AIR INGESTION AND TRANSPORT TESTING IN A ROTATING DRUM RAW WATER STRAINER

摘要

McGuire Nuclear Station (MNS) is performing ongoing work upgrading components and systems in their raw water supply (RN) system. The RN system supplies the cooling water for residual heat removal and represents the assured source of heat removal during emergency shut-down. At MNS, the main source of cooling water is Lake Norman but an additional ultimate heat sink pond, the Standby Nuclear Service Water Pond (SNSWP) represents the assured source of cooling water during an accident which has Lake Norman unavailable as a source for cooling water. When the RN system shifts suction from Lake Norman to the SNWP, the suction piping becomes nearly a half mile long and the inlet pressure to the strainer, which is installed on the suction side of the RN pump drops to vacuum conditions, depending on flow rate. The RN strainer is a rotating drum strainer, spinning at a constant 4.5 RPM. The motor and gear-box connection for the spinning filter element is at the top of the strainer, sealed by four rings of packing. Over time, the packing seals can wear and allow water to leak out through the packing since the strainer operates at about 10 psig (positive) under normal operating conditions. Under vacuum conditions, the water leakage out of the drum will stop and air ingestion into the drum will begin. A wide range of strainer and backwash system performance tests was conducted during the summer of 2011 at Alden Laboratory using a full-size raw water strainer taken out of service from MNS. Part of these tests concerned a quantitative determination of the rate of strainer air ingestion and an assessment as to the path of transport (if any) for the ingested air. Testing revealed that a sizeable air bubble forms on the interior side of the drum strainer, downstream of the filter elements. The air bubble appeared stable in testing under constant operating conditions and was largest at low system flow rates. When the test flow rate of the system was increased, the new stable, lower bubble volume was achieved quickly and the difference in bubble volume was transported downstream quickly. Such sudden transport of air has the potential to impact pump performance. However, a review of the plant system and operation reveals that the transient responsible for the sudden transport of air downstream is not possible. Nevertheless, the understanding gained from testing regarding the transport and accumulation dynamics of the ingested air has been helpful in better understanding what the likely impacts of air ingestion are on system performance.