The features of the development and maintenance of water-chemical mode of low-pressure steam boilers using membrane technologies

Ivan Tikhonov

The water – chemical mode (WCM) of steam boilers must provide effective and reliable operation of a boiler house.  In fact, this is achieved by creating conditions that exclude possible processes of scale formation or various corrosion damage of equipment and pipelines. These conditions are created with the help of water treatment systems, as well as the organization of proper operation of the steam boiler and thermal deaerator.

20 years ago the main technology of water treatment which was used in low-pressure steam boilers was ion exchange technology. Na-softening technology is the most widely used due to the simplicity of operation, low cost of equipment and the availability of reagents. Na-softened water potentially contains a large amount of alkali and effectively counteracts carbon dioxide corrosion during boiling, which favorably affects the operation of steam boilers. At the same time, this technology has a number of significant drawbacks. There is a large number of highly mineralized effluents, and softening does not reduce, but rather slightly increases the salt amount in water. H – OH ion exchange technologies are also common. But these technologies are quite complex in operation, require the use of extremely dangerous reagents and produce a large number of aggressive wastewater. These technologies can be used only in sufficiently large boilers with qualified service personnel and its own laboratory. Looking ahead, I would say that, in my opinion, in practice, the use of the H-OH ion exchange technologies for low and medium pressure steam boilers as the main stage of water treatment is not justified and should be replaced by reverse osmosis water desalination technology.

This cannot be said about the Na –softening technology. As mentioned, Na-softened water contains the potential for alkali, but it has a large amount of salt and promotes the formation of a large amount of carbon dioxide in the steam, which is obtained from such water. However, before the advent of reverse osmosis desalination technology, it was considered and still is considered by many that Na-water softening for low-pressure steam boilers is almost an ideal technology. This approach requires reconciliation with some serious disadvantages of this technology, such as extremely high corrosion activity of steam and condensate and high value of continuous blowdown of the steam boiler. The latter condition requires the utilization of secondary steam and cooling and utilization of a sufficiently large amount of boiler water, which greatly complicates the thermal scheme of the boiler house. Thus it is necessary to be prepared for high corrosive activity of condensate, recurring costs for replacement or repair of equipment and pipelines of steam and condensate line.

Here it should be noted that the author considers the modern low-pressure steam boiler house as having the simplest and most reliable thermal scheme, with almost complete automation, while having high efficiency and, accordingly, the lowest possible cost of produced steam (heat).

I must say that every boiler room is as individual as a person’s fingerprints. And this individuality is determined primarily by the composition of the source water, as well as technological features of the use of steam and the return of condensate. That is, the individuality is determined by physical-chemical changes in the water due to pretreatment, boiling, condensation conditions, conditions of return of condensate, etc. Thus, boiler houses operating on the same composition and source of the water, but with different schemas of the return condensate or no condensate return have different indices of water-chemical mode and face different challenges in the process of operation. This fact should be taken into account when new water treatment technologies are being introduced into the structure of WCM of the boiler house and we should not be satisfied only with the compliance of the prepared water with the requirements of normative documents developed in ancient times, in which such technologies were not intended to use. So, when the new technology of water treatment is introduced it is necessary to determine the possible impact of the prepared water on the entire boiler house including the condensate return system.

This article will try to determine the location of reverse osmosis technology of desalination of water in the structure of the WCM of the steam boiler, as well as to show the advantages and disadvantages of the exploitation of similar WCM.

In the last 20 years there has been an active development of reverse osmosis water desalination technology. This is due to the invention of rolled membrane elements with tangential water filtration. This technology has undeniable advantages over ion exchange technologies, but also a number of disadvantages. First of all, a large amount of waste water is generated. The classical approach to the operation of such systems says that 0.4-0.5 m3 of concentrate should be dumped into the sewer to obtain one cubic meter of demineralized water. In addition, if the water treatment of the steam boiler has only an osmotic installation and the source water does not contain Na+K ions, in this case, even if the condition is met that the feed water hardness is not more than 0.02 mg-eq/l, an extremely high corrosion of the steam boiler will be observed. If you do not take any corrective action, boiler will require major repairs in a very short time. The mechanism of this process will be discussed in more detail below. But we can say that the disadvantages of osmotic systems can also be attributed to certain difficulties in understanding the place of these systems in the structure of WCM of the boiler in comparison with a fairly well–studied technology of Na-softening.

Let us consider a low–pressure steam boiler using only Na-softening of make-up water. This is the most common WCM.

Main parameters of feed water quality for low-pressure steam boilers are:

– transparency, cm

– total hardness, mg-eq/l

– pH value

– dissolved oxygen, mg/l (ppm)

– total iron, mg/l (ppm)

– silica, mg/l (ppm)

– salinity, mg/l (ppm)

– total alkalinity, mg-eq/l

Transparency of water is a parameter that quantitatively characterizes the amount of suspended and organic substances in water. The standard parameter for this value is usually achieved by filtering water through a granular load.

The total hardness of water characterizes the tendency of water to form a solid precipitate (scale). Maintaining the standard for the total hardness of feed water less than 0.02 mg-eq/l eliminates the formation of solid deposits in the boiler.

The pH value is a parameter that determines the presence of free carbon dioxide in water or the presence of bicarbonates, carbonates or hydroxyl ion. The pH value of feed water should be more than 8.5.

Dissolved oxygen in the feed water contributes to corrosion of the boiler feed path, the boiler and the steam condensate path. This corrosion occurs with oxygen depolarization. Dissolved oxygen in feed water should be less than 50 µg/l (ppb).

Total iron in feed water must be below 0.5 mg/l. Iron does not create solid deposits in the boiler, but at high pH values of boiler water is coagulated in boiler water (especially in the presence of organic matter and high evaporation rate) and causes clogging of the measuring electrodes, level control glasses and continuous blowdown line in the boiler.

The amount of silica in the feed water of low-pressure steam boilers is not directly regulated in the Russian standards, but it is indicated that the ratio of caustic soda to silicic acid in the boiler water should be 1.5. Since the presence of hydroxyl ion in the boiler water (the pH of the boiler water is more than 10.3) supports silicon in the ionized state and does not allow the formation of solid deposits of silicic acid. At this pH value talc will be formed.

Water salinity and total alkalinity do not have specific standards. These values are determined by two parameters. This is the relative alkalinity of feed water and the value of continuous blowdown of the steam boiler.

Some boiler manufacturers also indicate in the operating manual that the value of bound carbon dioxide in water must be less than 25 mg/l [1]. This means that the total alkalinity of the feed water should not exceed 0.6 mg-eq/l.

For example, let us consider the WCM of a low-pressure steam boiler. Water from a surface source is taken as a source of water supply.

The initial (make-up) water must be pre-filtered through a granular load to meet the requirements for transparency. Modern granular loads can provide a filtration rating of up to 20 microns, which means that all particles larger than 20 microns will be removed from the water. Thus the water will remain all organic matter which breaks down in the boiler and may cause scum and foaming of the boiler water. However, water coagulation is not provided for low-pressure boilers due to the complexity of the process.

Then the clarified (make–up) water is passed through the Na-softening unit. Passing through ion exchanger, the bivalent ions Ca and Mg are replaced by a monovalent Na ion. The number of ions in this process does not change, but the hardness ions causing scale formation in the boiler are replaced by sodium ions. In this process, the anionic composition of the water does not change. Accordingly, after the installation of Na–softening, the ionic composition of water will consist of sodium bicarbonate, sodium chloride and sodium sulfate.

The pH value of water immediately after Na-softening does not change, because the ratio between bicarbonate ion and free carbon dioxide does not change.

In order to obtain deep–softened water Na-softening water is carried out in two stages.

Then the softened (make-up) water is directed to the column of the thermal deaerator. In the deaerator column, oxygen and free carbon dioxide are removed from the make-up water. The efficiency of this process actually determines the efficiency of the entire boiler house. The fact is that the pH value of water is determined by the ratio of different forms of carbonic acid. In the deaerator column, free carbon dioxide is removed, but the bicarbonate ion (the associated form of carbon dioxide) remains. As a result, the ratio between bicarbonate ion and free carbon dioxide increases, and the pH of water increases and with proper operation of the deaerator column pH of water reaches 8.4-8.5. Thus, in deaerated water there should be no not only oxygen, but also free carbon dioxide.

In case of condensate return, make-up water is mixed with condensate in the deaerator column. This water is the feed water for the steam boiler.  The returned condensate must also meet the feed water requirements.

Feed water, getting into the boiler, begins to evaporate. As a result, the salt content in the boiler water is constantly increasing. Part of the boiler water is removed through continuous and periodic blowdown, thereby maintaining the salinity of the boiler water at a level of not more than 3000 mg/l. There is a requirement that the continuous blowdown of the steam boiler cannot be more than 10% of its steam capacity. This means that the salinity of the feed water should not exceed 300 mg/l. If the salinity of make-up water after sodium softening exceeds 300 mg/l, it is necessary to provide a desalination plant as part of the water treatment system. If feed water with a salinity of more than 300 mg / l is mixed with condensate to produce feed water with a salinity of less than 300 mg/l, such water can be supplied to the steam boiler and it is not required to reduce the salinity of make-up water.

In fact, such WCM is quite inefficient and deficient, but is used everywhere because of the simplicity and cheapness of the technologies used.

The fact is that such a WCM does not take into account such a parameter as bound carbon dioxide. The bound carbon dioxide (bicarbonate ion) in feed water passes into free carbon dioxide in the boiler and is carried out with steam, and then passes into condensate. Free-form carbon dioxide in condensate causes active corrosion of the steam condensate path and equipment. The returned condensate contains large amounts of dissolved iron up to 2 or more mg/l. For low-pressure boilers, taking into account the cost and complexity of the process, condensate treatment is not provided. The use of ammonia in steam to bind carbon dioxide into bicarbonate (thereby reducing the corrosion properties of condensate) in the vast majority of cases for low-pressure steam process boilers cannot be applied. Accordingly, iron from the condensate with feed water enters the boiler, where it evaporates and precipitates. The operation of the boiler with such WCM causes considerable difficulties that are caused by the constant outages of the measuring electrodes, level control glasses, blockage of feed and blowdown lines of the boiler, etc.

The bound carbon dioxide (bicarbonate ion) must be taken into account in the requirements for feedwater and its value in the feed water must be not more than 0.6 mg-eq/l. This requires either a large proportion of the condensate return or the application of technology to reduce the alkalinity in the makeup water. H- cation of water can be applied for this. But practice has shown that the operation of such devices in low-pressure steam boilers is almost impossible. It requires careful control over such installations and there is a high probability of corrosion damage to the boiler and the feed path.

In summary, the advantages of WCM with Na–softening can be attributed to the simplicity and cheapness of treatment, and the disadvantages can be attributed to the formation of a large amount of salty wastewater, a significant heat loss with the blowdown of the boiler, significant corrosion of equipment and pipelines of steam and condensate line.

WCM of the boiler using reverse osmosis technology desalination of water has virtually none of these disadvantages. This technology allows to remove the salt from the make-up water without using chemical transformation of the dissolved substance in water. In fact, this is a physical method of desalting water. The main disadvantage of the process of reverse osmosis desalination is the formation of a large number of slightly mineralized wastewater. In fact, the source water by means of a semi permeable membrane is divided into 2 streams. One stream is the permeate and another is the concentrate. The permeate is desalinated water, the concentrate is water with a high salinity discharges into the sewer. The process of water desalination by reverse osmosis provides almost complete absence of salts in permeate, i. e. permeate has a salinity of about 1-5 % of the original salinity. As a result, the salinity of permeate is on average from 5 to 20 mg/l and, what is especially important, the amount of bicarbonate ion in permeate is from 0.05 to 0.3 mg-eq/l. Thus, reverse osmosis provides virtually no heat loss with continuous blowdown of the boiler at any proportion of the condensate return, as well as the minimum amount of carbon dioxide in the steam and condensate. Nevertheless, it is necessary to understand what technological features will be present in WCM of the boiler room when using reverse osmosis systems.

The author’s first experience in the operation of reverse osmosis in a steam boiler house was unsuccessful, but it showed how to organize the technological scheme of water treatment in the future. The fact is that during the reconstruction of the steam boiler house with DE and DKVR boilers it was supposed to change the Na-softening of water to reverse osmotic desalination of water. The replacement was made and six months later the steam boiler DE 6.5-14 required major repairs.  Numerous holes were found in the tube bundle of the boiler. In order to understand what caused such active corrosion of the boiler it is necessary to understand how free carbonic acid and oxygen behave in the water at each stage of water treatment.

I must say that there was no deaeration of water in the boiler house. Water after reverse osmosis desalination was sent to the water reserve tank with steam bubbling. Deaerator column on the tank was absent. The water temperature in the tank was maintained at the level of 75-80 C to prevent damages of the feed pumps. Thus, all the free carbon dioxide and oxygen in the source water passed into the water reserve tank. In a tank there are no conditions for effective removal of these gases, and they got to the boiler in quantity much more than normative. In the boiler there was intense carbon dioxide and oxygen corrosion. But there is a question why before installation of reverse osmosis the boiler had worked on Na-softening for 8 years and had not required repairs under the same conditions of lack of proper deaeration. The fact, that the Na–softened water increases the pH value under the condition of the removal of carbon dioxide from water (1). The more intense the carbon dioxide distillation is, the higher the pH value becomes, due to the formation of NaOH. This condition completely eliminates the occurrence of carbon dioxide corrosion in the boiler and partially reduces oxygen corrosion.

NaHCO₃<---->NaOH+CO₂^gas      (1)

As can be seen, the sodium bicarbonate in the feed water turns into caustic soda and raises the pH of the boiler water.

If only reverse osmosis desalination system is used for water treatment, the formal value of permeate hardness may be less than 0.02 mg-eq/l, but in such water there will be virtually no sodium bicarbonate, only calcium bicarbonate will be present. When such water is boiling, the pH value of the boiler water will not be higher than 8.5. Hydroxyl ion is not formed (2).

Ca(HCO₃)₂ <--> CaCO_3(precip)+H₂CO₃      (2)

As can be seen, the bicarbonate of calcium precipitates in the form of calcium carbonate with the formation of dissolved form of carbon dioxide in the boiler. Free carbon dioxide is removed with steam, but the pH of the boiler water does not rise above 8.5 – 8.8. When feed water with a low pH enters the boiler, intense carbon dioxide corrosion of the boiler occurs at high temperatures. Since, there are no hydrates in the boiler water to bind carbon dioxide, coming from the feed water, into the bicarbonate (equation 1).

Simply put, for the proper WCM of the boiler it is necessary that the feed water must contain sodium bicarbonate. The amount of sodium bicarbonate (A_feed, mg-eq/l) must correspond to the amount of salt of feed water (S_feed, mg/l). The ratio of bicarbonate to the salinity of feed water is called the relative alkalinity of feed water (A_raf, %).

A_raf=40*A_feed*100/ S_feed ,  %   (3)

If the relative alkalinity is less than 5%, active carbon dioxide corrosion of the steam boiler occurs. If the relative alkalinity is more than 50 %, alkaline intergranular corrosion of the boiler can be observed.

The relative alkalinity of feed water for the described case with the DE boiler was 1.5 %.

It can be said that reverse osmosis desalination of water in the structure of low-pressure steam boilers should be used in combination with Na-softening. That is, reverse osmosis desalination is not a complete replacement for Na – softening for low-pressure boilers.

When it works on the combined scheme of water treatment (reverse osmosis + softening), the value of relative alkalinity will be the same (approximately 15 to 40 %) as when working only on softening. That is, the value of the relative alkalinity will be optimal and the water has a low salinity, which significantly reduces the amount of boiler blowdown, while the quality of steam and condensate increases.

For high-pressure boilers, which require recharge with demineralized water, the technology of two-stage osmosis with subsequent correction of permeate is quite applicable.

We need to make a small digression. If an artesian well is used as the source water for the boiler house, this water is likely to contain sodium and potassium along with calcium and magnesium. This will allow using only reverse osmosis for water treatment, if the membrane will provide a standard hardness value below 0.02 mg-eq/l. But in practice it is difficult to achieve such a low hardness value in one stage of osmosis and it is necessary to use Na–softening at least to soften the permeate.

In my practice, I used reverse osmosis desalination of water without Na – softening for a small steam generator (0.5 t/h) running on artesian water. Due to the objective impossibility of organizing a complex water treatment system and the lack of maintenance personnel in the steam generator, it was decided to use only an automated reverse osmosis unit. Water had the following composition: salinity about 650 mg/ l, hardness about 7 mg-eq/l, alkalinity-6,0 mg-eq/l, pH-7,8 and oxygen content about 3,5 mg/l. Condensate return was absent. Using only Na-softening in this case was impossible. As a result of the work of osmosis, the following WCM of steam generator was received. Osmotic water (permeate) had the following composition: salinity 11 mg/l, hardness about 0.05-0.06 mg-eq/l, pH 6.2-6.3. Since the deaerator was not provided, and its installation was objectively impossible, I strongly recommended to add caustic soda to the permeate tank. There was no one to do it and, accordingly, it had not been done. Nevertheless, after two weeks of operation of the steam generator on osmotic water from the initial artesian water, the pH value of the boiler water was kept at 9.8-10.5. The feed pipes of the steam generator and the permeate tank are made of polymer material and are not subject to corrosion. During first few days there was a very large amount of iron during the purge of the control-level glasses, which indicated a significant corrosion of the boiler. But as soon as the pH of the boiler water increased to 10.3, active corrosion processes stopped. Currently, this boiler has been operating for more than 5 years. Despite the fact that its “predecessor”, working without water treatment, completely clogged with calcium carbonate about once a month and required constant boiling with citric acid, not to mention the huge gas overruns. Nevertheless, the resulting WCM is not normative and there is a possibility of through corrosion of the heat pipe and the failure of the steam generator, before the specified service life.

It is necessary to consider another point related to the operation of reverse osmosis desalination systems: why the permeate has a low pH and how it affects the WCM of the boiler room.

When water is filtered through a semipermeable membrane, ions dissolved in water practically do not pass through the membrane, because they are in a hydrated state, i.e. they have a connection with water molecules. As a result, the diameter of the hydrated ion is greater than the diameter of the pores in the membrane, taking into account the bound layer of water on the surface of the membrane, the hydrated ion is not transferred through the membrane and remains in the concentrate. Dissolved gases pass freely through the membrane as they have no charge and are not subject to hydration by water. The size of the gas molecule is smaller than the pore size in the membrane. Thus, the permeate contains the same amount of carbon dioxide as in the source water, and the amount of bicarbonate ion is significantly less (about 1-5% of the initial amount of bicarbonate). In the physical sense, we can say that the bicarbonate ion is an alkaline residue from the dissolution of carbonic acid. Accordingly, if we remove the alkaline component due to filtration, while leaving the acidic (carbon dioxide passes through the membrane), in this case the pH of the permeate decrease significantly. This is well shown by the equation of Henderson-Hasselbalch for the dissociation of carbonic acid H₂CO₃<->H+НCO₃

    (4)

For example, if the amount of bicarbonate in the feed water is 2.0 mmol/l, in permeate 0.12 mmol/l, the amount of carbon dioxide in the feed water is 10 mg/l (0.227 mmol/l), the pH value of the permeate can be calculated by the formula (4).

The pH value of the source water:

As you can see, the permeate has a low pH due to reducing the amount of bicarbonate and a constant amount of carbonic acid. The total amount of bound carbonic acid in the form of bicarbonate ion decreased from 2 mmol/l to 0.12 mmol/l. This means that the steam and condensate will contain about 95% less carbon dioxide than when using only Na–softening unit. That is an important advantage of using of the osmotic systems in WCM of boilers.

Then it is only necessary to avoid carbon dioxide corrosion of the feed path of the steam boiler. To do this, osmotic water should be sent to the deaerator column to remove carbon dioxide and oxygen. At the same time, the thermal deaerator should work properly, not just as a water reserve tank.

If the pH value of deaerated water is less than 8.4-8.5, it is necessary to find out the reasons for this, and not to start dosing caustic soda into deaerated water. Caustic soda will bind free carbon dioxide into bicarbonate, which in the boiler will turn into carbon dioxide and leave with steam.

The reasons for the low pH of deaerated water may be:

• insufficient heating of make-up water before the deaerator column;
• insufficient steam flow on the mirror;
• insufficient steam flow to the bubbling;
• condensate return is not in the deaerating column and deaerating tank;
• the flow of treated water through the deaerator column is greater than the design.

Knowing the features of reverse osmosis water desalination systems we can offer two concepts of their application in the structure of WCM of low-pressure steam boilers.

In the first scheme, the source water passes through a filter clarification, then it is fed to the installation of the ion exchange unit, and then softened water is fed to the reverse osmosis desalination. After osmosis, the permeate is supplied to the thermal deaerator (Fig.1).

In the second scheme, the source water, passing through the clarification filter, is immediately sent to the reverse osmotic desalination, then the permeate is fed to the Na–softening unit. After softening the water enters the thermal deaerator (Fig. 2). This option requires additional tank and a pumping station.

In both schemes, the resulting composition of make-up water fully complies with the requirements of standards and allows conducting WCM of boiler with a minimum amount of carbon dioxide in the condensate and very low heat losses with the blowdown of the boiler.

However, these two schemes are fundamentally different in terms of the organization of the work of the reverse osmosis system.

In the first scheme there is a preliminary softening of water. Scale-forming ions are extracted from the water. Accordingly, the formation of calcium carbonate deposits on the membrane is almost completely excluded. This condition can significantly reduce the amount of concentrate discharged from the reverse osmosis plant. The amount of concentrate can be only 10 % of the amount of source water. In this case, it is necessary to ensure the absence of coagulant substances in the source water.  So the iron content in the source water should be no more than 0.05 mg/l. The main drawback of the first option is a large consumption of salt for the regeneration of the softening unit.

In the second scheme, the initial water, usually in a state of carbon dioxide equilibrium, will begin to concentrate in the membrane by salts, which will immediately increase the concentration of calcium bicarbonate without increasing the concentration of carbon dioxide. Accordingly, a solid precipitate of calcium carbonate on the membrane will begin to form. This circumstance requires the use of sedimentation inhibitors. It is worth saying that quality inhibitors are quite expensive. In this case, anyway the discharge of the concentrate will be at least 25% of the amount of source water. The main advantage of the second option is the minimum cost of salt when operating the softening unit as a corrective stage. The second scheme is also applicable if the source water has a high hardness and relatively low alkalinity. In this case, there will be high costs associated with softening. It is more efficient to use the dosing of hydrochloric acid to destroy carbonates before osmosis. This will ensure the absence of deposits on the membrane. In this case, it will be necessary to provide for the stage of decarbonization of permeate. It is possible to use a membrane contactor for this purpose.

The effectiveness of the first scheme can be improved by using not the expensive tableted salt, and technical salt (halite) for the regeneration of the softening unit of. This requires a simple technical solution for pre-filtering of the resulting saturated salt solution. Article: Ivan Tikhonov- The use of technical salt (mineral halite) in the regeneration technology of modern automatic Na-softening filters www.tiwater.info. In this case, the cost of technical salt will be commensurated with the cost of the sedimentation inhibitor. Moreover, the system under the first scheme is much more variable and it has a number of significant advantages, which are described in a separate article. Article: Ivan Tikhonov –  Water degassing using reverse osmosis membranes tiwater.info.

It is necessary to take into account the fact that when make-up water (permeate) is supplied directly to the deaerator, after osmosis there may be excessive back pressure in the make-up line before the deaerator (up to 2 bar). Under these conditions, in order to maintain the capacity of the desalting unit for permeate, it is necessary to provide a reserve for the pressure of water at the inlet to the desalting unit. It is necessary to choose a more powerful pump to compensate for back pressure and maintain permeate performance.

In any case, as it has been mentioned above, each boiler is quite individual and in each case requires a thorough analysis of all factors and the choice of the most optimal option. At the same time, the use of reverse osmosis desalination together with Na – softening in different variants of their combination seems to be the most applicable modern technology of water treatment in the structure of WCM of low-pressure steam boiler plant.

Conclusion:

1. The use of reverse osmosis technology of water desalination in the structure of steam boilers allows to ensure efficient and trouble–free operation WCM of the boiler house and such WCM has a number of significant advantages compared to the use of only Na-softening technology.
2. The reverse osmosis technology of water desalination in the structure of WCM of low-pressure steam boilers is not a competitor to Na-softening technology. These technologies should work together to ensure maximum efficiency of each other and the entire WCM of boiler house.
3. The reverse osmosis WCM of boiler is incorrectly compared with Na – softening WCM. Na-softening technology does not provide a number of important parameters for the management of WCM of boiler.
4. Mismanagement of the process of thermal deaeration and decarbonization of make-up water, when the membrane technology is applied, may be the cause of corrosion damages of the elements of the feed circuit of the steam boiler, as well as to be the cause of high iron content in the boiler water.
5. The use of alkaline reagents to increase the pH value of feed water is likely to indicate improper operation of the thermal deaerator.

Figure 1.

 

Figure 2.

References

1. Viessmann VITOMAX 200-HS Operating And Service Instructions