Don t Repeat Mistakes! An SWRO Plant Case Study

Transcription

1 Don t Repeat Mistakes! An SWRO Plant Case Study Authors: Jan Olsson and Malin Snis Presenter: Malin Snis Abstract The paper describes a failure investigation of high-pressure piping made of 904L from a major seawater reverse osmosis (SWRO) plant on the coast of the Mediterranean Sea. It describes the plant and the process conditions, i.e. the total amount of dissolved salts (TDS) and temperature of the feed, and the result of a laboratory investigation. The piping had suffered extensive pitting and crevice corrosion due to inadequate corrosion resistance to the environmental conditions, i.e. the high level of chloride ions in combination with the temperature of the feed. Furthermore, the result is discussed in terms of corrosion resistance of grades commonly used for SWRO plants and experience reported from other plants of similar size, working under similar conditions, i.e. plants larger than 40,000 m 3 /day along the coasts of the Mediterranean Sea, the Red Sea, the Arabic Gulf and Canary Islands. International Desalination Association World Congress: SP

2 I. INTRODUCTION The history of corrosion failures in SWRO plants is just as old as the history of large SWRO plants itself. It started with the old Jeddah plant commissioned in 1979 where the high-pressure piping made of 316L showed to be inadequate already after a few years, Figure 1 [1,2]. Figure 1. Crevice corrosion under Victualic couplings on 316L high pressure piping in the old Jeddah SWRO plant. The Jeddah plant was followed by a series of SWCC (Saline Water Conversion Corporation) plants in Saudi Arabia during the 1980 s where the same type of stainless steel was used, i.e. either 316L or the slightly better 317L; Al Birk in 1983, Umm Lujj in 1986, and Duba and Haql in The performance of the HP piping was reviewed at the IDA conference in Washington in 1991 and corrosion problems were reported for Umm Lujj and Haql [3]. Corrosion problems have also been experienced for the even later Medina-Yanbu plant, the largest SWRO plant in the world when it was erected in the mid-1990 s [4]. The same story was repeated for the first plants in Malta, i.e. for Ghar Lapsi, Tigne and Cirkewwa, erected in 1986 and 1988 respectively [5] and also for the first plants on the Canary Islands. The poor experience from all these plants implied a need of better materials and 254 SMO (or similar 6Mo austenitic grades) became the most common material for large as well as small plants in the Arabian Gulf area, along the coasts of the Mediterranean Sea and on the Canary Islands. However, the striving for less costly solutions has resulted in the use the less resistant grade 904L (ASTM 904L/N08904) in several plants. This paper describes a failure analysis of 904L high-pressure piping in a Mediterranean Sea plant where pitting and crevice corrosion of the same type as previously described for plants with 316L and 317L piping have occurred. II. THE PLANT - It is an SWRO plant with a capacity of 50,000 m 3 /day. - The TDS of the feed is 41,600 ppm implying a chloride ion level of around 23,000 ppm. - The temperature of the feed is around 18 C in March and 27 C in September. - The reject has a TDS of 64,500 ppm. - The pressure is 70 bar. International Desalination Association World Congress: SP

3 III. FAILURE HISTORY The plant started to produce water in June 2003 and corrosion problems appeared already after a few months of service. Two types of corrosion could be established, one being crevice corrosion in 1½ high pressure connectors underneath Victualic couplings, Figure 2, i.e. the same type of problems that have been common in 316L and 317L high pressure piping as shown in Figure 1 above. Seven out of 700 such connectors were reported to have suffered this type of crevice corrosion after 4 months only, but the problems have continued and a remedy to end the problem is under discussion. The second type was pitting corrosion in elbows close to circumferential welds in 10 piping as illustrated in Figure 3. Two such failures were reported after 4 months of service. Figure 2. Crevice corrosion on 904L under Victualic couplings. Figure 3. Pitting close to a weld in 904L high-pressure piping. IV. METALLURGY The grade 904L is a highly alloyed austenitic stainless steel with the typical chemical composition given in Table 1. Table 1 also contains some other stainless steels commonly used or being future candidates for the high pressure piping in SWRO plants. International Desalination Association World Congress: SP

4 Table 1. Typical chemical compositions of some stainless steels for SWRO plants including 316L as reference. Outokumpu Stainless EN ASTM Cr Ni Mo Others PRE 1/ L L N Cu SMO 2/ S N 43 SAF / S N PRE = % Cr x % Mo + 16 x % N. 316L and 904L normally contain around 0.05 % of nitrogen. 2. Registered trade names by Outokumpu Stainless and Sandvik respectively. As can be seen from the pitting resistance equivalent formula, PRE, it is necessary to have high contents of chromium (Cr), molybdenum (Mo) and nitrogen (N) to achieve the optimum resistance to pitting, and also crevice corrosion resistance. Neither 316L nor 904L are alloyed with nitrogen, i.e. having a nitrogen content above 0.1 %. The reason is that these grades were developed long before the positive influence of nitrogen was known and 904L was actually developed once upon a time to resist dilute sulphuric acid. On the other hand, the 6Mo austenitic 254 SMO and the duplex SAF 2507 were designed to resist chloride induced corrosion such as pitting and crevice corrosion and they have not only higher PREnumbers than the others, their critical pitting temperatures are also much higher, see Table 2. Table 2. Critical pitting temperatures (CPT) in 1 M NaCl (ASTM G 150). Grade CPT, C 316L L SMO 85 SAF Such pitting temperatures cannot directly be used for engineering since they only reflect a ranking based on laboratory testing, but they can be used to evaluate the possibilities to use an alternative grade when combined with service experience. There are, however, also engineering diagrams available showing safe temperatures for different steel grades in chloride containing water, including chloride levels corresponding to seawater, Figure 4 [6]. There are no guidelines for SAF 2507 in this figure but it should approximately coincide with the lines for 254 SMO. Also if being slightly conservative, this diagram clearly shows that the use of 904L implies a pronounced risk of crevice corrosion and possibly also pitting when being used for seawater service. International Desalination Association World Congress: SP

5 Figure 4. Engineering diagram showing the risk of pitting (p) and crevice corrosion (c) for different stainless steels in chloride containing water of different temperatures. The dotted line for 254 SMO phe gives limitations for plate heat exchangers, illustrating the more severe crevice geometry in such applications. V. FAILURE ANALYSIS The samples, i.e. two pieces of the 1½ piping (Figure 2), were investigated at Avesta Research Centre. The other failure shown in Figure 3 was investigated at the plant and ascribed to insufficient post weld cleaning. 5.1 Chemical Analysis The chemical compositions of the pipes were determined by XRF (X-Ray Fluorescence) and combustion analysis (carbon and sulphur). The results are presented in Table 3. The compositions are within the limits for 904L also if the levels of chromium and molybdenum are rather close to the minimum levels, 19.0 and 4.0 % respectively. Table 3. Chemical analysis of the investigated 904L pipes. Sample C Si Mn P S Cr Ni Mo Cu Visual inspection The samples were visually examined in a stereomicroscope. Crevice corrosion was observed in areas where the pipes had been connected with compression couplings, Figure Metallographic examination Cross sections of the pipes were polished prior to etching in V2A at 55 C. After the etching the microstructure was evaluated in microscope. The structure is normal, Figure 5. International Desalination Association World Congress: SP

6 Figure 5. Polished and etched structure of a cross section of the 904L pipe. The grain size was determined to ASTM index 5, corresponding to an average grain diameter of 62 µm. 5.4 Failure investigation summary - The chemical composition fulfills the requirements for 904L. - The microstructure is normal. - The pipes have suffered crevice corrosion. - A grade with better resistance to crevice corrosion, e.g. 254 SMO or SAF 2507 should be used. VI. DISCUSSION According to the engineering company, they have good experience of 904L high pressure piping from other plants e.g. on the Canary Islands, which justified this grade also for this plant. However, there are also SWRO plants where 904L has suffered mainly crevice corrosion, both in the Arabian Gulf and the Mediterranean Sea areas [7]. This mixed experience from 904L installations and the engineering diagram shown in Figure 4 indicate that the selection of 904L implies a certain gambling when specifying this grade for SWRO plants. Another indicator of this is the pitting experienced in 10 piping shown in Figure 3. When this failure case was discussed initially only two out of 49 elbows had suffered corrosion, but the risk that more such problems will appear in the future is obvious. One remedy, which must be emphasised, is that a thorough post weld cleaning has to be performed on all items exposed to seawater or brine unless the welding method implies a proper gas protection of the weld area. It must also be emphasised that this gas protection has to be maintained until the work piece has cooled down to a temperature where no heat tint will be developed, i.e. below 200 C. A more resistant grade, such as 254 SMO or SAF 2507, would also widen the safety margin implying less risk of pitting and crevice corrosion, but the success of these grades can be jeopardised if the temperature of the feed considerably exceeds 30 C as shown in Figure 4. International Desalination Association World Congress: SP

7 VII. SUMMARY AND CONCLUSIONS 1. The material used fulfils the requirements for 904L in terms of chemical composition and microstructure and consequently also annealing process. 2. It is necessary to ensure a clean weld area when the material will be exposed to air saturated seawater or brine, either by applying an appropriate inert gas protection during welding or by a thorough post weld cleaning. 3. The specification of 904L implies a certain risk when used for SWRO plant high-pressure piping. 4. A higher safety margin will be achieved by the use of a stainless steel grade design to resist chloride containing water, i.e. 254 SMO or SAF REFERENCES 1. Wojcik C.K. Desalination of water in Saudi Arabia by reverse osmosis, performance study. Proc. First world congress on desalination and water re-use (DECHEMA, Florence, 1983), Vol. 3, pp Hassan A.M., Al-Jarrah S., Al-Lohibi T., Al-Hamdan A., Bakheet L.M. and I-Amri A. Performance evaluation of SWCC SWRO plants. Proc. Fourth world congress on desalination and water re-use (IDA, Kuwait, 199), Vol. 2, pp Hassan A.M., Al-Abanmy A.M. and Al-Thobiety M. Performance of SWCC SWRO plants. Proc. IDA World conference on desalination and water reuse (Washington D.C., 1991), Vol Malik A.U., Andijani I., Mobin M., Al-Muaili F. and Al-Hajri M. Investing weld leaks at Medina-Yanbu RO plant. Desalination & Water reuse 2005, 14(4), pp Lamendola M.F. and Tua A. Desalination of seawater by reverse osmosis, the Malta experience. Desalination & Water reuse1995, 5(1), pp Outokumpu Stainless material data sheet High Performance Austenitic Stainless Steel (2004). 7. Olsson J. and Cosic K. Stainless steels for SWRO plants high pressure piping, properties and experience. Proc. IDA World congress on desalination and water reuse (Bahamas, 2003). International Desalination Association World Congress: SP