On one of my previous posts, we have discussed about Boiler chemical dosing and control and explained how to test the water and dose the chemicals for water treatment. As the water treatment chemicals tend to settle the impurities, these will form deposits at the bottom of the boiler or will float at the surface of the water and need to be removed.

Always operate the boiler in accordance with the instructions provided by the manufacturer of the boiler, which should contain techniques for blowing down the boiler. The boiler will be equipped with either a surface skimmer valve or a bottom blowdown valve.

In order to keep the operations of a boiler steady, it is vital to manage blowdown control in the appropriate manner and there are a number of different considerations that go into determining how blowdowns should be carried out. The following are the primary goals:

• • Bottom blowdown for the prevention and removal of sludge deposits in the boiler
  • Management of material that is floating on the surface – skimmer
  • Controlling dissolved solids, preventing carryover, and reducing corrosion are all achievable goals with either approach.

If the boiler operates, especially on shore water, calcium phosphate sludge deposits may form on the bottom of the boiler, making it necessary to perform periodic bottom blowdowns. These will frequently settle at the bottom of the boiler, and the periodic operation of the bottom blowdown will assist in the removal of any deposits that may have formed there. In the event that this is not done, the boiler runs the risk of developing excessive amounts of sludge deposits, as well as the potential for the heating surfaces of the boiler to get overheated.

There is a possibility that not all boilers have surface skimmers. If the water is distilled or extremely soft, the requirement for bottom blowdowns may be reduced, and the surface skimmer may be able to control the amount of dissolved solids in the water instead. Conductivity in the boiler water can be automatically maintained by some boilers thanks to built-in automatic devices.

The word “Cycle of concentration” refers to the ratio of the amount of dissolved solids that is present in the feed water to the amount that is present in the boiler water. There are a few approaches to this calculation; one of the more prevalent methods involves measuring the levels of chlorine in both the feed water and the boiler water. A fundamental illustration of a boiler with a 5% blowdown can be seen in the figure below.

In this particular scenario, there are a total of 20 cycles of concentration (10 t / 0.5 t). This figure will often fall somewhere in the range of 10 to 30 most of the time.

In addition to an adequate pre-treatment of the boiler feed water, controlled reserves of treatment chemicals need to be maintained in the boiler water. This is necessary in order to guarantee that any traces of deposit-forming compounds, such as salts of calcium, iron, silica, copper, and magnesium, are prevented from forming hard scales or baked-on sludges. Scale buildup can be avoided by the use of a variety of different treatment strategies.

The carbonate cycle control method of treatment is only suggested for package boilers up to 10 bar that do not have any external feed-water treatment. This might potentially provide us with calcium hardness in the feed water of approximately 40 ppm. By adding sodium carbonate, the objective is to keep the carbonate alkalinity in the boiler water at a level of no less than 250 ppm. Because of this, any calcium that may have been present will now precipitate in the majority of the boiler water rather than becoming baked-on scale on the heat transfer surfaces. After that, the fine precipitate is extracted using the blowdown of the boiler. An excessive amount of carbonate will eventually decompose, resulting in the formation of hydroxide, alkalinity, and carbon dioxide. In the event that silica is present in the boiler water, magnesium will either precipitate as magnesium hydroxide or magnesium silicate. It is essential to include dispersants in this program in order to guarantee that precipitated compounds will remain in suspension throughout the blowdown process. This will make the process more simpler.

The phosphate cycle control treatment approach relies on good quality pre-treatment, (usually sea water evaporators) plus the addition of soluble phosphate and hydroxide alkalinity to the boiler-water. These react with any trace calcium, magnesium and silica impurities to form fine precipitates of: Calcium Hydroxyapatite, Serpentine and Magnesium Hydroxide. These compounds have an exceptionally low solubility, which means that they will precipitate in the boiler water. They can then be removed by blowdown after they have done so. Again, it is essential to include dispersants as part of the treatment program to guarantee that precipitated compounds are kept in suspension throughout the treatment process and to make it easier to remove them using blowdown.

When the chemistry of the phosphate cycle is used, it is essential to keep enough amounts of OH alkalinity in the solution. This will ensure that magnesium will precipitate as either magnesium hydroxide or as hydrated magnesium silicate, both of which are inert compounds. In order for us to reach these circumstances, we need to work toward achieving an alkalinity ratio of 0.4:1 for silica to OH and a ratio of 1:10 for phosphorus oxide to OH. Overdosing of phosphate must also be avoided to prevent the formation of phosphate scales.

The application of polymeric conditioning treatments is able to more than adequately maintain control of deposition in situations in which one can rely on appropriate and constant pre-treatment of boiler feed-water. These are often formulated with long chains of negatively charged polymers and co-polymers, and they have an excellent stability at the high temperatures that are present in boiler fluids. These formulations are generally considered to be proprietary.

All polymer treatments can be described to inhibit scales by the following mechanisms:

• • Crystal modification – The dispersant acts on the surface of the scale as it is formed to prevent the formation of large angular crystals which are adherent to the heat transfer This action causes the scale to form in smaller, more rotund particles which are less adherent to surfaces.
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  Example of crystal modification

  • Dispersion – The negatively charged polymers attach to boiler metal and surround particles in the boiler-water. This mechanism sets up repulsive forces that inhibit the particles from agglomerating to form scale or sludge deposits.
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  Dispersation

  • Complexation – Negatively charged polymers can form weak sub-stoichiometric complexes with calcium, magnesium and iron which allow these impurities to exceed their normal solubility levels and acts to inhibit deposition at heat transfer surfaces. For best scale control it is important to maintain an adequate reserve of free polymer in the boiler water at all times.

Typical boiler damages and defects and auxiliary boiler defects on pressure parts are typically related to mechanisms such as:

• • Active local pitting corrosion from the water/steam side
  • Overheating due to deposits, oil, scales, low water level, flame impingement, etc.
  • Poor workmanship during fabrication
  • Soot fires on fin/pin type water tube exhaust gas boilers
  • Cold corrosion from gas side

It has been observed that the majority of boiler defects that are reported are caused by corrosion, which arises out of probable factors related to an inferior water condition, most often as a result of insufficient maintenance. This is the case because corrosion arises out of probable factors related to an inferior water condition.

It would appear that the absence of a stable and passive magnetite layer (oxide) on the water/steam side of metal surfaces is the primary contributory mechanism that causes many of the documented faults.

A smaller number of defects are related to other factors or operational issues.

It has been observed that many ships are struggling to allocate time and arrange acceptable materials and resources to repair the defects after they have occurred or are observed, making the situation even worse.

The following are the most significant things that have been learned as a result of inspections and surveys experience:

• • Enhanced focus on water treatment: The risk of active local and general corrosion of the internal surfaces (steam and water side) is reduced to a minimum by utilizing prescriptive methods to initiate and sustain a passive magnetite layer on steel surfaces, as well as by increasing the frequency of monitoring the water condition. This keeps the risk of corrosion to a minimum. In addition, the heat transfer barriers can be decreased by maintaining the ideal state of the heat transfer surfaces. This can be accomplished by avoiding the accumulation of scale and other impurities, for example. This therefore results in:
    • • Improved fuel efficiency
      • Avoidance of thermal strains that could lead to cracks in the material
      • Preventing the wall of the furnace, the top plate, and the screening tubes from becoming overheated
  • Monitoring and maintenance of the boiler plant: Placing a greater emphasis on maintenance and conducting internal inspections reduces the likelihood that other contributing factors may result in a defect or make the likelihood of one occurring more likely (flame impingement from burner, etc.).
  • The risk of water side contamination/excessive dissolved oxygen and defects related to the gas side is minimized by optimizing the design of the feed water system (using things like salinometers, oil content sensors, and hotwell temperatures, for example), as well as monitoring the differential pressure across the exhaust gas boiler.
  • A more flexible class survey – As a consequence of this, a portion of the scope during each alternative boiler survey – which is not connected to the main class renewal survey – can be credited based on the chief engineer’s inspection report, which reflects evidence of a satisfactory internal examination. This is because the main class renewal survey is not connected to the alternative boiler survey.

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Source and Bibliography:

• WSS Water Treatment
• DNV