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Oil content monitoring - Working principles & procedure for ship service systems

In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer. The separator was shut down if there was any evidence of oil carry over, but problems with observation occurred due to poor light and accumulation of oily deposits on the inside of the glasses.

Present-day monitors are based on the same principle. However, whilst the eye can register anything from an emulsion to globules of oil a light-sensitive photo-cell detector cannot. Makers may therefore use a sampling and mixing pump to draw a representative sample with a general opaqueness more easily registered by the simple photo-cell monitor. Flow through the sampling chamber is made rapid to reduce deposit on glass lenses. They are easily removed for cleaning.

Bilge or ballast water passing through a sample chamber can be monitored by a strong light shining directly through it and on to a photo-cell (Figure 1). Light reaching the cell decreases with increasing oil content of the water. The effect of this light on the photo-cell compared with that of direct light on the reference cell to the left of the bulb, can be registered on a meter calibrated to show oil content.

Another approach is to register light scattered by oil particles dispersed in the water by the sampling pumps (Figure 2), Light reflected or scattered by any oil particles in the flow, illuminates the scattered light window. This light when compared with the source light increases to a maximum and then decreases with increasing oil content of the flow.

Fibre optic tubes are used in the device shown to convey light from the source and from the scattered light window to the photo-cell. The motor-driven rotating disc with its slot, lets each light shine alternately on the photo-cell and also, by means of switches at the periphery, causes the signals to be passed independently to a comparator device, These two methods briefly described, could be used together to improve accuracy, but they will not distinguish between oil and other particles in the flow.

Methods of checking for oil by chemical test would give better results but take too long in a situation where excess amounts require immediate shut down of the oily water separator.

Monitor for oily water using direct light
Figure 1: Monitor for oily water using direct light

Monitor based on scattered light (courtesy Sofrance)
Figure 2:Monitor based on scattered light (courtesy Sofrance)

Tanker ballast

Sampling and monitoring equipment fitted in the pump room of a tanker can be made safe by using fibre optics to transmit light to and from the sampling chamber (Figure 3). The light source and photo-cell can be situated in the cargo control room together with the control, recording and alarm console. The sampling pump can be fitted in the pumproom to keep the sampling pipe short and so minimize time delay.

For safety the drive motor is fitted in the machinery space, with the shaft passing through a gas-tight seal in the bulkhead. Oil content reading of the discharge is fed into the control computer together with discharge rate and ship's speed to give a permanent record

Seres monitoring system for tanker ballast
Figure 3: Seres monitoring system for tanker ballast

How to deal with tanker slops

Most tanker charter parties provide that slops will be dealt with as the charterers direct. In practice the charterers have two options:

1. to load on top, which is only practicable if the slops are compatible with the loaded cargo. The charterers will pay freight on the slops but are entitled to demand that they are discharged with the cargo at the destination port. The charterers effectively buy the slops at the cost of the freight; or

2. to keep the slops segregated from the loaded cargo, either by arranging for slops to be discharged to shore before loading, or keeping slops on board, but segregated, and paying freight on them plus the deadfreight incurred as a result of a full cargo no longer being carried to the discharge port.

If the charter party gives the charterers the option of dealing with slops as they decide, but the vessel arrives at the loading port with more slops than can be carried (segregated) along with the full contemplated cargo, the owners may be in breach of contract.

Summarized below some of the basic procedure of machinery service systems and equipment :
  1. Ballast arrangements

  2. The ballasting of a vessel which is to proceed without cargo to the loading port is necessary for a safe voyage, sometimes in heavy weather conditions. On arrival at the port the large amount of ballast must be discharged rapidly in readiness for loading....

  3. Cargo ships bilge systems

  4. The essential purpose of a bilge system, is to clear water from the ship's 'dry' compartments, in emergency. The major uses of the system, are for clearing water and oil which accumulates in machinery space bilges as the result of leakage or draining, and when washing down dry cargo holds. The bilge main in the engine room, has connections from dry cargo holds, tunnel and machinery spaces.....

  5. Bilge system layout details

  6. All bilge suctions have screw down non-return valves with strainers or mud boxes at the bilge wells. Oily bilges and purifier sludge tanks have suitable connections for discharge to the oily water separator or ashore. The system is tailored to suit the particular ship......

  7. Domestic water system

  8. Systems using gravity tanks to provide a head for domestic fresh and sanitary water, have long been superseded by schemes where supply pressure is maintained by a cushion of compressed air in the service tanks....

  9. Reverse osmosis

  10. Osmosis is the term used to describe the natural migration of water from one side of a semi-permeable membrane into a solution on the other side. The phenomenon occurs when moisture from the soil passes through the membrane covering of the roots of plants,....
  11. Salinometer features

  12. The condensate or product, if of acceptable quality, is delivered to the appropriate tanks by the distilled water pump. Quality is continuously tested by the salinometer both at start up and during operation. If the device registers an excess of salinity it will dump the product and activate the alarm using its solenoid valves. The product is recirculated in some installations......

  13. Sewage systems

  14. The exact amount of sewage and waste water flow generated on board ship is difficult to quantify. European designers tend to work on the basis of 70 litres/person/day of toilet waste (including flushing water) and about 130-150 litres/person/day of washing water (including baths, laundries, etc.). US authorities suggest that the flow from toilet discharges is as high as 114 litres/person/day with twice this amount of washing water......

  15. Sewage zero discharge system

  16. A retention or holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when the vessel is proceeding on passage at sea, usually beyond the 12 nautical mile limit. ...

  17. Biological sewage treatment

  18. A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process. Basically this consists of oxygenating by bubbling air through or by agitating the surface. ....

  19. Sterilization system

  20. Sterilization by the addition of chlorine, is recommended in Merchant Shipping Notice M1214. A later notice, M1401, states that the Electro-Katadyn process in use since the 1960s, has also been approved. Another problem with distilled water is that having none of the dissolved solids common in fresh water it tastes flat. It also tends to be slightly acidic due to its ready absorption of carbon dioxide (CO2). .....

  21. Treatment of water from shore

  22. There is a risk that water supplied from ashore may contain harmful organisms which can multiply and infect drinking or washing water storage tanks. All water from ashore, whether for drinking or washing purposes, is to be sterilized. When chlorine is used, the dose must be such as to give a concentration of 0.2 ppm....

  23. Water production low pressure evaporator

  24. A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler. Steamship consumption for the propulsion plant and hotel services can be as high as 50 tonnes/day.....

  25. Flash evaporator system

  26. The evaporator , boils sea water at the saturation temperature corresponding to the uniform pressure through the evaporation and condensing chambers. With flash evaporators the water is heated in one compartment before being released into a second chamber in which the pressure is substantially lower......

  27. Oil content monitor system

  28. In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer......

  29. Oily water separator

  30. Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges. They also find service when deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation....

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