How to Protect Open, Recirculating Cooling Systems
To keep dissolved and suspended material within acceptable limits, some of the recirculating water must be periodically removed, or "blown down," and fresh water added. The frequency and volume of blowdown directly affects cooling system operating costs. Besides the fee for the replacement water, treatment chemicals must be replenished and the blowdown may have to be treated or incur wastewater surcharges. The effectiveness of the treatment program determines the cycles of concentration needed to keep the system in proper balance. Open recirculating cooling systems typically operate at three to 10 cycles of concentration, with higher cycles preferred for economy.
However, blowdown alone cannot protect recirculating cooling systems against inorganic deposition, biofouling and corrosion, the most common problems affecting cooling system performance. Failure to control these problems directly impacts productivity and profitability by reducing heat transfer rates, forcing unscheduled downtime for cleaning and repairs, damaging heat exchange equipment and favoring the growth of pathogenic organisms like Legionella bacteria.
Hardness Scale ControlIf the makeup water contains high concentrations of inorganic salts, mineral scale can form insulating deposits on heat-exchange equipment. Because the precipitation of inorganic salts is favored by alkalinity, scale formation is more common with high cycles of concentration that increase pH levels. Sulfuric acid will reduce alkalinity and help prevent scale formation, but safety and environmental concerns restrict its use. When acid cannot be used in an alkaline system, an alternative scale control agent must be part of the water treatment program.
Fouling of heat transfer surfaces by calcium and/or magnesium hardness salts can be prevented by the use of polymeric dispersants, sequestrants and crystal growth modifiers. The modifiers also are called threshold inhibitors because they limit the growth of inorganic salt crystals when applied at substoichiometric levels.
In years past, various phosphonic acid compounds were used for scale control. These phosphonates had solubility and/or stability limits themselves and precipitated under high pH, high temperature or high hardness conditions. Also, to varying degrees, they were degraded by oxidizing biocides like chlorine and bromine. This precipitation or degradation rendered them ineffective for control of mineral scale and resulted in the fouling of heat transfer surfaces.
An all-organic scale control agent based on alkylepoxycarboxylate (AEC) technology is available that avoids many of these problems. This scale inhibitor is not a phosphonate nor does it contain phosphate. It exhibits superior stability and solubility compared to traditional phosphonates, is effective in alkaline systems and is not degraded by halogen-based biocides. The elimination of phosphate and the reduction in blowdown volume make AEC-based programs environmentally preferred as well as cost effective.
Effective deposit control requires observation of appropriate solubility indices such as Langelier's Saturation Index (LSI) for calcium carbonate. This ensures that the cooling system is operated in a pH range and at cycles of concentration that will permit successful treatment. For example, certain water treatment programs can only control recirculating silica levels up to 150 ppm (mg/L). Other programs extend the limit to 200 ppm (mg/L).
Similarly, alkaline cooling systems treated with conventional phosphonate inhibitors are routinely limited to an LSI of less than +2.5. Scale inhibitors based on AEC technology can maintain true solubility of calcium carbonate up to at least LSI +3.0. These extended limits represent opportunities to reduce operating costs while improving reliability.
Microbiological Fouling ControlAn effective microbiological control program requires consideration of a system's volume, blowdown rate, water chemistry, nonbiological contaminants and microbial infestation. The biocides selected must be effective against the target population and comply with all regulatory requirements.
Microbiological fouling can be controlled by means of oxidizing biocides as well as by nonoxidizing biocides and surfactants. Blends of nonoxidizing biocides have been developed that are synergistic with oxidizers and control microbial populations more rapidly and economically -- and for longer periods of time -- than single active products.
The oxidizing biocides most commonly used are chlorine (from gas, liquid bleach or chlorine dioxide) or bromine (from solid halogen donor products or liquid sodium bromide activated by chlorine). Free chlorine residual levels of only a few tenths of a part per million will control many bulk water organisms.
Bromine may be more effective than chlorine in some applications such as alkaline or ammonia-contaminated cooling waters. Several "stabilized liquid bromine" products have been introduced such as DBNPA, which is widely available. These products provide active bromine from a single source, thereby eliminating the need to activate bromide by co-feeding it with a stronger oxidant. These products are convenient to feed but generally are much more expensive than sodium bromide and bleach fed together.
Nonoxidizing biocides are organic molecules that disrupt specific microbial structures or interfere with essential microbiological processes. Such biocides are especially critical for systems contaminated by process leaks that use high-efficiency film-type tower fill or that rely on intermittent oxidizing biocide feed. Nonoxidizing biocides help control microbes living on surfaces in biofilms where they are protected from oxidizers.
Conventional methods for monitoring microbial populations work by culturing microorganisms, a time-consuming procedure that requires days or even weeks of incubation before the results are known. Biofouling can be well-advanced before treatment begins. In addition, it is believed that this technique does not detect the full range of microbial life present in cooling systems. A more effective product is ATP-based technology, which makes possible real-time assessments of microbial populations. It measures the light produced when enzyme reagents react with ATP, a substance found in all living cells. Testing can be performed, results read and corrective actions initiated within minutes of sample collection.
Corrosion ControlCorrosion of metal surfaces in cooling systems is minimized by adding inhibitors to the recirculating water as well as by managing the water chemistry itself. The selection of corrosion inhibitors is complex. The size and operation of a system, chemistry of the recirculating water, environmental discharge regulations, system metallurgy and program costs all must be considered when selecting a treatment program.
Open recirculating cooling systems can include mild steel, galvanized steel, stainless alloys, copper and copper alloys, and aluminum. These metals are subject to a range of corrosion problems such as generalized corrosion, galvanic attack, pitting, crevice attack and stress cracking.
The basic corrosion reaction is a well-defined electrochemical process. The corrosion cell is an electrochemical cell. At the anode of the corrosion cell, metal dissolves into the water as metal ions, as shown in the equation:
M represents the metal
n is the valence of the corroding metal
e represents the liberated electron(s)
The anodic reaction is an oxidative reaction. The electrons lost at the anode flow through the metal to the cathode. At the cathode, electrons are removed by reducing reactions.
A wide range of corrosion inhibitors is available for use in cooling systems. Table 1 summarizes typical treatment dosages and uses. Inhibitors generally are classified as anodic or cathodic according to which electrochemical process they affect. Anodic inhibitors interfere with the dissolution of metal at the anode of the corrosion cell. Cathodic inhibitors interfere with the cathodic process. Numerous inorganic materials have been used as corrosion inhibitors. These include metals with relatively low toxicity such as molybdate and zinc as well as nonmetallic, inorganic materials such as phosphate, silicate and nitrite salts.
Organic corrosion inhibitors primarily are azoles used to protect copper and copper alloys by forming thin films that stabilize the metal and limit or prevent oxidative reactions.
Metallic Corrosion InhibitorsChromate. For many years, the most commonly used corrosion inhibitors were chromate salts because of their excellent anodic corrosion inhibition for mild steel and copper. However, regulatory action has severely restricted chromates, and they are no longer used for corrosion protection in U.S. cooling water systems.
Molybdate. Molybdate salts are chemically similar to chromate salts and act as anodic corrosion inhibitors in much the same way. Because of their low toxicity,they are easier to handle and more environmentally acceptable.
In open recirculating systems, water losses make treatment programs with high levels of molybdate uneconomical. However, some system operators claim that low levels of molybdate (~10 ppm [mg/L]) have a synergistic action when used in combination with other corrosion inhibitors. In such applications, molybdate also serves as a tracer to facilitate program monitoring.
Zinc. Zinc is widely used as a corrosion inhibitor, particularly in alkaline cooling water programs. In mildly to strongly alkaline aerated water, zinc hydroxide precipitates at the cathode of the corrosion cell, forming a physical barrier that blocks the reaction.
Zinc fed at rates of less than 5 ppm (mg/L) often is used in combination with anodic inhibitors to prevent both anodic and cathodic corrosion reactions. Although zinc has relatively low toxicity, its direct discharge to public bodies of water has been restricted in some regions.
Nonmetallic Inorganic Corrosion InhibitorsPhosphate. For many years, the use of phosphates was frustrated by an inability to maintain effective levels of soluble phosphate. Phosphate precipitated as calcium phosphate, resulting in corrosion as well as generalized fouling. The problem was solved by the development of a polymer technology that significantly increased calcium phosphate solubility. This polymer technology now is used widely and has become the standard for soluble, phosphate-based corrosion control programs.
Phosphates such as orthophosphate and polyphosphate salts -- frequently applied in combination -- are by far the most widely used nonmetallic inorganic corrosion inhibitors. Orthophosphates can provide either anodic or cathodic protection, depending on dosage and system pH levels. Polyphosphates at concentrations of less than 5 ppm (mg/L) can contribute to both cathodic and anodic corrosion inhibition.
Silica. Silica is an effective inorganic corrosion inhibitor when applied at 25 to 50 ppm (mg/L) above naturally occurring background silica levels. Nontoxic and inexpensive, it functions by slowly forming a
corrosion-resistant film at the metal surface.
Nitrite. Nitrite salts commonly are used as anodic corrosion inhibitors in closed recirculating systems but rarely, if ever, in open recirculating systems. Although they are easy to use and environmentally acceptable, nitrites are readily metabolized by various microbes that rapidly deplete residuals, resulting in a loss of corrosion protection. The tendency toward high microbial levels in open cooling systems, combined with frequent use of oxidizing biocides, means that nitrites are metabolized or chemically oxidized to nitrates and rendered ineffective for corrosion control.
Organic Corrosion InhibitorsThe most common types of organic corrosion inhibitors are azole-based corrosion inhibitors. These are widely used in cooling systems to protect copper and copper alloys by forming a barrier film between the metal surface and water. In the case of mercaptobenzothiazole (MBT) and benzotriazole (BZT), the azole film can be quite thick (greater than 100 molecules in thickness). Tolyltriazole (TTA), the most widely used azole, produces a much thinner film - usually only a few molecules in thickness. These azole films do not interfere with heat transfer.
Azole films generated with MBT, BZT or TTA are degraded by halogen-based biocides and must be replenished continuously by the addition of azole to the bulk water. Oxidizing biocide requirements also increase. The increased levels of dissolved copper when corrosion protection fails contribute to mild steel pitting and environmental discharge concerns. In addition, the azole decomposition products generate a characteristic and objectionable odor.
Recent advances in azole technology have led to a halogen-resistant product for more-effective yellow metal protection. This modified azole is not degraded by halogen residuals. Because there is no breakdown reaction, effective azole and halogen residuals can be achieved at lower feed rates. The protective film formed by the modified azole is longer lasting than those produced by conventional azoles -- even in the absence of halogens. Environmental impact also is reduced because less halogen and copper are discharged.
Replacement of conventional chlorine-based programs by stabilized liquid bromine also has been proposed as a way to avoid the problem of azole degradation by chlorine, as well as the problem of halogen attack on organic-, phosphonate-based scale inhibitors. The choice is between using water treatment chemicals that are inherently halogen stable or replacing inexpensive commodity biocides with more costly stabilized liquid bromine.
Neutral and Alkaline Cooling ProgramsMost cooling systems operate best in a pH range of 6.8 to 9.0. For any given system, the operating pH range must be controlled (typically, +/-0.20 pH units of the target pH) to ensure optimal results from the treatment program. Cooling water pH control can be achieved by feed of acid or caustic or by controlling cycles of concentration. Table 2 shows a comparison of neutral and alkaline systems.
Neutral pH programs (pH less than 7.8) commonly rely on sulfuric acid for pH control. These programs permit the highest cycles of concentration to be achieved and are often the most economical. Neutral pH programs are especially well suited for makeup sources containing high levels of alkalinity combined with hardness and other dissolved solids.
Neutral pH programs provide a certain level of inherent protection against scaling. They also provide excellent corrosion protection as long as effective inhibitor levels and good pH control are maintained. However, corrosion can result from low pH excursions while the loss of acid feed, coupled with moderately high levels of anodic inhibitors, can lead to treatment-related fouling.
Alkaline treatment programs provide natural corrosion protection by slowing the production of hydroxyl ions and the overall cathodic reaction. With significant amounts of calcium hardness, calcium carbonate precipitates, forming a barrier film at the cathode.
Such programs carry a higher risk of calcium carbonate scale problems; however, advanced alkaline treatment programs can eliminate those concerns and allow some cooling systems to run entirely without acid for pH control, minimizing treatment costs and handling problems. The recent development of highly stable and soluble nonphosphonate-based scale inhibitors and halogen-stable corrosion inhibitors has dramatically improved the reliability and economics of such alkaline water treatment programs.