Basics of the water chemical mode of low-pressure steam boilers

Ivan Tikhonov

In the article, as simply as possible (as it seems to the author), the basics of the water-chemical mode of low-pressure fire-tube steam boilers are presented.

The water-chemical mode (WCM) of a steam boiler is an operating complex of technological solutions aimed at ensuring trouble-free and efficient operation of a steam boiler.

In fact, it is quite easy to achieve an efficient functioning of a steam boiler. To begin with, the main thing is not to drown in incorrect and ineffective (and sometimes harmful) information, which currently fills the entire information space.

First, a few words about water. Water has the property of entering a state of equilibrium with the surrounding system or with the conditions surrounding it. That is, the water in a river comes into contact with air, land, groundwater, and surface runoff. As a result, a certain balance is achieved between water and the conditions surrounding it. In the process of achieving this equilibrium, water can dissolve various substances in itself. For example, water is saturated with carbon dioxide from the air or from organic oxidation processes (decay). As a result, the water becomes acidic and begins to dissolve limestone. By dissolving limestone, the water is getting saturated with calcium bicarbonate while increasing the pH value is increasing. Therefore, water contains various types of ions and at the same time it has a complex gas composition. And all of these are in balance for the given environment. If, for example, you calculate the Langelier index for a full-flowing river, it turns out that it will always be around zero.

Now let us imagine that this water enters the boiler. The environment is changing. The water heats up, begins to boil, and all gases, including carbon dioxide, are  actively being removed from it. As a result, reaching a new equilibrium for the given environmental conditions, the water begins to release ions and organic matter in the form of scale and sludge. At the same time, heating increases the mobility of oxygen and therefore increases the corrosive activity.

Thus, the task of the water chemical treatment is to ensure the preparation of water in such a way that, under the conditions of a steam boiler, the water does not cause corrosion of the equipment and does not release scale, and at the same time it determines how efficient the boiler is.

Currently, boilers are divided according to the pressure of steam. For high-pressure boilers, the water requirements are very high. But at the same time a whole team of specialists monitors the fulfillment of these requirements. But what about a small enterprise with a low pressure steam boiler? It makes no sense to create a chemical service for the sake of one boiler. There is no outsourcing market in this area, at least in Russia.

The problem is aggravated by the fact that the requirements for low-pressure boilers are in many ways similar to high-pressure boilers. There is a clear tendency that the requirements for low-pressure boilers were simply copied from high-pressure boilers (with the exception of the requirements for steam quality).

As a result, we get very strict requirements for the organization of the WCM of the boiler house, but in practice, for small enterprises of a non-energy profile, we observe an almost complete absence of such.

In this case, the following process is observed. About 80% of low-pressure boilers generally operate without water treatment or with “some” water treatment, in which case, as they say, it is better without it. Moreover, sometimes a boiler functioning without any water treatment can be operated, but with water treatment it fails. After all, in the absence of proper operation, the success of a boiler depends on the source of the water supply and the quality and proportion of condensate return.

In these conditions, I would like to outline the basic principles under which a low-pressure steam boiler can work quite successfully without requiring “excessive” attention.

First of all, you need to understand what happens to the water in a steam boiler. In the boiler, carbon dioxide begins to be effectively removed from the water with steam. As a result, carbonates begin to appear in the water. As a result, calcium carbonate is released from the water in the form of scale. This process happens first. If sulfates are present in the water, calcium sulfate (gypsum) may precipitate, because the boiler water is evaporated and the concentration of sulfates and other salts increases. But the precipitation of gypsum occurs quite rarely (only in cases of their high concentration in the water supply source in the absence of a water treatment system). Because if calcium is present in the water, then the most of it precipitates with carbonates and there is simply not enough for sulfates. However, the water also contains magnesium, which will also precipitate with sulfate hydrates.

The precipitation of gypsum in the boiler is also possible due to the dosage of sodium sulfite in the feed water in order to bind oxygen. As a result, the concentration of sulfates in the water increases and if the softening unit stops working and a sufficient amount of calcium appears in the water, both carbonate and calcium sulfate may precipitate.

Therefore, the most important thing that needs to be done to prepare water is to soften it. As a result, sodium ions appear in the water instead of calcium and magnesium. Sodium ions will not precipitate and when carbon dioxide is removed from the water, sodium carbonate (soda) will be obtained, not calcium carbonate. The boiler water will hydrolyze sodium carbonate to form a hydrate (OH). As a result, the pH of the water in the boiler will rise. This is the main advantage of pre-softened water.

The boiler water will have a high pH value. If calcium gets into such water, it will be immediately transferred to the sludge. Not into scale, but into sludge. And it can be removed with periodic blowdown. If carbon dioxide gets into such water, it will be immediately converted into bicarbonate and driven off with steam. Thus, the boiler water formed from the softened feed water acts as a kind of protective buffer for the boiler. It transfers the hardness salts into sludge, and neutralizes carbon dioxide and removes it with steam. In this case, only oxygen corrosion is possible. A big plus is that if the boiler water contains hydrate, then the silicic acid completely dissociates and does not give scale or it is released in the form of a removable sludge.

As a result, the following activities can be recorded for the successful WCM of a low-pressure steam boiler.

  1. Water softening
  2. Maximum condensate return
  3. Deaeration and decarbonization of feed water
  4. Reverse osmosis desalting
  5. Constant automatic control of the pH values ​​and electrical conductivity of boiler water

Let’s consider each item in more detail.

  1. Water softening
    Water softening is the main water treatment process that must be implemented for a low-pressure steam boiler. Of course, you can completely deminerate the water, and then dose caustic soda into it. But in the absence of proper chemical control, this is tantamount to committing boiler murder. The process of water softening for low-pressure boilers has another very big advantage, which will be discussed in the fifth paragraph.
  2. Condensate return is an extremely important component of a successful WCM. The condensate receives a lot of carbon dioxide from the decomposition of bicarbonates in the boiler. As a result, carbon dioxide gradually passes into condensate and decreases its pH in the condensate tract. As a result, the corrosion of the condensate path begins to occur and the condensate is filled with corrosion products. The lower the temperature of the condensate is, the more carbon dioxide will dissolve in it and, accordingly, more iron will pass into the condensate. Iron will cause increased turbidity in the boiler water with all the negative consequences. Therefore, when designing a condensate return system, it is extremely important to avoid lines where condensate stagnation and chilling is possible. The condensate must be returned to the deaerator head. The advantages of this are: firstly, mixing condensate with the make-up water, heats it up and deaeration takes place more efficiently; the secondly, carbon dioxide is distilled off from the condensate in the deaerator head. If the condensate is returned directly to the deaerator tank, the carbon dioxide will react with the water hydrate in the deaerator tank. The pH value of the feed water will drop and more steam is required for bubbling to ensure the required pH of the feed water after the deaerator.
  3. Deaeration and decarbonization of feed water takes place in a deaerator. At the same time, it is necessary to note the difference in the processes of deaeration and decarbonization. Deaeration effectively occurs only in the deaerator head. Thus, for effective deaeration the water temperature on the last (lower) plate must be at least 100 ° C. This is difficult to achieve in the absence of pre-heating of the make-up (softened) water. Therefore, if cold softened water enters the head, a large steam consumption is required in order to obtain the required quality of feed water for oxygen. Especially under variable performance conditions, which is typical for low-pressure industrial boilers, such a mass transfer unit as a deaerator operates with very low efficiency.
    It seems to be obvious that it is necessary to organize the preliminary heating of softened water in front of the deaerator. But what looks just like a heat exchanger with steam and water supply on paper, in practice is a very serious task.
    Firstly, softened water is extremely corrosive due to the shift in the balance of the carbon dioxide towards dissolution. Therefore, when such water is heated, something which can be called the explosive corrosion is observed. Therefore, it is permissible to use only stainless steel.
    Secondly, the organization of the heat exchanger operation also requires a very serious approach. The fact is that heating occurs with steam. Make-up water passes through the heat exchanger irregularly. Accordingly, when the water flow through the heat exchanger stops, and the steam regulator does not have time to shut it off, for example, the plates of the heat exchangers immediately collapse from the fact that the water in the water line simply boils.
    All this requires a very serious approach to solving a seemingly simple heat-engineering problem.
    As a result, usually for low-pressure boilers there is a significant excess of oxygen in the feed water. But even in this case, it is primarily the steel economizer and the feed path of the steam boiler that are susceptible to corrosion. Getting directly into the boiler, oxygen is immediately carried away with the steam. In the case where deaeration is completely absent, oxygen corrosion of the steel economizer, the feed path and, possibly, the return pipes of the boiler will most likely be observed.
    The decarbonization of feed water is primarily the removal of free carbon dioxide. Free carbon dioxide is easily removed directly from the deaerator tank. The fact is that carbon dioxide cannot be removed to the level of the so-called zero. With a decrease in its concentration in water, carbonates and, accordingly, hydrates begin to appear. Hydrates bind free carbon dioxide into bicarbonate. As a result, this will help to avoid hydrogen (acid) corrosion of the feed path and the boiler economizer. In any case, about 90% of the bicarbonates in the boiler will be converted into carbon dioxide, which will be removed with steam.
  4. Reverse osmosis demineralization of make-up water is not a mandatory requirement for the operation of low-pressure steam boilers in the case of a large proportion of condensate return and (or) low salt content of the source water. However, I highly recommend using reverse osmosis in conjunction with water softening. As a result, we get deeply softened water with low salt content. In this case, we have a small boiler blowdown. The condensate becomes much less aggressive. Therefore, even an ineffective organization of the condensate return system is not critical. The absence of organic matter in the water does not cause sludge appearance in the boiler. All this leads to a significant increase in the reliability of the entire boiler house. As a result, the enterprise does not incur losses due to the absence of downtime of the boiler house and, accordingly, the entire technological chain.
  5. Studying the boiler water, as the blood of a person, you can find out almost all diseases at once. If not almost all of them, then chronic once for sure.

The main monitored parameters of boiler water are alkalinity for phenolphthalein and methyl orange and salinity. Alkalinity analysis requires a technician and laboratory. The salinity of the boiler water can be measured with a conventional conductometer. Based on the total alkalinity of the boiler water and the salinity, a certain relative alkalinity of the boiler water is calculated. There is probably a certain misunderstanding here. The concept of relative alkalinity, i.e. the ratio of total alkalinity to salinity, came from high-pressure boilers. For high-pressure boilers, this parameter is important because the feed water is completely chemically demineralized and caustic soda is dosed into it to raise the pH in order to avoid carbon dioxide corrosion. As a result, if more caustic soda is dosed into the water than required, this can lead to boiling and removal of boiler water with steam, which is critical for power boilers. For low-pressure boilers, where water demineralization is not required, this parameter is not informative. It does not show anything and therefore is harmful, because many are guided by it.

For low-pressure boilers, the pH value of the boiler water and its electrical conductivity are important. Because due to the absence of strict requirements for the quality of steam, for low-pressure boilers, only water softening is sufficient. The fact is that depending on the composition of the source water and the rate of evaporation in the boiler, in the case of completely softened water, the pH value of the boiler water will be only one and it will absolutely clearly correspond to one value of the electrical conductivity of the boiler water. Thus, we can predict the dependence of the boiler water pH value on its electrical conductivity for a particular water supply.

Accordingly, if the pH value of the boiler water drops, but the electrical conductivity value increases or remains the same, this indicates that hardness salts begin to enter the boiler and a check of the softening system is required.

This control technique is described in more detail in the article: Tikhonov I.A., “Control of the water-chemical regime of low-pressure steam boilers using the pH value of the boiler water.” Link –

I understand that this technique is indirect, but it can be fully automated. In this case, control can be carried out remotely and continuously. As a result, all changes in the water chemistry of the boiler can be surprisingly clearly tracked by simply looking at the smartphone screen.

Taking into account the peculiarities of the operation of the water softening plant (if something is wrong, it starts to work, periodically giving out hard water, which will immediately be recorded in the form of a drop in the pH value of the boiler water), using this technique it is possible, albeit indirectly, but it is easy to understand whether softening works and whether more precise chemical control is required to adjust the filter cycle of the water softener. In most cases, at the beginning of the operation of the boiler room, this makes it possible to understand whether the personnel add salt to the feeder. It also happens, and quite often.

For remote objects, this is simply irreplaceable.

In conclusion, I would like to note that a efficient WCM for low-pressure steam boilers is, first of all, finding the balance of water depending on the ambient conditions (conditions in the boiler and the feed path) when using mass transfer processes (softening, osmosis, thermal degassing). Because mass transfer processes do not require constant monitoring and operate successfully under relatively stable initial conditions. But dosing of various reagents as the main tool for conducting WCM is highly undesirable. If you really want to, you can use the dosage of reagents in very small concentrations as corrective and (or) stabilizing measure. But in almost all cases, in the absence or improper operation of the main water treatment system, dosing of reagents will lead to rather negative consequences. So phosphates and sulfates can only be dosed into deeply softened water. Organic reagents can simply muddy the boiler. Sodium sulfite requires constant renewal of the solution, because it consumes oxygen from the air. Dosing of ammonia eliminates the use of steam in food production, etc. And at the same time, all this requires serious control, which is rarely organized in small non-energy enterprises.

Respectfully yours,

Ivan Tikhonov


4 Replies

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  1. Ринат says:

    Спасибо за статью. Прочитал на одном дыхании.

    Котлы низкого давления редко бывают большой производительности. Обратный осмос это роскошь для таких котельных конечно.

    • Иван says:

      Спасибо за комментарий.
      По поводу осмоса, здесь конечно внимательнее надо быть.
      Как мне кажется, сейчас необходимо в любом случае проводить оценку экономической эффективности его применения, конечно с учетом смогут ли его эксплуатировать на объекте. Может оказаться что применение осмоса значительно снизит эксплуатационные издержки, а может и наоборот.
      Еще раз спасибо за комментарий.