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There may be multiple closed-loop cooling systems at your power plant. Chances are good that they cool or control temperature on some very critical components. The two that are most likely to exist are the so-called bearing cooling water system (which takes care of more than just bearings) and the stator cooling system, for those plants that have a water-cooled stator. Closed-loop cooling systems can also be found in air coolers on the intakes of combustion turbines.
By its very nature, when a closed-loop system remains closed and operates properly for an extended period of time, it is often forgotten—or at least neglected. Small changes in the chemistry or the flow rates and differential pressures throughout the system may not be noticed. However, once corrosion processes get a foothold in these systems, it can be very difficult to correct them. In the meantime, critical data equipment may be damaged to the point where it affects the ability of the plant to operate.We begin with some general principles and practices for closed-loop cooling water systems before looking at the stator cooling water system, which is a special case.
Understanding Closed-Loop Cooling Systems
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.
1. Oxygen pitting in a closed-loop cooling water system. Courtesy: M&M Engineering
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.
Chemical Treatments for Closed-Loop Water Cooling
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.
Monitoring Closed-Loop Cooling Water
Key to keeping your closed-loop system functioning properly is regular monitoring. Whatever the active agent is in your treatment (nitrite, molybdate, or polymer) the concentration must be regularly monitored. Generally, weekly testing is sufficient unless the levels of the treatment are dropping. (You won’t know that if you are not monitoring regularly.) Because the carbon steel and copper corrosion treatment are typically blended into one product, low levels of treatment may affect more than just the carbon steel piping.
The pH of the water should also be tested regularly. Considering the amount of pH buffering in chemical treatment, the pH of the water should be rock solid. Drops in pH may indicate bacterial contamination, particularly with the nitrite-based treatments. Another thing that can drop the pH is leaks in the system, which bring in fresh demineralized water makeup.
Be on the lookout for other signs of bacterial contamination, such as slimy growth in any sightglass or flow indicators, or septic smells when the sample is collected. Plate and frame heat exchangers have a very large surface and small spacing for heat exchange between the plates. Bacterial contamination can not only seriously affect heat transfer, but it also can cause pinhole leaks in the stainless steel plates. Depending on the pressure of the closed-loop versus open-loop system at this point, the bearing cooling water may leak out, or the open cooling water may leak in.
Remember that it is much easier to prevent bacterial contamination than it is to try to recover from a system that is severely contaminated.
Stator Cooling Water Systems
The stator cooling water system is a very special closed loop for a couple of reasons. First, it protects one of the most critical pieces of equipment—the generator. There is only one metal of concern in this system: copper. And this system must remain very clean, even pristine. Small temperature increases in stator cooling bars can restrict load on or even shut down the generator. Therefore, this system requires special understanding, attention, and monitoring
■ A head tank containing the deionized water that provides suction to the pumps
■ Circulating pumps
■ Heat exchanger
■ Filters (cartridge filters, mesh strainers, or both)
■ Mixed-bed deionizer
■ Monitoring for flow, temperature, conductivity, dissolved oxygen, and, in some cases, pH
There are often two deionizer vessels and two sets of filters to allow one to be valved out to replace the filter cartridge or for replacement of the mixed-bed resins.
The cooling loop removes heat from the stator bars and conveys it away through heat exchangers. The water is continuously passed through a mixed-bed polisher that removes any soluble ionic contaminants that enter the water. These impurities are generally dissolved carbon dioxide and ionized (dissolved) copper corrosion products.
The ion exchange resins may also trap fine particles of copper oxides, though this is better done by the cartridge filters. The ion exchange resin can become exhausted over time (as indicated by increasing conductivity). But it is more common for differential pressure across the resin bed (caused by the collection of corrosion products in the resins) to require the resins to be replaced.
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