Most power plants using closed-loop water cooling for mechanical systems (rather than for the steam cycle) have several subsystems. The bearing cooling water system generally provides cooling for critical pump bearings and seals, hydrogen coolers for the generator, lube oil, and air compressor coolers. Other closed-loop cooling systems can include chilled water systems for air chillers used at the air inlet to the gas turbines at a combined cycle power plant and the chemistry sample panel.
A closed-loop cooling system can exchange heat with the main cooling water system in conventional tube and shell heat exchangers or plate and frame heat exchangers. Chilled water systems (air chillers) exchange heat with the compressor, which in turn uses a cooling tower to throw heat back into the environment.
Generally, demineralized water is used for closed-loop cooling water makeup, but chemical treatments are required to prevent corrosion and, in some systems, freezing. Most commonly, the piping in a closed-loop system is carbon steel. Heat exchange surfaces, such as air chiller assemblies, may be copper or even aluminum. Plate and frame heat exchangers are often made of stainless steel plates. Care and keeping of these systems requires that you pay attention to all the metals.
In a closed-loop system, oxygen pitting is the most common type of corrosion (Figure 1). Symptoms of oxygen pitting may be rusty water or recurring maintenance on bearings due to the abrasion caused by the corrosion products against the seal surfaces.
In order for oxygen pitting to occur, there must first be a deposit that covers a portion of the metal surface, creating a differential between the oxygen content underneath the deposit and the oxygen content in the bulk water. The oxygen-deficient area underneath the deposit becomes the anode, and the area around the deposit that is exposed to the bulk water becomes the cathode. This “big cathode, little anode” configuration causes concentrated and accelerated pitting in a confined area, producing pinhole leaks.
If bacteria are allowed to propagate inside the closed-loop system, they can create a “living” deposit. The byproducts of bacterial respiration are often acidic, and respiration also consumes oxygen, causing the base of the biofilm to be conducive to corrosion of the base metal. This further encourages some types of bacteria, as they use the oxidized metal in their metabolism.
When a closed-loop cooling system is tight—experiencing no water loss—the chemical treatment that is applied can last for weeks or months before it needs to be refreshed. This can lead to complacency. On the other hand, closed-loop cooling systems that have leakage—and which have significant water loss—can be nearly impossible (and sometimes very expensive) to maintain at the proper treatment levels. Improper treatment levels will always lead to corrosion of these systems.
Below we list of few options that you can successfully use for treating closed-loop cooling systems such as the bearing cooling water system or closed-loop air chiller system. Generally, you find a treatment program that works well for the various metals in your system and system requirements (for example, determine if you need freeze protection) and then stick with it.
Regardless of which of the three chemical treatments you choose, they are likely to also contain pH buffers (caustic and sodium borate are common) to maintain an alkaline pH, which is conducive to minimizing corrosion in carbon steel. If there is copper in the closed-loop system, an azole may be added to the treatment to maintain a protective chemical layer on top of the exposed copper metal surfaces.
Sodium Nitrite. Sodium nitrite has been in use for many years to prevent corrosion in a wide variety of closed-loop systems. Nitrite is an oxidizer and essentially stops corrosion by “corroding” everything evenly. This seems counterintuitive, but when everything becomes the cathode and there is no anode, corrosion stops.
A constant supply of nitrite in the system ensures that any bare spots that are created quickly become passivated. However, if there is insufficient nitrite in the chilled water loop, an anode can form in the piping, and again we have the big cathode/little anode corrosion cell. The general guidelines for nitrite-based treatments are for a minimum of 700 ppm of nitrite.
Nitrites are utilized by some bacteria as an energy source. If the closed-loop system becomes contaminated with these bacteria, the nitrite level can decrease rapidly. The bacteria also generate biofilms, which create deposits producing areas that are anodes to the rest of the piping. Adding more nitrite only further accelerates the reproduction of the bacteria, making the problem worse. Systems using nitrite should be regularly tested for the presence of bacteria. In some systems, nonoxidizing biocides such as glutaraldehyde or isothiazoline are added to the treatment to prevent bacterial growth.
Sodium Molybdate. Sodium molybdate is generally classified as an anodic oxidizing inhibitor. Molybdate works with the dissolved oxygen in the water to form a protective ferricmolybdate complex on the steel.
Molybdate treatment levels can be anywhere between 200 ppm and 800 ppm as molybdate. Closed-loop systems that use demineralized water makeup would tend to be on the lower end of this range. Unfortunately, the world supply of molybdate metal tends to be concentrated in areas of historical political unrest, and over the years, molybdate prices have varied dramatically. That price variability can make molybdate treatment competitive with nitrite—or far more expensive.
Ironically, in closed-loop systems that are very tight, dissolved oxygen levels can drop, and thus minimize the effectiveness of a molybdate treatment (which requires dissolved oxygen to form a passive layer). Experts recommend a minimum of 1 ppm of dissolved oxygen in molybdate-treated systems.
Polymer Treatments. Polymer treatments have been used for many years to prevent scale and corrosion product accumulations in open cooling towers. Similar polymers are also now sold for use in closed-loop systems. It appears that the polymer acts as a dispersant for any corrosion products or scale that might form, so it prevents corrosion by keeping the surface clean and ensuring that any dissolved oxygen in the water attacks all surfaces evenly. This produces a general, but overall low level of corrosion.
One of the advantages of this treatment is that it is thought to be very environmentally benign, although as long as the closed-loop system remains closed, there should be no impact on the environment.
CLO2
Chlorine dioxide is one of the fastest growing Chlorine alternatives in water treatment application. This technology offers the benefit of effectiveness at high pH, being unaffected by ammonia, and demonstrating significant advantages over Chlorine in systems susceptible to high levels of organic contaminants also for systems wherein minimum oxidant levels in water is serious concern in view of effluent discharge. Because chlorine dioxide is a selective oxidizer, its ability to control microorganisms in water at very low dosages makes it an especially cost effective solution.
1) Chlorine Dioxide is one of the fastest growing Chlorine alternatives in water treatment.
2) This technology offers the benefits of effectiveness at high pH, hence significant advantages over Chlorine in systems having high levels of organic contaminants.
3) Control microorganisms in at very low dosages makes it an cost effective solution.
Most effective against micro-organisms like
Protozoa,
Cryptosporidium,
Legionella
Giardia
E-coli
Polio- virus
Rota-virus
SRB
NRB,
IRB
Chlorine dioxide has excellent bactericidal and algicidal properties hence used as dis-infect water and inhibit growth of algae.
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