Module 7 : Oily Water Separators Flashcards
Conditions for discharge
- Proceeding en-route
- Mixture has been processes through oil filtering equipment meeting the requirements of the code
- Oil content does not exceed 15 ppm
- Oil mixture does not originate from cargo pump-room bilges on oil tankers
- Oily mixture is not mixed with oil cargo residues
Special allowances
- When necessary for saving lives, securing the safety of the vessel or to prevent loss of the vessel
- Accident of navigation not due to poor seamanship and where all reasonable actions were taken before and after to minimize the amount of pollutant discharge
- Small and unavoidable loss of oil from underwater machinery
- Controlled and approved scientific research
Pollution Prevention Equipment
Equipment must be approved by a proper authority, receive a type test certificate.
Ships over 400 gross tons must have at least 1 tank to hold oily residue/sludge from oil and fuel purification and from all machinery leakages. Capacity is enough to hold normal amount of discharge for type of machinery and length of voyage. Must have 1 pump capable of discharging oily residues and sludge to a shore reception facility. The piping for this discharge will terminate on the weather deck, will have a stop valve and a standard outlet connection. The pump must also have a stop device located in close proximity to the discharge so that the pump may be stopped in event of leakage from the deck connection or transfer hose.
Oily Water Separation
- The difference in densities of the two liquids. Normally oil is lighter than water therefore it will tend to separate in the upward direction and float on the surface of the water. The larger the difference between the densities the faster the separation will be. Unfortunately, some residual oils are as heavy as or sometimes heavier than water making separation difficult if not impossible.
- The lower the viscosity, the faster the oil will move upwards in the water because there is less resistance to movement. Since temperature affects the viscosity of the oil, OWS’s are normally fitted with heaters to control temperature and consequently improve separation
- Oil particles float in water in globular form. The larger the globule the more gravitational force is applied on it to separate from the water. Conversely small globules will have a slower separation velocity within the water.
Characteristics of OWS
- The separator entry point is usually fairly large to allow for a slow flow rate
- The use of centrifugal pumps imparts high velocity to the oily water mixture causing agitation and end up breaking the oil particles into very small globules. Consequently, it is preferable to fit low capacity positive displacement pumps that do not agitate the mixture. Sometimes, the pump is placed on the out-going side of the separator and does not play a role in disturbing the size of the oil globule.
- High velocity in the pipes and roughness of the pipe interior can cause problems as well. Laminar flow within the system provides better separation efficiency. This type of flow is achieved to varying degrees by numerous design considerations including appropriately sized pipes, low flow resistant valves and suitably sized pumps.
- Overloading the separator can cause an increase in liquid velocity and will be avoided by following the suggested flow rates provided by the separator’s manufacturer.
Coalescing Type OWS (construction
Consists of a cylindrical steel tank fitted with baffles. The tank is coated with a special paint that prevents corrosion and does not inhibit the ability of the separator to function properly. The top of the tank has a solenoid valve that will discharge accumulated oil. Approximately 1/3 of the distance down the tank is the oily bilge water entry and clean water exits from the bottom. Fitted in the lower 2/3 of the tank is a series of inverted plates (usually steel but could be plastic) arranged around a central pipe. Above these plates is affixed a circular plate with two pipes that extend into the upper portion of the separator. A vent valve and two sample valves are fitted into the top of the separator. The vent valve will be used to remove trapped air and the sample valves are fitted to physically determine how much oil is in the top of the separator.
Coalescing Type OWS Operation
Oily bilge mixture enters the separator and flows across and down through the center of the circular plate. The top of the central pipe is capped off so that water much travel to the periphery and down through the baffles.
The baffles are spaced closely together and as the mixture passes through them, small oil globules separate out. Since oil globules tend to float towards the surface, the upward sloping baffles will collect small oil globules on their undersides. As the oil globules are collected on the bottom side of the baffles, they coalesce with other globules to form larger particles. These larger particles can now move more quickly along the baffle in an attempt to reach the top of the separator.
The oil globules now travel to the outside of the separator and up into the space below the top plate. The accumulated oil travels up through the two pipes and into the upper portion of the separator.
Clean water travels out the bottom of the separator.
As oil accumulates in the top of the separator, the interface between water and oil starts to lower. If this interface gets too low there is a risk of the separated oil ending up in the baffles and going to discharge. Fitting into the top of the separator is a capacitance probe. This is hooked into the control system and is used to measure the location of the interface. When the interface reaches a predetermined level, the solenoid valve is energized and oil is dumped to the waste oil tank
Sigma OWS
Consists of a single tank in which several oil separation zones are created. The tank is square in shape and manufactured from steel coated to resist corrosion. Located in the lower portion of the separator are 2 square filters containing beads. The beads attract oil and repel water and are commonly made of a special plastic. A baffle is fitted in the top part of the separator to direct inlet water to the perifery of the tank. The baffle is commonly made of plastic. A dam is fitted around the inlet pipe to aid in separation.
A progressive cavity pump is fitted to the water discharge pipe of the unit so the separator operates in a vacuum.
Water enters the separator through a non-return valve and the inlet baffle redirects the flow of water away from the upper bed, providing an extended retention time for the water in the upper portion of the tan and allowing the larger droplets of oil to separate from the oily water mixture. The inlet weird separates the accumulated oil from the inlet flow to prevent re-entrainment of the oil.
Oil droplets accumulate on the beads in the beds. As the oil droplets grow in size, they become more buoyant and break away collecting in the upper portion of the tank. The lower coalescer bed functions like the upper one but it is thinner and operates as a polishing filter. The water space between the beds separates them and prevents contamination of the lower bed by the upper one.
Backflushing:
1. When a separated batch of oil accumulates in the upper portion of the tank, it is detected by a capacitance probe
2. The pump stops, which stops the discharge overboard
3. There is a tee fitted between the separator and the pump, which is used as an inlet for clean water from the ship’s domestic water system. A solenoid valve is activated which allows clean water to enter at the tank bottom backflushing the coalescer beds and displacing the accumulated oil to a collection tank
4. Once the oil has discharged and clean water contacts the correct level of the interface sensor, the backflush stops
5. The pump restarts and discharge overboard resumes
2 Stage OWS
In order to receive the desired level, a second stage is fitted to coalescing separators.
The 2nd stage is a polishing filter that will remove fine particles of oil and dirt from the water.
It consists of a cylindrical tank fitting with 2 fine mesh filters, normally composed of plastic or fiber. Discharge water from the first stage enters the 2nd stage where it passes through a filter element. Any dirt or debris in the water is trapped by the filter element, and the material further promotes coalescing of the oi. Oil collects in a space above the first element where it can be manually or automatically drained off.
The water then passes to a second element where the process is repeated. Oil from this filter accumulates in the top of the tank where it is drained by the same method. Clean water passes to the outlet and overboard.
Oil content meter
Continuously monitors how much oil is in the water that is pumped out of the discharge line and will not allow the oil concentration to exceed 15 ppm. If this happens, an alarm will sound and fluid is recirculated back to the bilge or the oily bilge tank until the water reaches an acceptable level or the system is shut down.
The OCM uses light beams to determine how much oil is in the product water. A light source is fitted adjacent to a clear glass tube. On the opposite side of the tube iss a photocell and product water travels continuously through the tube.
The meter will gauge the oil concentration present in the product water based on the intensity of the light hitting the photocell. The more oil in the product water, the more opaque it will be, the less light will hit the photocell. Eventually a threshold will be reached that sends the sensor into alarm. Modern oil content meters have data logging systems that can store the concentration for more than 18 months.
Sometimes the OCM will give a false reading and this is caused by the glass tube being fouled. Clean water is run through the sensor cell or it may be cleaned with a brush. Some products like soap can also give false readings. Any products used in the machinery space must be OWS friendly.
