Fitness-for-purpose assessment of repaired feedwater heater

Abstract

The leakage of tube-tubesheet joint of feedwater heater on the inlet required redesign and repair by additional tube welding. The original design took into account only the sealing, and subsequently after some period leakage occurred due to tube displacement in tubesheet, caused by thermal and mechanical overload. Repair procedure included cutting out the heater header along the welded joint, preparing and welding the tubes to the tubesheet and repeated welding of the header. The header and tubesheet are made of carbon steel. Welding procedure anticipated an austenitic soft interlayer and local heat treatment for residual stress relaxation before final joining. Fitness-for-purpose assessment has been issued and models were developed by finite element analysis according to the design solution and condition after repair. The results of mechanical and thermal responses of model confirmed that repaired feedwater heater is fit for purpose.

Pricurivanje na spoju cevi i cevne place na ulazu u zagrejač vode je zahtevalo rekonstrukciju i popravku dodatnim zavarivanjem cevi. Prvobitnim projektom je u obzir uzeto samo zaptivanje i posle nekog vremena u radu je došlo do curenja zbog pomeranja cevi u cevnoj ploči, izazvanog termičkim i mehaničkim preopterećenjem. Postupak popravke je predvideo isecanje glave zagrejača po zavarenom spoju, pripremu i zavarivanje cevi za cevnu ploču, i ponovno zavarivanje glave. Glava i cevna ploča su izrađeni od ugljeničnog čelika. Postupak zavarivanja je predvideo austenitni meki sloj i lokalnu termičku obradu za otpuštanje zaostalih napona pre završnog spajanja. Ocena podobnosti za upotrebu je zahtevana i razvijeni su modeli prema projektnom rešenju i stanju posle popravke analizom konačnim elementima Rezultati mehaničkog i termičkog odgovora modela su potvrdili da je popravljeni zagrejač napojne vode podoban za upotrebu.