Air Entrapment Issues in Piping for a Small Hydroelectric Station in Western North Carolina

A team from Western Carolina University’s Rapid Center in Cullowhee, North Carolina visited a local fish farm to determine why a previously installed stream-powered, electricity generating system was not working correctly. The team met the owner and a local Agricultural Extension Office Manager (AEOM), at the site. The AEOM provided the team with a tour of the primary structures and equipment. The system inspected consists of four water-powered impulse type turbines connected to 1320 feet of 8-inch black plastic, seamless pipe. The pipe inlet provides 98 feet of head (42.2 psig) to power the turbines. The turbines provide wattage generation based upon delivered water flow at a specific pressure. The higher the water flow and pressure, the higher the turbine wattage output, up to 1 kilowatt each, maximum output as installed. The turbines appeared to operate correctly, but the water pressure was measured to be less than optimum, affecting the turbine wattage output. The team inspected the length of the pipeline and found issues of concern. First, the pipe inlet suction structure was not totally flooded. It appeared that air might be introduced into the pipeline through the suction opening by the venturi effect. Secondly, the static pressure readings taken at the turbine building were not as expected. The expected static pressure, considering the 98 feet of head was 42.4 psig. The actual reading was 22 psig. The team discussed the static pressure issue and inspected the pipeline, finding two sections were not flooded with water and contained air. There were sections of the pipeline, which had slipped off their support structures. Since no major leaks were detected, air entrapment was considered as the cause of the head pressure issues. The sections of the pipeline which were out of position and had slipped off their support structures could trap air. This paper discusses the measures taken to correct the inlet water piping to the stream powered turbines. Introduction A team of engineers and graduate students from Western Carolina University’s Rapid Center visited a trout fish farm near Robbinsville, North Carolina to determine why a previously installed stream powered, electricity generating system was not working correctly. The Team met the owner and a local Agricultural Extension Office Manager (AEOM), at the site. The AEOM provided the team with the history of the system and a tour of the primary structures and equipment which had been installed by a professional solar, hydroelectric power installation contractor. The Rapid Center is a university-based research and development center. The Center’s industryexperienced faculty and staff work with business partners to address their specific needs and to remove obstacles to product commercialization and process improvement. The Rapid Center fosters a culture of collaborative innovation that helps clients refine existing products, develop new ones and improve business practices. At the same time, the Center’s engineering and technology students, working with faculty mentors and course instructors, work to address realworld problems for project sponsors while they gain hands-on experience during the twosemester senior capstone project. [1] Discussion The turbine system inspected consists of four water-powered, impulse type turbines connected to about 1320 feet of 8-inch seamless black plastic pipe. The pipe inlet provides 98 feet of head (42.2 psig) to power the turbines. The turbines provide a certain wattage generation based upon delivered water flow at a specific pressure, see Table 1. The higher the water flow and pressure, the higher the turbine wattage output. The turbines appear to operate correctly, but the supply water pressure was measured to be less than optimum, affecting the turbine wattage output. Maximum output for each turbine is 1900 watts each, but for the system as installed, 1000 watts each is expected. For this system, 8” pipe will convey 700 gpm per Crane No. 410 [3] with less than 2 psig pressure drop in 1300 feet. At 98 feet of head, each turbine can produce 1000 watts (1 KW) of power at 12 volts, direct current. The required flow rate for the four turbines combined is 448 gpm total, well within the capabilities of the plastic supply line. Stream Engine Output in Watts (Continuous) Flow rate: Liters per second (LPS) Gallons per minute (GPM) 0.67 1.33 2.5 5 6.67 7.5 9.5 Meters Ft 10 20 40 75 100 112 150 3 10 ** 20 40 75 100 130 150 6 20 15 40 80 150 200 250 350 15 49 45 100 200 375 500 650 800 30 98 80 200 400 75