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A heat exchanger tube gets all steamed up
Engineering Failure Analysis 8 (2001) 133±134
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A heat exchanger tube gets all steamed up A. van Bennekom School of Process and Materials Engineering, University of the Witwatersrand, Private Bag 3, P.O. WITS 2050, South Africa Received 10 June 1999; accepted 26 October 1999
Keywords: Corrosion; Heat-exchanger failures; Plastic deformation; Pressure dierence
This technical note serves to communicate a very interesting phenomenon encountered during the course of a failure investigation which dealt with the thinning and ultimate perforation of a type 316L stainless steel heat exchanger tube, 15 mm in diameter. The tube in question had an original wall thickness of 0.8 mm and failed after a relatively short time in service. The relevant operating conditions of the secondary heat exchanger, in which the tube failed, are presented in Table 1. Examination of the failed heat exchanger tube showed that it had thinned to perforation due to general corrosion from the inside of the tube (see Fig. 1). This mode of failure was con®rmed by the complete absence of corrosion on the outer surface of the tube as was seen by the presence of the original circumferential markings on the outer surface. The cause of failure was thus quite obvious, i.e., corrosion caused the wall of the tube to thin to the point of perforation. The pressure dierence between the inside and outside of the tube would then be expected to result in plastic deformation of the lips adjacent to the perforated region, as can be observed in the ®gure. Since the lips of the perforated region have been pushed (deformed) inwards, this would indicate that greater pressure must have existed on the outside of the tube. The operating conditions were, however, such that greater pressure was present on the inside of the tube (see Table 1), and as such, the thinned portions of metal should have been pushed outward by the pressure dierence. This abnormality resulted in the operating pressures of the heat exchanger being checked, but these were found to conform to the ranges speci®ed in Table 1. After much thought and pondering, the reason for this abnormality became clear and may be explained as follows. Since the tubes carry super-heated water at high pressure, any perforation of the tube will allow this water to escape, and if the pressure outside the tube is less than inside the tube and/or if the temperature outside the tube is greater than inside the tube, the water will instantaneously vaporise. The very rapid expansion of the steam escaping from the tube will result in the lips around the perforation E-mail address: [email protected] (A. van Bennekom). 1350-6307/01/$ - see front matter 7 2001 Published by Elsevier Science Ltd. PII: S 1 3 5 0 - 6 3 0 7 ( 9 9 ) 0 0 0 4 2 - 4
134
A. van Bennekom / Engineering Failure Analysis 8 (2001) 133±134
Table 1 Relevant information pertaining to the secondary heat exchanger from which the failed tube originated Operating conditions for the secondary heat exchanger Tube side Temperature Minimum Maximum Pressure Maximum Solution type Softened water Shell side Temperature Minimum Maximum Pressure Maximum Solution type Thermic oil
Ambient 140±1608C 6.5 bar Ambient 2508C 0.5 bar
being pushed towards the inside of the tube since the thermic oil will not be able to immediately accommodate the volume of escaping steam from the tube. It is thus, clear that the observed plastic deformation is not simply the result of a dierence in the operating pressures of the shell and tube sides of the heat exchanger, but is rather a result of the increased pressure that was encountered due to the vaporisation of the super-heated water stream that escaped from the perforation in the tube.
Fig. 1. The heat-exchanger tube: exterior view (left-hand side); interior view (right-hand side).
www.elsevier.com/locate/engfailanal
A heat exchanger tube gets all steamed up A. van Bennekom School of Process and Materials Engineering, University of the Witwatersrand, Private Bag 3, P.O. WITS 2050, South Africa Received 10 June 1999; accepted 26 October 1999
Keywords: Corrosion; Heat-exchanger failures; Plastic deformation; Pressure dierence
This technical note serves to communicate a very interesting phenomenon encountered during the course of a failure investigation which dealt with the thinning and ultimate perforation of a type 316L stainless steel heat exchanger tube, 15 mm in diameter. The tube in question had an original wall thickness of 0.8 mm and failed after a relatively short time in service. The relevant operating conditions of the secondary heat exchanger, in which the tube failed, are presented in Table 1. Examination of the failed heat exchanger tube showed that it had thinned to perforation due to general corrosion from the inside of the tube (see Fig. 1). This mode of failure was con®rmed by the complete absence of corrosion on the outer surface of the tube as was seen by the presence of the original circumferential markings on the outer surface. The cause of failure was thus quite obvious, i.e., corrosion caused the wall of the tube to thin to the point of perforation. The pressure dierence between the inside and outside of the tube would then be expected to result in plastic deformation of the lips adjacent to the perforated region, as can be observed in the ®gure. Since the lips of the perforated region have been pushed (deformed) inwards, this would indicate that greater pressure must have existed on the outside of the tube. The operating conditions were, however, such that greater pressure was present on the inside of the tube (see Table 1), and as such, the thinned portions of metal should have been pushed outward by the pressure dierence. This abnormality resulted in the operating pressures of the heat exchanger being checked, but these were found to conform to the ranges speci®ed in Table 1. After much thought and pondering, the reason for this abnormality became clear and may be explained as follows. Since the tubes carry super-heated water at high pressure, any perforation of the tube will allow this water to escape, and if the pressure outside the tube is less than inside the tube and/or if the temperature outside the tube is greater than inside the tube, the water will instantaneously vaporise. The very rapid expansion of the steam escaping from the tube will result in the lips around the perforation E-mail address: [email protected] (A. van Bennekom). 1350-6307/01/$ - see front matter 7 2001 Published by Elsevier Science Ltd. PII: S 1 3 5 0 - 6 3 0 7 ( 9 9 ) 0 0 0 4 2 - 4
134
A. van Bennekom / Engineering Failure Analysis 8 (2001) 133±134
Table 1 Relevant information pertaining to the secondary heat exchanger from which the failed tube originated Operating conditions for the secondary heat exchanger Tube side Temperature Minimum Maximum Pressure Maximum Solution type Softened water Shell side Temperature Minimum Maximum Pressure Maximum Solution type Thermic oil
Ambient 140±1608C 6.5 bar Ambient 2508C 0.5 bar
being pushed towards the inside of the tube since the thermic oil will not be able to immediately accommodate the volume of escaping steam from the tube. It is thus, clear that the observed plastic deformation is not simply the result of a dierence in the operating pressures of the shell and tube sides of the heat exchanger, but is rather a result of the increased pressure that was encountered due to the vaporisation of the super-heated water stream that escaped from the perforation in the tube.
Fig. 1. The heat-exchanger tube: exterior view (left-hand side); interior view (right-hand side).