Cooling Tower Passivation Explained

New evaporative cooling systems, typically cooling towers, ammonia condensers and fluid coolers are manufactured with several options for materials of construction, each with their own benefits and draw backs. Hot-dipped galvanized steel (steel that has been coated with zinc) has become the most common choice for construction as it provides structural strength at a relatively low cost and enhanced corrosion resistance over that of uncoated steel. However, the zinc coating is not well-suited for locations that are in constant contact with water, and requires passive oxidation to be protected from corrosion when exposed to cycled cooling tower water. Passivation is the process by which this zinc layer is oxidized slowly under controlled conditions to produce a passive layer of zinc oxide on the exterior surface of the coating, which provides superior resistance to corrosion.

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While passivation has always been required on galvanized materials in evaporative coolers, it has increasingly become a more prevalent topic of discussion. Previously, cooling units were manufactured ahead of time in standard sizes, and were left outside until a unit of that size was needed. During this time, the towers would be repeatedly exposed to the elements including rain and condensation, which naturally passivated the galvanized surfaces.

Advances in manufacturing techniques have improved the time required to design, build, and assemble each unit. Today, almost all cooling towers begin manufacture at time of order and as a result, most units being installed have not had the opportunity to passivate naturally. Therefore, a passivation program must be implemented as soon as possible after installation before the system sees cycled tower water under load.

White Rust Formation

When an un-passivated galvanized surface is exposed to cycled cooling water, which will typically be oxygen-rich and have an elevated pH, white rust will begin to form. White rust is a corrosion product of zinc and is composed of a mixture of zinc, oxygen, water, and carbonate. White rust can form, remain in place, and not grow in extent or coverage. If this is the case, the main disadvantage is the unwanted appearance. White rust formation will not always cause premature failure or issues with the cooling tower. However, if this corrosion process continues, the carbon steel underneath the zinc coating can become exposed. If this occurs, the zinc will no longer offer protection, and the carbon steel will corrode rapidly, resulting in premature failure. The basin is most susceptible to this mode of failure, as the basin will remain fully wet during system operation. Thus, it is vital that the zinc be properly passivated to ensure maximum protection from corrosion and to extend the life span of the equipment.

Considerations

When installing a new cooling tower, there are several important factors that must be considered with regards to passivation.

First, a stainless steel basin should be considered when purchasing the cooling tower. As mentioned previously, basins are the most susceptible to failure by white rust. While more expensive than a galvanized basin, stainless steel does not require passivation, and cannot fail due to white rust formation. Generally, a stainless steel basin will offer a far superior lifespan as compared to a galvanized basin.

If a galvanized basin is selected, then the following considerations are required to ensure proper passivation to protect your newly acquired asset. First, and most importantly, the local makeup water chemistry will play a critical role. Systems with water supplies that are high in dissolved minerals or pH may require an acid feed system to maintain the proper water chemistry to promote passivation. To promote passivation of the zinc coating, it is recommended to maintain specific water chemistry and have the galvanized surfaces regularly exposed to that water. The general water chemistry guidelines are as follows:
  • High hardness levels (>100ppm)
  • Neutral pH (~6.5-7.3)
  • Low total dissolved solids
  • Presence of ortho-phosphate
cooling tower inspection
Secondly, wherever possible systems should be started under no or very low load conditions.  If the system will be under little to no load, it may be possible to maintain a passivating condition using the natural water chemistry of the makeup water, with addition of phosphate as required. Under these circumstances, uncycled water can be sprayed intermittently in the tower, causing wet-dry cycles which allow for the most effective promotion of passivation. However, if the system will be under load while passivation is being attempted, it may not be possible, or economical, to maintain uncycled water to passivate naturally through bleed, and acid feed may be required to keep pH close to neutral as required. This can be costly both in material costs for acid and phosphate, as the system will be using a substantial amount of water depending on the system size, and in equipment costs, as acid feed requires specialized chemical feed and control systems to safely inject acid and maintain pH levels.  In addition, the reaction rate of white rust formation will be accelerated with increases in temperature.  The warmer the system the faster the rate of zinc corrosion.  This will typically be of the most concern in condenser and fluid cooler systems as the heat source is in the galvanized tube.

A successful passivation program will require cooperation between the water treater, mechanical contractor and end user to achieve the conditions required to promote passivation.

Conclusions

When the galvanized coating on carbon steel is maintained on cooling tower or other evaporative cooler surfaces, it functions to essentially slow down the rate of corrosion of the underlying steel. This promotes acceptable performance of the steel and maximizes the life span of the equipment while minimizing the overall cost of the material. However, the steel must remain wholly covered with the galvanized zinc coating to maintain this corrosion inhibition. White rust formation on these galvanized surfaces may eventually lead to the coating being compromised, resulting in rapid, local corrosion at the site of the exposed carbon steel beneath.  Properly passivating the zinc coating on these surfaces can greatly improve the corrosion resistance of the coating, extending its protection of the steel structure. This process can be repeated on cooling evaporative systems that have already been in service to re-passivate the surface, restoring the passive zinc oxide layer. All galvanized surfaces should be regularly inspected for white rust formation, and the process repeated as needed. Talk to your water treatment specialist for any specific questions you may have.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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