How to Choose the Best Boiler Feed Water Treatment Technologies for Your Plant

When it comes to choosing technology for boiler feed water treatment, knowing the feed water source quality in relation to the water quality required for your specific boiler is essential, as inadequate water treatment can lead to the scaling, corrosion, and fouling of the boiler and downstream equipment. We’ve mentioned this before in some of our previous boiler feed water treatment articles, but this bears repeating as some of these issues, if neglected, can be pretty serious.

Although the water quality and makeup quantity is a complex calculation that needs to be analyzed by a boiler feed water treatment expert, there are some common characteristics for boilers and their recommended feed water treatments that can indicate different technologies that might be useful in your system.

Here’s how to be sure you choose the best boiler feed water treatment technologies for your plant:

Know the quality of water feeding your boiler

Since water absorbs more heat than any other inorganic substance, it is often used to create energy. However, when water is being used in a boiler to create a large amount energy in the form of steam, any impurities in the water can be a detriment to the boiler itself and equipment down the line. For this reason, it’s essential to know the impurities present in the water and treat them accordingly.

Boiler feed water is a combination of the boiler makeup water (what is required to replace any lost water in the boiler due to evaporation or water loss in blowdown and processing steam) and condensate return water (the distilled water created when the boiler is producing steam that condensates on the internal areas of the boiler):

Boiler makeup water

Depending on how often your boiler is blown down (rid of any impurities that occur as a result of the steam-making process) or how much water is lost to evaporation and steam generation, the quantity of makeup water needed to replenish this loss might fluctuate.

Pretreatment of makeup water is important, though, especially in the case of higher-pressure boilers that require extremely pure water.

Choosing your boiler makeup water source critical in determining the treatment options that will go into the makeup of your system. These sources might include city water, city-treated effluent, in-plant wastewater recycle (cooling tower blowdown recycle), well water, or any other surface water source.

Some common impurities include:

Iron Copper Silica Calcium Magnesium Aluminum Hardness Dissolved gasses Total dissolved solids Suspended solids and organic material

The presence of these contaminants can cause a scale to form inside the boiler pipes and parts. This is a hard deposit that can decrease the efficiency of the boiler, promote local overheating, and be extremely damaging to the system. Others promote corrosion, fouling, and loss of steam purity and need to be removed to maintain the integrity of the system.

Some common treatments to remove these types of contaminants include:

Coagulation/chemical precipitation filtration and ultrafiltration deaeration/degasification ion exchange/softening membrane processes such as reverse osmosis and nanofiltration

So, again, depending on the impurities present in your water, any combination of these treatments might best suit your facility and makeup your treatment system.

Condensate return water

When steam is produced inside the boiler, the water particles collect and condense, then are recycled and used as part of the boiler feed water again. Technically, the condensate that the steam-making process produces is distilled, pure water, but dissolved gases such as oxygen and carbon dioxide are sometimes present. The chemical reactions due to the presence of these dissolved gases can cause severe corrosion on boiler pipes and parts.

The gases are typically removed with deminerization, advanced deaeration devices, or chemical scavengers, so if you have these present, chances are your boiler feed water system will incorporate some form of these technologies.

Know the quality of water needed for your boiler

The quality of feed water needed for your individual boiler depends on many factors, but the primary element to consider is the pressure at which you need to run your boiler in relation to the amount of water you need to process per day and how fast (this is your required peak gallons per minute, or GPM). For certain pressures, there is a maximum level of contaminants to you can feed into the boiler, and as you increase the pressure in your boiler, it becomes more critical for thorough water treatment that yields higher quality water.

Low-pressure boilers (600 PSI and lower) and water with a low amount of total dissolved solids. Typically the technology used for lower-pressure boilers includes simple filtration to make sure no dirt gets into the boiler and a water softener to take out the hardness. As the water chemistry might dictate, or as the pressure increases, you might use a water softener in addition to a dealkalizer for a lower alkalinity feed. High-pressure boilers (600 PSI and higher). Treating your feed water for a higher-pressure boiler usually requires some type of demineralization, ion exchange, or electrodeionization (EDI) polishing. Resin-based sandwich or mixed-bed polishing devices can also be used, and these technologies can be permanent (regenerable in place) or portable (requiring an exchange service from an outside provider). Reverse Osmosis is a very popular technology for power plants when used in combination with a polishing technology. They are typically used on high-pressure boilers in power plants or refineries where extremely high purity water is desired.

Also note that boiler/turbine manufacturers each have their own requirements for water quality, so be sure to check with your manufacturer what their recommendations are.

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2 Important Drains in the Ship’s Engine Room

There are several drains provided in the ship’s engine room in order to prevent water clogging and damage to any machinery. Though all these are equally important, there are few that require regular attention.

Two most important drains in engine room are:

The Air bottle moisture drains and Water Drains from fuel oil settling and service tanks

Most of you would know this for sure, but the fact remains that several mishaps happen due to improper handling and infrequent draining of these tanks. Mentioned below are two instances which explain the importance of these drains in ship’s operations.

Lesson 1

On one of our previous ships, we had an anchorage of 24 days and in this period everyone in the engine room forgot to drain the water from Fuel Oil settling and service tanks in the ship’s engine room.

We then suddenly received orders to move the ship. After a few hours, the engine slowed down and an overload alarm appeared on the engine control room panel.

We immediately tried to find out the reasons for the overload alarm and realized that the viscometer was showing “high viscosity” level. After some brain-storming, it was decided to check the service tank drains.

The junior engineer, who was assigned the task of checking the drains, came back saying that oil and not water was coming from the drains. However, when asked to drain more, a large amount of water discharge was found.

Luckily the generators were on Diesel oil (D.O) which helped prevent the blackout on the ship. Moreover, we had two settling tanks, so removal of water was much faster, but we missed the convoy at Suez as the engine RPM did not increase until “good quality” oil came in the line and the viscosity reached near normal.

Lessons Learnt:

We should have drained the tanks every day until we saw (Collecting the drain in suitable container- half cut soft drink can) no water coming. In our case, the drain pipe end was ending too deep in the funnel so the contents could not actually be seen. It is thus extremely important that what is coming out of the drain is clearly seen.

In dry dock, while cleaning Fuel Oil service tanks, a senior engineer must carefully check the drain from both inside and outside. The steam heating coils must also be pressure tested at this occasion.

Lesson 2

Air bottles are important for supplying air to the marine engine air starting system and for other important purposes. On a new ship, it was found that we were draining water not from the Air bottle bottom drain but from a drain on the filling line.

Below the filling line drain, a funnel was installed which was quite familiar to the air bottle drain funnel. The motorman was so sure about the drain that no senior officer bothered to cross check. Then, one day we got a doubt about the quantity of water draining from the funnel, which was of course not of the quality we expected.

Upon opening the air bottle manhole door it was found that the water was filled up to quite a high level and luckily it did not find its way into the main engine, else there could have been a massive damage to the engine or even injury due to water hammer. The Air bottle was then cleaned and the defect was rectified.

Lessons Learnt

Nowadays, it is often seen that the tracing of pipelines in ship engine room is not done properly, mainly because of shortage of crew members and fast turnarounds. Efforts should thus be made to trace the pipelines once in a while.

While opening air to main engine, moisture must be drained from the bottom most point of the line. Air driers and control air line filters must also be regularly checked.

These days quarantine inspection has also become important. The cold room holding provisions thus must be kept clean. Meat room and fish room drains should be checked (put a handful of salt in them), along with the unit cooler pan heater and drain.

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Procedure for Starting and Stopping of Sewage Treatment Plant on a Ship

Any machine on the ship requires a proper procedure to be followed for starting and stopping it. Failure to follow this step-by-step procedure will lead to either failure in starting or stopping the machine or lead to some fault.  

Sewage treatment plant is one such equipment on the ship which requires a step-by-step procedure to be followed for starting and stopping it. In this article we will go through the procedure of starting and stopping a sewage treatment plant.

Starting of a Sewage Plant

Sewage plant is generally running all the time during sailing, but it might need to be started when the ship is installed with a new sewage treat plant which needs to be stopped at regular interval of time for improving its performance and maintenance procedures. Below are the points that need to be followed for starting a sewage treatment plant.

1. Make sure if any maintenance is carried out on the sewage treatment system, all the openings have been closed properly before starting.

2. The sewage plant is be filled with fresh water inside the chamber.

3. At this stage, there are no aerobic bacteria inside the chamber, but the sewage has started coming to the plant. Thus, in order to increase efficiency and starting rate of the plant bio pac is added to the plant by flushing the amount specified in the manual.  This bio pac is mixed with warm water which helps in growth of these bacteria and also efficient functioning of the plant.

4. If the bio pac is not added, the plant might take up to 5 to 7 days to be completely functional. However, with the bio pac it becomes functional within 24 hours.

5. Start the air compressor or open the air valve as per the design of the plant. The pressure is maintained as per the manual. Generally 0.3-0.4 bars.

6. Open the sewage overboard valve and close holding tank valve when the ship is out of restricted waters.

7. The plant is continuously monitored and checked for the flow through the transparent plastic tubes.

8. The sample is taken for checking for suspended solids and chlorine content.

Stopping of the plant

Stopping of the sewage treatment plant is generally done either before entering the dry dock or in case some maintenance has to be carried out inside the treatment plant.

1. For stopping the system, close the inlet valve to the sewage plant and close the overboard valve and let the sewage go overboard.

2. Empty all the three chambers of the plant i.e. aeration, settling and chlorination chambers. If the chambers are not emptied, it will lead to growth of anaerobic bacteria which forms the toxic H2S gas.

3. If entry has to be made inside the tank, the later should be checked for hydrogen sulphide gas H2S with the help of dragor tube by taking a continuous sample from the plant. Entry is made with the help of mask and rubber gloves should be put on.

4. In case the ship is going to dry dock the overboard should be connected to shore reception facilities.

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Maintenance and Checks for Sewage Treatment Plant on Ship

An efficient running of a sewage treatment plant on a ship requires periodic maintenance and daily checks of the system. Failure to do so can lead to an output that cannot be discharged into the sea, blockage of pipelines, and even failure of some parts.

There are several factors that results in smooth working of a sewage treatment plant on a ship and this article enumerates all the maintenance and checks for that smooth running.

Routine Checks

1. During daily rounds the pressure of the system should be checked and should be within the limits.

2. The air lift return should be checked to make sure the system is working properly. This is usually checked by the flow through the clear plastic pipe present on the installation. A clear sludge can be seen flowing through the tubes back to the aeration chamber.

3. Over a period of time, the sludge content in the aeration tank due to the recycling of the sludge from settling tank and fresh sewage increases. This sludge content or suspended solid particle is measured in mg/liter. The method of checking it is to take sample in a conical flask provided by the manufacturer and filling it up to 1000ml mark. The sample is then allowed to be settled and reading of sludge content is checked.

 

The sludge content should not be above the 200 mark, but if it is above the 200 mark, the tank has to be emptied for increasing the performance. In some ships this is checked by filtering the sample through a pre-weighed pad which is dried and re-weighed. This also depends from manufacturer to manufacturer, but is done every week.

4. Also the bio-pac is added every week to the plant to increase efficiency. The bio-pac contains aerobic bacteria which get activated when mixed with hot water.

5. The chlorination of the sample should be between 1-5 ppm and accordingly the dosing has to be increased or decreased.

6. The level of biological oxygen demand (BOD) is also checked and it should not be above 50 mg/liter. The sample is checked by incubating the sample at 20 degrees and well oxygenating the same. The amount of oxygen absorbed over a period of five days is measured. This is done to check the oxygen required for full breakdown of sewage after it has been treated by aerobic bacteria.

7. The internal coating of the sewage treatment plant should be checked for cracking and blistering. If any kind of damage is found then we first need to empty the tanks and then necessary repairs to be performed. Special precautions should be taken before entering the tank as it may contain toxic gases that cause suffocation. The gases should be checked by dragor tube, a special tube in which samples of various gases are taken before entering.

When it is made sure of the absence of toxic gases, entry is made with the mask and gloves. After completion of work the area has to be disinfected. Later, hands should be properly scrubbed and overalls be thoroughly washed.

8. If the sewage treatment plant is fitted with UV disinfectant system instead of the chlorination system, the UV lamp has to be changed as recommended by the manufacturer.

9. High and low level limit switches should be checked for auto cut-in and cut-out of the discharge to over-board pump.

10. Make sure the stand-by sewage discharge pump is put on auto during the working of the sewage treatment plant.

Maintenance

In case of a blockage of the sewage line there is a connection for back flushing which uses sea water. This is to be used to unclog the sewage pipelines; however, it is to note that all valves are closed and only the necessary valves are open, for it might back flush from WC of the cabins.

Generally, stewards are instructed for using chemicals provided by various manufacturers such as Drew Marine and Unitor during washing so that no blockages of lines are caused. However, there shouldn’t be any overuse of these chemical as it would lead to killing of aerobic bacteria which will decrease the efficiency of the plant and other problems.  The amount of chemicals is to be as per manufacturer recommendation.

Reference: From marine auxiliary machinery by Mc George

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4 Important Terms Related to Sewage Treatment Plant on Ships

Sewage on board ships needs to be treated before it is discharged to the sea. Sewage treatment plant is used to treat the sewage and make it less harmful for the sea.

Marine engineers must know the operation of the sewage plant before using the same in order to comply with the rules and regulations of discarding sewage.

While operating the sewage plant, engineer must know:

However, apart from the above mentioned aspects, marine engineers should also know four important terms while dealing with sewage treatment plants on ships. They are:

Biochemical Oxygen Demand (BOD) Coliform Count Recommended levels of pumping out solids Bio-chemical digestion of sewage

1. Biochemical Oxygen Demand

Biochemical oxygen demand is a test to identify biological decomposable substances and to test the strength of the sewage. BOD depends on the activity of bacteria in the sewage. These bacteria feed on and consume organic matter in the presence of oxygen.

 

BOD can also be defined as the amount of oxygen required by the micro-organisms in the stabilization of organic matter. The results are generally expressed as the amount of oxygen taken by one litre sample (diluted with aerated water) when incubated at 20 degree for five days.

BOD of raw sewage is 300-600 mg/litre. IMO recommends BOD of less than 50 mg/litre after treatment through sewage treatment plant.

 

2. Coliform Count

Coliform is a type of organism which is present in human intestine and is recognized as indicator organisms of sewage pollution. Presence of these organisms in water is an indication of pathogen (pathogen count), which are diseases causing bacteria responsible for cholera, dysentery, typhoid etc.

The number of coliform organisms present in sewage on ship is very large, with each person contributing around 125 billion in winters and 400 billion in summer.

IMO recommends faecal coliform count of less than 250 faecal/100 ml. of affluent after treatment.

 

3. Recommended levels of pumping out solids

Dissolved solids – Solids which are dissolved in the solution

Suspended solids – Solids physically suspended in sewage that can be removed by laboratory filtration and are relatively high in organic matter.

Settle able solids – Suspended solids that will subside in quiescent liquid in a reasonable period of time (usually around an hour)

Suspended level of raw sewage – Around 300-400 mg/litre; IMO recommends 50 mg/ litre after treatment.

Residual disinfectant – After treatment residual disinfectant should be as low as possible. IMO recommends use of ultra violet exposure for chlorination method.

 

4. Biochemical digestion of sewage:

Anaerobic process

Anaerobic bacteria can only multiply in the absence of free oxygen as they utilize chemically bound oxygen to survive. Anaerobic bacteria break down the organic matter into water, carbon dioxide, methane, hydrogen sulphide and ammonia. This process is called putrefaction.

The products thus produced out of this process are noxious and toxic. The effluent is of poor quality and by-products are highly corrosive.

Aerobic process

Aerobic bacteria require free oxygen to survive. They break down the organic matter to produce safe products such as water, carbon dioxide, inert residue, and energy to synthesize new bacteria.

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How To Classify Oil Tanks?

Oil tanks are ideal containers used to store crude oil or other kind of oil, they are widely used in refinery, oil field, oil tank farm, or other industrial production. Oil tanks play a very important role in the process of oil and gas storage and transportation. What’s more, oil tanks come in various of sizes and types according to different classification standards so that there are some differences in their installation to some extent.

Divided by structure of oil tanks

According to the structure, oil tanks can be divided into crude oil tanks, fuel tanks, lubricating oil tanks, edible oil tanks, etc. According to the characteristics of the stored oil, they can be divided into heavy oil tank and light oil tank. According to the depth of burying, they can be divided into aboveground oil tanks, underground oil tanks, and semi underground oil tanks.

Divided by the material of oil tanks

According to the material, oil tanks can be mainly divided into steel oil tanks, concrete oil tanks, and plastic oil tanks.

steel oil tanks concrete oil tanks plastic oil tanks

Steel oil tanks can be divided into vertical oil tank (including fixed roof tank and floating roof tank), spherical oil tanksand horizontal oil tank (cylinder tank).

Vertical fixed roof oil tanks, which are composed of fixed tank roof and vertical cylindrical tank wall, are mainly used for storage of non volatile oil, such as diesel oil and the similar oil. The most commonly used volume of fixed roof oil tanks right from 1,000m³ to 10,000m³. Floating roof oil tanks, which are composed of a floating roof that floats on the oil surface and a vertical cylindrical tank wall, are suitable to store volatile oil, like gasoline and the similar medium, and the volume of floating roof oil tanks are generally larger. The floating roof of oil tanks can increase or decrease following with the increase and decrease of the stored oil production in the oil tanks, a ring seal is installed between the outer edge of floating roof and tank wall. What’s more, the medium in the oil tanks is always covered by the internal floating roof directly, in order to reduce the evaporation of medium.

Steel oil tanks Vertical cylinder fixed roof oil tanks

The capacity of this kind of oil tanks is generally less than 10,000m³, the tank wall adopts sleeve connection method (fillet weld). Jacking method is the most used erection method for vertical cylinder fixed roof oil tanks, that is erecting the tank start from the tank top and install the tank wall from top to bottom layer by layer, with the help of jacking system to raise the tank. Compared with the traditional method, which is erecting the tank start from the tank bottom shell, then erecting the tank wall from top to bottom layer by layer, jacking method is safer as it reduces the risk of aerial work.

Vertical cylinder floating roof steel oil tanks

Floating roof steel oil tanks are equipped with double floating deck roofs or singe deck floating roof which can float up and down, the double deck floating roof can reduce the heat radiation effect, therefore, the oil evaporation loss is small. But when the capacity is larger (more than 10,000 m³), it is generally preferred a single deck floating roof in order to reduce the cost. This kind of oil tanks should be paid more attention on choosing of reasonable sealing device, which required excellent sealing performance, convenient installation and maintenance.

Vertical cylinder internal floating roof steel oil tanks

This kind of oil tanks consist of both doom roof and internal floating roof, internal floating roof can float up and down upon the liquid level. In addition to the characteristics of external floating roof tank, internal floating roof tank can also ensure the cleanliness of the stored oil.

Spherical steel oil tanks

This kind of oil tanks can withstand the working pressure ranging from 0.45MPa to 3MPa, the capacity is generally between 50m³ and 2,000m³.Spherical steel oil tanks are generally used to store liquid petroleum gas.

Horizontal steel oil tanks

The capacity of horizontal steel oil tanks is generally less than 50m³, it can store gasoline and volatile oil products.

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Sewage Treatment Plant on a Ship Explained

Discarding sewage produced onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to same him and his shipping company from heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and it for this reason it has to be discharged into the sea.

Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging of sewage that needs to be followed. Sewage on sea is generally the waste produced from toilets, urinals and WC scuppers. The rules say that the sewage can be discharged into the sea water only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.

But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of surrounding water.

Generally, ships prefer treating sewage before discharging to save themselves from any type of embarrassment. There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco friendly. It is to not that each sewage treatment system installed onboard has to be certified by classification society and should perform as per their requirement and regulations. 

Working of a Biological Sewage Plant

The basic principle of the working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed off in the sea. Air is a very important criterion in the functioning of the biological sewage plant because if air is not present, it will lead to growth of anaerobic bacteria, which produces toxic gases that are hazardous to health.Also, after decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discoloration of water which is not accepted for discharging. Thus in a biological sewage treatment plant the main aim is to maintain the flow of fresh air.

Division of Processes

The biological sewage plant is divides into three chambers:- 

Aeration chamber

This chamber is fed with raw sewage which has been grinded to form small particles. The advantage of breaking sewage in small particles is that it increases the area and high number of bacteria can attack simultaneously to decompose the sewage. The sewage is decomposed into carbon dioxide, water and inorganic sewage. The air is forced through diffuser into the air chamber. The pressure of air flow also plays an important role in decomposition of the sewage. If pressure is kept high then the mixture of air and sewage will not take place properly and it will escape without doing any work required for decomposition. It is for this reason; controlled pressure is important inside the sewage treatment plant as this will help in proper mixing and decomposition by the agitation caused by air bubbles. Generally the pressure is kept around 0.3-0.4 bars. 

Settling tank

The mixture of liquid and sludge is passed to settling tank from the aeration chamber. In the settling tank the sludge settles at the bottom and clear liquid on the top. The sludge present at the bottom is not allowed to be kept inside the settling tank as this will lead to growth of anaerobic bacteria and foul gases will be produced.The sludge formed is recycled with the incoming sludge where it will mixes with the later and assist in the breakdown of sewage. 

Chlorination and Collection

In this chamber the clear liquid produced from the settling tank is over flown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-coli bacteria present in the liquid. To reduce these bacteria to acceptable level chlorination is done. Moreover, to reduce the e-coli, the treated liquid is kept for a period of at least 60 minutes. In some plants disinfection is also done with the help of ultra violet radiation. The collected liquid is discharged to overboard or settling tank depending on the geological position of the ship. If the ship is in restricted or near coastline then the sewage will be discharged into the holding tank; otherwise, the sewage is discharged directly into the sea when high level is reached and is disposed automatically until low level switch activates.

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