Development of Prototype Valve to Separate Two-Phase Air-Water Flows

A certain nuclear power plant uses a siphon to bring cooling water into the plant during emergency operations. This siphon was “hardened” by adding a nuclear grade vacuum system to the siphon high points. A float valve, sized for the required air flow rate, was used to prevent water carryover into the vacuum system. In order to pass the required flow rate, the valve’s orifice size would be the largest produced by any manufacturer to date. Initial system testing revealed that the valve was not fully opening due to the high vacuum and the design of the valve opening mechanism. Analysis and testing developed a solution to the opening problem. However, a new problem was introduced — excessive liquid carryover. The liquid carryover rate was estimated at 6 gallons per minute. The liquid carryover was postulated to be due to droplet entrainment as high velocity air bubbles entered the valve body, ruptured the air-water interface, and carried some of the resultant droplets through the valve outlet as the valve opened due to falling water level. Additionally, valve operating mechanism changes resulted in it responding slower. Using information learned from modifying the original valve, a new valve design was conceived that would be based on the following 3 principles: 1) liquid momentum must cause it to separate from the air upon entrance into the valve body; 2) enough distance and space must exist inside the body to allow gravity to act upon any liquid droplets to allow them to fall back into the liquid pool instead of being carried out of the valve toward the vacuum system; 3) valve operating mechanism must respond quickly enough to close the valve upon rapid liquid influx. The valve manufacturer reviewed the conceptual design and produced a prototype. The prototype valve was successfully tested with zero water carryover plus better flow versus pressure loss performance than the original valve. The valve also functioned acceptably during shaker table testing to simulate earthquake conditions.