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Boiler water treatment

Steam Condenser on Board Vessels: Operation, Maintenance, and Troubleshooting

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The steam condenser is a critical component on board vessels that plays a crucial role in the efficiency and operation of boiler and steam-powered systems. It enables the conversion of exhaust steam from the consumers or main propulsion or power generation systems back into water, allowing for reuse and maximizing energy efficiency. In this comprehensive article, we will delve into the correct operation of a steam condenser, highlight the importance of regular maintenance, and provide a troubleshooting guide for common operational malfunctions.

    1. Correct Operation of a Steam Condenser

The correct operation of a steam condenser involves several key steps and considerations:

1.1. Cooling Water Circulation

Adequate cooling water circulation is vital for the efficient operation of the steam condenser. Cooling water is typically drawn from the sea or a freshwater source and circulated through the condenser tubes, absorbing the heat from the exhaust steam. It is crucial to ensure a sufficient flow rate and temperature difference for effective heat transfer.

1.2. Air Extraction

To maintain optimum performance, it is essential to remove non-condensable gases, such as air, from the condenser. This is achieved through air extraction systems that continuously remove air from the condenser, preventing its accumulation and reducing the risk of performance degradation.

1.3. Vacuum Creation

Maintaining a vacuum within the condenser is vital to enhance the efficiency of steam condensation. A vacuum is created by condensing steam, reducing its pressure, and evacuating the condensed water. Efficient vacuum creation is crucial for maximizing power generation and optimizing overall system performance.

    1. Maintenance of a Steam Condenser

Regular maintenance is essential to ensure the reliable and efficient operation of a steam condenser. Neglecting maintenance can lead to reduced performance, increased energy consumption, and even catastrophic failures. Key maintenance activities include:

2.1. Tube Cleaning

Over time, fouling and scaling can accumulate on the condenser tubes, reducing heat transfer efficiency. Regular tube cleaning, using methods such as mechanical brushing, chemical cleaning, or high-pressure water jetting, is necessary to remove deposits and maintain optimal heat exchange.

2.2. Inspection and Repair

Routine inspections of the condenser, including visual checks and non-destructive testing, help identify any corrosion, tube leaks, or structural damage. Prompt repair or replacement of damaged components is crucial to prevent further degradation and ensure the condenser operates at its designed capacity.

2.3. Cooling Water Treatment

Effective treatment of cooling water is essential to prevent scaling, corrosion, and biological growth within the condenser. Regular monitoring and adjustment of water chemistry parameters, such as pH, alkalinity, and dissolved solids, help maintain water quality and prevent detrimental effects on condenser performance.

2.4. Air Extraction System Maintenance

The air extraction system must be inspected regularly to ensure proper functioning. This includes checking air extraction pumps, vacuum breakers, and associated valves for leaks or malfunctions. Any issues should be addressed promptly to maintain efficient air removal from the condenser.

Maintenance is of paramount importance for steam condensers due to the following reasons:

    • Efficiency and Performance: Regular maintenance ensures that the condenser operates at optimal efficiency, maximizing heat transfer and energy conversion. This results in improved overall system performance, reduced fuel consumption, and increased power generation or propulsion efficiency.
    • Equipment Longevity: Proper maintenance helps extend the lifespan of the steam condenser. Regular inspections, cleaning, and repair of components prevent premature wear, corrosion, and deterioration. This not only reduces the risk of costly breakdowns but also contributes to the longevity of the condenser and the entire steam system.
    • Energy Savings: A well-maintained steam condenser can significantly impact energy savings. When the condenser operates efficiently, more steam is converted back into water, reducing the need for additional steam generation. This leads to lower fuel consumption and operational costs, resulting in substantial energy savings over time.

3. Troubleshooting

Despite proper maintenance, steam condensers may experience operational malfunctions. Here is a troubleshooting guide for common issues:

3.1. Insufficient Cooling Water Flow:

      • Check the cooling water intake for any blockages or restrictions.
      • Verify that the cooling water pumps are operating correctly.
      • Inspect and clean the cooling water filters to ensure unrestricted flow.

3.2. Inadequate Vacuum:

      • Check for air leaks in the condenser or associated piping and repair any leaks.
      • Verify the proper functioning of vacuum pumps and vacuum breakers.
      • Ensure proper condensate removal to maintain the desired vacuum level.

3.3. Tube Fouling or Scaling:

      • Conduct a thorough cleaning of the condenser tubes using appropriate methods.
      • Review and adjust cooling water treatment to minimize scaling and fouling.
      • Consider implementing online cleaning systems for continuous tube maintenance.

3.4 Corrosion or Tube Leaks:

      • Perform regular inspections to detect any signs of corrosion or tube leaks.
      • Promptly repair or replace damaged tubes or components.
      • Consider using corrosion-resistant materials or protective coatings where applicable.

In conclusion, steam condensers are integral to the efficient operation of boilers and steam-powered systems on board vessels. Correct operation, regular maintenance, and timely troubleshooting are essential for optimal performance, energy efficiency, and prolonged equipment life. By following proper procedures, conducting routine maintenance activities, and promptly addressing operational malfunctions, ship engineers can ensure the reliable and efficient operation of their steam condensers, contributing to the overall success and safety of their vessels.

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What to do in case of oil contamination of marine boilers

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Marine boilers play a vital role in powering vessels, providing heat and energy necessary for various operations at sea. However, the occurrence of oil contamination within these boilers can have severe consequences, jeopardizing both the safety and efficiency of maritime operations. In this blog post, we will explore the causes, impacts, and essential steps to address marine boiler oil contamination, ensuring the smooth operation of marine vessels.

Understanding Marine Boiler Oil Contamination

Marine boiler oil contamination refers to the unwanted presence of oil or oil-related substances within the boiler system. This contamination can arise from multiple sources, including fuel supply contamination, leakage from seals or pipes, improper maintenance practices, or even accidental spills. Regardless of the source, the effects of oil contamination can be detrimental and require prompt attention.

Many of you must’ve been heard or encounter an oil contamination of the boiler due different machinery or systems breakdown. The contamination has an immediate and consequential impact on boiler operation and is needless to say that the adverse effects of oil contamination on boiler steel and the plant can never be deemed overemphasized. In some situations, large areas of heat transfer lead to the development of cracks, which in turn leads to a loss of the material’s integrity. The majority of them lead to costly repairs that require a significant amount of time and effort, such as the replacement of pressure parts, chemical-mechanical cleaning, and downtime.

Boiler oil contamination

The most prevalent sources of oil pollution detected on boilers come from leaking heating coils fitting in fuel oil tanks, fuel/lube oil heaters, cylinder lube oil from reciprocating steam engines for pumps, and heating coils in DB tanks allocated for sludge/waste oil tanks. It is not unheard of for cargo tank heating coils and tank cleaning heaters installed on the cargo side to be a contributing factor in some instances of contamination.

On the other hand, main propulsion boiler plants that use a segregated saturated steam system as their primary source of heating medium have the lowest risk of oil contamination.

Minor cracks in HFO heating coils or in other heaters in the engine room or in other places on the ship where steam is utilized for heating can sometimes be the cause of oil leaks that become more severe over time. There are several different pathways by which oil can seep out of an engine room in today’s advanced machinery. It is not usually possible to see very thin coatings of oil. According to past experiences, oil pollution at a level of 15–20 ppm (parts per million) will not be noticeable. If there is an oil pollution level of 25 parts per million, then the steam drum will collect roughly 12 kilograms of oil every single day. The boiler has a capacity of 20 tons of steam per hour. There is a potential for localized overheating of the material and potential damage to the boiler if there are thin layers of oil on the tubes of the boiler or on any of the other directly heated surfaces of the boiler.

Example of oil heater

 

Impacts of Marine Boiler Oil Contamination

Foaming and carryover in oil-fired boilers due to increased tension at the water surface are two of the immediate effects of oil contamination. Other immediate effects of oil contamination include malfunctioning boiler water level controls and even protective shutdown systems. Carry-over of water and moisture with the steam may even reach the intensity of priming in the worst-case scenarios, wreaking widespread destruction on consumers such as turbines, super heaters, steam pipework, and associated gaskets.

The presence of severe oil pollution causes a decrease in the rate at which heat is transferred through the steel of the boiler, which contributes to the metal temperature being greater than the design value. Even an oil film or deposit as thin as 0.5 millimeters thick on the water side of an auxiliary boiler rated at 7 bar (g) can easily increase the metal temperature on the furnace side from the design value of 250 degrees Celsius to well above 600 degrees Celsius under normal operating conditions. This occurs when the metal temperature on the furnace side exceeds the design value of 250 degrees Celsius. Because of this, there is a domino effect that leads to an exponential decrease in the material’s yield strength. This continues until the pressure parts that are subjected to active heat transfer fail.

In circumstances in which the decrease in strength does not result in an immediate failure, the boiler steel may nevertheless be subjected to a time-dependent creep zone that is difficult to evaluate (if the temperature is higher than 380 degrees Celsius), unless alloying is taken into account during the design stage.

In the case of exhaust gas water tube boilers with an extended surface area that forms part of the system for the generation of steam by forced circulation, this may, in the worst cases, lead to soot fires due to a lack of heat transfer from the gas side and a rise in the temperatures of the metal due to the uncooled boundaries. In addition, this may cause a decrease in the efficiency of the steam generation system. As a result of the differential expansion of the overheated tubes in comparison to the shell, smoke tube exhaust gas boilers are prone to developing cracks on the tube terminations (see the image below for an example).

It is also important to be aware that other long-term impacts include local corrosion of the area that has been exposed to the acidic nature of oil deposits. This is something that should be kept in mind. When hydrocarbon deposits are subjected to high temperatures in the presence of water, they have a propensity to undergo an acidic transformation.

Addressing Marine Boiler Oil Contamination

As an immediate precautionary measure, derating the boiler’s steam generating capacity by reducing the firing rate/heat input in conjunction with the design working pressure is highly  recommended.

Depending of the degree of oil contamination on boiler, sometimes can lead to a requirement of the permanent restoration of the heat-transfer surfaces on the water and steam side prior to the boiler being put back into service.

Rebuilding boiler tube nest

To mitigate the negative impacts of marine boiler oil contamination, it is crucial to follow a systematic approach. Here are the key steps to be taken:

      1. Detection and Confirmation: Regular monitoring and analysis of water samples can help detect contamination. Suspicious characteristics or abnormal levels of impurities should be investigated further to confirm the presence and extent of the contamination.
      2. Isolate the Contaminated System: To prevent the spread of contamination, it is necessary to isolate the affected marine boiler system. This may involve shutting down the boiler or bypassing it temporarily until the issue is resolved.
      3. Identify the Source: Thorough inspection and investigation should be conducted to identify the root cause of the oil contamination. Addressing the source is essential to prevent its recurrence and implement appropriate preventive measures.
      4. Clean the System: Cleaning the contaminated marine boiler system is vital to remove oil residues and contaminants. The cleaning process typically involves flushing the system with specialized cleaning agents or solvents designed for marine boiler systems.
      5. Rinse and Drain: After the cleaning process, the system should be thoroughly rinsed with fresh water to eliminate any remaining cleaning agents or loosened contaminants. Complete drainage is necessary to ensure a clean and oil-free environment.
      6. Inspect and Replace Components: Inspect all components of the marine boiler system for damage or residual contamination. Replace compromised or heavily contaminated components to ensure the integrity and reliability of the system.
      7. Test and Restore: Thorough testing of the marine boiler system is necessary to ensure its proper functioning. Conduct pressure tests, verify temperature control, and monitor performance indicators to restore optimal operation.
      8. Preventive Measures: Implementing preventive measures is crucial to minimize the risk of future oil contamination incidents. This includes regular maintenance, monitoring, and sampling of the marine boiler system, along with adequate crew training in boiler operation and maintenance best practices is essential.

Boiling out the water side of the boiler using recommended chemicals and/or mechanical cleaning are normal procedures undertaken to facilitate satisfactory cleaning. This may be additionally supported by hardness checks and a hydrostatic pressure test at 1.5 times the design working pressure to ensure the expected safety factor at the design temperature.

Boil-out chemicals are highly caustic. Caustic Soda ash will produce a violent flash if introduced to water too rapidly. It is needless to say that the crew involved into this operation and handling the chemicals must wear protective equipment like, goggles, gloves, aprons, and an emergency shower should be nearby. Vinegar can be used as an antidote.

Below there is a recommended boil-out procedure that may be followed:

      1. If the boiler is equipped with prismatic type gauge glasses, replace them with the temporary boil-out glass to prevent chemical attack on the operational gauge glasses.
      2. Remove all manholes and handholes’ covers to verify that tubes and nozzles are not plugged with foreign materials.
      3. Wire brush any heavy scale on drum surfaces and remove them out.
      4. Close all inspection openings such as manholes, handholes, etc.
      5. Fill the boiler to the lower level of the water gauge glasses.
      6. Blow-down valves, scum valves, and gauge cocks should be checked and closed.
      7. Add the chemical and water mixture to the unit slowly and in small amounts to prevent excessive heat and turbulence. Add the mixture through the chemical feed or feed-water connections to a level just above the bottom of the gauge glass.
      8.  Fire the boiler at a very low firing rate.
      9. When the boiler begins to produce steam (as seen through the open vents), allow the unit to steam freely for at least four hours. Watch the level in the gauge glass and always maintain normal water level (midpoint of the gauge glass). It will be necessary to add more boil-out solution when the water level falls.
      10. Close all vents.
      11. Keep the drum pressure at aprox.1 bar.
      12. After 8 hours, increase the pressure up to 20 % of the nominal working pressure.
      13. Continue boil out for at least 48 hours. During this period, open the blowdown valve intermittently and drain an amount of solution equal to one-half of the gauge glass every eight hours. Then refill the unit to the midpoint of the gauge glass with the boil-out solution.
      14. The boil out procedure should be continued until clean blowdown is observed.
      15. If clean blow down is observed, the boiler should be stopped and cool down gradually.
      16. Drain the water in accordance with local, national and international rules and regulations.
      17. Flush the boiler with clean water for at least 2 times.
      18. Fill the boiler with clean distillated water and start the firing up procedure.
      19. Start using treatment chemicals as per manufacturers’ instructions.

For stubborn oil deposits or heavy contamination, manual mechanical cleaning may be required. Use appropriate brushes, scrapers, or cleaning tools to remove the oil residues from internal surfaces, tubes, and components. Be careful not to damage the boiler surfaces during this process.

It’s important to note that the above steps provide a general guideline, and the specific cleaning process may vary depending on the type of marine boiler and the severity of contamination. It is recommended to consult the vessel’s operational and maintenance manuals, follow manufacturer recommendations, and seek expert assistance to ensure a safe and effective cleaning process tailored to your specific marine boiler system.

In conclusion, marine boiler oil contamination poses a significant threat to the efficiency, safety, and environmental sustainability of maritime operation. Timely detection, isolation, and remediation are vital to address this issue effectively. By following a systematic approach and implementing preventive measures, vessel operators can safeguard their marine boilers, reduce operational costs, and ensure a safe and smooth voyage. Prioritizing the elimination of oil contamination is not just an imperative for individual vessels but also for the overall well-being of the marine industry.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World, Telegram Chief Engineer’s Log Chat or Instagram and will try to answer to all your queries. You can use the feedback button as well!

If you like my posts, please don’t forget to press Like and Share. You can also Subscribe to this blog and you will be informed every time when a new article is published. Also you can buy me a coffee by donating to this website, so I will have the fuel I need to keep producing great content! Thank you!

Boiler operation, chemical scale control and defects

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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.

Example of boiler skimmer 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.

Example of overheated areas in marine boilers

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.

Example of cycle of concentration

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.

  • 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.

  • 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.

Complexation

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.

Example of passive and stable magnetite layer

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.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries. You can use the feedback button as well!

If you like my posts, please don’t forget to press Like and Share. You can also Subscribe to this blog and you will be informed every time when a new article is published. Also you can buy me a coffee by donating to this website, so I will have the fuel I need to keep producing great content! Thank you!

Source and Bibliography:

  • WSS Water Treatment
  • DNV

Boiler chemical dosing and control

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As discussed in one of my previous posts which can be found if you click in here, analytical tests and chemical treatment must be carried out in line with the chemical manufacturer’s recommendations. To keep the chemical levels within an acceptable range, the treatment must be added, but caution must be exercised, as excessive treatment can frequently cause more severe harm than minimal treatment. The results of the chemical analysis on the boiler water are recorded, and the effects of the added treatment can be tracked over time.

Following the analysis of the boiler water, a decision must be made regarding the amount and type of chemicals to be added to the boiler feed water, if any.
The treatment is, usually, added to a chemical injection tank and from there, it is routed to the boiler feed water lines. Chemicals for direct injection into boilers are combined with water in chemical injection tank. The combination is injected into the boiler water feed line immediately after the feed water control valve by a pump unit and the auxiliary boiler and the exhaust gas boiler share the same feed tank and treated feed water. Chemicals can also be put to the chemical injection tank and pushed from the tank into the boiler feed water lines using pressurized feed water.
The addition of chemicals must be done in line with the manufacturer’s recommendations.

There are several possible dosing points for a boiler system, and the choices depend on several factors like system configuration, boiler pressure and product combinations. Figure below shows typical dosing points for a low pressure system. All chemicals should be dosed with a suitable metering pump.

Example of chemical dosage equipment

This will allow continuous dosage of the products and will minimise the handling of chemicals. Batch or slug dosing is never recommended.

Dosing point 1 is the dosing into the hotwell which is common, and can be used for non-volatile chemicals. Sulphite, alkalinity and scale inhibitors can be dosed here. Neutralising amines and oxygen scavengers based on DEHA, hydrazine or carbohydrazide should not be dosed here as there will be some vaporization in the hotwell. The dosage point should be below the water level, preferably close to a water inlet that can provide some mixing.

Dosing point 2 is the preferred point for volatile oxygen scavengers and neutralising amines. This will provide protection from dissolved oxygen as early as possible in the system, avoiding excessive vaporization in the hotwell.

Dosing point 3 is the dosage point which is used where separate dosing to multiple boilers is necessary. Scale inhibitors and some combined treatment are sometimes dosed here. Keep in mind that when dosing on the pressure side of the feed water pump, the metering pump need to be designed for pumping against a higher pressure.

Because of the constantly changing load on a boiler, daily monitoring of the chemical levels is important to make sure that the system is in good condition. For a small system sampling is typically taken from the boilers and the condensate return. More complex system would require samples from the feed water as well.
All sampling points should have a sample cooler to ensure the sample is at a temperature of 20-25°C when sampled.

Example of a sample cooler

This is important because it will prevent flashing of the volatile components like the amines and DEHA, yielding lower results than actual when sampling. Additionally, this will prevent burn incidents of the personnel. The water sampling procedure and cooler use explanation can be found in here.

When sampling, there should always be used clean sampling bottles. A good practice could be to have pre-labelled bottles so that the same bottle is used for the condensate each time. Trace amounts of boiler water from last sample may very well ‘ruin’ a condensate sample if bottles are mixed. Ideally, the bottle should be flushed with the water to be sampled a few times before the samples are collected.
Cleanliness is important when analyzing the water. Dirty hands and working benches may contaminate the samples. As an example, human sweat contains app. 6000 ppm chlorides, so there is not much needed to contaminate a condensate sample with chloride.

Oxygen control

After due consideration of the feed system the operation of the deaerator (if installed) it is still necessary to apply a chemical oxygen scavenger to eliminate oxygen residuals and assist in the passivation of metal surfaces. There are various types of oxygen scavenger available to carry out this task and selection of the best approach is a function of the amount of oxygen present, risk, feed system design, economics and any particular limitations required by the process using the steam.

For oxygen control the most known chemicals that can be used:

      • Sulphite – Sodium sulphite (Na2SO3) is widely used for oxygen scavenging. Sodium sulphite has been found satisfactory at pressures up to about 62 bar. Above these pressures decomposition products such as H2S and SO2 can affect steam purity.
      • Hydrazine – Hydrazine, unlike sodium sulphite, does not increase the dissolved solids content of the boiler water. Hydrazine is very volatile and should be injected at the earliest possible point in the feed system.
      • DEHA – (DiEthylHydroxylAmine) is an organic oxygen scavenger and metal passivator, enhancing formation of a protective magnetite layer. It is significantly more volatile than hydrazine, resulting in increased protection in the steam and condensate system. Being an amine, it has also some neutralising properties. It is more thermally stable than hydrazine and can be used for all types of boilers from low to high pressures.
      • Carbohydrazide – is a ‘combined form’ of hydrazine. It was designed to minimise exposure to hydrazine vapours during handling. Carbohydrazide and its reaction products will add no dissolved solids to the water. Carbohydrazide can be used as an oxygen scavenger and metal passivator at both high (230 °C) and low (65 °C) temperatures. Carbohydrazide can be applied to boilers up to 170 bar.
      • Erythorbic acid – is another effective oxygen scavenger and metal passivator, it is the only non-volatile scavenger which can be used for spray attemperation. It does not add measurable solids to the boiler water, is non-volatile, and will not jeopardies steam purity. Erythorbic acid can be used in boilers up to 122 bar.

pH control

      • In low and medium pressure boilers it is usual to maintain a level of free OH alkalinity to aid in the prevention of corrosion of steel. The recommended level of free OH alkalinity is dependent on boiler pressure and heat flux and can be found in manufacturer’s manual.
      • In high pressure boilers where there is a risk of caustic concentration and subsequent caustic attack it is common to apply a coordinated or congruent phosphate control program.
      • Hide-out – In high pressure water-tube boiler it is sometimes observed that the concentration of soluble salts, notably phosphate salts, do not rise in line with other salts and when the boiler load reduces their concentrations suddenly rise. This is phenomena termed ‘hide-out’ and is due to the reduced solubility of sodium phosphate at temperatures above 250 °C. As load and temperature at heat transfer surfaces increases then some of the sodium phosphate will precipitate and measured PO4 reserves will fall. When load and temperature reduce the PO4 salts resolubilize and the reserve is seen to increase. When phosphate hide-out occurs there is a risk of permanent scale deposition and/or localized evolution of free caustic which in turn could lead to severe corrosion.

So for pH control, if there is a history of boiler deposits or phosphate hide-out is a recognized problem, it may be prudent to consider an All Volatile Treatment approach (AVT). This approach uses entirely volatile solids free chemicals such as Hydrazine, Carbohydrazide, Erythorbic acid and neutralising amines (Ammonia, Morpholine, Cyclohexylamine) to maintain the boiler pH at a level to high enough to control corrosion and give good passivation of metal surfaces. All steel systems are normally controlled at a pH of 9.2 – 9.6 and those containing copper or its alloys at a pH of 8.8 – 9.2. A drawback is that the boiler water is relatively un-buffered and if contamination occurs the boiler pH can be reduced dramatically. Additionally it is important to be aware of the amount of silica in the system as there is no free OH alkalinity to handle it correctly. High levels of silica in the feed-water will preclude this treatment approach.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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Source and Bibliography:

  • WSS Water Treatment

 

Boiler feed water system, water sampling and treatment systems explained

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The feed water system of the boilers is the component of the steam producing plant that circulates feed water from the cascade tank into the oil-fired auxiliary boiler and the exhaust gas boiler via the boiler feed water pumps and feed water regulators.

A comprehensive video regarding feed water system. Source and credit: International Engineering Training

The feed water regulating valve automatically controls the feed water flow to each boiler in line with the change in water level in the boiler, in order to keep the water level constant. The regulating valves are, generally of pneumatic actuated globe valve type. In the video below you can see how these valves actually works.

How pneumatically valves work. Source and credit: saVRee

Two boiler feed pumps take suction from the cascade tank and provide main and auxiliary feed lines to the oil-fired auxiliary boiler and the exhaust gas boiler. Each boiler’s main lines are equipped with a feed water control valve, which automatically regulates the flow of water to the boiler in order to maintain the proper water level. The auxiliary feed pipes allow for direct feed input in which case human control of the boiler’s water level is required.
Before the feed pump discharge valve, a little amount of water is diverted back to the cascade tank from each feed pump output, thus the discharge line to the drain tank features a number of aperture plates to minimize water pressure.
This water discharge ensures that even when the boiler feed control valve is closed, water flows via the running feed pump.

The boiler feed water is sampled and treated to prevent corrosion and scale formation in the auxiliary and exhaust gas boilers, as well as the degradation of steam quality. Incorrect or insufficient boiler water treatment will severely damage the boilers, necessitating frequent testing and treatment to prevent the risk of damage. Even when distilled water is utilized for boiler feed, there is a risk of corrosion. In service, the pH of the water fluctuates, and oxygen might dissolve in the water where the feed system is exposed to the atmosphere. Although keeping the feed water temperature reasonably high, above 60°C, will reduce the amount of dissolved oxygen, the problem is always present.
Water sampling connections are provided on the auxiliary and exhaust gas boilers, with the outlet from these being sent to a sample cooler that is chilled by water from the service cold water system. Usually, the sample cooler is placed in the workshop or in the close proximity of the boiler. In order to acquire a fully representative sample of water from the boiler, the water must be allowed to run from the boiler for a minute before being sampled. The boiler’s sampling valve is positioned to produce a representative sample, but old water in the pipes and cooler must be purged before the testing sample is drawn. Every day, the boiler water must be tested. To guarantee that the boiler water is properly treated, the directions provided by the water treatment test kit vendors must be strictly followed.

Taking water sample from the boiler. Source and credit: wareboilers

The procedure of taking a water sample from the boiler can be described as follow:

  • Check that the cooling fresh water is available for the water sampler.
  • Open the sample cooler cooling water outlet and inlet valves and check the flow of fresh cooling water through the sample cooler.
  • Open the water sample outlet valve on the sample cooler and
  • Slowly open the sampling valve on the boiler from which a water sample is required and allow boiler water to flow through the sample cooler. Ensure that water is leaving the sample cooler outlet and not a mixture of steam and water. If the temperature of the boiler water leaving the sample cooler is too high, reduce the flow of boiler water to the sample cooler.
  • After the boiler water has been flowing for one minute, collect a sample of the boiler water for analysis.
  • Close the boiler sampling valve and then close the sample cooler cooling water valves and the sample inlet and outlet valves.
  • Analyze the sample of boiler water in accordance with the instructions of the chemical treatment supplier and record the information. Add chemical treatment to the boiler feed water as required.

Analytical tests and chemical treatment must be carried out in line with the chemical manufacturer’s recommendations. To keep the chemical levels within an acceptable range, the treatment must be added, but caution must be exercised, as excessive treatment can frequently cause more severe harm than minimal treatment. The results of the chemical analysis on the boiler water are recorded, and the effects of the added treatment can be tracked over time.

Following the analysis of the boiler water, a decision must be made regarding the amount and type of chemicals to be added to the boiler feed water, if any.
The treatment is, usually, added to a chemical injection tank and from there, it is routed to the boiler feed water lines. Chemicals for direct injection into boilers are combined with water in chemical injection tank. The combination is injected into the boiler water feed line immediately after the feed water control valve by a pump unit and the auxiliary boiler and the exhaust gas boiler share the same feed tank and treated feed water. Chemicals can also be put to the chemical injection tank and pushed from the tank into the boiler feed water lines using pressurized feed water.
The addition of chemicals must be done in line with the manufacturer’s recommendations.

If the level of boiler water dissolved solids is too high, these can be removed on a regular basis using the scum valve on each boiler, whereas dissolved solids can be minimized by blowing some of the water out of the boiler and replacing it with fresh distilled feed water. This is known as boiler blowdown, and it is achieved by opening the boiler blowdown valve for each boiler. The scum and blowdown lines link to the same blowdown pipe, which connects to an overboard discharge placed below the waterline of the ship.

Boiler blow down instruction example. Source and credit: go2atp

The blowdown procedure is as follow and must be performed during boiler low load:

  • Check with the bridge that it is safe to blow down the boiler if the ship is in port.
  • Open the ship’s side blowdown valve
  • Ensure that the boiler is filled to the high water level.
  • Slowly open the boiler scum valve and reduce the water level to the normal position, then close the scum valve.
  • Refill the boiler to the high water level position and blow down the boiler using the blowdown valve. After the blowdown of the boiler, close the boiler blowdown valve and then close the line and ship’s side valves.
  • Test the boiler chemical concentrations and adjust as necessary.

In conclusion, the following precaution must be taken when you are dealing with boiler feed water and treatment chemicals:

  • Caution must be exercised as the sampling lines from the boiler are under boiler pressure and the temperature of the water being drawn from the boiler is high.
  • Care must be taken when operating the sampling equipment. The cooling water supply must be confirmed to be flowing before the boiler sample valve is opened and valves must be opened slowly.
  • Care must be taken when handling boiler water treatment chemicals. Protective equipment must be used.
  • When blowing the boiler down the overboard discharge valve must be opened before the boiler or scum blowdown valves, as opening the boiler valves first will subject the blowdown line to full boiler pressure.
  • When turning down one of the boilers for a short length of time (for example, the exhaust gas boiler while the ship is in port), it is critical to ensure that the water in the boiler has been properly treated. After a shutdown boiler is restarted, a water test should be performed as soon as practicable.
  • It is essential that details of water analysis are recorded together with details of the treatment added. Only with detailed information is it possible to determine the cause of possible future problems.

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