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...!