The Impact of Tube Plugging on Closed Feedwater Heater Operation and Maintenance

Abstract

Failures of tubes in closed feedwater heaters are due to localized thermal, mechanical and chemical processes, in combination with the susceptibility of the tube material involved. Experience has demonstrated that changes in operating practice such as feedwater chemistry, frequency of exposure to atmosphere on shutdown and other factors often modify the environmental conditions and introduce unanticipated causes of tube failure. Good practice calls for immediate corrective action, whenever tubes fail, to arrest extension of the problem. Such corrective action is based on an accurate determination of the location of each failure with respect to the tube pass, position in the tube field and distance from the face of the tubesheet. Nondestructive examination and individual tube hydro test of adjacent and other suspect tubes for incipience of failure is required, including the examination of failure specimens, if they can be recovered. More than one failure mechanism may be in process in a heater and no problem potential should be treated casually. Analysis of tube plugging should consider the affect of each failure on heater performance and other equipment in the system, the probability of chronic, continuing failures and the ultimate number of tubes that can be plugged before plant capacity is curtailed. Once the tubes that should be plugged have been determined, plugging systems and installation techniques must also be addressed, with emphasis on preventing secondary damage from failed tubes and the avoidance of any failure caused by the plugs themselves.