By Peter Tallman CTI Industries, Inc. And Jim Smith NRG
It is common experience that many failures of condenser tubes in the electric utility industry occur within the first six inches of the bundle. Frequently encountered forms of such tube end damage are erosion, impingement attack, stress corrosion cracking and pitting/crevice corrosion. The damage variously manifests itself as tube thinning, grooving, localized pitting and cracking, all of which can ultimately lead to leakage and tube failure.
In the past, the accepted solution for repairing this highly localized tube damage has been full retubing, even though usually more than 95% of the tube bundle length remains largely unaffected. However, this is a radical solution, which is very costly and time consuming. Today, with the emergence of deregulation of electric utilities, a proven, cost-effective alternative to retubing must be strongly considered by plant engineers.
Historically, the most familiar stratagem for protecting damaged tube ends against further erosion or erosion-corrosion, and thus extend condenser tube life, is the use of plastic inserts. These inserts or tube protectors are only partly effective because they are not pressure tight and therefore unsuited for restoring leaking tubes to service. Also, plastic inserts can cause end step erosion and tend to become dislodged over time, especially during cleaning and back-flushing operations.
With the objective of dealing with these shortcomings, a new restoration technique was developed in the mid-1970's. It makes use of thin-walled metallic inserts, variously referred to as I.D. tube shields, or sleeves, which are expanded into the existing tube ends, adding years of additional service to the exchanger.