FWG and Jacket Cooling water System

Question 1

What are the causes of loss of vacuum in fresh water generator ?

Answer 1

first of this will cause The shell pressure of the freshwater generator to increase and the rate of freshwater produced decreases.

The reasons for this are:

• Air leaks into the evaporator shell in large quantities and air ejector cannot cope.
• The cooling water flow through the condenser is reduced or cooling water temperature is high. This cause saturation temperature and hence saturation pressure within the condenser to rise.
• Malfunctioning of the brine/air ejector. like for instance failure of the ejector nozzle due to fouling or erosion
• Flow rate of the heating medium (fresh water) is increased and excess water vapour produced. Since this excess vapour can not be condensed, the pressure of the shell increases and the vacuum drops.
• Vacuum Breaker which is used for releasing Vacuum when we shut down the FWG is faulty so air is leaking in and vacuum is being lost

Question 2

With reference to a low-pressure fresh water evaporator:

a) Explain why the water at the evaporator outlet is unsuitable for drinking; (8)

b) State the treatment that may be used to make the water potable. (8)

Answer 2

a.) at evaporator (FWG) outlet water has been boiled but isn’t safe or suitable to to drink because its been boiled at a much lower temperature than 100deg (usually about 55) this doesn’t sterilise it and so bacteria are still present, meaning water is harmful to human health. Additionally water isn’t tested before it enters the FWG or evaporator only after. Before water can be consumed it must treated to remove the bacteria. normally water is boiled at 100deg at atmospheric pressure to kill of bacteria and therefore make water sterile. In a FWG or in other words evaporator pressure is lowered an water is boiled at a lower temperature. this is done by inducing a vacuum to lower the boiling point enough so that temperature of jacket cooling water is high enough to boil the sea water seawater and by condensing produced vapour fresh water is generated. in addition to having bacteria present the water lacks any minerals and nutrients so its unpleasant to drink but its more about the fact that when the fresh water is produced it will have mineral salts removed during the evaporation process. this makes it distilled water that lacks nutrients and if consumed over time human body will leech minerals causing an electrolyte imbalance.

b.) As previously mentioned, as the water is generated at a lower temperature and so bacteria can still be present within the potable water and it lacks nutrients. Therefore we are required to treat the water to ensure it is safe to be consumed.
This can be done utilising different treatment methods like chlorination, electro-silver ionisation and ultra violet.

Chlorination method

Chlorine is dosed into the produced water at a rate to
achieve a free chlorine level of 2 ppm approx. While in
the storage tank chlorine will slowly begin to evaporate. It is a requirement to have a residual amount in the FW distribution system of 0.2ppm. Chlorine as a disinfectant requires around 20 minutes contact time to react and kill off the bacteria and make water sterile. A test kit should be used to check the chlorine level ensuring a residual amount is left and that water is sterilised. a chemical injection unit otherwise known as an automatic chlorination unit is added to the FWG system, this supplies a small amount of chlorine to the condenser tubes to sterilse the distilled water being produced.

Electro-silver Ionisation

Silver ions which are dispersed in the water will attach themselves to bacteria and kill them. Dosing requirements are 0.1ppm dosed at maximum flow. the minimum time required for the silver to take effect is 4 hours after passing through the unit. This should ensure to leave a measurable residual amount of 0.08ppm.

Ultra violet sterilisation

Ultra violet sterilisation will instantaneously kill any bacteria within the potable water system, however there is no residual measurable properties so there’s no way to tell if bacteria have been killed off therefore this method is used in conjunction with an additional means of treatment. UV units should be installed so that the direction of flow is vertical to keep the deposits in the tubes to a minimum. The water should be continuously circulated in the system through the UV unit. There should be a means to measure the intensity of UV radiation, and a switch-off mechanism with an alarm should be fitted in the event of UV radiation being too weak. The performance of the tubes should be regularly monitored.

Apart from these methods remineralisers are also aded into the distrbution system, when distilled water flows through these add essential salt minerals such as calcium and magnesium back into the water.

Question 3

What would happen if the vacuum reaches 100% in fresh water generator ?

Answer 3

This increases the salinity content because of agitation. The reason for the agitation is that boiling rate of water becomes very high. To prevent this condition, open the vacuum breaker to maintain 93% vacuum. The vacuum must be maintained at 93% to have the boiling point of water at around 55degC or at least the temperature of whatever the jacket water. The more you increase the vacuum the more you lower boiling point of water and so rate of water produced becomes too high. if alternatively the vacuum was lowered then boiling point of water becomes to high until the point where you cant use jacket water to heat the sea water.

Question 4

Why fresh water generator is fitted on ships ?

Answer 4

To produce the high purity distilled water from sea water and thereby provide make up water for boiler and potable water for drinking and domestic use and save costs, alternative option being getting water in a port. Considerable amount of freshwater is used on ship. Crew uses about 100litres per day. Potable water (water safe for ingestion) can be taken in port for crew and passengers. The quality of this water is too poor for use in water tube boilers or filling expansion tanks and or any other associated engine systems. Therefore usually a ship would take the minimum supply needed for crew and passengers and the rest is produced by FWG.

Question 5

What are semi permeable membranes ?

Answer 5

“semi” means some or partial, and “permeable” means to pass through. So, a semi-permeable membrane is a membrane that only allows certain materials to pass through. Semi-permeable membranes are different from permeable membranes, which allow the passage of all materials through the barrier.

the materials used for these are

Hollow fine fiber (aromatic polyamide or cellulose acetate spurn to form hollow fiber)

Spirally wound (cellulose acetate for fresh water and polyimide or polysulphate for sea water)

Question 6

what types of FWGs are used onboard and what is meant by performance ratio Pr

Answer 6

There are various types of FWG available on-board. They can be split into two main categories single effect plant and double effect plants. Single effect plant meaning the evaporation takes place at one pressure only. The plant normally have more than one evaporator and evaporators are arranged in parallel. Double effect plant means evaporation takes place at two different pressures and evaporators are arranged in series.
Regardless of the type of fresh water generator that is used on-board the essential requirement for the plant is efficiency.

(Pr) performance ratio is essentially how efficiency can be measured. its defined as the ratio between the kg of vapour produced to kg of steam supplied.

Single effect plants have a value of about 1.1 and double effect plants have a value of about 1.9. Although Pr is good basis which is used to compare plants to find out how efficient they are it cant tell you everything. This is because the heat source for a FWG is usually waste heat (energy) from either jacket water or steam, meaning there hasn’t been any useful work (energy) before arrival at the evaporator.

Question 7

how is performance of FWG affected?

Answer 7

Performance of a FWG is affected by scale formation. When scale formation is rapid, heat transfer is reduced and Pr decreases. It is also very important that you do not operate the plant in polluted water. Fresh water must not be produced from polluted water, as the produced water will be unsuitable for human consumption. Sea water that is to be treated on ships should be taken from areas relatively free from pollution, including air pollution. It is advisable and safe to start fresh water generator when the vessel is at least 20 nm from the shoreline. Generally this is considered to be a safe distance but it may be necessary to have ship further than this. Judgment should be used based on a risk assessment which should include consideration of the possible effect that this water might have on ships equipment due to the quality of the water.

to maintain performance

The seawater inlets (sea chests) should be located forward and if possible on the opposite side of the ship from the overboard waste water and ballast tanks discharge outlets. Sea water passes through suitable filters before entering the FWG. The manufacturers’ operating instructions should be clearly posted in the plant room and strictly followed. By-passes should not be installed around treatment units except where necessary as part of the treatment process. There should be an adequate store of spare replacement parts particularly for any vital or fragile parts. Distillation units should indicate low range salinity levels, operational temperature levels and have an automatic discharge. They should also have an alarm with trip setting or equivalent.

Question 8

List the types of FWG used onboard

Answer 8

Types of FWG normally used on-board

1.Submerged tube type FWG
2.Plate type FWG
3.Reverse osmosis plant

Question 9

Explain the working principle of a submerged tube type FWG with the aid of a sketch

Answer 9

Working principle of a submerged tube type FWG ( low pressure single effect plant)

This FWG works by lowering the boiling point of water, which is done by reducing the pressure within the shell of the FWG. By maintaining a low pressure water is boiled at around 50 degrees Celsius, which is approximately the temperature of the jacket water from the main engine. This being the main source of heat for most FWG’s but some use steam. Seawater is evaporated by using a high vacuum ( which allows water to be evaporated at a lower temperature . This type of FWG consists of two sets of shell and tube heat exchangers one acting as a evaporator, the other as a condenser. A vacuum with the right conditions is created in the evaporation chamber. Vacuum being a space without any matter (anything that can be weighed), with a limited amount of air or no air. When it comes to a vacuum you have to consider how much air is inside the space, which is why there such a thing as a high vacuum and a low vacuum. High vacuum having less air inside and a low vacuum having more air inside. Space for example has a nearly perfect vacuum so pressure is almost 0. the lower the amount of air inside a vacuum the lower the pressure. To get a vacuum inside the evaporation chamber it has to be leak proof.

Seawater is pumped in by the ejector pump and delivered to the air/brine ejector to take out brine (concentrated seawater) and air and there by creating a vacuum. Jacket cooling water from main engine is supplied to the evaporation chamber and feed rate is controlled by an orifice fixed at feed inlet. The jacket water transfers heat to the seawater so it can be evaporated due to a high vacuum and turned into vapour. Water spray and droplets are then removed from the vapour by the deflector mounted on top of the evaporator and built in demister. Separated water droplets are then extracted by the air/brine ejector. The Vapour after passing through the demister continues onto the condenser where it condensed by incoming seawater and turned back into a liquid. The liquid is now distilled water ( water that has been boiled into vapour and then condensed to a liquid). distilled water then is taken out by fresh water pump (distillate pump) and controlled by salinometer and solenoid valve. If the salt content is high in the distilled water then the solenoid valve diverts the water back to shell side of FWG and alarm goes off. To get better suction distillate pump is placed at lowest possible location in FWG plant. This is because FWG shell is at low pressure so distillate pump can get maximum suction. The salinometer is connected to a remote alarm so in the event of high salinity alarm goes off in ECR.

Question 10

Explain the working principle of a plate type FWG with the aid of a sketch

Answer 10

Plate type FWG

This FWG functions essentially the same as the submerged tube type FWG. The only difference is instead of using shell and tube heat exchanger this uses a plate type heat exchanger. The condenser and evaporator are both plate type heat exchangers. Heat is once again used from the diesel engine cooling water to evaporate a small fraction of the seawater feed in evaporator. Water that hasn’t been evaporated is discharged as brine by air /brine ejector. The evaporated water passes through the demister to the condenser. After condensation it is discharged to fresh water storage tank by distillate pump. During operation the feed rate to the evaporator is controlled by the orifice plate at the feed inlet to the evaporator. In the event of high salinity of fresh water exceeding a predetermined value (maximum usually 4 ppm) the solenoid controlled dump valve diverts the flow back to the shell side of FWG. This prevent contamination with the fresh water ready for use. Excess salinity can be caused by many factors including leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, and many other reasons. There will be a fraction of what cannot be condensed at the condenser these are called ‘incondensable gases’ like air for example these gases are continuously ejected out by air/brine ejector. This way the shell of fresh water generator is maintained at high vacuum, which is a requirement to boil water at a low temperature of around 50 degrees Celsius.

Question 11

with the aid of a sketch explain the working principle of a reverse osmosis plant?

Answer 11

check sailors notebook

Question 12

explain how and what maintenance is carried out on a FWG

Answer 12

Pre-Maintenance Preparation

before carrying out maintenance on a FWG the following steps have to be carried out:

Shut down the freshwater generator system and isolate it from the power source.

Ensure all valves and pipes are closed to prevent water leakage.

Use appropriate personal protective equipment (PPE) such as gloves, goggles, and masks.

The plate heat exchanger is a critical component of plate type freshwater generators. Regular cleaning is necessary to maintain its efficiency.

Follow these steps:

Remove the end covers and access plates from the heat exchanger.

Measure the holding bolts distance from the end tip to the end plate in order to have a tightening reference.

Soak the plates into an mild acid base solution (e.g. Sulphuric acid) and keep them soaked for few hours, as it will help to easily remove the salt scaling.

Use a soft brush or sponge to gently clean the plates, ensuring the removal of any fouling, scale, or corrosion.

Rinse the plates thoroughly with clean water to remove any residue.

Inspect for any signs of leakage or gasket damage. Replace damaged plates and/or gaskets if necessary. Use rubber glue if required for securing the gaskets.

Reassemble the heat exchanger, ensuring proper alignment and tightness of bolts.

Pressure test the plate assembly to ensure that there are no abnormal leaks detected.

Valves and pumps play crucial roles in regulating water flow. Regular inspection and maintenance is essential:

Check all valves for proper functioning, tightness, and freedom from leakage.

Lubricate valve stems and ensure smooth operation.

Inspect pumps for signs of wear, leaks, or abnormal noise. Replace worn-out parts if necessary.

Verify pump impeller clearance and adjust if required.

Check and re-adjust, if necessary, the feed water regulating valve.

Check and clean, if required the feed water nozzle.

Filters and strainers prevent contaminants from entering the freshwater generator system. Regular maintenance is essential:

Remove and clean intake filters, strainers, and mesh screens.

Inspect for clogs, damage, or excessive fouling.

Replace or clean the filters as per manufacturer guidelines.

Ensure proper alignment and tightness during re-installation.

Inspect Seawater Supply:

Check for clogged or malfunctioning seawater intake filters, valves, or strainers.

Check for feed water regulating valve adjustment

Check the water level in the sight glass.

Check the system vacuum and shell temperature.

Check the brine ejector for proper operation.

Monitor Pressure Gauges: Ensure proper pressure readings within specified ranges. Low pressure may indicate a blockage or fouling in the system.

Misaligned Components: Check alignment of pumps, motors, and other rotating elements. Realignment may be required to reduce noise and vibration.

Loose Mounting: Inspect mounting brackets, bolts, and fasteners. Tighten as needed to minimize vibration.

Question 13

explain how and what maintenance is carried out for reverse osmosis plant

Answer 13

Before starting maintenance on a reverse osmosis (RO) freshwater generator, the following steps must be carried out:

Isolate the system from the power source and shut off the seawater supply.

Open the system to relieve pressure.

Wear appropriate PPE to protect against chemicals and ensure safety.

Cleaning the RO Membranes

The RO membranes are the heart of the reverse osmosis system and require regular maintenance to optimize performance.

Perform the following steps:

Prepare a cleaning solution as recommended by the membrane manufacturer.

Flush the system with clean water to remove any loose particles.

Circulate the cleaning solution through the membranes for the recommended duration.

Rinse the system with clean water to remove residual cleaning solution.

Inspect the membranes for signs of fouling, scaling, or damage. Replace if necessary.

High-pressure pumps are vital for maintaining the required pressure in RO systems. Regular inspection and maintenance is therefore crucial:

Check the pump’s suction and discharge valves for proper operation and tightness.

Inspect the pump for leaks, vibrations, and unusual noises.

Verify the pump’s pressure and flow rates. Adjust as per manufacturer guidelines.

Lubricate pump bearings if required, following the manufacturer’s instructions.

Proper functioning of instrumentation and controls is essential for the overall performance of the RO system. Follow these steps:

Inspect pressure gauges, flow meters, and control valves for accuracy and freedom from blockages.

Calibrate instrumentation devices if necessary.

Verify the performance of automatic control systems and alarms.

Test emergency shut-down systems to ensure their functionality.

Question 14

Troubleshooting For Reverse Osmosis Systems

a.) insufficient FW production what are your actions?

b.) Excessive Freshwater Salinity

c.) Leakage or Water Purity Issues

Answer 14

a.) Insufficient Freshwater Production

• Check the seawater flow rate and pressure. Adjust as required by valves, open SW inlet and outlet slightly in small increments and observe parameters for any changes.
• Inspect and clean clogged filters or strainers, then monitor pressure for any changes
• Evaluate the condition of RO membranes for fouling or scaling. Over time, membranes can lose their efficiency due to wear and tear. Monitor the performance of the membranes and consider replacing them if they are significantly aged or damaged.

b.) Excessive Freshwater Salinity

Verify the system’s seawater flow and pressure. Adjust if needed.

Inspect the RO membranes for damage or fouling.

Review and adjust the operating parameters of the RO system, such as pressure, flow rate, and recovery rate, as per manufacturer guidelines. Optimizing these parameters can enhance the membrane’s performance in removing salt. If the salt level remains high after adjusting the operating parameters, perform a thorough chemical cleaning of the membranes to remove any accumulated deposits that may be hindering their performance.

Check salinity sensor, clean it and replace it as found necessary. Be aware that the sensor must be cleaned with a clean dry rag and must avoid to be touched by bare hands.

c) Leakage or Water Purity Issues

Inspect valves, pipes, and fittings for leakage or improper sealing. Repair or replace as necessary.

Check for loose or damaged connections.

Examine gaskets and seals for wear or degradation. Replace if needed.

Question 15

what are the reasons for the following?

a.) Salt Water Carry Over

b.) Gradual Increase in Level of Brine

c.) Increase in Salinity of Freshwater

Answer 15

a.) During the operation of the freshwater generator salt water can be carried over in large quantities. This is called priming. General reasons of the priming are:

Level of salt water inside the shell is high. When water level is high agitation due to boiling occurs and salt water may carry over along with the vapours. When there is a high level of water agitation due to boiling and salt water may be carried along with the vapours. Rate of evaporation is also increased.

b.) A constant level of brine must be maintained in the shell for the satisfactory operation of the freshwater generator. Brine is the concentrated water of the sea after the release of water vapours. This brine is gradually extracted from the shell. Typically, this is obtained by the combined air-brine ejector. It extracts air as well as brine from the shell of FWG. Any fault with the ejector causes the brine level to increase.

c.) Possible causes are:

Brine level inside shell too high.
Leaking condenser tubes or plates.
Operation of evaporator near shore with contaminated feed water.
Shell temperature and pressure too low.
Increased solubility of CO2 generated from the salt water due to reduced sea water temperature. This dissolved CO2 makes water acidic and conductivity of water increases. Salinometer therefore shows increased salinity, which is a measure of conductivity and non-salt presence.

Question 16

In some cases, the quality of the produced freshwater may not meet the desired standards. describe what actions to take should this occur

Answer 16

Evaluate Feed water Quality:Check the quality of the seawater being fed into the system. High levels of contaminants or unusual seawater conditions can affect the quality. Address any issues with the feed water source, such as pre-filtration or pre-treatment, to improve the incoming water quality.

Inspect and Clean Pre-filtration Systems:Examine and clean the pre-filtration systems, including filters and strainers, to ensure they are effectively removing larger particles and contaminants from sea water

Check Chemical Dosage:Review the dosage of chemicals, such as antiscalants or biocides, used in the system. Incorrect dosing or expired chemicals can impact quality. Follow the manufacturer’s recommendations for proper chemical dosage and replace expired chemicals.

Question 17

what is meant by atmospheric pressure?

Answer 17

It’s the force exerted on a surface by the air as gravity pulls it to earth. The pressure value at which air around you will press against everything. This pressure value is called atmospheric pressure (pressure within atmosphere or earth)

Question 18

describe the materials used in construction for a FWG

Answer 18

Materials used in construction for a FWG

The FWG shell is usually fabricated steel (or non-ferrous metal like cuprous-nickel) which has been shot blasted then coated with some form of protection. One type of coating is sheet rubber which is rolled and bonded to the plate then hardened afterwards by heat treatment. Heat exchangers are made with aluminium brass tubes and muntz metal (copper alloy composed of 60% copper and 40% zinc) tube plate in the case of shell and tube type fresh water generator. For plate type, titanium plates are used for condenser and evaporator. Demister is made of a layered wire of monel metal (nickel alloy).

Question 19

what is the procedure for starting a FWG?

Answer 19

Fresh water Generator Starting Procedure

(pre checks)

• Before starting the fresh water generator we have to check that the ship is not in polluted water and is 20 nautical miles away from the shore. This is done because near the shore the
  effluents from factories and sewage are discharged into the sea this can get into the fresh water generator and damage the equipment .
• Check whether engine is running above 50 rpm, the reason for this is that at low rpm the temperature of jacket water which is around 60 degrees and not sufficient for evaporation of water.
• Check the drain valve present at the bottom of the generator is in close position.

start up

1.Make sure seawater ejector pump suction, discharge and overboard valves are open. Start the ejector pump. Seawater pressure at the air ejector must be 3 bar or more.

2. Close the vacuum breaker located on top of the evaporator. Then wait for vacuum to build up inside fresh water generator shell. (About 92 % vacuum) check by pressure gauge.
3. Open the feed valve to feed seawater to the evaporator and observe and adjust the feed water pressure. Normally marking is provided on the pressure gauge for desired feed water pressure.

4.Open main engine jacket cooling water inlet and outlet to the evaporator gradually and close by pass valve

5. Switch on the salinity alarm panel for measuring purity of the freshwater produced. There will be a sight glass provided at the suction line for the distillate pump. Through the sight glass provided in the evaporator shell, observe flashing of water. once confirmed that water is being boiled, then open JCW inlet and outlet fully.

6.Make sure condensed (distilled) water is see in sight glass and . then start the distillate pump and open discharge valve to lead generated water to specified fresh water storage tanks. However if this is first time operating the fresh water generator and your unsure of quality of water its best to first direct produced fresh water to bilge. after the salinity is confirmed to be ok then direct the water to the desired fresh water storage tanks. make sure to switch salinity alarm and distillate pump to auto.

7.Start to check all pressure gauges, temperatures and any usual associated equipment that is checked when doing a safety round. check for the brine level inside. It should not be too high or too low. Ensure FWG Shell temperature is around 50 deg Celsius. Check seawater inlet and outlet temperature to the condenser. Ensure seawater pressure at air ejector inlet is more than 3 bars. Check distillate pump pressure and water flow meter. Check salinity of fresh water produced. Check level and flow of dosing chemical. Check ampere of ejector pump and distillate pump motor.

Question 20

explain how to regulate the capacity of a FWG?

Answer 20

Regulating the Capacity of FWG

Capacity of a fresh water generator means the quantity of fresh water produced by it per day. The capacity of fresh water generator can be varied by reducing or increasing the amount of jacket cooling water to the evaporator. The quantity of jacket cooling water to the evaporator can be adjusted by adjusting the bypass valve provided. When the temperature of jacket cooling water is low, the flow rate to the evaporator needs adjusting. At the same time cooling seawater pressure to the condenser also to be adjusted.

During very low seawater temperatures, evaporation temperature can falls to a lower value. In this case, adjust vacuum valve to control vacuum inside the shell. Then cooling seawater flow rate to the condenser can be reduced to increase the evaporator temperature.

During high seawater temperatures, evaporation temperature can go up. In this case, increase the flow rate of seawater to the condenser to reduce evaporation temperature. High evaporation temperature causes scale formation in the heat exchanger. On the other hand, too low evaporation temperature results in seawater carry over which increases salinity of fresh water produced.

The distillate pump discharge valve to be throttled so that pump cannot run dry. The rate of distillate pump discharge and rate fresh water produced in the condenser should match. When distillate pump is not able to extract the freshwater at the rate of production, level of freshwater increases in the condenser and effective cooling area of the condenser reduces. This results in reduced evaporation quantity.

Question 21

when should the FWG be stopped and what is the procedure for stopping a FWG?

Answer 21

Fresh water Generator Stopping Procedure

When ship approaches port, shallow water, etc. it is desirable to stop the fresh water generator. This is because the seawater may contain harmful bacteria which can enter the freshwater produced. Operation of freshwater should be carried out in consultation with bridge watch keeper.

Following procedure stops fresh water generator.

1) Slowly open bypass valve for main engine jacket cooling water.

2) Ensure that main engine jacket cooling water temperature is within normal limits.

3) Close jacket cooling water inlet and outlet valves for the freshwater generator, normally inlet is closed first then outlet valve

4) Close the feed water treatment valve (chemical dosing valve).

5) Stop the distillate pump and shut discharge valve.

6) Switch off salinity meter.

7) Close filling valve to freshwater tanks.

8) Wait for evaporator shell temperature to drop below 50 deg Celsius.

9) Close the feed water valve to evaporator.

10) Stop ejector pump (SW pump). Shut fresh water generator overboard valve.

11) Open the vacuum valve to make shell side pressure equal to atmospheric pressure.

12) Open the drain valve of the evaporator to drain all the seawater from the fresh water generator.

Question 22

what are Precautions for Operation of Fresh water Generator

Answer 22

Seawater pressure at the inlet of air ejector must be 3 bar or more.

The pressure at ejector outlet should not exceed 0.8 bar.

Never start fresh water generator distillate pump in dry condition.

Operate jacket cooling water valves to the fresh water generator gradually to avoid thermal shock to the main engine.

Feed water to be supplied for a few minutes to cool down the evaporator before stopping.

Never open the drain valve of evaporator before opening vacuum breaker.
Otherwise atmospheric pressure causes seawater inside to hit and damage the deflector.

Question 23

explain how scale formation occurs in a FWG

Answer 23

How Scale Formation Occurs in Fresh Water Generator

The performance of fresh water generator reduces with the formation of scales because of reduction in heat transfer efficiency. Three scales which are normally found in fresh water generators are:

Calcium Carbonate, CaCO3
Magnesium Hydroxide, Mg(OH)2
Calcium Sulphate, CaSO4

Calcium carbonate and magnesium hydroxide scale formation mainly depends on the temperature of operation. Calcium sulphate scale formation depends mainly on the density of the evaporator contents or brine.

If the sea water in the fresh water generator is heated to a temperature below 80 degree Celsius, calcium carbonate scale occurs.

If sea water is heated above 80 degree Celsius, magnesium hydroxide scale is deposited. If the density of evaporator contents is in excess of 96000 ppm, calcium sulphate scales are formed. But fresh water generator brine density is normally 80000 ppm and less. Therefore scale formation due to calcium sulphate is not a problem. Therefore it is recommended to operate fresh water generator at its rated capacity, not more.

More production of water than rated capacity means higher concentration of brine and more scale formation. Similarly higher shell temperatures result in formation of hard scales which will be difficult to remove. All these together will reduce the plant efficiency drastically.

Question 24

Explain How to Minimize Scale Formation in a FWG

Answer 24

Scale formation in fresh water generator can be minimized by continuous chemical treatment. sodium polysulphate with anti foam is often used on ships. Examples of these chemicals include Vaptreat by UNITOR or Ameroyal by DREW CHEMICALS.
These chemicals minimize calcium carbonate scale formation and possibility of foaming. The chemical is non toxic, non acidic, and can be used in fresh water generator producing water for drinking purposes. It would be continuously fed into the feed line using a pump, by gravity or by adding a dose to the expansion tank. Amount of chemical to be dosed depends on the capacity of fresh water produced. Important thing is that this chemical is effective only on low pressure fresh water generators.

Question 25

what are the typical faults that occur in a FWG and what effect do these have

Answer 25

The typical faults that can occur are loss of vacuum or over-pressurisation of FWG Shell, this causes the shell pressure of the fresh water generator to rise and then rate of freshwater produced decreases. This can happen due to large amounts of air loss because of air leaks in the evaporator shell and or failure of air/brine ejector.

Another typical issue is the cooling water flow rate through the condenser is reduced or cooling water temperature is high. This causes temperature and pressure within the condenser to rise.

One other issue that can occur is flow rate of the heating medium is increased and excess water vapour produced. Since this excess vapours cannot be condensed, shell pressure increases or vacuum falls.

Question 26

what is the purpose and primary functions a feed water regulating valve?

Answer 26

Controls the flow of seawater into the fresh water generator. The operation of this valve is finely adjusted to maintain the desired pressure and flow rate, ensuring optimal conditions for the evaporator to produce fresh water. The valve operates based on a pre-set loaded spring that monitors the system’s conditions, adjusting the flow of feed water as needed to maintain stable performance.

Primary functions

Regulating Flow: The valve regulates the flow of seawater, ensuring it doesn’t exceed the system’s capacity or drop below the required feed rate for optimal performance.

Pressure Control: The spring-loaded mechanism allows the valve to adjust according to changes in system pressure, maintaining a steady flow regardless of variations in seawater pressure.

Preventing Overload: In the event of a sudden increase in seawater pressure, the valve can close partially to prevent overloading the fresh water generator.

Question 27

what maintenance is carried out for feed water regulating valve?

Answer 27

Regular Inspection: conduct routine inspections of the valve to check for signs of wear, corrosion, or damage. Any issues should be addressed promptly.

Lubrication: Lubricating the valve components, such as the spindle and seat, ensures smooth operation and reduces friction that can lead to wear.

Cleaning: Over time, the valve may accumulate marine fouling or deposits. Periodic cleaning is necessary to maintain its efficiency.

Calibration: Calibration of the valve is essential to ensure it operates within the specified parameters. This may involve adjusting the pressure settings or flow rates as needed.

Replacement of Parts: As with any mechanical component, parts of the valve may need to be replaced periodically due to wear and tear.

When issues arise with the feed water regulating valve, on-board marine engineers are responsible for diagnosing and addressing the problems promptly.

Pressure Fluctuations: If pressure within the system fluctuates, engineers may need to inspect for leaks, blockages, or a malfunctioning valve.

Inconsistent Flow: Inconsistent feed water flow can be a result of valve wear or a misalignment of components. This requires a careful examination and possible adjustment.

Corrosion and Fouling: Engineers should check for corrosion and fouling regularly. If detected, cleaning and potential replacement of corroded parts are necessary.

Valve Sticking: If the valve sticks, it may not open or close as required. This could be due to debris or wear and may necessitate cleaning or repairs.

Leakage: Leakage is a serious concern and may require immediate action to prevent damage to the equipment and environmental contamination.

Question 28

explain the function and working principle of a Brine/air ejector

Answer 28

Ejector accomplishes its work by transforming the potential energy stored in the pressure of high-pressure motive fluid into velocity. After that, work is done on the suction fluid with high velocity fluid that has been released from a motive nozzle. This work takes place in the diffuser inlet as well as the suction chamber. After then, the leftover energy from the velocity is converted into pressure on the other side of the diffuser. In simpler words, high-pressure motive fluid is utilized to raise the pressure of a fluid that is currently at a pressure that is significantly lower than the pressure of the motive fluid. Pumping effect is achieved by using a medium like gas flow this generates a dynamic pressure drop in the motive nozzle, the resulting negative pressure accelerates the suction medium by impulse transmission in the mixing nozzle and pulls it along

According to the laws of thermodynamics, a high velocity can be attained by adiabatically expanding the motive fluid over the converging/diverging motive nozzle, which lowers the pressure of the motive fluid relative to the pressure of the suction fluid. At the exit of the motor nozzle, the supersonic velocities that are caused by the expansion of the fluid over the nozzle are seen. The converging section of a diffuser reduces velocity as the cross-sectional area is reduced. The diffuser throat is designed to create a normal shock wave. A dramatic increase in pressure occurs as flow across the shock wave goes from supersonic, to sonic at the shock-wave, to subsonic after the shock wave. In a diffuser diverging section, cross-sectional flow area is increased and velocity is further reduced and converted to pressure.

Fluidpressures that are at or above a predetermined minimum have been designed into the system by the manufacturer to ensure that it continues to operate in a steady manner. If the pressure of the supply of motive fluiddrops below what was designed for, then a motive nozzle will let through less fluid. In the event that this occurs, the ejector is not supplied with an adequate amount of energy to compress the suction fluid to the discharge pressure that was designed for it.
It is possible for an ejector to function in an unstable manner if it is not provided with an adequate amount of energy to enable compression to its design discharge pressure. Dramatic shifts in the ejector’s operating pressure are one of the signs of a unstable ejector.

The Brine Ejector is a device that has been developed specifically to be used with seawater in its operation. It is used for brine elimination from the lower side of the FWG shell and for creating and maintaining the necessary vacuum required for FWG to boil the water by using the jacket water temperature of 70 – 90 ºC and shell temperature of 40-60 ºC.

By forcing sea water to pass through the air/brine ejector and sea water supplied by the ejector pump to be delivered to the ejector, the combined air/brine ejector is able to produce an evaporator chamber vacuum condition. This allows the brine (concentrated seawater) and air to be ejected from the evaporator chamber. Ejectors are hydro-pneumatic generators with motor fluid that utilize the transfer of mass and energy between an agent with a high energy potential (pressure) and the working fluid, which has a lower pressure.

The ejectors have a disadvantage in that they produce poor yields, but they have an advantage in that they are highly reliable because there are no moving components in them.

Question 29

what are some reasons as to why an ejector might fail?

Answer 29

Maintenance

Although ejectors are very reliable, it might happen that sometimes it will under perform (poor ejection performance) due different reasons.

These reasons can be:

Lower than designed motive fluid (sea water) pressure – in this case there is something wrong with the sea water ejector pump and need to be investigated (fouled filter, worn impeller, damaged mechanical seal etc.)

Higher than designed motive fluid (sea water) pressure – outboard valve need to be checked, same as outlet piping for any obstacles. If everything is fine the motive fluid nozzle can be replaced with another nozzle designed for a higher pressure.

Damage of the suction chamber – same need to be opened and investigated for any abnormalities (cracks, corrosion, cavitation etc.). It might be temporary repaired, by using bronze putty or ceramic putty, if spare parts are not available.

As part of their maintenance, ejectors have no moving parts in contact with the process fluids and, as such, they are reliable andrequire very little maintenance.

Question 30

with reference to Water storage and distribution system

describe the Design and maintenance requirements

Answer 30

Water storage and distribution system

Design and maintenance requirements

Non contaminating coatings should be used inside storage tanks

Piping should be colour coded and marked clearly with direction of flow

System should be completely separate from non potable water system
Pipelines should not be directed through tanks containing any other liquid

Lines should not be directed through bilges

System should be arranged to avoid ‘dead legs’

FW tanks should be:

Entered only by staff wearing clean overalls

Opened, inspected and if required should be washed prior to refilling

Cleaned with chlorine based wash, filled with chlorinated water and flushed out yearly

Filled with super concentrated freshwater solution and left in place for 12hours during dry dock process which happens every 5 years

Checked for legionella bacteria. Showers should be paid attention to as there part of FW system and are a potential source of bacteria especially if shower has not had frequent use

Question 31

a.) under what conditions is it safe to drink Water made from sea water by a distillation plant

b.) How much drinking water must be available at all times?

Answer 31

a.) Water made from sea water in distillation plant on board is safe to drink under the following conditions:

1. When its boiled at temperature of 75degrees Celsius. Most of the low pressure operating plants operate at temperatures ranging from 40 to 60℃ and therefore these plants are fitted with potable water treatment units like chlorine before the water is discharged to drinking water tanks
2. chemical additives used for diesel engine cooling water must not be harmful, those that aren’t allowed to be added for health reasons are chromate’s. reason being that they are harmful to human health.
3. FWG cant be used within 20 nautical miles from the coastline. According to MCA 20 miles is considered as a safe distance but isn’t always the case because its dependant on the vessels location and in some cases the water is still not considered safe. Therefore a risk assessment should be carried out when FWG has to be used and vessel is within this range or if its close like for instance 21 or 22 nautical miles.

b.) At least 2 days worth of water that is suitable for drinking and use should be available

Question 32

with reference to jacket water cooling system;

what is the function a expansion tank?

Answer 32

1. Expansion tanks are fiitted to a jacket water cooling system and they are designed to allow for thermal expansion of fresh water. As water heats up, it expands, and without a proper means of allowing for this expansion, the system can experience excessive pressure build-up. the expansion tank within a jacket cooling water system acts as a resorvior allowing the fresh water to safely expand and contract without compromising the integrity of the cooling system. When the water expands, it enters the tank, reducing the pressure exerted on other components of the cooling system. This pressure regulation prevents potential damage to pipes, valves and pumps.
2. Cooling water expansion tanks also minimize air within the cooling system. this is done by having the expansion tank equipped with an air vent, allowing trapped air to be released from the system. Removing air prevents airlocks but and enhances the efficiency of heat transfer, as air has a lower heat capacity compared to water.
3. Expansion tanks also have filtration systems that prevent contaminants from entering the jacket cooling water system.

Question 33

what kind of maintenance is preformed on expansion tanks

Answer 33

1 Regular Inspections: Regular inspections of cooling water expansion tanks are necessary to identify any signs of wear, damage, or leaks. Inspect the tank’s exterior for physical damage such as dents or corrosion. Additionally, check the tank’s connections, including inlet and outlet pipes, for any signs of leakage. Early detection of issues can help prevent costly repairs or potential system failures.

2. Pressure Monitoring: Monitoring the pressure within the cooling water expansion tank is crucial for its proper operation. Utilize pressure gauges installed on the system to ensure the pressure remains within the recommended range. If the pressure consistently exceeds or falls below the recommended levels, it may indicate an underlying issue in the cooling system that requires investigation and correction.
3. Ventilation and Air Release: Cooling water expansion tanks often incorporate ventilation systems or air vents to remove trapped air from the system. Ensure that the vents are clear and functioning properly to prevent airlocks, which can hinder the cooling system’s performance. Regularly inspect and clean the vents to maintain their effectiveness and promote efficient heat transfer within the system.
4. Water Quality Maintenance: Water quality plays a vital role in the longevity and performance of cooling water expansion tanks. Implement appropriate water treatment methods, such as filtration and chemical treatment, to prevent the accumulation of contaminants that could lead to corrosion or blockages within the tank and the cooling system as a whole. Regularly monitor water quality parameters, such as pH levels and dissolved solids, and perform necessary maintenance and treatment actions based on the results.
5. periodic Flushing and Cleaning: Over time, sediments, debris, and scale may accumulate within the cooling water expansion tank. Regular flushing and cleaning of the tank will help remove these deposits, ensuring optimal performance. Follow manufacturer guidelines and industry best practices when conducting flushing and cleaning procedures, and use appropriate cleaning agents that are compatible with the tank’s material.
6. Maintenance of Tank Supports and Mounting: Cooling water expansion tanks are typically supported by brackets or mounting systems. Periodically inspect these supports to ensure they are secure and in good condition. Any signs of wear, rust, or damage should be addressed promptly to prevent potential tank displacement or failure.

Question 34

what is a jacket cooling water system

Answer 34

Diesel engines burn fuel and as a by-product of the combustion processes, waste heat is produced and exhaust gas boilers (economizer) are used to collect some of the energy that would have been lost as waste heat and put it to use in the production of steam. It is necessary to dissipate the bulk of the heat that is created by combustion in order to avoid the moving parts of the engine, such as the pistons, valves, and associated seals, from overheating and failing. the cooling of the the combustion chamber and the cylinder head is achieved by using the jacket cooling water system, which is a closed cooling system and allows for heat to be disspated. Engine Jacket Cooling Water Systems (JCW) are used for providing process cooling water to engine jackets to prevent damage to the engine. If cooling water comes from distilled water which is circulating in the jacket cooling water system, then scaling is not typically an issue because this water does not include any salt minerals that can cause scale formation.

Question 35

With reference to Jacket Cooling Water System

what are the typical scaling contaminants?

Answer 35

Typical scaling contaminants are:

 Calcium carbonate– scale appears as a pale cream. yellow deposit and is formed by the thermal decomposition of calcium bicarbonate at heat transfer surfaces.

 Magnesium silicate– scale is a rough textured, white deposit found in cooling circuits where sufficient amounts of Magnesium are present in conjunction with adequate amounts of silicate ions. Silicate deposits are also a risk where silicate additives are used as corrosion protection for aluminium metal in the cooling circuit. the silicate is maintained in solution by maintaining a relatively high pH in the cooling circuit (9.5 – 10.5). If this pH is reduced due to ingress of shore-water or sea-water then the silicate will precipitate in the cooling water (as magnesium or calcium silicate) and cause fouling.

 Iron oxidedeposit – This type of red/brown iron oxide deposit is found because of active corrosion occurring somewhere within the cooling system.

 Copper– The presence of copper deposits in a cooling system presents a very serious corrosion risk because of possible galvanic corrosion mechanisms with steel. The steel in the cooling system is anodic to the copper and rapid loss of metal will occur. It is usual to incorporate specific copper corrosion inhibitors.

Question 36

what factors effect corrosion rate?

Answer 36

The factors that effect corrosion rate are:

temperature– the pace of most chemical reactions increases by a factor of two for every 10 degree Celsius when the temperature is raised. Since of this, a rise in temperature will hasten the rate at which corrosion occurs because the processes that take place at the cathode will happen more quickly. Up to about 80 degrees Celsius, there is also an increase in the rate of oxygen diffusion. Because oxygen is less soluble in an open system, the rate of corrosion is beginning to decrease, which is why the solubility of oxygen is decreasing.

pH/alkalinity –as was said earlier, the rates of corrosion that metals experience with regard to pH might differ based on the electrochemical nature of the metal in question. For instance, the rate at which steel corrodes is significantly increased in acidic environments because these environments hinder the production of a protective oxide layer. As the pH increases up to roughly 13, the decreased solubility of iron oxide causes the corrosion rate of mild steel to decrease. This is because iron oxide is more soluble in lower pH levels. Copper behaves in a similar manner. Because of their amphoteric nature, aluminum and zinc both have elevated rates of corrosion when exposed to extremely low or high pH.

chloride ions– at the anodic sites, small negatively charged ions like chloride ions have a tendency to congregate in order to electrochemically balance the positive ions (Fe2+ etc.) that are created as a result of the oxidation of the metal. These ions boost the locally enhanced conductivity, and as a result, the voltage gap between the anode and the cathode. Because of this action, the atmosphere becomes one in which corrosion can advance more quickly. When pollution of sea water has taken place, it is common practice in the maritime environment to make recommendations for increased amounts of corrosion inhibitor.
Stainless steels are the most common materials to experience the phenomenon known as stress corrosion cracking owing to chlorides. The chloride ions in this situation are of a size that allows them to enter the atomic matrix of the metal, and the concentration of these ions speeds up the corrosion process and causes fissures to propagate further in the metal. Failure on a catastrophic scale is frequently the result of corrosion mechanisms of this kind.

galvanic corrosion– when two different types of metals are joined together and then subjected to an environment containing water, a kind of corrosion known as galvanic corrosion can develop. Because of this, what is known as a “galvanic cell” is created when one metal becomes anodic and the other becomes cathodic. When compared to the cathodic metal, the anodic metal is more likely to show signs of corrosion. The most typical manifestation of this kind of corrosion happens when copper and mild steel are joined together in water at the same time. The corrosion of the mild steel occurs as a result of the mild steel’s transformation into an anodic state, which occurs due to its greater propensity than copper to give up electrons.

cavitation– the immediate creation and collapse of vapour bubbles in a liquid that is subject to quick and strong localized pressure changes is an example of cavitation. In essence, this is localized boiling that does not involve an increase in temperature. Cavitation damage occurs when this phenomenon act at the metal surface and the hydrodynamic forces caused by the collapsing vapour bubbles form microscopic “torpedoes” of water, which are capable of reaching speeds of up to 500 meters per second, and when they make contact with a metal surface, they remove the protective oxide layer and deform the metal itself. Cavitation is common in the feed-pumps that are used on steam boiler systems. Additionally, it is becoming more common in engine cooling systems as engine manufacturers move toward producing more compact high efficiency diesel engines with smaller, more compact cooling circuits where the risk of localized boiling is increased. This is because cavitation can cause the water in the cooling circuit to boil more quickly.

corrosion fatigue cracking– this particular kind of corrosion develops as a consequence of a combination of being exposed to an environment that is corrosive and being subjected to recurrent tensile stress. The application of stress in a repetitive and cyclical manner leads to metal fatigue, which, when combined with exposure to a corrosive environment, ultimately results in the metal cracking. These fractures typically manifest themselves as groups of fine to large cracks that run perpendicular to the direction in which the tension is being applied and are filled with thick corrosion product. The rate of damage that occurs as a consequence is higher than it would be if only the stress or corrosion mechanisms were taking place. Any type of metal can experience corrosion fatigue, and it can happen in a wide variety of corrosive situations.

Question 37

explain how corrosion is controlled for jacket cooling water system?

Answer 37

The majority of systems are designed and manufactured in such a way that a chemical inhibitor is required in order to control corrosion. This is the case despite the fact that there are many different options available for minimizing corrosion through improved design, the selection of materials, and improved construction techniques. Any chemical treatment that is put to the water and stops the anodic reaction will halt corrosion, and any inhibitor that prevents the cathodic reaction will minimize corrosion. This applies to both types of treatments.

On the basis of the manner in which they influence the corrosion cell, corrosion inhibitors are divided into the following three categories:

Anodic inhibitors
Cathodic inhibitors
Combination inhibitors/organic inhibitors

Nitrites, which are good anodic corrosion inhibitors for mild steels, are by far the most frequent type of corrosion inhibitor utilized in engine cooling systems. Nitrites cause mild steel surfaces to oxidize, which results in the formation of a layer of corrosion product that is exceedingly thin yet highly tenacious. Because of the very large dose rates that are necessary, nitrites are almost exclusively utilized inside of closed systems due to the economic benefits associated with this application method. Once an appropriate corrosion inhibitor has been chosen and applied to an engine cooling circuit, general testing of the water quality would be carried out to measure the levels of inhibitor, pH and chlorides (as a check for contamination), and hardness salts. This would be done in order to determine whether or not contamination has occurred. Bacteria are uncomplicated forms of life that can be found in practically all of the water that can be found on Earth, which if they are already present in engine cooling circuits have the ability, under certain conditions, to adapt so that they can feed on the nitrite-based compounds that are employed in corrosion inhibitors and also on oils that are introduced into the cooling circuit as impurities. As a result of the bacteria’s actions, nitrite is converted to nitrate, which results in the loss of corrosion inhibition. This circumstance can result in rapid population increases of bacteria, which can then lead to the creation of insulating biofilms on the internal surfaces of the cooling system as well as the obstruction of filters and control devices. Because of the existence of the deposit, the effectiveness of the cooling system will decrease, and there will be an increased danger of corrosion as a result of the depletion of the corrosion inhibitor. Scale deposits produce the same effect. The typical manifestation of this condition is the depletion of the nitrite reserve along with a steady or rising trend in the conductivity of the system. This is because the nitrate that has been generated continues to contribute to conductivity. The high temperatures and pH levels that are generally experienced in engine cooling circuits contribute to the low incidence of microbial fouling that can occur. In the event that microbiological fouling is discovered, a specialized biocide should be used to eliminate the bacteria that are causing the problem and prevent further growth.

Question 38

(a) Make a labelled diagram of a Jacket cooling water system. (8)
(b) State the purpose of the major components shown in the sketch in Q1(a) (8)

Answer 38

a.) SEE EOOW ORAL/IAMI Sketch Pack

b.) drain tank - fitted to avoid losing treated water, used during maintenance

JCFW pump - circulates water through the system into the engine to cool components like the cylinder liners for example.

preheater - used to heat jacket water to normal operating temperature and keep it at this temperature when the engine is in stand still (not working)

FW Coolers - Remove heat from the freshwater by utilising sea water

de-aerating tank - removes oxygen bubbles from the system

FWG - Produces fresh water that is then used in the system

expansion tank - allows for thermal expansion of jacket water.

3 way regulating control valve - keeps the main engine cooling water outlet at a fixed temperature level, independent of the engine load.

Question 39

With reference to a main engine jacket water cooling system:

a) Explain, with the aid of sketch, how the jacket water temperature is maintained at its optimum value; (8)

b) State the possible effects on the engine if the jacket water temperatures are maintained outside the desired value. (8)

Answer 39

SEE EOOW ORAL/IAMI Sketch Pack

a.) A thermostatically controlled regulating valve is located at the inlet or outlet of the jacket water cooler, and keeps the main engine cooling water outlet at a fixed temperature level, independent of the engine load. This valve receives a remote variable set point from engine control system (ECS) which is controlled by a PID (proportional integral derivative) controller. PID controller maintains the parameters according to the desired value set by the engineer. PID controller controls the three-way valve according to the jacket water engine outlet temperature. If the temperature at the outlet is less than the desired value, the controller bypasses the cooler with the use of the 3-way valve by doing so it helps to maintain the jacket water temperature of the engine at the desired value this being between 80 and 85 degrees.

b.) HT/Jacket water cooling systems remove the heat generated within the engine.

• Too high temperature will raise the internal temperature of the components within the engine leading to failure of those components.
• too low a temperature can lead to condensation forming on the inside of the cylinder liners and thereby washing off the protective cylinder lubrication leading to acid attack of the liner and piston ring metal to metal contact with the liner.

Question 40

With reference to Jacket Cooling Water treatment of a diesel engine:

a) state the chemical tests carried out; (6)

b) state the frequency of testing; (2)

c) explain why it is necessary to keep the test results within certain limits(4)

d) state the action to be taken to rectify an abnormal test result. (4)

Answer 40

a.) PH test - carried out to check alkalinity level and acidic level. if there is too much acid or alkaline the jacket water will become corrosive. water can become acidic by mixing with seawater or exhaust gas. if water is transparent this indicates seawater.

chloride test - to check for sea water contamination. sea water contains salts and some salts are hardening and chloride is also corrosive. hardened salts can cause scale formation in engine jacket water cooling pipeline. max value for chloride test is 50ppm.

nitrite test - standby reaction neutralise test. this test is preformed to ensure nitrite level isn’t too low. if it gets too low this will cause corrosion and scale formation therefore nitrate chemical base treatment must be maintained. Jacket water cooling systems are treated with Nitrite based chemicals to prevent corrosion within the pipework and engine casing.

b.) these tests must be carried out weekly or according to C/E orders. refer to manual as well for more guidance and PMS.

c.) too much acid or alkaline the jacket water will become corrosive. if nitrite level is too low this will cause corrosion and scale formation. corrosion and scale formation damages main engine components associated with the jacket water cooling system.

d.) for any of the tests if reading is below the acceptance value according to operational manual and manufacturer guidance then use chemicals provided like liquidewt (drew marine). ensure to add the correct amount to not make jacket water more alkaline.

Question 41

what additives are used for cooling water?

Answer 41

Those generally used are either anti-corrosion oils or inorganic inhibitors. If pistons are water cooled an anti-corrosion oil is recommended as it lubricates parts which have sliding contact. The oil forms an emulsion and part of the oil builds up a thin unbroken film on metal surfaces. This prevents corrosion but is not thick enough to impair heat transfer. Inorganic inhibitors form protective layers on metal surfaces guarding them against corrosion. It is important that the additives used are not harmful if they find their way into drinking water – this is possible if the jacket cooling water is used as a heating medium in a fresh water generator. Emulsion oils and sodium nitrite are both approved additives, but the latter cannot be used if any pipes are galvanised or if any soldered joints exist. Chromates cannot be used if the cooling water is used in a fresh water generator and it is a chemical that must be handled with care.

Question 42

why is it important to treat jacket water and how is this done?

Answer 42

Its just as important to look after the jacket water or otherwise known as diesel engine cooling water. This is because it cools various machinery on the ship and is the feed water for the boiler. Effective cooling is dependant upon efficient transfer of heat through the metal wall of the surfaces surrounding the combustions space to the water circulating on the other side of the metal. If scale or other contaminants are allowed to precipitate out of the water solution and settle on the metal surface, an insulating layer forms and the ability of the water to remove the heat is reduced. In extreme cases the build up of scale, debris or metallic particles can lead to cracking in engine cooling channels. The debris circulating in the water can also effect the condition of other components such as O-rings and reducing their lifetime expectancy.

How the cooling water can have an effect on the main engine

If the cooling water becomes acidic. This can be due to sulphur or nitrogen products or any other products of combustion combining with the water can form acids strong enough to cause corrosion

If oxygen is able to build up in the system then this can combine with the water and corrode the metal in the cooling system

If erosion occurs due to non laminar flow of water through the system. This occurs due to design faults, vibration from machinery or a build up of debris causing turbulence

The jacket water cooling system will cause premature corrosion to engine if it not treated and monitored to stop any of the problems mentioned above from occurring. Its therefore compulsory to carry out regular tests of jacket water.

In addition to this you must ensure the following characteristics are met;

The pH value is stable and is between the values of 7 and 10 at 120 degrees Celsius

Corrosion inhibitors concentration level is adequate

The total dissolved solids (chloride concentration) are within adequate level (50ppm), an increase in this levels is an indicator of sea water contamination

It also important to stop any bacteria from contaminating the system. This is monitored by checking the nitrite level. If this drops without loss of water then an investigation should be carried out. The company that supplies the chemicals for treatment will supply all the equipment necessary for testing and monitoring the water. Monitoring the water can determine the chemical dosage required by observing parameters and then adding more chemical when necessary. However normally manufacturers guidance will outline chemical dosage requirements. Testing of cooling water is done once a week or in accordance with manufacturers and chief engineers guidance. Samples should also be sent ashore for further testing every 3-4 months. For engines that have aluminium components its important to complete the protection process and maintain a low pH value. The modern products in use also form an oxide film on metal surfaces which prevent electrolytic corrosion. Its also important of course to follow manufacturer specifications closely this is for any machinery. With nitrite based protective systems the concentration of nitrite is important. The level must be high enough to build up and maintain a thin protective layer over the metal components. A low water level will lead to water increasing in conductivity and therefore enhancing corrosion. If chemical being added into system is in powder form it should be mixed thoroughly before used in cooling system. if there is a cooling water header tank you must ensure inhibitor is circulated around entire system. When handling any chemical correct PPE must be worn in accordance with chemical specification sheet.

If its required to drain and flush the system to refill with fresh water. This could be because the water is too contaminated and best action is full replacement. When carrying this out entire system should be fully drained and refilled with clean fresh water, that is circulated round the system and heated up to about 60 degrees Celsius to dissolve any grease left over. All necessary valve should be opened to have water flow in every section of system. Check for any leaks. The flushing water is drained from the lowest point in system. If its become necessary to use an acid wash to de-scale a system then you need to keep flushing the system until a neutral pH value is obtained this is recorded at the drainage point. The fresh water is added from the desalinated supply and correct initial charge of chemical added with accordance to manufacturer guidance. Then carefully monitor the system especially for the first 24 hours.

Question 43

what type of cooling water is allowed to be used for cooling the engine?

Answer 43

The cooling water of the engine should be only demineralized (distilled) water with proper treatment, which is necessary for keeping effective cooling and preventing corrosion of the system.

Sea water or fresh water contaminated by sea water even in small amount is not allowed to be used as cooling water of the engine due to high risk of severe corrosion and deposits formation in the system.

Rainwater is heavily contaminated and highly corrosive in general, which is also not recommended as cooling water.
Tap water (drinking water) is not recommended as cooling water due to risk of chalk deposit formation inside the cooling system.

However, if the distilled water, for example from freshwater generator, is not available, tap water may be used as cooling water after softening and some other treatments according to the ingredients.

If test result shows that the contents of cooling water changes suddenly or gradually, the cooling water system should be checked to trace the cause.
Some of the changes may indicate the cause as follows:

Chloride content increasing:

• Check possibility of seawater penetrating into cooling water.
• Check the system which includes sea water, for example freshwater cooler cooled by sea water.

pH value decreasing or sulphate content increasing:

• Check if cooling water is contaminated by exhaust gas.
• Check cylinder head by hydraulic pressure test.

Question 44

what are the two Marine notices that govern the standards of ships water supply and what are the main points contained in these M notices?

Answer 44

There are two M notices that govern the standards of ships water supplies, MGN 525 and MSN 1845

The main points covered in these M Notices are the following

• They contain information on the most effective means of ensuring the safety of the fresh water supply. This is through the use of a risk assessment and management approach that covers the whole process from loading to delivery at the tap and includes a planned maintenance system (PMS).
• information on Fresh Water Safety Plan (FWSP), which should be incorporated into the ship’s planned maintenance system. A FWSP should be based on the following;

 A System assessment which is carried out along with hazard analysis (including an assessment on the source of water loaded onto the ship)

 A Management plan and control measures, (the selection and operation of appropriate treatment processes);

 Monitoring and corrective actions to take in a range of various scenarios. for instance in the event of potable water becoming unfit for human consumption then the tank(s) and distribution system should be drained, super-chlorinated and flushed

Question 45

what is one difference between high pressure distillation plants and low pressure distillation plants?

Answer 45

High pressure distillation and reverse osmosis plants are highly effective in removing micro- organisms and making water sterile. They can therefore be employed as a single treatment so long as they remain effective. They can however be combined with the application of a low level of residual chlorine or other equivalent disinfectant. Low pressure evaporators operate at lower temperatures, this type of plant should be fitted with an automatic disinfecting unit, generally chlorine is used, before it is pumped to the storage tanks.

Question 46

what are some general checks made on a FWG

Answer 46

General Checks

• Through the sight glass provided in the evaporator shell, observe flashing of water.
• Also check for the brine level inside. It should not be too high or too low.
• Shell temperature must be around 50 deg cel.
• Make sure shell vacuum is more than 90% from the vacuum gauge.
• Check seawater inlet and outlet temperature to the condenser.
• Ensure seawater pressure at air ejector inlet more than 3 bars.
• Check for distillate pump pressure and water flow meter.
• Check salinity of fresh water produced.
• Check level and flow of dosing chemical.
• Check ampere of ejector pump and distillate pump motor.
• The salinometer is designed to alarm, automatically shutting the freshwater discharge valve to the storage tank, and dumping the distillate to bilge when maximum salinity is exceeded. It is always best to check the diverter valves are all operating as these can stick due to heat/coating of salts.
• The distillate level in gauge glass should be monitored and always maintained at half gauge glass
  level when the distillate pump is running
• Remember to check the main engine jacket water cooling temperatures once the freshwater
  generator has settled down

Question 47

With the aid of a sketch describe the working principle of a multistage flash type evaporator?

Answer 47

• Multistage stage flash distillation for desalination is carried out in a pressure vessel that is
  divided into numerous sections that have decreasing pressures and temperatures. The
  sections each have a set of coils that have seawater pumped through them. These coils
  serve two purposes: condensing the steam produced by the flash evaporation and
  preheating the seawater to almost the required temperature for distillation.
• The seawater then passes through an inline steam heater, taking it up to a temperature
  between 70ᴼC and 100ᴼC; the seawater now being considered to be brine. Multi-flash
  distillation plant engineers will sometimes reduce this temperature to decrease the
  formation of scale.
• The brine now passes into the different stages and, as it is at a higher temperature than
  the sections, some of it flashes to steam. This is condensed by the seawater coils at the
  top of the sections, with the condensate dropping down onto the collection trays under the
  coils.

This continues to the final stage when the brine is pumped to waste and the collected
distilled water is delivered to storage tanks. There is a salinometer on the discharge line to
the storage tanks that operates a tank isolating valve/dump valve in the event of a high
ppm alarm.

As the brine enters each stage, its temperature is above the boiling point at
the pressure of the stage, and a small fraction of the brine water boils
(“flashes”) to steam