Freshwater Generation on ships

Ever wondered how the seafarer's get the freshwater onboard the ships? It intrigued me when I was about to join this career. Many of us have watched the movies where protagonist searches for drinking water and finds it empty. Well all my questions are now answered, after I studied about the Freshwater generation plant on board the vessels. 

Freshwater Generator on ships works on a very basic principle. When we evaporate Sea water, it produces vapors, which can be cooled to produce distilled water. When added with required salts and minerals, it is fit for consumption. A typical freshwater must therefore contain an evaporator, a condenser, a mineralizer and a unit to check the consumption fitness of the produced water. 

Before we go in depth study of the freshwater generator, lets be aware with the basic line diagram which will facilitate our learning. 

Let's understand the whole working principle of the FWG system:

• Firstly the sea water is taken from the sea chest, and pumped through an ejector pump (explained later). The sea water in this state is at ambient sea water temperature which can be used as a cooling medium, thus, to increase the overall efficiency this sea water goes into the condenser unit of the Freshwater Generator. This causes the seawater to be in a preheated situation and also the water vapor coming from evaporator to cool down into distillate form. 

• The hot sea water coming out from the condenser unit, is sent into the evaporator unit, and is also sent to the Air/Brine Ejector (Ejector pump) as the main working fluid. While sending it into the evaporator unit we pass it through the orifice plate which, fixes the feed rate of the sea water going into the FWG.

• Now the preheated sea water is in the Evaporator unit. We have to evaporate the sea water in order to produce the distillate, thus we need a heating medium. To increase overall efficiency the Main Engine Jacket Cooling water (JCW) system provides for the heating medium of the evaporator unit. 

• Here one thing is very important to understand. The JCW system has freshwater at 80 degree Centigrade. To boil the sea water, we need either to elevate the temperature or decrease the boiling point. To decrease the boiling point we remove air from the unit using the Air/Brine Ejector. You can see a line coming from the top connected to the Ejector pump. 

• Thus vacuum is created inside the Freshwater generator, which causes the sea water to boil at a reduced temperature of 80 degree centigrade. Now the vapors, go up through the Demister, and the left salty mixture (Brine) is settled in the bottom of the Evaporator unit. Demister ensures that no salts by any means go into the condenser unit from the evaporator unit to ensure so, it restricts the salt and only allows the vapor to go from evaporator to condenser. 

• Now the vapor in the condenser exchanges heat with the sea water (as explained above), and the distilled water is formed in between the plates of the heat exchanger. The distillate is then taken out using a distillate pump which pumps out the distilled water prepared inside the generator. ***

• After this additional salts and minerals are added into the distillate to make it suitable for consumption in a Mineralizer. Following which the obtained freshwater is sent to the Freshwater Tank and is ready for consumption. 

*** SALINOMETER 

Salinometer is a device which keeps a check on the salinity levels of the distillate produced. The reading in the Salinometer is kept below 10 ppm. Usually the Salinometer is placed before mineralizer unit and after the distillate pump. As shown in the daigram:

Salinometer

Some Important Topics: 

1. What is an Ejector Pump?
2. Why do we have a safety relief valve and a air purge valve at the top?
3. Do we have any Pressure or Temperature sensors on FWG? If so, why?

Let's answer them one by one. 

Ans 1) Ejector pump basically works on the principle of Venturi effect. A high velocity fluid moving through a small cross sectional region, creates a vacuum which can be used to create a suction from a pipe attached in the same region. High velocity of working fluid creates an area of low pressure near the nozzle which creates a suction in the inlet region. 

A simple Ejector Pump 

Ans 2) The production of vapors and vacuum inside the FWG creates a region of variable pressure, which exposes a risk of rupture and crumbling under the pressure. A safety valve is necessary to remove excess pressure from inside of the FWG, similarly an air purging valve is use to fill the FWG with the air from atmosphere. 

Ans 3) The FWG is also equipped with Pressure and Temperature sensing units, which continuously monitors the abnormal growth of pressure and temperature in the FWG unit. 

HOW TO BATTLE SCALING IN FWG....?

One of the major problem in the FWG is formation of scales, which can minimize the heat transfer thus decreasing the overall efficiency of the system, and also result in less water production. Major scales forming compounds in FWG are: 

• CaCO3
• Mg(OH)2
• CaSO4

To minimize the scale formation it is advised to run the FWG on its rated condition. Chemical treatment and regular inspection and maintenance reduces chances of Fouling and scaling inside the tubes in FWG. 

I've tried to keep it short and cover all the topics in the blog. Please ask if I need to add anything....

Thank you for reading, Have a great day..!

ENTANGLED: GHOST GEAR

Ghost nets are the discarded nets in the ocean left accidentally or intentionally. The ghost gear includes the fishing nets, pots, traps, fishing lines, hooks and ropes. The problem lies in the fact that it is in a significant amount in the seabed. The problem with the ghost gear is that it not only lasts long since it is made from plastics, it also keeps doing its operation for which it was made, that is catching aquatic life. 

Image source: Canadian Geographic

According to WAP (World Animals Protection) committee 1 lakh whales, dolphins, seals are stuck to death or injuries in these gears per year. The UN estimates that the discharge of this ghost gear is still increasing at an alarming rate of 640,000 tonnes per year, which is roughly around 1 ton per minute. 

This heavy discharge ensures the fact that the 10 % of the total plastic waste in the ocean is the fishing gears lost or discharged. Loss to the marine environment can be estimated by the fact that in 2018  more than 300 turtles were killed stuck in the fishing gears in Mexico, the periphery of the USA. 

Previous Symposiums 

The issue is not new. Archives show that the first ever talk was done nearly 35 years ago in the 16th session of FAL in 1985. Till then, a lot of NGOs and local communities have taken their shoulders up for the change. The problem with the ghost gear is pretty simple to understand. The plastic content enables the gear to last long (approximately 600 years and then decomposing into microplastics) and also it continues to catch up the fishes and other aquatic life. 

The cycle is pretty simple to understand. A small fish gets caught in the net attracting big fishes and thus a very huge fish ends up being caught in the ghost nets and ropes. Which gives them either death from suffocation or everlasting injuries. Gill nets are the worst in these scenarios. They are designed to catch the floating fish’s head or gill area. When left in the ocean this wall of drift net or gill net catches up the fish or turtles which either kills them by fatigue of dragging the net or extreme wound infections. 

In 2019, extremely endangered species North Atlantic Right Whale was killed due to these nets and thus in the US and several other countries around the world have banned the production of these drift nets. 

North Atlantic Right whale entangled in Ghost gear 
Image courtesy: Radio Canada International

What needs to be done

Semi-Biodegradable and Bio friendly fishing gears: 

This particular solution can not stop the nets from hunting the marine life but it can shorten the lifespan of the nets. The semi-biodegradable nets are basically composite materials and have some bio friendly polymers composition. The composition is some synthetic polymers polybutylene succinate (PBS) resin or polybutylene adipate-co-terephthalate (PBAT) resin. The semi biodegradable quality enables the economic feasibility of the fishing gears and thus helps in degradation of the fishing gears at the sea bed. 

Providing Incentives: 

One of the best ways is to provide the incentives to the fishing communities to bring back the damaged cables or nets and other items which are thrown overboard to amplify the Ghost fishing. Similar approaches have been applied in South Korea from 2003, the ghost nets are received and money is given hand to hand as per the weight of the ghost cable.  

Image Source: Olive Ridley Project

Segregation and cleansing 

Governments across the world must provide the maritime communities, some incentives weather in the form of money or any other goodies so as to create an awareness among the people associated with the maritime world like fishing communities and diving communities to collect those plastics which they find in the ocean. Nearly 2 to 3 years ago in a movie presented by oceanweek.eu presented the fact that fishing communities many a times encounter with the ghost gear and when approximated it comes to be in 60 to 40 ratio, later being plastic items. The fishers thus can be educated and provided those important incentives to bring back that 40%, as it also makes a change. 

Another idea is to launch attractive programs as South Korea launched back in 2003 the “Derelict gear buyback program” or “Marine debris buyback program”  in which the government provided fishers the money in return of the ghost gear they provided. Similar programs can be launched for all kinds of marine plastic debris. Not only this, a separate program launched by NOAA allows the fishers to deposit their discarded ghost gear to the ports which accept them at no cost and thus facilitates the fishers to deposit that material which could be a pollution in upcoming years.  

(Sources : The Hindu, Insider, The Dodo, National Geographic)

Machinery space bilge management

One of the most biggest challenge for a marine engineer is to manage the machinery space bilges on a vessel. This involves stringent regulations which needs to be followed to prevent the pollution from the oil. To understand the bilge management we should have a clear idea some basic questions which we will try to answer through this page:

• What is Bilge?
• What is Oily water separator?
• Use of Incinerator 
• What are Bilge wells ?
• What is Bilge holding tank, how it is different from Bilge wells?
• What is sludge and how it is different from bilges?

Let's start with bilges. Machinery space has a number of oils involved at various places to serve in different manners. All these oils are used and circulated to account for the overall economic operation of the vessel. But, due to some leakages and discrepancies in the system, if the oil leaks from the lines and other designated places it comes down in tanks called as Bilge wells. 

Usually the bilge wells are arranged in 4 corners of the machinery space. as given in the figure. This increases the chance of the leaked oil to end up in the bilge well. Naturally, the bilge wells are the most lowest tanks in the machinery space. 

Also, the bilge doesn't only contains oil, a major portion is water, which also leaks from cooling systems and pipelines. 

So the bottom line is, Bilge is the accumulated leaked water and oil, from the machinery space. 

MACHINERY SPACE BILGE SYSTEM 

Given below is a simplified version of the bilge system. Let's comprehend the full cycle. 

Machinery Space Bilge Pipeline Layout
(Simplified)

• So the oil and water coming from the machinery space fills up the bilge wells. Whenever the bilge wells are fully filled up the high level bilge alarms glows which is a bad sign for a marine engineer specially when near a port since there are restrictions, as we can not pump the bilges directly overboard. Its a crime. So what do we do, when the bilge wells are topped up? 

• We open the Bilge pumps, to pump out the bilges out of the wells, which goes into the Decanting tank. Decanting tank as the name suggests, separates the water and the oil, as oil floats over water and water stays in the bottom area. The water from the Decanting tank goes into the Bilge holding tank, whereas the oil is directly put into sludge tank. 

• Bilge holding tank also contains water and oil as a mixture. The rules for the overboard discharge of bilge water is, The oil content in the bilge must be less than 15 ppm, to ensure which we have a oily water separator (OWS) in the vessels. The bilge water from the bilge holding tanks is treated in the oily water separator, which separates the water from the oil, and a mechanism to check the content of oil in the water coming out from OWS keeps a check that the water sent overboard has oil content less that 15 ppm. To understand working of OWS click here. 

• The oil coming out from the OWS gets into the sludge tank. The sludge is a left over mixture of oil with a little water content. Sludge can't be burnt to produce enough power to propel the vessel or to provide power. Thus it is burnt in the incinerator and is smoke is discharged through the funnel. 

In this manner the cycle is completed. 

DISCHARGE REGULATIONS 

The discharge of bilges directly into the oceans is prohibited, from the MARPOL Annexure 1. Following conditions must be met before discharging the bilge overboard. 

• The ship is en-route. This means that ship is moving between established point A to another established point B. It can not discharge while moving in a non fixed voyage or while at anchorage or zero speed.

• The oily mixture is filtered through an oil filtering equipment as proposed in regulation 14 of the annex 1. Regulation 14 Annexure 1, puts an upper limit to the concentration of the oil in the bilge which is up to 15 ppm. That is the oil content should not be more than 15 ppm. 

• The oil mixture should not be from or should not be mixed with cargo pump room bilges and oil cargo residues. That is, we can not mix the cargo p/p room bilges with the bilge from machinery space, also, the residual oil left in the oil tanker tank after tank cleaning can not be sent overboard through the machinery bilge system. 

Other regulations are:

• The discharge from the vessel should not contain traces of chemicals or other substances in any quantities. Any chemical could result in environmental hazards. 

• The oil residues which are prohibited to discharge must be kept onboard stored in a proper manner, which may be discharged at the port reception facility. 

I have tried my best to keep it short and comprehensive at the same time. If you want something to add or if you have any questions, please comment below. Thank you for reading...!