Troubleshooting Condenser Vacuum Issues at North Omaha Station Unit 1

Abstract

In 2004, North Omaha Station Unit 1 (NOS 1) experienced multiple condenser vacuum upsets. At least one of them resulted in a unit trip. The upset conditions occurred over relatively short periods of time with no clear indication of the initiating mechanism. Overall, condenser vacuum was low. Various methods were employed to combat the vacuum issues and upset conditions. These included operating both sets of holding steam jet air ejectors (SJAEs) above 600 psig (50% above design), using the hogging SJAE during unit operation and, operating the condensate pumps in a recirculating mode. Air inleakage was a known problem on NOS 1. The air inleakage was no longer measurable since it exceeded the scale of the installed instrumentation (20 scfm). Besides air inleakage, tube fouling of the condenser tubes was also contributing to degraded condenser vacuum. NOS 1 had a history of fouling due to calcium carbonate plateout on the condenser tubes. During the January 2005 outage, major sources of air inleakage were identified and fixed. Leaking tubes in the SJAE intercondenser/aftercondenser were plugged. The condenser tubes were scraped to reduce fouling. Although condenser vacuum improved, problems persisted at low loads. Following a vacuum upset in June 2005, the hogging SJAE was placed into service. Helium testing in July 2005 indicated high air inleakage. The problems continued to persist and, in February 2006, the unit tripped on low condenser vacuum. At that time, the unit had been operating at about 58 Mwe. In order to maintain the unit on line at a reduced load of 40 Mwe, both the hogging SJAE and one set of first stage and second stage holding SJAEs had to be deployed. An attempt to remove the hogging SJAE from service was unsuccessful since it resulted in rapid decrease in air-removal capability of the holding SJAEs. This paper describes the methodology used to troubleshoot the condenser vacuum issues for NOS 1 and remedies proposed for proper performance and reliable operation.