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Chilled water loop protection with nitrite

How well do we understand it?

| | Sep 15, 2019 | 10:16 pm
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As many of you probably know, I frequently visit chilled water and District Cooling plants in the GCC region and in the United States. Just recently, I visited a recently commissioned District Cooling plant that had come into service.

My team and I were about to take over its O&M services and, prior, were chatting with the main contractor responsible for the defects liability period (DLP) and were taking a look at the chilled water loop analysis reports. As you would probably know, the entire purpose of water analysis and chemical treatment in chilled water systems is to maximise the efficiency and longevity of the heat-transfer surfaces. This means maintaining the wetted surface in a clean condition. The surfaces need to be consistently kept free of corrosion, scaling and fouling.

On being handed over the report, I was shocked to notice that it showed a nitrite level reading of 315 ppm. And what was more, the water treatment company stated their recommendation as maintaining it at that level.

I was concerned, to say the very least, because I knew this was a mistake and would cause extensive corrosion in the chilled water loop. Having started and commissioned the loop with such a low level of nitrite, I also knew, was a major issue, as the loop would not have had a passivation protective film established to prevent corrosion.

What amazed me most was that here was a “water treatment company” giving bad and dangerous information to the client and the contractors. It was obvious to me the water treatment person representing the company did not know what he was talking about and that his information was very dangerous – thus the reason for the focus on nitrite in this part of License to Chill.

Sodium nitrite, mixed with sodium borate, is a standard offering among chemical sales companies. However, nitrite treatment is a poor choice for closed systems for a number of reasons. First, nitrite is an environmental toxin. Second, it is aggressive to copper and brass. Third, at levels above target concentrations, it hardens rubber gaskets and forms abrasive crystals at evaporation sites, wearing seals and valves. At levels below target concentrations, it accelerates corrosion rates, making it worse than having no treatment at all. Fourth, nitrite is a ready source of food for the microbes that cause fouling. Having pointed out all these limitations of sodium nitrite, it is, at the very least, important to start up, passivate and maintain chilled water loops with the required protection levels.

Sodium nitrite has been used as a corrosion inhibitor for closed loop water systems for many years. Sodium nitrite functions as an anodic corrosion inhibitor. Nitrite works to form a protective gamma iron oxide film on a metal surface. This layer is formed by the reaction of nitrite and dissolved oxygen and then kept in repair by the nitrite alone. Nitrite is not consumed to any practical extent, since very little is needed to form the film. It is the film that protects the metal surface from corrosive attack.

Sodium nitrite offers excellent corrosion protection for ferrous metals. Nitrite functions best when used in the pH range 9.5 – 10.5, and formulations for closed chilled water systems often make use of borate buffers. In addition to buffering the pH into the desired range, borates promote passivation by facilitating the absorption of oxygen, and also provide some microbiological control. The concentration of nitrite required for inhibition increases with temperature.

Sodium Nitrite is not considered a good corrosion inhibitor for copper or copper alloys; in fact, as stated earlier, it can be aggressive to copper and brass.

ASHRAE’S WORDS ON NITRITE

ASHRAE has this to say about nitrite: “Whether nitrite is used alone or in conjunction with pH buffering agents, relatively high concentrations are needed to establish a protective film, usually on the order of 700 – 1,200 mg/L to completely inhibit pitting corrosion. Once the protective film has been established, the nitrite concentration can be lowered slightly to 700 – 1000 mg/L . Some sources have stated that the required nitrite level
is influenced by the amount of chloride and sulphate present in the water, because they can affect the stability of the magnetite layer. As with

all anodic inhibitors, severe pitting can occur at low concentrations (<500 mg/L as NaNO2). In other words, too little nitrite is actually worse than none at all, because low levels of nitrite will speed up the corrosion process. Loss of nitrite can occur via electrochemical and biological processes. In the former case, if corrosion continues, nitrite can be reduced at the cathode to form ammonia according to the equation:

NO–2 + 5H+ + 6e– NH3 + 2OH–

“In chilled water loops, exposure to bacteria has the potential to oxidise nitrite to nitrate or reduce it to ammonia or nitrogen. Controlling biological activity is difficult, because oxidising biocides (like chlorine) will oxidise the nitrite to nitrate, and the efficacy of non-oxidising biocides tends to be less certain. Difficulty in preventing biological degradation of nitrite has always been a serious limitation.”

Other concerns with nitrites are it also has reduced effectiveness when chlorides and sulphides are present in the water, and the chilled water loop in question was filled with water, which did have a high chloride value. General recommendations are maintaining no less than 500 ppm of sodium nitrite (preferably 1,000 ppm), keeping levels of at least 350 ppm above the total concentration of chloride and sublet present in the closed loop system.

While the use of sodium nitrite in closed water systems has merit as an inexpensive and very effective corrosion inhibitor, very careful consideration should be given to the proper application of the programme to avoid the potential downsides discussed in this article.


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