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Marine bilge drainage system

26.02.2021

Marine bilge drainage system

In order to keep the holds dry and pump out excess water, ships must have a so-called drainage system (bilge line).

Where does the water come from in the holds? This can be pipe leaks, rain (if the vessel is open or during loading). A constant source of water is condensation, when warm air comes into contact with a cold surface (the inner surface of the sides, for example, or cold cargo). This is a completely normal situation, the water will be pumped out and will not harm anything.

The engine room usually has its own separate dehumidification system for oily water. The drainage system usually consists of three "lines of defense":

  1. Small pump for pumping out small amounts of water supplied daily. Dirty bilge water with oil ("bilge water") is pumped into the bilge tank, and from there through a separator, after cleaning, overboard. Or it is pumped into a separate settling tank.
  2. Powerful pump for pumping bilge water directly overboard in case of emergency.
  3. Direct pumping of bilge water by the main engine cooling pumps with their enormous capacity. Used in emergency situations.

The drainage system is an important part of the ship's security system, its design and location are regulated by law. So, according to the SOLAS rules, the drainage system, fire extinguishing system and ballast line should be three independent systems that can replace each other if necessary. The drainage system should be periodically checked and the results recorded in the logbook. This is serious.

 

Manual way

To determine the amount of water in the hold, there are two ways - manual and instrumental, using level sensors. But the sea is the sea - despite all the gauges, according to the rules, it is still necessary to regularly check the level manually. This is done manually - a metal measuring tape with a weight at the end (called a “sounding tape”, yes, you guessed it) is lowered into a special tube in the hold (which for some reason is called a "sounding tube") until the weight hits the bottom. The tape is lifted and looked to what level it is wetted, determining the level of the liquid. And to better see the border of the liquid on the tape, a special paste is applied to it, which changes color upon contact with the liquid.

With remote control, a sensor with a float (flood sensor) is placed in a collection (drain) well or other place, which gives a signal if the liquid has risen to this level and the float has floated. This activates an alarm or automatically activates pumpdown.

 

Air bubble method

Accurate level measurements can be made with diaphragm pressure gauges, an electric level gauge, a special radar or an air bubble system - yet another neat invention used on ships. Air is pumped into the tube, immersed to the bottom of the liquid, while the pressure is measured. When the air reaches the end of the tube, overcoming the pressure of the liquid column, and begins to escape, the pressure stops growing. This is the pressure at the bottom; it is used to determine the liquid level.

Will it help?

The capacity of the pumps of the bilge drainage system is from 100 to 300 m 3 / hour. Calculations show that a hole in the side of 5x5 cm at a depth of 5 m will deliver 90 m 3 / hour to the hold , and twice as large in area already 180 m 3 / hour. So there are chances!


What the dehumidification system consists of:

Bilge pumps


They should always be ready, although they can be used for other purposes. Bilge pumps are self-priming (usually piston), the line does not need to be filled with water before use.

Fitting


The dehumidification system uses conventional valves - safety valves, plugs, filters, distributors, etc. Suction lines are connected to the common drain line via the open-close valves. These lines have non-return valves so that water from flooded compartments cannot penetrate into dry ones.

Main hold line


It is located in the engine room and starts from the manifold on the suction side of the pumps. Suction lines from the ship's compartments are connected to the manifold. The bilge line equipment in the engine room includes one obligatory pump and one additional (reserve) pump. The suction pipes of the cooling system pumps of the main engine are also connected here. In the event of a significant leak, these high performance pumps can act as an emergency bilge pump. A manual emergency valve with a large red handwheel that enables emergency pumping is located in the engine room.

Suction lines and collecting chambers

Cargo holds have 4 collection wells, one in each corner of the hold. Each is connected to a separate suction line, which, in turn, is connected to a drying line. Which well collects water depends on the design of the vessel, how it is loaded, and trim.

The bilge, or modular, well consists of two compartments separated by a bulkhead. The bulkhead reaches half or three quarters of the height of the well. The well itself is closed with a lid with small holes, which acts as a coarse filter. As soon as the water inside the well reaches the level of the top of the bulkhead, it begins to flow into the second compartment, where the suction pipe is located. Above the grate there are suction nozzles for emergency pumping out from the cooling pumps.

Ejector

A vacuum is created in the ejector due to the high speed of the water flowing through it. This mechanism can also be used to pump out holds such as chain boxes. The flow of water through the ejector is created by a fire pump that creates a lot of pressure. The bilge water is sucked in and flows overboard together with the ejector water.

Bilge water. Cleaner. Separator

According to MARPOL (International Convention for the Prevention of Pollution from Ships, 1973), bilge water from the engine room cannot be pumped directly overboard without purification. A separator is used to separate the oil that may be in this water. Dirty oil is collected in a separate tank.

Such separators, with an oil-in-water meter and a warning system, are mandatory for installation on ships with a displacement of 1000 GT (Gross Tonnage). Pumped overboard water may contain no more than 15 ppm (parts per million).


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