Studies on performance of indices in cooling water system

Transcription

1 Indian Journal of Chemical Technology Vol. 19, January 2012, pp Studies on performance of indices in cooling water system S Sathish Kumar, A Suriyanarayanan & B S Panigrahi * Fast Breeder Test Reactor, Indira Gandhi Centre for Atomic Research, Kalpakkam , India Received 22 December 2010; accepted 5 December 2011 The usefulness and applicability of water indices such as Langelier saturation index, Ryznar stability index and Practical scale index for predicting the corrosive/scale forming tendency of chemically treated open recirculating cooling water has been studied. The study is based on the obtained corrosion and scale data for the period from May 1998 to December 2007, carried out at Fast Breeder Test Reactor, Kalpakkam, India. All the indices are calculated using ph, calcium, total dissolved solids, temperature and alkalinity of cooling water. Practical scale index and Ryznar stability index are found to be more informative than Langelier Saturation Index in keeping track of the corrosive/scaling tendency of the cooling water. Keywords: Cooling water, Fast breeder test reactor, Indices, Langelier saturation index, Practical scale index, Ryznar stability index A 40 MWt Fast Breeder Test Reactor (FBTR), situated at Kalpakkam, India, uses plutonium and uranium mixed carbide as fuel and liquid sodium as coolant in both the primary and secondary circuits. The cooling water system of FBTR comprises service water system and condenser cooling water system. Service water system forms the terminal heat sink for various process heat exchangers (HX) including preheating and emergency cooling (PHEC), biological shield cooling (BSC), primary and secondary thermofluid HXs, emergency diesel generator (DG) coolers, air compressor, nitrogen plant coolers and steam water system auxiliary coolers. Three centrifugal pumps (175 m 3 /h each) of 50% capacity are provided to extract the heat load of the process HXs. Condenser cooling water (CCW) system caters to main condenser, dump condenser, turbine oil cooler, generator air cooler and condensate cooler. Two CCW pumps (1900 m 3 /h) are provided in this system. Both the systems share a common induced draft cooling tower, cooling water pit, corrosion monitoring set up, chlorinator and side stream filtration unit. Corrosion control measures adopted in cooling water system of FBTR has already been reported 1. Influence of sodium hexa metaphosphate antiscalant on the corrosion of carbon steel in industrial cooling water system has also been studied 2. *Corresponding author. bsp@igcar.gov.in Scale deposits are formed by precipitation and crystal growth on a surface in contact with water. Precipitation occurs when solubilities are exceeded either in the bulk water or at the surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubility with temperature 3. The scaling or corrosivity of the cooling water is generally predicted qualitatively by using various water indices 4-7. The most commonly used indices are Langelier Saturation Index (LSI), Ryznar Stability Index (RSI) and Practical Scale Index (PSI). However, these indices are designed to be predictive tools for calcium carbonate scale only. A coating of 1 mm of scale on the heating surfaces creates an insulation effect which increases heating costs by about 10 per cent 8. Scaling also depends on cycles of concentration (COC) and some technologies are adopted for high COC cooling water treatment 9. Though these indices are widely used, their efficacy in predicting actual conditions in cooling water systems differs from one plant to another. In this study, these indices have been measured for a period of one decade in our cooling water system to find out their suitability in predicting scaling/deposition and corrosive tendencies of cooling water. LSI is an empirically calculated number used to predict the CaCO 3 stability of water, that is whether CaCO 3 will precipitate, dissolve or be in equilibrium with water. Langelier had developed this index for predicting the ph s at which ph water is saturated in

2 76 INDIAN J. CHEM. TECHNOL., JANUARY 2012 CaCO 3. The LSI is probably the most widely used indicator for predicting cooling water scale potential. It is purely an equilibrium index and deals only with the thermodynamic driving force for CaCO 3 scale formation and its growth. LSI is expressed as LSI = ph a - ph s (1) where ph a is the actual ph of water; and ph s, the saturation ph of water with respect to CaCO 3. ph s is calculated as ph s = (9.3 + a + b) (c + d) (2) where a,b,c and d are the empirical constants based on total dissolved solids (TDS, ppm), water temperature (ºC), calcium hardness (CH as CaCO 3, ppm) and total alkalinity (TA as CaCO 3, ppm). If ph a <ph s, the system water is undersaturated with respect to CaCO 3 and hence it has the urge to dissolve the protective CaCO 3 layer already present on the pipelines and exposing them for the corrosive attack. Hence, if LSI is negative, water is corrosive. Similarly, if ph a >ph s, the system water is supersaturated with respect to CaCO 3, and hence it has the urge to throw out the excess CaCO 3 present in the water on the pipelines as scales. Hence, if LSI is positive, water is scale forming. The greater the deviation of actual ph from ph s, the more pronounced is the instability 10. LSI is determined by using five major factors considered for water balance, namely (i) ph a, (ii) TA, (iii) CH, (iv) temperature, and (v) TDS. As TDS increases the ph s increases and therefore LSI decreases, implying that the water is more corrosive; this is evident for instance in comparing tap water and marine water. Sea water gives faster rust attack than tap water due to high TDS content. With increase in temperature, the solubility of calcium salts decreases. These salts precipitate out forming scales. Temperature also plays a role on ph s of water. Increase in temperature decreases ph s. This is due to the fact that the empirical constant for temperature decreases with increase in temperature. The hardness of water is due to the presence of calcium and magnesium. Calcium hardness describes the water quality with reference to the presence of calcium compounds that may deposit scale in various areas. More hardness contributes to more scaling. Alkalinity refers to the buffering effect of dissolved compounds that resists changes in ph values. Alkalinity of water is due to the presence of carbonates, bicarbonates and hydroxides that are dissolved in the water. Ryznar stability index has its basis in the concept of saturation level. Ryznar attempted to quantify the relationship between CaCO 3 saturation and scale formation, as shown below: RSI = 2*(pH s ) ph (3) The empirical correlations of the Ryznar stability index are: RSI > 6.0 leading to corrosive tendency and RSI < 6.0 leading to scale forming tendency of water. Puckorius developed the practical scale index, which provided a more accurate indication of the calcium carbonate scaling tendency of cooling water. The basis of this index is the ph of saturation of CaCO 3 and equilibrium ph, as shown below: PSI = 2* ph s - ph e (4) where ph e is the equilibrium ph. Which is defined as ph e = * log 10 (alkalinity) (5) PSI <6 implies scale forming tendency and PSI >6 implies corrosive tendency. While 0 is the equilibrium point in LSI, 6 is the equilibrium point in RSI and PSI. All these indices indicate the corrosive or scale forming tendency of water qualitatively. In FBTR, the cooling water system chemistry is maintained by dosing proprietary formulations comprising corrosion inhibitor, inorganic dispersant, bio-dispersant, chlorine activator and biocides along with chlorination. The chemistry parameters of the makeup water are ph , conductivity 250 µmho/cm, TDS as CaCO ppm, total hardness as CaCO 3 90 ppm, calcium hardness as CaCO 3 60 ppm, magnesium hardness as CaCO 3 30 ppm, total alkalinity as CaCO 3 60 ppm and chloride as Cl - 60 ppm. To monitor the cooling water chemistry, water indices like LSI, RSI and PSI are calculated by using necessary parameters. Experimental Procedure Corrosiveness and scale forming properties of water can be predicted by using various scale indices. The main objective of using these indices is to adjust the cooling water chemistry parameters to a noncorrosive condition and keeping the system in slightly scale forming side. The most common scale forming salts are CaSO 4, CaCO 3 and Ca 3 (PO 4 ) 2. Due to retrograde solubility, they form extremely tenacious heat insulating deposits particularly in high heat flux areas. The rate of formation of deposits depends mainly on the temperature of water, its alkalinity and

3 SATHISH KUMAR et al.: STUDIES ON PERFORMANCE OF INDICES IN COOLING WATER SYSTEM 77 concentration of scale forming salts in the water. To monitor corrosion rate against the FBTR technical specification of less than 3 mpy, a corrosion rack was installed in the system and the rate of corrosion was measured by weight loss method using MS coupons exposed continuously for one month 11. The corrosion rack is installed at the inlet to cooling tower where the temperature varies from 29ºC to 36ºC, depending on the power operation of the reactor and heat load of heat exchangers. Total alkalinity 12 of water is determined by titration with standard solution of a strong acid by using methyl orange as the indicator. Calcium hardness 13 of the sample is determined by titration of a suitably buffered sample with EDTA solution using murexide as the indicator. Alkalinity and calcium hardness values are used to calculate water indices values. Results and Discussion In FBTR, the major material of construction is mild steel having pipelines of length of about 1000 m of different diameters. The heat exchanger materials are mostly copper based alloys like cupronickel, admiralty brass and brass. As the heat exchanger materials are more cathodic than iron, mild steel corrosion is predominant. This has been verified experimentally by installing brass coupons in the corrosion monitoring set up and exposing them continuously for six months and evaluated by weight loss method. It is observed that corrosion of brass is less than 0.5 mpy for the past 25 years. Also, no leak in heat exchangers has taken place so far confirming the integrity of copper based heat exchangers. Therefore, the observed corrosion rate here is mainly for the mild steel. The corrosion rates observed for a decade with the mild steel coupons in the corrosion monitoring rack of the cooling water system of FBTR are shown in the Fig. 1. Analysis of deposit from cooling water system of FBTR using high performance ion chromatograph has been reported earlier 14. The LSI, RSI and PSI calculated for the cooling water based on its chemical parameters are shown in Fig. 2. During the months of November 98 and April 99, the corrosion rates were 5.8 and 10 mpy respectively. The LSI was between and respectively indicating scaling tendency and therefore, there should not have been any significant corrosion as per LSI philosophy. As the indices are qualitative parameters, the scale forming tendency or corrosive tendency cannot be measured quantitatively. On these occasions LSI could not predict the actual Fig. 1 Corrosion rates of mild steel coupons exposed to chemically treated cooling water during Fig. 2 PSI, LSI and RSI of FBTR cooling water system during

4 78 INDIAN J. CHEM. TECHNOL., JANUARY 2012 corrosive state of the cooling water. The other two indices PSI and RSI were greater than 6, implying corrosive tendency of water as discussed above. A strainer has been provided at the inlet to corrosion rack to remove the debris that finds access through the open cooling tower. This strainer is cleaned periodically. However, as optimum flow could not be maintained continuously, corrosion rate of about 10 mpy was observed even though this severity was not shown by PSI and RSI. During the period from November 98 to April 99, the high corrosion rate of 10 mpy was observed only once and this might be due to actual corrosion due to flow disturbance or poor quality of the particular coupon installed for that month. During both the months RSI and PSI indices were in agreement with the observation qualitatively, whereas LSI was not. However, as shown in Fig. 2, the corrosive tendency as per PSI was more than that shown by RSI. Again during the period October February 2001 the corrosion rates were above 3 mpy. The LSI was between 0.9 and 2.3. The PSI was between 7.1 and 6.2 in October 2000 and the RSI was more than 6.0 for more than half of this month. This implies that as per PSI and RSI the water during October 2000 was having corrosive tendency, whereas as per the LSI it was only in the scaling zone. During January-February 2001 again the LSI was in the scaling zone. The PSI during this period was above 6 for more than 15 days but the RSI was above 6 only for 3 days. Therefore, during this period also LSI could not predict the observed corrosion and PSI was more indicative than RSI about the corrosive state of cooling water. During , both PSI and RSI have predicted relatively scale forming tendency of the cooling water for more number of days, whereas LSI has predicted only scaling tendency. In 2000, the reactor was in high power operation for four months (April, October, November and December). During the last two months of 2000, the PSI and RSI were below 6, implying scaling tendency and similarly, the LSI was around 2. During the same period the primary thermofluid heat exchanger was found to be chocked. In October 2000 the corrosion rate was also high. This is due to the fact that the problems of scaling and corrosion are interrelated and no one problem can be isolated from the other. For example, scaling occurs more rapidly in the corroding system and under-deposit corrosion can lead to rapid fouling. The magnitude of these phenomena depends on the process dynamics. During August-September 2002, the reactor was operating at 14 MWt and temperature difference across the cooling tower was 6 C. During this period also the LSI was 2.4, PSI was 5.0 and RSI was 4.6. All these indices suggested a scaling tendency of the cooling water. Occasionally observed corrosive tendency by PSI and RSI could have also contributed to the observed scaling due to displacement and redeposition of corrosion products elsewhere within the circuit. In the month of January 2003, heavy deposit was reported from the secondary thermofluid heat exchanger (admiralty brass tubes) and dump condenser (70/30 cupronickel tubes). Since 1998 to 2003, the cooling water system was under proprietary chemical treatment wherein the total phosphate was maintained around ppm during reactor operation and ppm during reactor shutdown period in the cooling water. In addition, the LSI during this treatment period was always above 1, irrespective of the reactor power operation. The deposit was collected from the heat exchanger and chemically analysed. Calcium (16%), magnesium (7%), phosphate (18%), iron (0.5%) and acid insoluble (24%) were found to be present in the deposit. The deposit collected from the dump condenser was having higher concentration of calcium (33%). During reactor operation, if the turbine is not valved-in then the entire steam is dumped in dump condenser instead of routing through the main condenser. Being a research reactor, in FBTR a dump condenser has been provided to dump the entire steam during the non-availability of turbine. This provision is unique in power plant engineering since the reactor can be operated even without turbine for irradiation experiments. Therefore, the high concentration of calcium in dump condenser deposit is understandable as the temperature in the dump condenser is significantly higher than that in the thermofluid heat exchanger. Calcium salts having retrograde solubility were deposited in dump condenser in large amount. The low iron content in both the deposits pointed to the insignificant corrosion in the system. This was further corroborated by the observed low corrosion rate of less than 3 mpy (Fig. 1) of the mild steel coupons installed in the corrosion rack in the cooling water system. Similarly, the higher amount of calcium, magnesium and phosphate found in the deposits clearly indicated that the choking was mainly due to the scaling only. It was felt that the higher concentration phosphate used in

5 SATHISH KUMAR et al.: STUDIES ON PERFORMANCE OF INDICES IN COOLING WATER SYSTEM 79 the treatment program was probably responsible for the scaling observed. In the middle of 2003, the existing proprietary chemical treatment was replaced by another proprietary chemical treatment, wherein the total phosphate concentration in water was maintained at around 8-10 ppm. Afterwards, none of the indices except LSI indicated significant scaling tendency of the water till today though LSI was always maintained on the scaling side. However, in 2005, primary thermofluid heat exchanger having admiralty brass tubes was found with heavy deposit. The deposit was collected from a heat exchanger and chemically analysed. Calcium and magnesium (28% as CaCO 3 ), phosphate (2%), iron (34%) and acid insoluble (6%) were found to be present in the deposit. The point to be noted here is that in the earlier instance when calcium concentration in the deposit was significant, the iron concentration was highly insignificant. This indicates that both the corrosion products and scale ingredients were present in the deposit. Apart from that significant concentration of sulphate reducing bacteria (SRB) (10 5 CFU/g) was also noticed. SRB were assayed using postgate medium. The inoculated plates were incubated in anoxic conditions, SRB colonies were counted after 96 h of incubation. Medium used for isolation of SRB is tryptone 10.0 g, sodium sulphite 1.0 g, sodium sulphate 1.0 g, ferric citrate 0.5 g and agar 15.0 g. The inoculated culture plates were incubated at 45 C to simulate the temperature of the heat exchanger. The corrosion products could have been carried away from some other location to this location depending on the flow velocity at different locations and afterwards it could have helped in depositing scale ingredients further. Iron content was more in the deposit indicating corrosion and during this period both PSI and RSI were in corrosive zone. All the indices are calculated for the global temperature of water. However, in FBTR condenser cooling and service water systems have many heat exchangers with highly varying heat fluxes. The indices measured for the global temperatures may indicate slight scale forming tendency of water, whereas the same water while passing through a high heat flux heat exchanger becomes severely scale forming. The true indications from indices will come only when heat fluxes of heat exchangers are nearly the same. If not, at one location the water may be having corrosive tendency, whereas at another location in the same system it may be having scale forming tendency. Conclusion In case of FBTR cooling water system the water is chemically treated to minimize the corrosion and scaling. The corrosion rates were generally maintained below 3 mpy for mild steel coupons and 0.5 mpy for brass coupons. The LSI was always on the scaling side and could never indicate the observed high corrosion rates on few occasions. Other indices like RSI and PSI were helpful in indicating both the corrosiveness and scale forming tendencies of water. PSI was better indicative than RSI though the trends indicated by both the indices were similar. Whenever PSI and RSI indicated the scaling tendency, the deposits collected comprised mostly scale forming ions. However, with PSI and RSI in the corrosive zone, the deposits comprised mainly iron and the corrosion products could have initiated the deposition. Experience with FBTR cooling water system shows PSI to be a better index compared to LSI and RSI. Probably, equilibrium ph and the empirical constants used for calculating the equilibrium ph make the PSI better represent the actual corrosive or scale forming tendencies than the actual ph that is used in the calculation of LSI and RSI. References 1 Faizal V A, Suriyanarayanan A, Subramanian K G & Panigrahi B S, Corrosion control in cooling water system of FBTR, paper presented at CORCON 2010, Goa, September Kumar H & Chaudhary R S, Indian J Chem Technol, 17 (2010) Handbook of Industrial Water Conditioning, 9 th edn (Betz Laboratories, Inc., Trevose, USA), 1991, Puckorius P R & Brooke J M, A new practical index for calcium carbonate scale prediction in cooling tower systems (Paper No.99), paper presented at the NACE-Corrosion 90 Conference, Las Vegas, April Modern Power Station Practice: Vol.E Chemistry & Metallurgy, 3 rd edn (Pergamon press, Oxford, UK), 1992, Handbook of Industrial Water Conditioning, 9 th edn (Betz Laboratories Inc., Trevose, USA), 1991, Puckorius P R & Brooke J M, Effectively evaluating cooling water treatment programs (Paper No.17), paper presented at the NACE-Corrosion Conference, St Louis, USA, March Introduction to Water Treatment, Vol.II (American Water Works association), 1984, Singh S, Proc of the National Symp on Water & Steam Chemistry in Power Plants & Industrial Units (SWASCH- 2000), edited by G Venkateswaran (Applied Chemistry Division, BARC, Mumbai), 2000, 548.

6 80 INDIAN J. CHEM. TECHNOL., JANUARY NALCO Water HandBook, 2 nd edn (McGraw-Hill, New York, USA), 1988, Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens, (ASTM G 1 03-E) (American Society for Testing and Materials), Standard Methods For the Examination of Water and Waste Water, 14 th edn (American Public Health Association, Washington, USA), 1976, Standard Methods For the Examination of Water and Waste Water, 14 th edn (American Public Health Association, Washington, USA), 1976, Suriyanarayanan A, Subramanian K G, Rajendran C, Jambunathan D & Panigrahi B S, Analysis of corrosion products of service water system of FBTR using high performance ion chromatograph, paper presented at the National Symposium on Electrochemistry in Nuclear Technology (NASENT 98), Kalpakkam, April 1998.