On ship, Purification is done for Fuel Oil and Lube Oil separately. For Lube oil, there is only purifier to purify but for Fuel oil, there are settling tank and Centrifuges arrangement for gravity and centrifugal separation respectively. In settling method, time required is not permissible and it can’t separate all types of particles and impurities.
- Purification is nothing but the separation of impurities like water and sludge.
- This separation is done on the basis of density difference, i.e. if density of oil and water will similar then it will be very difficult to separate them. But we know density of water is higher than oil.
- (Density of Solid > Density of Liquid) and (Density of water > Density of oil), so we can conclude that: sludge density > water density > oil density.
- Denser substance (having more density) goes lower by gravity force and lighter substance (less density) goes upper, and this is the basic fact behind gravity separation.
- Separating Force is defined by Stoke’s law as : Fs = π/6 × D³ (ρw – ρo) × g
- In gravitation separation, g is responsible for separation speed, which is fixed.
- Now if we replace the gravitational acceleration g with centrifugal acceleration (ω² r) then we can increase the separation force by increasing angular velocity and effective radius.
- Thus if settling tank is replaced by a rotating cylinder the separating force and hence the speed of separation can be increased. This is called centrifuge.
Some terms :
e-Line : It’s the oil-water interface line.
G-value of Separation : The ratio of centrifugal force to the gravitational force is known as G-value.
Retention Time : It’s the amount of time a liquid stays in a vessel (centrifuge).
Centrifuge consists of a bowl, this bowl may be tubular or disc type. But because of better performance and ease in cleaning, Disc (wide) type bowl is used now-a-days.
Disc stack : Discs are separated by a vertical distance of 0.05-0.06 mm by a series of caulks fixed on the upper side of discs. Due to angular stacks, less rpm is required for separation.
Gravity discs : It is fitted at the top of disc stack. It is used to create oil-water interface, also called as dam ring. For different density of oil, interface line may vary so we have to select correct gravity disc. Selection of gravity disc is advised by the manufacturers, by using Monogram.
It is used to vary the position of e-line and this line should be as nearer as possible to the disc edge between oil hole and end of disc.
Paring disc : It is a stationary impeller, in a chamber at the neck of the bowl. It converts the rotational energy into pressure energy.
Blind disc : Blind means no hole. It is used in conversion of purifier into clarifier. It is at the bottom of disc stack.
Inlet/Outlet : There is one Oil inlet from upper part and one water inlet/operating water inlet from bottom part of bowl. There is one outlet for clean oil and one outlet for water.
Rotating shaft (Bowl spindle) : For centrifugal acceleration, bowl is driven by motor shaft directly or by using bevel gear.
Spring loaded valve : It is provided for de-sludging process.
Power transmission from the electrical motor to the shaft takes place through automatic clutch system. Bowl rotates at operating speed 5-8 thousand rpm, depending upon size and design of purifier. Complete bowl stack assembly is placed on the bowl body and locked. The top disc cover, bowl hood, butterfly nut, gravity disc etc. are placed and then covered with hitch cover and locked with stationary purifier body.
Generally for separating solid impurities, mainly Clarifier is used and for separating water, Purifier is used. There are some basic difference in Clarifier and Purifier, but basic principle remains same and purifier can be changed into clarifier. Clarifier has one oil inlet and one clarified oil outlet. It doesn’t have dam ring (gravity disc) but having a sealing ring instead of that. But Purifier has one oil inlet, one water outlet and one purified oil outlet. It contains gravity discs (dam rings).
Rotational speeds are of the order of 7000 rpm. Efficiency is increased by discs stacks. When oil is supplied, centrifugal force separates oil and dirt in discs actually. And dirt is pushed towards the bowl periphery due to higher density. Oil is pushed towards the outlet connection.
For conversion of Purifier in to Clarifier:
- Simply, Open purifier and set blind disc at the bottom of disc stack.
- Block the water outlet by a seal on gravity disc.
- Block the water inlet. Finally it acts as a clarifier.
Now, purification process is based on types of purifier used : (1) Non-continuous operation type purifier (2) Continuous operation type purifier.
In Non-continuous operation type purifier, sludge is cleaned manually after operating some time. And In Continuous operation type purifier, sludge is cleaned automatically at regular intervals, it is also called as self-cleaning purifier. Non-continuous operation type is still suitable for lube oil system.
(1) General Oil Purifier – (sludge is cleaned manually)
The impure oil after passing down through the central passage carried by centrifugal force towards the periphery of the bottom bowl. And find the oil hole at the disc and moves upward through the disc stack. Particularly towards the upper side of the disc because of oil feed pump pressure. Actual separation between the different density takes place in the lower side of the bowl and in the channel formed between the two discs.
Sludge cleaning process :
- Open the sealing water
- Wait till water come out of the overflow line
- Close the operating water
- Open the de-sludging water and then close.
(2) Self-Cleaning Purifier – (sludge is cleaned automatically)
To remove the accumulated sludge without stopping, without loosing oil – self cleaning purifier is used on every ship now a days. Sealing and Ejection process is used to desludge.
There are 3 bowls keyed to the central drive shaft (spindle). Upper bowl is arranged such that it can move up-down with rotational speed. But two lower bowl secured with spring loaded valves. Purifier is first brought upto 5000-8000 rpm.
For desludging, first oil supply is shut off, then sealing water is provided in purifier sealing space to remove remaining oil from purifier.
Now operating water is supplied through solenoid valves. Due to hydraulic pressure, spring loaded valves are opened. And water from all bowl and sludge are discharged through sludge port.
(3) ALCAP Purifier – (for high density oil)
It is manufactured by Alfa Laval.
It’s construction is same but it has auto-control panel with the timer. Desludging interval can be set which is normally at 4 hours.
It has a flow control disc instead of gravity discs, that’s why suitable for different fuel densities.
It uses transducers to detect water in oil side and oil in water side and according to that signals the microprocessor to desludge automatically by detecting change in interface.
Safeties in Purifier
- Low pressure switch in the outlet of clean oil
- Emergency brakes – for speed regulation
- High pressure switch in the clean oil outlet
- Water transducer to avoid water mixing