A Case Study: Corrosion Failure of Tube Heat Exchanger

Transcription

1 doi: /me A Case Study: Corrosion Failure of Tube Heat Exchanger Ibrahim M Ghayad, Z. Abdel Hamid, N. Gomaa Central Metallurgical Research & Development Institute (CMRDI), P.O.Box:87 Helwan, Cairo, Egypt ighayad@yahoo.com Abstract A failed heat exchanger was sent to CMRDI to investigate the possible causes of leaks occurred in the tubes of the heat exchanger. The heat exchanger tubes were made of stainless steel 316 straight type. The heat exchanger served to heat up water basin (desalinated water) during the cold winter time. The water contains a high chloride level. Corrosion testing indicated susceptibility towards pitting corrosion especially at high temperature. It was suggested that controlling chloride level and proper adjustment of ph are the optimum solutions to avoid corrosion of heat exchanger pipes. Keywords Tube Heat Exchanger;Corrosion;Failure Analysis Introduction A shell and tube heat exchanger is a class of heat exchanger designs [1 5]. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed of several types of tubes: plain, longitudinally finned, etc. There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tube sheets. The tubes may be straight or bent in the shape of a U, called U tubes. To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresseswill occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, ph, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally conductive, corrosion resistant, high quality tube materials, typically metals, including copper alloy, stainless steel, carbon steel, non ferrous copper alloy, Inconel, nickel, Hastelloy and titanium [3]. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross contamination and possibly loss of pressure. Process line heat exchanger failures can be very costly or even catastrophic for a business. Metallurgical failure analysis and metal corrosion testing and investigation can help minimize and eliminate the undesirable aspects of unexpected down time. When the root cause of the metallurgical/corrosion failure is identified, results are to be communicated and recommendations are to be made to help prevent future events.metallurgical failure analysis of a component can prevent recurring failures, help with preventative detection, or suggest remedies [6 11]. Service History Tube heat exchanger was made of 316 stainless steel straight type. The heat exchanger served to heat up cold basin water in Sharm Elshaikh, Egypt. Basin water was supplied from desalinated water which contained high chloride level as high as 800 ppm. Temperature of the heat exchanger tubes reached as high as 80 o C. 57

2 Journal of Metallurgical Engineering (ME) Volume 4, 2015 Performed Investigations The following investigations are performed on the heat exchanger tubes: Visual examination Chemical analysis Corrosion testing Microscopic investigation Results and Discussion Visual Examination The heat exchanger (Fig.1) was cut to remove the tubes for examination. Several tubes were taken, cleaned and examined by naked eyes. There were no holes seen on the examined tubes. After that, a cross section of the heat exchanger was also cut to examine the contact points between the stainless steel tubes and the heat exchanger head. We were also unable to observe leak points. This may be due to the fact that there is large number of stainless steel tubing gathered in one bundle which makes it difficult to determine the leak points. Chemical Analysis FIG. 1 IMAGE OF THE HEAT EXCHENGER The chemical analysis of the tube material was performed using the Spectro lab (optical emission spectrometer) device to determine the main elements of manufactured material. Table 1 shows that, the tube material is made of 316 stainless steel as we have been informed by the company. Corrosion Testing TABLE 1 CHEMICAL COMPOSITION OF TUBE MATERIAL Element C Mn Cr Si Ni Mo Fe wt% Bal. Specimens from the tubes were subjected to electrochemical corrosion (cyclic anodic polarization test) testing using computerized potentiostat in solution containing 1000 ppm chloride ions (slightly in excess chloride ions than exist in the basin water) prepared by dissolving 1.37 g of NaCl in distilled water. The test was performed at room temperature and at 80 o C to simulate working conditions. The test gave important information about the susceptibility of material towards pitting corrosion attack. At room temperature, current remained passive (about 3x10 5 A cm 2 ) until 0.6 V and then increased rapidly (see Fig.2). The potential at which potential was increased was taken as the pitting potential. Upon reversal of the potential scan at about 1 V (blue line) a considerable loop was observed indicating that the alloy takes time till restoring passivation. This phenomenon was associated with materials which suffered from pitting corrosion. At high temperature (80 o C), current remained passive (about 3x10 5 A cm 2 ) until 0.4 V and then increased rapidly (See Fig.3) Upon reversal of the potential scan at V (blue line) a considerable loop was observed indicating that the alloy takes time until restoring passivation. It was shown that the pitting potential is decreased while the passive current is increased upon the increase of temperature. The loop area was also increased at high temperature. 58

3 Potential / V FIG. 2 CYCLIC ANODIC POLARIZATION CURVE OF TUBE MATERIAL IN 1000 PPM CHLORIDE ION AT ROOM TEMPERATUE AND 3 MV/S SCAN RATE. Potential / V FIG. 3 CYCLIC ANODIC POLARIZATION CURVE OF TUBE MATERIAL IN 1000 PPM CHLORIDE ION AT 80 O C AND 3 MV/S SCAN RATE After performing cyclic anodic polarization experiments and determining pitting potential, tube specimen was fixed at potential slightly higher than that potential for a sufficient amount of time (2 h). This test was named potentiostatic test and it supplied conditions which favor the occurrence of pitting corrosion so that it can be seen under the microscope. Steady currents of 6x10 4 A cm 2 and 5x10 3 A cm 2 were obtained at room temperature and 80 o C, respectively (Figs.4&5). The increase in temperature led to the increase of steady state current obtained and hence the susceptibility toward pitting corrosion Time /s FIG. 4 POTENTIOSTATIC POLARIZATION CURVE OF TUBE MATERIAL IN 1000 PPM CHLORIDE SOLUTION AT 0.65 V FOR 2 H AT ROOM TEMPERATURE Time /s FIG. 5 POTENTIOSTATIC POLARIZATION CURVE OF TUBE MATERIAL IN 1000 PPM CHLORIDE SOLUTION AT 0.45 V FOR 2 H AT 80 O C. Fig.6 shows that pitting corrosion was visually seen on specimen tested at 80 o C. The absence of pitting corrosion on the specimen tested at room temperature does not mean the absence of possibility of occurrence of pitting corrosion but it means that the specimen needs more potentiostatic testing time. Pitting FIG. 6 PITTING CORROSION ON SPECIMEN TESTED AT 80 O C 59

4 Journal of Metallurgical Engineering (ME) Volume 4, 2015 Corrosion Mechanism Corrosion is expected to occur on the basin water side of the heat exchange tubes. This comes from the fact that the basin water contains quite high chloride levels (~ 800 ppm) which is due to the use of desalinated water (R.O. technology) and also the use of hydrochloric acid to adjust the ph of the basin water. The corrosion resistance of stainless steel alloys relies on the formation of a surface protective film formed mainly of Cr(III) oxide. Stainless steel alloys are subjected to pitting corrosion when electrochemical or chemical breakdown exposes a small local site on a metal surface to damaging species such as chloride ions. The sites where pits initiate include scratches or surface compositional heterogeneities (inclusions). The pit grows if the high current density (the area of breakdown initiation is exceedingly small) involved in the repassivation process does not prevent the formation of a large local concentration of metal ions produced by dissolution at the point of initiation. If the rate of repassivation is not sufficient to choke off the pit growth, two new conditions will develop. First, the metal ions produced by the breakdown process are precipitated as solid corrosion products such as the Fe(OH)2 which usually cover the mouth of the pit. This covering traps the solution in the pit and allows the buildup of positive hydrogen ions through a hydrolysis reaction. Then, chloride ions diffuse into the pit to maintain charge neutrality. Consequently, the repassivation becomes considerably difficult because the solution in the pit is highly acidic, containing a large concentration of damaging ions and metallic ions, and a low oxygen concentration. Thereby the rate of pit growth accelerates. The pit is the anode of an electrochemical corrosion cell, and the cathode of the cell is the non pitted surface. Since the surface area of the pit is a very small fraction of the cathodic surface area, all of the anodic corrosion current flows to the extremely small surface area of the breakdown initiation site. Thus, the anodic current density becomes very high, and penetration of a metal structure bearing only a few pits can be rapid [Uhlig, 2011]. Conclusions and Recommendations Tube material is stainless steel which confirms the company information. Visual examination could not reveal the leak points in the heat exchanger. This may be due to the fact that there is large number of stainless steel tubing gathered in one bundle. This does not allow observers to determine the leak points. Corrosion is expected to occur on the basin water side of the heat exchange tubes. This comes from the fact that the basin water contains quite high chloride levels (~ 800 ppm) which is due to the use of desalinated water (R.O. technology) and also the use of hydrochloric acid to adjust the ph of the basin water. Cyclic anodic polarization and potentiostatic polarization experiments reveal the susceptibility of tube specimens towards pitting corrosion at high chloride levels, especially at high temperature. Microscopic examination revealed pitting corrosion in the specimen tested at high temperature. The chloride level in the basin should be controlled to be less than 300 ppm. This may be done by controlling the desalination process (reverse osmosis).desalination can be repeated until the desired level of chloride is obtained. Avoiding the use of hydrochloric acid (HCl) in adjusting the ph of the basin water to avoid the increase of chloride content of the water. Sulfuric acid can be used safely. It was noticed that some of the heat exchanger tubes were blocked to accelerate water flow. We think it is better to use the full number of tubes and to block none of them. REFERENCES [1] Shah, R.K., Sekulic, D.P. (2003) Fundamentals of Heat Exchanger Design. Publ. by John Wiley & Sons, Inc., Hoboken, New Jersey, USA, 939p. [2] Benonysson, A., Boysen, H. (2002) Optimum Control of Heat Exchangers, Danfoss A/S, Nordborg, Denmark, p. 60

5 [3] SadikKakaç and Hongtan Liu (2002). Heat Exchangers: Selection, Rating and Thermal Design (2 nd Edition). CRC Press. ISBN [4] Perry, Robert H. and Green, Don W. (1984). Perryʹs Chemical Engineersʹ Handbook (6th Edition). McGraw Hill. ISBN [5] Rajiv Mukherjee (2004) Practical Thermal Design of Shell and Tube Heat Exchangers. Series in Thermal & Fluid Physics & Engineering, Begell House, ISBN : [6] Otegui, J.L., Fazzini, P.G. Failure analysis of tube sheet welds in cracker gas heat exchangers. Engineering Failure Analysis, 2004, v.11(6), p [7] Brooks, C.R., Choudhury, A. (1993) Metallurgical Failure Analysis. New York.: McGraw Hill, USA. [8] Vasauskas, V., Baskutis, S., Failures and fouling analysis in heat exchangers, ISSN MECHANIKA Nr.5(61). [9] Usman, A. Nusair Khan, Failure analysis of heat exchanger tubes, Engineering Failure Analysis, 15 (1 2) (2008) [10] Jahangiri, M.R., Failure Analysis of Type 304 Stainless Steel Tubes in an Energy Production Plant Heat Exchangers, Pressure Vessel Technol. 133 (6), (Oct 19, 2011) (10 pages) doi: / [11] Uhligʹs Corrosion Handbook (2011), 3 rd Edition, R. Winston Revie (Editor) ISBN: , Publ. by John Wiley & Sons, Inc., Hoboken, New Jersey, USA. 61