Purifiers clean fuels and lubricants as well as processing bilge water and sludge, but onboard vessels they are mainly part of fuel and oil treatment system. Purifiers are usually under a junior engineer responsibility and, sometimes they proves to be real nightmares for some of them, mainly because of lack of knowledge regarding operating principle, lack of confidence and negligence.
Marine purifiers use separation technology involving a disk stack separator, known as a centrifuge.
This is used for centrifugation in which various phases of solids and liquids are isolated from one another based on difference of densities.
Similar to a settling tank, disk stack separators use the force of gravity to separate liquid with a specific density from other liquids and solids. Unlike a settling tank which uses retention time as the main parameter for liquids of different densities to divide into layers of and solids to precipitate to the tank, centrifugal separators use mechanical force to separate liquids and solids with different densities from each other. In an essence a disk stack separator is a settling tank who’s base is wrapped around the center line of the bowl. Conical plates are stacked, one on top of the other, inside the bowl, to provide more separation area and to increase separation efficiency. These stacks dramatically speed up the precipitation of the solids from the liquids.
Onboard vessels usually there are two or three centrifugal self cleaning Heavy Fuel Oil purifiers fitted (depending on the Main Engine size and fuel consumption), a Diesel Oil purifier, a Main Engine Lube Oil purifier and one or two Auxiliary engines Lube Oil purifiers. One of the HFO purifiers can be used as DO purifier as well, in case of emergency or when is needed. Normally one HFO purifier is in use with the second one on stand by or under maintenance process. Each purifier is equipped with a supply pump, duplex filter and steam heater and all the equipment is located in the purifier room.
Service air is supplied to the purifiers to control the supply of oil to the bowl and the automatic discharge facility, while fresh water is supplied for sealing and flushing purposes.
Normally the HFO purifier throughput is set to match the fuel consumption in the main and auxiliary engines and in the auxiliary boiler. If the throughput is exceeding the consumption the excess delivered to the service tank will overflow back to the settling tank when the service tank become full.
Heated liquid mixtures and solid-liquid mixtures enters the purifier and the centrifugal force created by the rotating bowl causes the liquid mixture to separate into its different constituents within the disc stack. The solid particles suspended in the oil settle on the underside of the discs and slide down into the solids holding space at the periphery of the bowl. The smooth disc surface allow the solids to slide down and provide self-cleaning of the discs. A regulating ring determines the position of the interface between the oil and the collected separated water and is set according with the density of the oil to be cleaned. Greater oil density requires a smaller internal diameter of the regulating ring. Two pumps are mounted on the outlet side of the purifier and these are a sensing pump which discharges the dirty water and a centripetal pump which converts the kinetic energy in the oil to a pressure for discharging to the receiving tank. Here below you can see the working principle of Alfa Laval S and P Flex separation system:
Purifiers can be used and operated in a purification or clarification mode. When operating in a purifier mode a chamber cover over the centripetal pump has holes and when operating in a clarifier mode the cover has no holes. However, modern purifiers like Alfa Laval’s ALCAP™ separator technology eliminates the gravity disc to allow for automatic adjustment to fuel density.
Nowadays, purifiers being of self-cleaning type, the accumulated solids within the holding space are ejected at predetermined intervals depending on the quality of the oil. The cleaning cycle is achieved automatically, a number of air actuated control valves act to allow the oil to bypass the purifier and to open the bowl for a set period of time. The self cleaning process takes place with either partial or total discharge of the bowl. For this purpose, the bowl must be first filled with displacement water following closure of the dirty oil inlet. Depending on the discharging process the bowl will be partially or completely filled with displacement water. Control water is introduced into the opening chamber of the bowl to open it, thus the pressure exerted by the control water moves the piston valve which opens the sludge discharging ports. The content of the bowl will be discharged totally or partially depending on the process.
Due high centrifugal forces and liquid velocity generated within the bowl expels almost all the sludge. Damaged seal O-rings or scratched/damaged surfaces of the sludge chamber and disks make the self-cleaning process more difficult and sometimes impossible this being signaled through a machine malfunction alarm. Control water flows out of the bowl during sludge discharge and the bowl is closed again. Here below there is an example where you can actually see the velocity generated due high centrifugal forces (this test it is done in workshop into a controlled environment):
Usually, there is a water monitoring system which controls the discharge of water and a sludge monitoring system which controls the discharge of sludge.
Water which is separated from the oil collects in the outer part of the bowl. The separating disc in the bowl continuously directs a small flow from the outer part of the bowl to the sensing liquid pump, which discharges this flow to a conductivity sensor. if the conductivity sensor detects the presence of water it means that a reasonable amount of water is present in the bowl. The control system that opens a solenoid operated valve to discharge the water through the dirty water outlet; the separator is operating as a purifier. When the water has been discharged the valve is closed and the separator resumes operation as a clarifier.
Desludging of the bowl normally takes place at timed intervals, which are changed to suit the quality of the oil being treated. A sensor in the sludge monitoring system detects the build-up of sludge-water in the sludge space of the bowl. if the amount of sludge becomes excessive, the system will activate the automatic desludging process even though a timed desludging operation is not due. Usually if there are more than two untimed desludging operations an alarm is activated and this requires the intervention of the engineer on watch responsible with the engine room safe operation.
The alarm can be visualized on the purifier operating panel where all relevant process data and alarm condition are displayed. The components which are controlled or monitored by the control system include, but are not limited to:
- oil inlet/bypass valve (three-way circulating valve)
- flushing water
- operating water
- circuit and water discharge valve
- water sensor
- oil inlet temperature
- audible alarms
Software assignment for each purifier is carried out in the factory and/or by commissioning engineer using a password function. Any alteration to the set parameters must be carried out by a qualified and authorized engineer. Changes in parameter settings are not an operational requirement of the purifiers and are not normally necessary once the system has been commissioned for the type of the oil being treated. If any change in parameter setting is required (for emergency reasons) this must only be carried out with the approval of the Chief Engineer and after consultation with control system manual. It is essential that the correct separating temperature is set for the grade of fuel or lubricating oil being centrifuged. Too low temperature can result in inefficient purification, but too high can have a cracking effect (especially on the lubrication oil).
Generally, the discharging time interval is initially set to that the bowl will open and discharge the sludge and water before the sludge space is filled. If the desludging time interval is too short there is excessive sludge and efficiency of the system can be negatively altered. Each purifier discharges sludge from the purifier to the sludge tank situated , usually underneath purifiers.
As a caution, engineers must bear in mind that purifiers operate on an automatic desludging system, but failure of the system to effectively discharge sludge can cause overload and subsequent breakdown of the bowl arrangement which rotates at high speed. After maintenance and cleaning, care is needed to ensure that the bowl is assembled correctly, as incorrect assembly can result in disintegration at high rotational speed. All operating and maintenance precaution mentioned by manufacturer in the maintenance manual must be duly observed. Moreover, hot oil and steam are present and can result in serious injuries or risk of fire if leakage occurs.
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