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Why Treat Condensate ?
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Why Treat Condensate ?

What causes corrosion in steam condensate systems?

Carbon dioxide and oxygen cause most condensate system corrosion. Carbon dioxide, dissolved in condensed steam, forms corrosive carbonic acid. If oxygen is present with carbon dioxide, the corrosion rate is much higher and is likely to produce localized pitting. Ammonia, in combination with oxygen, attacks copper alloys.

How is steam condensate corrosion prevented?

The general approach involves removing oxygen from the feedwater mechanically and chemically and providing pretreatment of the make-up water to minimize potential carbon dioxide formation in the boiler. Chemical treatment reduces corrosion potential further. Volatile amines neutralize carbonic acid formed when carbon dioxide dissolves in condensate. Volatile filming inhibitors form a barrier between the metal and the corrosive condensate.

How do chemical oxygen scavengers help control condensate system corrosion?

Deaerators (and feedwater heaters) remove oxygen mechanically. The best-designed and operated deaerators can reduce oxygen to as low as 0.007 PPM. Since very small amounts of oxygen can cause corrosion, complete oxygen removal often requires chemical treatment.


What is the basis for choosing between neutralizing and filming inhibitors?

The proper choice depends on the boiler system, plant layout, operating conditions and feedwater composition. In general, volatile amines are best suited to systems with low make-up, low feedwater alkalinity and good oxygen control. Filming inhibitors usually give more economical protection in systems with high make-up, air in-leakage, high feedwater alkalinity or intermittent operation. In most cases, a combination of these treatments may be the best to combat condensate corrosion.

What characteristics should a good condensate corrosion inhibitor have?

A good volatile neutralizing amine should have a favorable distribution ratio in steam and condensate so that it protects the entire steam-condensate system. It should have no insoluble reaction products and should be stable at high temperatures and pressures. A good filming inhibitor should be easy to disperse in water. It should be stable under usage conditions and form a thin, protective film without causing deposits in either the boiler or steam-condensate system

Condensate Corrosion--Factors and Control

Carbon Dioxide

Breakdown of carbonate and bicarbonate that enter the boiler is the main source of carbon dioxide. Left unchecked, this can result in low pH condensate. This has been observed as grooving in sections of condensate lines that are not completely filled with water.

Oxygen

Oxygen can enter a condensate system by many sources even if the deaerating heater is functioning properly. The oxygen, at its worse, can result in deep pitting of condensate lines. The combination of oxygen and carbon dioxide corrosion can be particularly troublesome in causing corrosion products to be produced and transported to the boiler.

Velocity

Although often not controllable, high flow rates within the condensate system can produce extremely severe corrosion conditions. This flow-assisted corrosion is accelerated at low pH and can be minimized by keeping the pH above 9.0.

Other gases

Other gases that can be corrosive and present in the condensate system include ammonia, hydrogen sulfide, and sulfur dioxide. The most common of these is ammonia. Copper corrosion can be as serious as iron corrosion and is made even more serious in the presence of copper complexing agents such as ammonia. Again, oxygen in combination with these gases increases copper corrosion.

Neutralizing Amines

Neutralizing amines, when fed to the boiler, volatile with the steam and enter the condensate system. These amines are weak bases and will therefore neutralize any carbon dioxide present and will raise the pH of the condensate. If oxygen levels are very low, these neutralizing amines can, by themselves, effectively control condensate corrosion. However, knowing which ones to feed, how much, and how to control can be a difficult and confusing decision for the typical user. There are more than a dozen amines in common use. Each amine is unique in certain characteristics, each of which affect how the amine functions at a given point in a given condensate system. Those amine characteristics include basically value, molecular weight, distribution ratio, and hydrolytic thermal stability. Since it is necessary to prevent corrosion from the point of initial steam condensation to the far ends and back of condensate systems, a blend of neutralizing amines is normally fed.

Volatile Passivating Agents

If oxygen is present, the neutralizing amines alone will not control corrosion. Fortunately, chemical treatments have been developed which will transport with the steam and will, in addition to possibly reacting with oxygen, function as Passivating agents to prevent corrosion. But as is often the case, such new alternatives bring with them a whole new set of potential problems and set of rules. Which product to use where, when, and how are questions that are best answered by an expert in water treatment chemistry who is also highly knowledgeable about your system. Control and testing are the main problems with these Passivating agents. That is why corrosion monitoring, always important, becomes even more so when treatment results depend on a passivation chemical.

Monitoring

Corrosion monitoring is a major task for any water system. Relative pure waters, like condensate, make the task even more difficult. Some of the methods used and recommended by Thermidaire Consultants include the following:

Test Coupons

Steel and copper corrosion coupons have been used in condensate systems for many years. While there are concerns as to how accurately they reflect the actual corrosion rate within a given system, the do provide a good relative measure of trends in long term corrosion. A consistent, continuing program using coupons at the same locations and for the same duration of time is therefore most meaningful. The locations and plumbing for the coupons must be carefully engineered to avoid meaningless results.

pH Monitoring

While not a direct measurement of corrosion, continuous measurement of condensate pH can be very helpful in systems that depend on neutralizing amines for pH control. Other monitors, such as for conductivity, are also helpful to guard against condensate contamination. Location of sample points and method of sampling are critical and should be established by a qualified water treatment engineer.

Iron and Copper Testing

Another old but proven effective standby is iron and copper testing. The sample points, method of collection, and analytical procedures are more critical in trace metal testing than in any other analysis. Incorrect results are much worse than no results at all. The sample program should be established to collect samples at a pre-determined interval. The final feedwater represents the corrosion products load actually entering the boiler and can be a good indicator of the expected cleanliness of boiler surfaces over a period of time. Composite as well as spot samples should be taken. However, since the slightest change in flows can make sample results worthless, any composite samples collected should be collected from a continuously flowing sample and with a proven condensate composite sampler.

Corrosion Test Monitors

Electronic corrosion test monitors have now been developed that can be effectively used in condensate systems. Older corrosion test meters were not capable of this because of the need for the water being tested to exhibit a minimum conductivity. As with any of the monitoring methods, results from these should be evaluated over a long period of time and compared with prior results versus actual inspections.

Monitoring of your condensate is essential toward protection of, not just the condensate system itself, but more importantly, to the continued reliability of the system boilers.

 

 

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