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A Marine Bilge Water Treatment System

An Allen White Paper

Copyright © 2009 Allen Filters, Inc.

All rights reserved.

Allen Filters, Inc.

P.O. Box 747

Springfield, Missouri 65801

United States

Toll Free: +1 800 865 3208

Phone: +1 417 865 2844

www.allenfiltersinc.com

Contents

4 General

4 Historical Data

5 The Behavior of Emulsions

6 Product Evolution

7 Description of the Treatment System

8 System Components

8 The Inlet Strainer

9 The Pump

9 The Heater

10 The Aero-Jet Mixer

4

General

A ship’s bilge water is a mixture of discharges and leakage from a wide variety of sources, which drain to the lowest compartment (bilge) of a vessel. Bilge water includes seawater accumulation, normal water leakage from machinery, and wash downs. It can also contain a variety of constituents such as solids, cleaning agents, solvents, fuel, lubrica-tion oil, and hydraulic oil.

Vessels can use a variety of bilge water collection systems such as:

Waste oil collection tanks partitioned by baffles into oily and clean sides y

Waste oil collection tanks without partitioning where gravity separation of oil occurs y

The water phase of the separated bilge water is usually pumped overboard as necessary, regardless of the distance from shore. The lower portion of the liquid in the oily water tank can be pumped overboard outside of 50 nautical miles (nmi) from shore.

In the wake of the Exxon Valdez disaster, the Oil Pollution Act amended the Clean Water Act to prevent oil dumping by ships. Oil discharge within 12 nmi from shore must leave no “visible sheen” and measure less than 15 parts per mil-lion (ppm). Beyond 12 nmi from shore, ships may release oily waste that measures less than 100 ppm. The remaining oily waste must be retained on board and disposed of at a shore-based facility. Ships must also record the disposal of oily residues and bilge water.

Historical Data

The following quantities of bilge water were taken from a report by the U.S. Navy Station at Pearl Harbor. They give an indication of annual quantities of bilge water generated by two types of vessels.

Table 1

Vessel Type 1 31,500 gallons (gal.) (119,240 liters (L)) to on-shore facilities

0 gal. (0 L) while transiting within 12 nmi

300,000 gal. (1,135,624 L) outside 12 nmi

Total Generated 331,500 gal. (1,254,864 L) per year, per vessel

38 gal. (144 L) per hour

Table 2

Vessel Type 2 54,000 gal. (204,412 L) to on-shore facilities

80,540 gal. (304,877 L) while transiting within 12 nmi

400,200 gal. (1,514,922 L) outside 12 nmi

Total Generated 534,740 gal. (2,024,211 L) per year, per vessel

61 gal. (231 L) per hour

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The total bilge water discharged by two vessels within the 12 nmi zone was 80,540 gal. (304,877 L).

Assuming that this discharge had an oil concentration of 15 milligrams per liter (mg/L), which is allowable, then the total amount of oil discharged into the oceans was 304,870 L x 15 mg/L = 4,573,045 mg or 4,573 kilograms (kg) of oil or assuming approximately 3 kg/gal. and 42 gal./barrel or 129 kg/barrel this is equivalent to 109 barrels of oil.

If it is assumed that the amount discharged outside 12 nmi was 800,200 gal. (3,029,013 L) with an oil concentra-tion of 100 mg/L (allowable), then the total amount of oil discharge into the oceans was 3,029,013 L x 100 mg/L = 302,901,300 mg or 302,901 kg of oil or approximately 2,404 barrels of oil.

The Behavior of Emulsions

Figure 1: A Close-Up of an Oil-In-Water Emulsion

Bilge water sloshes with the constant motion of the ship. This creates an extremely tight emulsion layer (see figure 1) which floats on top of the water phase. The emulsion consists of a mixture of oil, solids, and water (sea or fresh). The emulsion layer also holds a considerable amount of fine solids. The water phase contains suspended solids and the bilge tank bottom consists of settled solids, sludge, and microorganisms. In the emulsion layer oil droplets are stabilized by the solids that surround them as is shown in figure 2.

Figure 2: Impingement Angles of Solid Particles Onto an Oil Droplet

90°

Water

Oil

Solid Particle

150° 30°

The stable interfacial film which keeps the oil droplets from coalescing is achieved by the densely packed solids in the oil-water interface as is shown in figure 3.

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Figure 3: Solids Film Around an Oil Droplet

Water

Solid Particles

Oil

Product Evolution

The Allen Bilge Water Treatment System (BWTS) has evolved from the Allen Hydroscav Oil Purifier, which has been in production for over two decades. The Hydroscav is used world wide in the oil and gas industry for purifying and reusing rotating equipment lubrication oils. It does so by continuously separating water from oil, evaporating the water, and recovering the dehydrated oil for reuse.

The BWTS is a variation that is designed to handle heavily contaminated oil emulsions and large amounts of water with the advantage that it produces no liquid discharge; the water is evaporated and discharged into the atmosphere. A general assembly of a typical BWTS is shown in figure 4.

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Figure 4: The General Assembly of a Bilge Water Treatment System

Some items and/or dimensions have been omitted for clarity.

Description of the Treatment System

The bilge water tank consists of mostly water (sea or fresh), an emulsion layer consisting of a tight mixture of oil and water, and solids in addition to free floating oil.

The BWTS separates the water phase from the oil phase while straining out the solids. The water is evaporated into the atmosphere resulting in a zero liquid discharge. The oil phase is recovered and disposed of in an on-shore facility. It can be sold for burner fuel or further refined. Figure 5 shows a schematic flow diagram of the BWTS system.

The system pumps contents of the bilge tank via a positive displacement gear pump through a self-cleaning auto-matic strainer, which removes solids larger than 100 microns. The fluid is then heated by an immersion heater to a minimum of 250°F (121°C) after which it is pumped through an Aero-Jet Mixer. In the Aero-Jet Mixer the fluid is forced through a narrow orifice with high energy and speed. The oil-water emulsion is sheared by the high-energy input. Atmospheric air, which is drawn in by the Aero-Jet Mixer, is heated by the hot fluid and thoroughly mixed with it. In the subsequent Separator Vessel, the water flashes off as steam while the waste oil is pumped from the bottom of the vessel back to the bilge tank. Repeated circulation through the BWTS will evaporate the water leaving the waste oil to be disposed of later in an on-shore facility. By continuously evaporating the water phase of the bilge emulsion while the ship is at sea, the bilge volume is greatly reduced until only the oil phase remains.

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Figure 5: Flow Diagram of Bilge Water Treatment

Bilge Water Compartment

Strainer

Waste Solids

Heater Gear Pump Aero-Jet Mixer

Air

Separator Vessel

Water Vapor

System Components

The Inlet StrainerThe inlet strainer is a duplex in-line automatic strainer that uses two electric motor-driven rotating screens and rigid scraper bars to remove solids from the surface of the screens when the screens are rotated. A solids collection sump is also provided. Cleaning is initiated when the motor and the flush valve are actuated by the differential pressure switch. One strainer remains on line while the other one is cleaned.

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Figure 6: Inline Automatic Self-Cleaning Strainer

The PumpThe pump is an electric motor-driven gear pump. This type of pump is very reliable and the gears can process a cer-tain amount of solids without damage. The pump has an internal relief valve to prevent over-pressuring the system. A return relief line back to the bilge water compartment can also be provided.

Figure 7: Gear Pump

The HeaterThe heater is a multi-element immersion heater, which is encased in stainless steel and filled with a thermal fluid. This prevents the heating elements from coming in direct contact with the corrosive sea water. When calcium depos-its accumulate on the outside of the thermal fluid casing, it can easily be removed, cleaned, or replaced. The heater is controlled by a high temperature shut-off as well as by a programmable logic controller in the control panel.

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Figure 8 shows the insulated heater with the heating elements encased in a removable canister.

Figure 8: The Insulated Heater

In Out

Flange with Inner Tank (Must be Removable for Cleaning of Outer Surface of Tank)

Heating Elements

Thermal Transfer Fluid

Process Oil

Insulation

Flange with Outer Tank

Flange Holding Heating Elements

The Aero-Jet MixerFigure 9 shows a schematic of the Aero-Jet Mixer. This device imparts a high degree of shear to the bilge water, which breaks the emulsion by rupturing the attractive forces that bind the oil and water in a water-oil emulsion. It also draws in atmospheric air, which is mixed with the hot bilge water. The high degree of shear causes intimate mixing of the oil, water, and air.

The pressurized, heated mixture is depressurized in the Separator Vessel where the water flashes off as steam to be vented to the atmosphere and the waste oil is returned to the bilge water compartment.

The water phase of the bilge is thus continuously removed while the oil phase is returned to the bilge tank for sub-sequent on-shore disposal.

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Figure 9: A Schematic of the Aero-Jet Mixer

Jet nozzle in which oil is injected at a high velocity and laminar flow

Air is drawn in by the vacuum effect created by the oil jet

Primary spiral flow pattern mixing area where extreme turbulence creates an intimate mixing of oil, water, and gases

Point of tangential injection into the separator vessel for final separation of oil and water-gas vapor

Secondary mixing area where there is a gradual decay of turbulence moving towards final separation

Figure 10: A Typical Explosion-Proof Hydroscav Oil Purifier