ENGINEER'S NOTEBOOK COLUMN David Sellers Part 1 Net Positive Suction Head BY DAVID SELLERS, P.E., MEMBER ASHRAE Net positive suction head (NPSH) is a complex concept, and the piping on the project shown in Figure 1 suggested that the concept was not fully understood and embraced by the project team. It took me a while to feel comfortable with NPSH. This column is the fi rst of two parts, the goal of which is to share the NPSH lessons learned over the years in the hopes others might benefi t from the insights. FIGURE 1 Positive pressure at the pump suction will go a long way toward preventing NPSH problems. The fact that the pump suction line for this system rises above the cold basin level means that the pressure in the suction line is subatmospheric from the start. Pressure drops in the piping will compound the problem. Visit http://tinyurl.com/ASHRAE-NPSH01 to tour. The blue condenser water piping confi guration leaving the counterfl ow cooling towers in Figure 1 was just one component of the problem we observed on that recent site visit. The linked tour (http://tinyurl.com/ ASHRAE-NPSH01) will allow you to observe several other contributors, but none as obvious as shown in Figure 1. Pump Suction Piping Rises Above the Cold Basin Condenser Pumps Tower Cold Basin Equalizer Line Preventing Cavitation NPSH is all about preventing cavitation-a phenomenon that happens with pumps (and control valves and propellers on ships) due to the reduction in static pressure that occurs as a fl uid accelerates to pass through the smaller cross-sectional area represented by the eye (and immediate vicinity) of the impeller. It can ruin the pump impeller, control valve plug or ship propeller. Most people describe it as sounding like rocks bouncing around inside the pump. You can hear an example of it in the tour associated with Figure 1. Bernoulli's equation (a simplifi ed form of the steady fl ow energy equation) suggests if the fl uid needs to accelerate (increase kinetic energy), the energy to David Sellers, P.E., is senior engineer at Facility Dynamics Engineering. 22 ASHRAE JOURNAL ashrae.org J U N E 2 0 2 4http://tinyurl.com/ASHRAE-NPSH01 http://tinyurl.com/ASHRAE-NPSH01 http://tinyurl.com/ASHRAE-NPSH01 http://www.ashrae.org