (P_atm ± P_suction_gauge - P_vapor)/(ρg) + z_s - h_L,suction, where z_s is the static suction head (positive for a flooded suction, negative for a suction lift). The pump must be selected so that NPSH_available > NPSH_required, with a safety margin of typically 0.5 to 1.0 m for small pumps and larger margins for hot or volatile liquids. Affinity laws describe how pump performance scales with speed N and impeller diameter D: Q ∝ N, H ∝ N², P ∝ N³. Variable-speed drives are widely used to match pump output to demand, because reducing speed rather than throttling a valve saves substantial energy. Common pump types include centrifugal (radial flow, best for medium to high head), axial (high flow, low head), and positive-displacement (constant flow, high pressure).