Fresh Water Generators.docx

BERNHARD SCHULTEDSHIPMANAGEMENT CO. LTD CYPRUS M/T PROTEO IMO NUMBER: 9038830 CALL SIGN: 3FCV3

REVISED BY:

ELABORATED BY:

Carlos Diaz

Leonardo Araujo

Second Engineer

Engine Cadet

OCTOBER, 2019

INDEX

INTRODUCTION .................................................................................................. 3 FRESH WATER GENERATOR ............................................................................ 4 1.

TECHNICAL SPECIFICATION ................................................................ 4

2.

COMPONENTS OF THE DISTILLER ...................................................... 5

3.

PRINCIPLE OF OPERATION .................................................................... 5

4.

MAINTENANCE OF EVAPORATOR SECTION..................................... 8

5.

ASSEMBLY SCHEME FOR PLATE CHANNELS ................................... 9

CONCLUSIONS ................................................................................................... 12 BIBLIOGRAPHY ................................................................................................. 13

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INTRODUCTION On board merchant vessels, the generation of fresh water is produced by a freshwater distiller, which evaporates sea water by means of heat transfer using hot water from the main engine or steam, and then condensates it to generate freshwater, it is a very complex process and very important for all Engine officers onboard, the fresh water produced is used for various processes and it is a key element for the function of the vessel and for human consumption. The distiller uses plate heat exchangers in the evaporator and the condenser; those plates must be clean every two months approximately and in the process of the overhaul it s a must to know the correct assembly of the plates and their correct positions, due to the fact that if there is a mistake in the assembly the distiller will not work properly. Another important points when it comes to the distiller’s working conditions are to keep the parameters of the distiller in the correct status, such as: vacuum, temperature of the heating medium, content of salt in the outlet water, temperature in the casing, the water level inside the casing, etc… so it is essential for the engine officer in charge of the distiller be aware of all theses parameters and supply the proper conditions to the machine. Knowing the process of evaporation of sea water and the consequent condensation is important for keeping the freshwater distiller working properly and in good conditions, and also its maintenance.

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FRESH WATER GENERATOR 1. TECHNICAL SPECIFICATION The fresh water generator used on board MV/SS MT PROTEO is made by ALFA LAVAL DESALT company, there are two distillers on board, one works while the other one is in stand by condition, the technical specification are described ALFA LAVAL DESALT company: Table 1. Distillers’ technical specification Type

JWSP-26-C100

Freshwater production per 24 hours

30 m3

Heating medium: jacket water temp. min

78 ºC

Heating medium: steam of 4,0 kp/cm2

1.400 kg/h

Cooling medium: seawater temp. max 32 ºC

73 m3/h

Net weight empty

738 Kg

Weight in operation

838 Kg

Source: Instruction Manual for Plant Type JWSP-26-C100

In the current time, the distiller cannot reach 30 m3 Freshwater production per 24 hours, due to the prolonged time that the machine has been on service (around 20 years). It is important to empathize that according to the Safety Instructions of the maker, the distiller is not to be operated in polluted water or within 20 miles from the coast, and freshwater must not be produced from the polluted water, as the produced water will be unsuitable for human consumption

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2. COMPONENTS OF THE DISTILLER

The distiller consists of the following main components: 1. Separator Vessel. Separates the produced water from the brine 2. Evaporator Section. Consists of a pleat heat exchanger, and is enclosed in the separator vessel. 3. Condenser Section. Consists of a pleat heat exchanger, and is enclosed in the separator vessel. 4. Combined Brine/Air Ejector.

Extracts air and brine from the separator

vessel 5. Combined Ejector/Cooling Water pump. Supplies sea water for the condenser, jet water for the combined brine air ejector and feed water for evaporation. 6. Freshwater Extraction/transfer Pump. The pump extracts the produced water from the condenser, and transfers same to the freshwater tank. 7. Salinometer. The Salinometer checks continuously the salinity of the produced water. 8. Electric Panel. Contains starters for the electric motors and terminals for the Salinometer.

3. PRINCIPLE OF OPERATION The boiling point of the seawater is descended from 100 ºC to approximately 70 ºC by means of a vacuum inside the distiller, this is very important because there is no need to supply such amount of energy to the distiller to get 100 ºC for evaporating the feed water or sea water. The vacuum is reached by the Bernoulli principle, which states that a region of fast flowing fluid exerts lower pressure on its surroundings. The vacuum necessary for the evaporation is established and maintained by the combined brine/air ejector, which is driven by the Combined Ejector/Cooling

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Water pump, on board there are two pumps (located in the floor deck) which have this function. The vacuum in the distiller is measured in percentage from 0 % to – 100 % (1 bar to -1 bar), the normal status is under – 90 % or - 0,9 bar (Kg/cm2). The process stars when the feed water enter the evaporator section through an orifice, and distributes itself into every second plate channel (evaporation channels). The jacket water or steam (heating medium) distributes itself into the remaining channels thus transferring its heat to the feed water in the evaporation channels. Having reached the boiling point temperature the feed water undergoes a partial evaporation. The mixture of generated steam and brine enters the separator vessel, where the brine is separated from the steam. The brine is extracted from the distiller by Combined Brine/Air Ejector. Image 1. Aft view of the distiller

Source: Instruction Manual for Plant Type JWSP-26-C100

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The Steam goes through a demister (filter) the steam enters every second plate channel in the condenser section (condensation channels). The demister can be showed in the next picture. Image 2. Components of the Distiller

Source: Instruction Manual for Plant Type JWSP-26-C100 The last picture represents an external and internal view of the distiller, some of the main components of the Distiller can be showed, the Evaporator Section is in the low part while the Condenser Section is higher. The seawater supplied by Combined Ejector/Cooling Water pump distributes itself into the remaining channels (condensation channels) thus absorbing the heat transferred from the steam during the condensation. The steam changes into liquid water, the produced fresh water is extracted and transferred to the Fresh Water Tank by the Freshwater Extraction/transfer Pump. On board there are two fresh Water tanks (port side and starboard side), each one have the capacity of 185,8 m 3 (Total capacity 371,6 m3) On the delivery side of the pump an electrode unit is fitted, which together with the Salinometer continuously checks the salt content of the produced water, on board the alarm for maximum content of salt is activated at 9 ppm (mg/L) of salt. According to the Instruction Manual of the Distiller, the salt content of the produced water is so low that it meets all common requirements on boiler feed

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water. A solenoid valve is activated when the salt content is above 9 ppm and the produced water is rejected.

4. MAINTENANCE OF EVAPORATOR SECTION According to the Distiller’s Instruction Manual the overhaul interval for the evaporator section is every 8.000 operating hours (approximately once a year), but in the actual condition of the distiller, there are not chemicals to be used for conditioning feed water, that’s why the evaporator section gets unsuitable for working in a very short interval of time. The actual interval is around 700 operating hours. The procedure for maintenance of evaporator section is described as: 1- Remove bolts in front cover, and open. 2- Loosen 6 nuts in plate stack shifting between them, so that no not is carrying the entire load alone 3- Remove plate stack 4- Submerge plates completely in a hot, inhibited acid bath at maximum 50 ºC for 24 hours. 5- Examine plates and gaskets for possible damage, and remove damaged plates and/or replace damaged gaskets. 6- Reassemble evaporator section in accordance with assembly scheme. 7- Pressure test evaporator section before closing front cover in order to identify a leak. In the current time the acid used as a cleaning agent is called DESCALEX, its composition is Sulphamic Acid in a concentration between 60-100%, it is a very irritating acid. It is a powder that is mixed with water which the plates are submerged. It is very important every time this acid is used avoid any contact with eyes or skin, the correct PPE such as: gloves, goggles and suit must be used.

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5. ASSEMBLY SCHEME FOR PLATE CHANNELS The correct assembly scheme for plate channel in the evaporator section is described in the Distiller’s Instruction Manual. The plate channels for the evaporator section corresponds to the E plates, by the other hand the plate channels for the condenser section corresponds to the K plates. In the next picture you can see the correct assembly scheme for each one of the sections, you can tell that each one of the plates has a different direction for the lines, the lines are opposite so the heat transfer can be incited.

Image 3. Correct assembly scheme for condenser and evaporator sections

Source: Instruction Manual for Plant Type JWSP-26-C100

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According to the picture the first piece to be fitted in the condenser section is the plate stack followed by the KE, KD, KV, KD, KV… KS. The same assembly scheme is fitted for the evaporator section EE, ED, EV, ED, EV… ES. Table 2. Types of plates Type

Evaporator

Condenser

Section

Section

Start Plate

ES

KS

End Plate

EE

KE

Water Plate

EV

KV

Steam Plate

ED

KD

Source: Instruction Manual for Plant Type JWSP-26-C100 In the next picture you can see the correct positions for the gaskets in every plate channel, so if any of the gaskets gets damaged the correct position must be used for the fitting of the new gasket. It is important to tell that the EV plates have a small orifice which allows the inlet of feed water. It is a must to meet the correct position of the gaskets to avoid any mixture of seawater and the heating medium in the evaporator section, and also to evade any mixture of seawater and produced water in the condenser section. Image 4. Position of the Gaskets

Source: Instruction Manual for Plant Type JWSP-26-C100

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Another important fact to be metioned is that the plate channels of both sections are made out of titanium, so they can be very resistant to the corrosion of seawater.

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CONCLUSIONS The distiller produces fresh water from sea water by an evaporator section (which evaporates sea water) and a condenser section (which condensates steam into fresh water). The distiller can use jacket water or steam as a heating medium in the evaporator section. The boiling point of the seawater is descended from 100 ºC to approximately 70 ºC by means of a vacuum which is reached by the Bernoulli principle, which states that a region of fast flowing fluid exerts lower pressure on its surroundings. The vacuum is established by the Combined Ejector/Cooling Water pump. The exchanging of heat in each of the section is produced in plate channels which are made out of titanium, those plate channels have lines which are opposite so the heat transfer can be incited. If a gasket of any of the plate channels are damaged and must be changed, it is important to meet the correct position of the gaskets to avoid any mixture of seawater and the heating medium (evaporator section) or seawater and produced water (condenser section). The Salinometer continuously checks the salt content of the produced water, on board the alarm for maximum content of salt is activated at 9 ppm (mg/L) of salt. The overhaul interval for the evaporator section is every 8.000 operating hours, but in the actual condition of the distiller, there are not chemicals to be used for conditioning feed water, that’s why the evaporator section gets unsuitable for working in a very short interval of time. The actual interval is around 700 operating hours.

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BIBLIOGRAPHY

Instruction Manual for NIREX FRESHWATER DISTILLER Type JWSP26-C100. Maker: Alfa Laval Desalt. 1992

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