Closed Loop Systems Water Treatment

The problems found in a recirculating cooling water system usually fall into the following categories:

- Corrosion

- Scale

- Fouling Deposits

- Microbiological

Scaling is not as much of a problem as in open-loop systems. However, corrosion, fouling deposits and microbiological growth has equal potential to contribute to failure as in evaporative systems.

The presence of mixed metals (brass, copper, aluminum, stainless steel, etc. in addition to steel / iron) increases the potential for galvanic-induced corrosion. Thus, galvanized metals are normally avoided for use in closed loops due to their susceptibility to galvanic related attack caused by dissimilar metals.

When corrosion products accumulate, deposits are formed and collect in the lower flow areas where oxygen concentration differential under the deposit cap (tubercle) promotes additional corrosion.

Since there is no blowdown in a closed loop, the period of time the water is held in the enclosed piping can be quite long. Corrosion products can build up and create a condition that is ideal for microbial growth to worsen the situation. As the swimming bacteria (planktonic organisms) move from one area to another, the entire system becomes inoculated with a growing biofilm. Any scale that does form also becomes a developing problem due to hindered heat transfer. Limitations of chemical to penetrate the biofilm and deposits further complicate the treatment issues.

Fresh water make-up is the most common source of this biological contamination as it is not sterile (even if sanitized). Micro-biocides are added to water to effectively prevent organisms from becoming a problem. It is very important to be sure to add any biocides in sufficient amounts to kill bacteria that could be a developing growth. to prevent bacterial acclimation toward the chemical.

Treatment level should be monitored and supplementary doses are added based on a predetermined frequency to counter ongoing microbiological inoculation from make-up water dilution due to leaks and losses in the system and because, over time, all (environment friendly) biocides hydrolyze to non-hazardous by-products and lose effectiveness. The “half-life” of biocides is measured over hours.

Proper chemical treatment of any closed loop should begin before a new system is started up. Flushing and pre-treatment are necessary steps to clean and prepare the surfaces to receive corrosion inhibitors. System control can be lost long before any water is circulated due to incomplete pretreatment. This is never 100% visible. As an example, see the photo below. The center and right pipes show incomplete pretreatment and passivation. Preferred is the pipe at left.

  

Closed-loop inhibitor chemicals often involve blended corrosion inhibitors. Typical inhibitors might contain a blend of nitrite and borate (or other buffering agents), plus a copper corrosion inhibitor. Also found in many formulations are dispersants and organic scale inhibitors. Silicate is often added as protection for any aluminum present. But this will also require some additional techniques for total protection, such as pH targets and other inhibitors (e.g. organic, Molybdate).

Soft water removes any scale potential that might be present. Scale potential will be generally associated with heat, which might come from a hot water hydronic system or a high heat flux unit normally associated with furnaces. The latter unit can be normally identified with high-temperature metal-smelting related processes or waste-burning fuels such as a paper mill burning black liquor.

Removal of suspended contaminants can be accomplished using cartridge insertion type filters. (5 ~ 10μ mesh size) and 2 ~ 5% of pumping capacity (sufficient for one volume turnover every one to three days).

Microbiological counts should be held to a maximum of 103 CFU / ml of aerobic activity. Anaerobic activity should be held at close to zero. If one or 10 cfu are identified in the bulk water, there must be a biofilm growing and hiding somewhere in the system even if aerobic counts are ≤103 cfu/ml. Biological targets are intentionally held at lower limits than in open, recirculating systems due to the long holding times involved.

Corrosion coupons are installed on the system for monitoring the effectiveness of corrosion inhibiting- treatment chemicals. Typically, 90-day coupons are used for demonstration of program effectiveness.

 

System-conditions and exposure for the coupons shown in the photo above were identical, except for the monitored flow rates. A typical assessment for program effectiveness (closed loops only) is shown in Table 1

 

 

Any pitting-type corrosion is unacceptable since localized attack is extremely concentrated. Even at an acceptable mpy (mils per year) penetration, a localized rate failure may exist.

From “Closed System Maintenance and Treatment” by Mike Morgan