Recommendations for the development of steam condensate return systems

Ivan Tikhonov

The steam condensate returned to the boiler room is a high-quality feed water for the steam boiler in the absence of contaminants. Condensate pollutants include dissolved iron, hardness salts and dissolved aggressive gases (carbon dioxide, oxygen). Let’s take a closer look at how these contaminants can get into the condensate. Dissolved iron is the main contaminant of the condensate. Hardness salts mainly enter the condensate through the damaged heat exchange surface of the equipment. If there is such a problem, it is necessary to determine the source of contamination and repair the damage. But dissolved iron is a natural and extremely unpleasant contaminant in condensate. It’s pretty hard to fight it. The most frequently used solution to this problem is the dosing of ammonia into the feed water. Unfortunately, in practice, such a solution is not suitable for most low-pressure boilers due to the lack of a chemical service (ammonia dosing requires constant proper control), or the profile of the enterprise does not allow the use of ammonia. But even in the absence of ammonia dosing, it is possible to significantly reduce the iron content in the condensate by organizing a competent water-chemical mode of the boiler house and the condensate return system.

First, you need to understand how iron can get into the condensate. To do this, let us consider what composition the steam condensate has. The composition of the steam condensate is formed based on the chemical composition of the feed water, as well as the operating conditions of the steam condensate path.

Figure 1 shows a typical diagram of the formation of condensate and its return to the steam boiler as feed water. Make-up water contains sodium bicarbonate (NaHCO3). The steam boiler is fed by this water. During the process of steam formation in the boiler, most of the bicarbonate decomposes and turns into gaseous carbon dioxide (CO2). As a result, caustic soda (NaOH) is formed in the boiler water. The carbon dioxide is carried away with the steam. With a competent organization of the water-chemical mode of the boiler, the steam humidity will be no more than 1%. That is, with a salt content of boiler water equaling 3000 ppm, the salt content of steam will be equal to no more than 30 ppm. Therefore, the salt composition of the condensate will be insignificant and can be ignored as pollution. But the steam contains carbon dioxide.

Figure 1

The amount of carbon dioxide in steam can be estimated quite accurately using the following formula.

СО2_(steam)=44*k*A_. ,    ppm

Where,

k = 0,7-0,95 – coefficient of completeness of decomposition of bicarbonates in the boiler.

1. A_. – feed water alkalinity, mq-Eq/l

Example, if the alkalinity of the feed water is 2.0 mq-Eq/l, then the amount of carbon dioxide in the steam of a low-pressure steam boiler will be equal to

СО2_(steam)=0,9*2=1,8 mq-Eq/l =44*1.8=79,2 ppm.

That’s a pretty large amount of carbon dioxide. If 79.2 ppm CO2 is dissolved in water, then the pH of such water will be about 4.6. The water will be acidic and corrosive. The condensate will have a pH value of 4.6 if it cools down to a temperature of 25 ⁰C (at atmospheric pressure) and carbon dioxide is not removed from it beforehand.

Low pH condensate corrodes pipes with hydrogen depolarization. As a result, iron appears in the condensate.

Figure 2 shows a graph of the dependence of the solubility of carbon dioxide in water on temperature.

Figure 2

As you can see, the higher the water temperature is, the less carbon dioxide dissolves in it. It turns out that in order to prevent the transition of gaseous carbon dioxide into carbon dioxide of the condensate, the following conditions are necessary:

1. Keep the condensate temperature as high as possible throughout the entire condensate path.
2. Avoid places with possible stagnation and cooling of condensate.
3. Do not install intermediate condensate collection stations with open atmospheric tanks. While some of the carbon dioxide will immediately evaporate into the atmosphere in an open container, the temperature of the condensate will drop, and most of the carbon dioxide will dissolve in the water to form a corrosive environment.

To reduce the formation of carbon dioxide in the steam, it is also necessary to reduce the alkalinity of the feed water.

The alkalinity of the feed water consists of the alkalinity of the make-up water and the condensate. If the alkalinity of the condensate can be neglected, provided that the condensate is returned to the deaerator head (not to the deaerator tank), then the alkalinity of the make-up water must be reduced.

One of the most effective ways to reduce the alkalinity of make-up water is reverse osmosis water desalination. As a result of water treatment by reverse osmosis, almost all alkalinity is removed from the water. At the same time, the alkalinity value of the filtrate after reverse osmosis is about 0.05 – 0.2 mg-eq/l. This value of alkalinity makes it possible to obtain a sufficient amount of caustic soda in the boiler water to protect the boiler from corrosion and at the same time prevent a large amount of carbon dioxide in the boiler.

Example, in the deaerator of a steam boiler, condensate and make-up water are mixed in a ratio of 50/50. In the first case, let’s assume that make-up water only softens. Therefore, the alkalinity of make-up water remains equal to the alkalinity of the source water and is 2 mg-eq/l. Then the alkalinity of the feed water will be 0.5 * 2 = 1 mg-eq/l.

With such alkalinity of the feed water, the steam will contain СО2 =  0,9*1=0,9 mg-eq/l = 0,9*44 = 17,6 ppm.

If make-up water is passed through reverse osmosis, then the alkalinity of make-up water will be 0.1 mq-eq/l or less. Then the content of CO2 in the vapor will be equal to 0,9*0,05=0,045=1,98 ppm.

Reverse osmosis water treatment also has the advantage that the evaporation rate of the boiler water will be much higher than in the absence of reverse osmosis. This will reduce the quantity of purge water and, as a result, fuel savings will be observed.

As the practice of operating low-pressure steam boilers shows, in the case when the alkalinity of the feed water is insignificant and at the same time the condensate return system does not allow stagnation and cooling of the condensate, almost no iron is observed in the condensate. But if the condensate return system contains stagnant places where the condensate stagnates and cools, then even if technologies that reduce the alkalinity of the feed water are used, the condensate contains an increased concentration of iron.

The condensate return system is a complex technical solution that requires careful study to prevent excessive solubility of carbon dioxide in the condensate.