D. Yildiz, TÜPRAŞ, Batman, Turkey
Cooling systems are vital to remove heat from equipment or process. The function of a cooling system is to transfer the heat from one medium to another. Heat transfer from process fluid results in a rise in temperature in the cooling water. Many properties of cooling water are changed by temperature. The tendency of a process to scale, corrode or cause microbiological growth is also affected by cooling water temperature.
This work is focused on the effect of cooling water parameters on admiralty brass tubes. The effect of high free chlorine on the corrosion rate of brass tubes has been investigated by scanning electron microscopes (SEMs) and eddy current test (ECT) methods and the laboratory results of the cooling water were determined and discussed. Also, the tendency of the feed water scaling indices that is LSI, PSI & RSI was calculated. The results show that a dezincification corrosion mechanism was observed due to the high-level free chlorine on the admiralty brass tube. Additionally, scaling compound formation, such as calcium carbonate, occured on the tubesheet due to a lack of acid dosing. Scale build-up on the tubes affected operating efficiency and reduced the heat transfer rate. To remove the formation, an acid cleaning method was assessed.
Cooling towers. Cooling towers are important for refineries. The primary task of a cooling tower is to transfer heat to the atmosphere. The cooling water’s basin level decreases due to evaporation. Therefore, fresh water—referred to as make-up water—must be added to replenish water lost to evaporation. There are two main cooling tower systems: open- and closed-recirculating systems.
There are two types of cooling towers: mechanical draft and counterflow (FIG. 1). Hot water from the condensers is sent to the cooling tower’s basin. The exact level of cooling water exits the tower and is sent back to the equipment. A typical open recirculating cooling system is shown in FIG. 2.
One of the main obstacles of an open-recirculating system is that dissolved solids present in the water do not evaporate when the water evaporates. The concentration of dissolved solids in recirculating cooling water gradually rises. As a result of this, different types of corrosion and scaling are observed in the system. Generally, the shell side of heat exchangers and the cooling water piping system are made of carbon steel. However, heat exchanger tube materials are mostly made of brass.
In this system, water recirculates according to design cycles of concentration (COC). At this point, blowdown and chemical treatment are essential for this system. Control of the tower system’s blowdown ensures that conductivity is within process limits, minimizing scaling and ensuring a low corrosion rate on the surface of the exchangers and piping.
Scaling is the precipitation of dissolved mineral components such as calcium—primarily as calcium carbonate (CaCO3)—in large concentrations. The tendencies of scaling formation are pH, the surface temperature of heat exchangers, system temperature and make-up water quality. Scaling indices provide parameters to determine the make-up water in the cooling system. These data indicate the tendency of water scaling and are comprised of three indices: RSI, LSI and PSI.
Determining the scaling index. The calculation procedures are adopted from the corresponding ASTM (formerly the American Society for Testing and Materials) standards. Scaling indices are calculated using the total dissolved solids, temperature, calcium hardness, total alkalinity and pH of the water. In make-up water, the specific parameters are measured, and then the numerical values are selected from data listed in TABLE 1. After that, Eqs. 1 and 2 are used for calculation:
pHeq = 1.465(log(TA) + 4.54) (1)
pHs = (9.3) + (A + B) – (C + D) (2)
Obtain values A, B, C, D and pHeq from TABLE 1. The factors are defined as:
After calculating pHs and pHeq from Eqs. 1 and 2, the theoretical measure of scaling can be completed using Eqs. 3–5:
PSI = 2(pHs) – pHeq (3)
RSI = 2(pHs) – pHmeasured (4)
LSI = pHmeasured – pHs (5)
For cooling systems, the tendency of scaling and corrosion formation can be determined by using TABLE 2. The cooling water should not be aggressive (for corrosion rate and scaling) to maintain a protection layer on the pipeline surfaces.
Controlling the scaling index by pH (acid addition). pH is an independent factor that controls the scaling index. As shown in FIG. 3, CaCO3 production increases by pH, which also leads to higher carbon dioxide (CO2) solubility. System pH plays a significant role on scale deposition in cooling water systems. As pH increases, so does the scaling potential in cooling water, such as CaCO3, Ca and zinc phosphates, zinc hydroxide and magnesium silicate.
The solubility of CaCO3 vs. pH is shown in Eq. 6:
Ca2+ + HCO3– <--> H+ + CaCO3 (6)
By adding H+ as an acid, the equilibrium can be shifted to the left side to maintain CaCO3 levels. The LSI in the concentrate stream must be negative to control scaling. Adding an acid is useful to control scale. Conversely, adding acid may cause severe acidic corrosion of pipeline materials and protection layers.
In practical terms, the scaling index is set by the quality of the feed water. The make-up water’s sample analysis results are shown in TABLE 3.
CASE STUDY
According to TABLE 3, pHeq is calculated using the following steps:
pHeq = 1.465 (log(TA) + 4.54)
pHeq = 1.465 (log(144) + 4.54)
pHeq = 11.02
From TABLE 1:
A = 0.1, B = 2.3, C = 1.85 and D = 2.25
According to Eq. 2: pHs = (9.3) + (A + B) – (C + D)
pHs = (9.3) + (0.1 + 2.3) – (1.85 + 2.25) = 7.6
According to Eq. 3: PSI = 2(pHs) – pHeq
PSI = 2(7.6) – 11.02 = 4.18
According to Eq. 4: RSI = 2(pHs) – pHmeasured
RSI = 2(7.6) – 7.8
RSI = 7.4
According to Eq. 5: LSI = pHmeasured – pHs
LSI = 7.8 – 7.6
LSI = 0.2
After calculating the scaling indices parameters (PSI, RSI and LSI) according to TABLE 2, the feed water of the cooling tower has a scaling tendency. Therefore, adding acid into the cooling tower plays a significant role on pH control in a successful precipitation process.
Cooling water troubleshooting. Sulfuric acid is usually used for pH control in refinery cooling systems. For a while, acid addition could not be injected into the system in the absence of sulfuric acid.
COCconductivity and COChardness were compared during the pH control time. A large cycle difference between conductivity and hardness suggests that precipitation was present. As shown in FIG. 4, a sudden reduction was seen in both hardness measurements, which indicated that scaling occurred at the pH upset period. After dosing with acid, the trend was stabilized.
The effects of temperature. In cooling water, the balance between Ca, CO2, bicarbonate and carbonate are delicate, and any change that drives the reaction toward CaCO3 (such as increasing the temperature) will tend to cause precipitation. The most common mineral scale in water systems is CaCO3. It has an inverse temperature solubility—i.e., its solubility decreases as temperature rises. FIG. 5 shows CaCO3 precipitated on the tubesheet of the heat exchanger.
The sample obtained from the tubesheet was investigated by x-ray fluorescence (XRF) analysis. According to TABLE 4, the results of the XRF analysis indicated scale formation and degradation of the magnetite layer.
Galvanic corrosion can produce both iron and copper products. Theoretically, iron oxides form deposits, not scale. However, these deposits become quite hard and tenacious and are often referred to by plants as iron scale. The cause of iron precipitation is different from the cause of most other types of scale, but the problem is equally serious. Iron in the ferrous form (Fe+2) is soluble. When water containing ferrous ions is aerated or chlorinated (or when any other oxidizing material is added), the soluble ferrous ion is rapidly converted to the insoluble ferric form, which precipitates as iron(III) hydroxide [Fe(OH)3] or iron(III) oxide (Fe2O3) (FIG. 6).
From a practical point of view, iron oxide is not soluble in cooling water systems. The precipitation of iron in the system may be aggravated by the presence of iron-depositing bacteria that use the energy produced by the oxidation of Fe+2 to Fe+3. Most often, an iron deposit looks like rust.
Solution. The distillation column’s overhead reflux temperature increased because of high pH. As a result, heat transfer between the cooling water and the hot flow decreased. Acid cleaning is a useful method to control scaling. Hydrochloric acid was used as a solvent to remove the precipitation of CaCO3. After the acid cleaning, the overhead reflux temperature returned to a normal operational condition.
Condenser brass tubes. Brass tubes are the most common materials in water environments because of good heat transfer and the strength of the material. In the author’s company’s Batman refinery, heat exchanger or condenser tubes are mostly admiralty brass material (ASTM B111 C44300), which is also sometimes referred to arsenical brass. These seamless tubes are copper-zinc alloys with the addition of tin and arsenic for inhibiting the material.
Types of corrosion in cooling water service. There are many types of corrosion in brass materials (e.g., dealloying, erosion-corrosion, under-deposit corrosion, crevice corrosion, microbiologically influenced corrosion, galvanic corrosion, ammonia stress corrosion cracking, among others). The parameters that affect failure severity according to corrosion types are detailed in TABLE 5. Additionally, in this study, the effect of high free chlorine levels due to the dealloying corrosion mechanism was investigated by laboratory results.
Dealloying (selective leaching). Dealloying is the selective corrosion mechanism that occurs in the combination of alloying and the environment. There are many examples of this, as shown in TABLE 6. Brass materials containing > 15% zinc are highly susceptible to dezincification in environments consisting of different water (FIG. 7). Some parameters like pH, temperature, deposition and velocity influence the severity of damages. Stagnant conditions can promote dealloying, as well.
There are two types of dealloying: plug (localized) and layer (uniform through the cross section). Dealloying in brasses can be detected visually by a reddish, copper color vs. a yellow brass color. Towards the end of dealloying, the porous structure has fewer mechanical properties, so there is a possibility that catastrophic failure can occur suddenly.
Failure analysis of brass tubes. During an upset period, chemical dosage could not be injected, which caused a high pH. This led to a plant shutdown due to low heat transfer performance. The ECT method was used to display the severity of problems in the brass tube. The test results are shown in FIG. 8. An excessive corrosion rate was observed in the inner surface of the condensers, which had been in service for 9 mos.
Besides the corrosion rate, the brass tubes were studied by SEM-EDX methods due to the detection of red areas on the tube’s surface. The effect of high PH and high free chlorine on the brass tube was analyzed by spectrophotometer in laboratory. The analysis is illustrated in FIG. 9.
As shown in FIG. 9, the dezincification mechanism (reducing the zinc content) can be seen from Area-1 to Area-5 (surface of the tube). The rate of the corrosion mechanism increased from the first point to the end point. The original brass had a yellow color, while the dezincified area had a reddish color.
In copper-based alloys, the surface appears reddish or pink where the active component has dissolved. The results of the SEM-EDX analysis and ECT testing indicated the corrosion rate and dezincification on the brass tube increased because of high pH and high chlorine dosing. Chlorine is used to control biological growth in cooling water. The upper limit of free chlorine in cooling water is 0.5 ppm. The excessive chlorination of the cooling water accelerated the dezincification of the brass tube. The analysis results of chlorine levels in the cooling water are shown in FIG. 10.
The evolution of the corrosion rate on the brass tube was investigated by the weight loss method (corrosion coupon). Pre-weighed copper and mild steel coupons were used for monitoring corrosion in the cooling water. The coupon weight loss provided a quantitative measurement of the corrosion rate, and a visual appearance of the copper coupon provided information to assess the type of corrosion. Controlled water flowed past the pipe that was fixed on a corrosion rack. The corrosion rate was calculated using Eq. 7:
mpy = (Weight loss × 365 × 1,000) / [Density of copper coupon × Exposed area × Time (day) × 2.54] (7)
The results of FIG. 11 indicated the following:
Results and discussion. The following are results/takeaways from the analysis:
Dilan Yıldız is Process and Energy Management Lead Engineer at TÜPRAS Batman refinery. She has worked for TÜPRAS since 2018. Yıldız earned a BS degree in chemical engineering from Yıldız Technical University, and an MS degree in chemical engineering from Istanbul Technical University. She is currently pursuing an MBA at Koç University.