3.4.1 Tube preparation and installation
After receiving the test tubes, the protective plastic is removed before the outer surface is polished. Polishing removes any oxide layer on the outer surface of the tubes, whereas the inside of the tubes are relatively clean as a result of their trans- port plugs. Each test tube is then rigorously cleaned using acetone, and they are carefully installed into each heat exchanger whilst preventing any contamination from handling. The centralizing discs installed in the center of the heat exchang- ers (figure 3.5) are centered and held in place by a threaded union. Thereafter the o-rings are installed in the tube glands and they are tightened by hand to prevent over-tightening.
3.4.2 Heat transfer tests
Heat transfer tests are performed regularly while the fouling test is underway. This entails recording flow rates and temperatures over an approximate 90 min period. Usually this is performed early in the morning (08:00 AM) during which the fouling water temperature has negligible fluctuation and is deemed to be at steady state (expanded upon later). Owing to the design of the apparatus and non-intrusive ultrasonic flow meter, these tests do not interfere in any way with the fouling conditions within the test tubes and hence they undergo similar con- ditions compared to the actual tubes in the condenser.
3.4.3 Water quality sampling and analysis
Water samples are collected at a 20 mm tap-off valve, installed immediately after the y-strainers, before the fouling fluid enters the apparatus. This valve is fully opened and allowed to run for two minutes before filling a 500 mL polyethylene
CHAPTER 3. EXPERIMENTATION
sample bottle. The sample bottles are completely filled to minimize any interac- tion with air inside the container, and they are delivered to the analysis labora- tory within two hours of sampling.
Table 3.4 shows the pertinent results from analyzing water at two different points: the first sample point is at the top of the clarifier and the second is at the tap-off valve upstream of the apparatus (refer to figure 3.1). Water from the top of the clarifier is collected using a collection bottle fixed to a pole and this method is used by the station personnel to sample and monitor the cooling water quality. As indicated in table 3.4 the results from the two different sample points are comparable, despite being on different days and this suggests that the tap-off point of the apparatus has been suitably designed to use water representative of the actual cooling water within the condenser. The full results from the analysis are included in appendix F.
Table 3.4:Water analysis comparison
Sample point
Descriptor Limits Clarifier Apparatus
Sample date 2015 /12/2 4 2016 /1/27 2016 /4/2 2016 /8/5 pH 25◦C 8.1-8.6 8.85 8.64 8.59 8.88
Total dissolved solids mg/l 2040 1511 1638 Chloride (Cl) mg/l 400 174 268 172 173 Sulphate (SO4) mg/l 1000 688 946 727 695
Turbidity (NTU) 100 81.3 32.2 32.9 274 Total hardness mg CaCO|3/l 752 538 636
Suspended solids mg/l 60 43 374
LSI 0 0.92 0.65 1.17
RSI 6.5-7 6.79 7.29 6.55
Further scrutiny of table 3.4 reveals chloride and sulphate ion concentration levels less than the station limits. Therefore it is expected that corrosion levels are within acceptable levels. However the total hardness is very high, and there is a high potential for scaling. This is supported by the Langelier Saturation Index (LSI) and the Ryznar Stability Index (RSI) that describe the following ranges of scaling potential:
• LSI > 0: water is super saturated and tends to precipitate calcium carbon- ate.
CHAPTER 3. EXPERIMENTATION
• LSI < 0: water is below the saturation level and therefore tends to dissolve solid calcium carbonate.
• RSI < 6 supersaturated and water tends to form calcium carbonate scale. • 6 < RSI < 7 water is considered to be approximately at saturation equilib-
rium with calcium carbonate.
• RSI > 7 water is below the saturation level and therefore tends to dissolve solid calcium carbonate, although corrosion of mild steel becomes a prob- lem.
The turbidity (a measure of the cloudiness of the water) in table 3.4 varies be- tween 23 and 274 (which is greater than the station limit of 100). This variance is caused by an increase in the concentration of suspended solids and is indicative of a fluctuation in the makeup water or operational change of the clarifiers. In any event such variations are common to all the tubes because of the design of the apparatus and in this way fluctuations in water chemistry are common for all the tubes and can be identified as such when comparing tubes relative to their control tube. Moreover the water quality is so poor that the influence of minor fluctuations in the water quality do not detract from the fouling data, since the dominant fouling mechanism is clearly identified when comparing relative data. In fact the fluctuations are sufficiently small that the change in fouling factor is contained within the experimental uncertainty estimates.
3.4.4 Periodic maintenance
Fortnightly maintenance activities include cleaning the strainers, checking the heated water tank level, and backwashing the static pressure tappings. Macro debris is removed from the y-strainers, by isolating the strainers sequentially so as to not interrupt flow to the apparatus. Potable water is used to top up the level of the hot tank, and is used to backwash the static pressure tappings to remove any blocked debris.
3.4.5 Tube removal, drying and sectioning
Tubes are removed by simply loosening the locking nut of the tube glands that se- cures the o-rings in place. In this way the foulant layer is not altered during tube removal. Under sterile conditions the tubes are sectioned immediately and the foulant layer is scraped and swabbed for bacterial analysis. The sectioned tubes are then stored at room temperature for 48 hours on a slight incline to completely dry them. Finally they are transported to a laboratory for QEMSCAN®analysis; this scanning electron microscope analyzes the mineralogical composition of the deposits.
CHAPTER 3. EXPERIMENTATION