Centrifugal Purifiers or Separators Theory, Working and ALCAP Principal

Learn about centrifugal purifiers working, theory related to their working and about the new ALCAP principal that is used nowadays. Purifiers are one of the most essential machinery on board. Being high precision equipment, it becomes very important to be well versed with the principles of operations. Purifiers are often attributed to cause sleepless nights to 4th engineers.

Centrifugal Purifiers

This diagram gives a general overview of the purifier / centrifugal separator unit.
Purifier
                                                   Image Source : brighthubengineering.com

Theory Behind The Centrifugal Purifiers

Basically, a purifier separates water from oil. But isn’t that also done in Settling Tanks ?
Equation
Yes, but as we can see the separation efficiency depends upon difference in densities, particle size as well as ‘g’.
This principle is used in Settling Tanks. If we replace the ‘g’ with ‘ω’ , in case of centrifugal separation, the time required for separation drastically reduces as ‘ω’ is a lot greater than ‘g’ ( ‘g’ being acceleration due to gravity, a quantity we cannot change willingly. ‘ω’ being Angular velocity, a quantity
we can alter )
Centrifugal Action
Image
Courtesy : Marinediesels.info
A bowl containing impure fuel is rotated, the centrifugal force developed exerts a force on all particles in the fuel. The heavy particles like solids and water are thrown to the periphery of the bowl ( m ω2, angular velocity being constant, denser particles having more ‘mass’ experience more Force of Separation )

Centrifugal Purifiers Drive Mechanism

The Purifier can either be driven by a belt arrangement with and electrical motor or could be bevel geared with another shaft directly coupled with the motor ( Friction Clutch arrangement )
Centrifugal Clutch
Image
courtesy :  slideshare.net
Earlier, the design incorporated was called tubular stack purifiers, they needed very high RPM for obtaining better separation, as the Force was not enough to throw the particles all the way to the periphery ( therefore more angular velocity required )
Modern purifiers have eliminated the need for very high RPM by incorporating bowl shaped discs stacked on top of each other……..continued

Concept of Interface and Gravity Disc In Centrifugal Purifiers

Motion of fluid between two plates varies between the maximum at mid point and minimum approaching the plates.  A particle entering into the plates will tend to be pushed upwards by the fluid flow.
All the time centrifugal force tends to retard the horizontal component of the movement causing the particle to approach the underside of the top disc, velocity reduces as it approaches.The centrifugal force eventually overcomes the force acting on the particle due to fluid movement and the particle starts to move towards the outer rim
Purifier
 Care should be taken to maintain the ‘e line’ A line formed at the interface of oil and water and should be formed just inside the top disc outer circumference Shifting of e line outwards causes oil in water side Shifting of e line inwards causes water in oil side.
Nomogram
 Image
Courtesy : marineengineering.org.uk
Selection of Gravity Discs is very important for better purification; this graph called a Nomogram is used to ascertain the best possible gravity disc for given specific gravity and difference in separation temperatures
The gravity disc with the largest possible central bore diameter should be selected which not cause overflows. Reducing the flow rate to a purifier also increases the quality of the output.
 General
Arrangement for Purifiers on board is given in the diagram
Fuel Purification Arrangement
 Diagram
source : class4oral.blogspot.com
 

The Purification and De-sludging Operation Of Purifiers

Earlier, manual purifiers used to be stopped after a few hours of ‘batch operation’ and periodically cleaned. Desludging process can be manually operated or automatically timed depending upon the Manufacturer.
Self Cleaning Purifers are able to slide open the bowl and discharge accumulated sludge and water through discharge ports and close.Depending upon the time for which the bowl is kept open, the process is known as partial or total discharge.
In total discharge method, more clean oil is wasted
In partial discharge method, clean oil can be saved.
There is stationary centripetal pump (also called an oil paring disc), an impeller mounted on the light phase outlet (discharge – clean oil ). A discharge valve throttles the back pressure in the bowl, by changing the immersion depth of the lip of the impeller, which helps removes air from the light phase delivery chamber. This reduces chances of foaming.
Water and Oil Interface
Image
Courtesy : hfoplant.blogspot.com
During purification, centrifugal force acts on pilot valve therefore packing remains sealed, operating water remains filled in the chamber and bowl is kept pushed upwards to main sealing ring. Due to constant loss of water (evaporation) operating water is replenished by ‘make up
water’ or ‘replenishing water’.
For Desludging, bowl has to slide open, supply of operating water is stopped. ‘Desludging water’ is supplied which acts on the lower surface of the pilot valve having a larger surface area, thereby opening the Pilot Valve to the counter radial direction (opposite to centrifugal force).
This causes all the operating water to drain from the drain orifice which is normally kept closed by the pilot valve. The bowl slides down and accumulated sludge and water at the bowl periphery is thrown out. The desludging water is stopped and operating water is simultaneously started to fill the water chamber. Causing the bowl to lift and again return to normal working position.
And the pilot valve also looses water pressure which was forcing it to remain open in counter radial direction. Therefore the drain orifice is also closed by the pilot valve. This completes the Desludging operation.
Water is distributed by a water paring disc. Water sequence and timing are controlled by Solenoid Valves. Water from the Hydrophore or in some cases, Header tanks are used.
Purifier Operating Water
Diagram
Source : hfoplant.blogspot.com
The difference between partial and total discharge operations of Mitsubishi Self ejector series is only the time duration for which the bowl remains opened. This is achieved by supplying desludging water for shorter time duration.
In case of Alfa Laval Purifiers, Valve Springs are used instead of Pilot Valves.

The ALCAP Principle

 Alpha Laval ALCAP design claims it can purify high density residual fuels.
Its flow control disc eliminates the need to change gravity discs based on differing fuel densities.
It used Transducers to detect water in oil side and oil in water side and thereby signaling the microprocessor to desludge automatically upon detecting changes in the interface.
Alcap System, centrifugal purifiers
Image
Source : seperationequipment.com
Hopefully this helps in understanding the principles of operations involved in Purifiers Centrifugal Separators. Did we miss something ? Let us know in the Comments !

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Arnav Bhattacharya

Hi this is Arnav Bhattacharya. Marine Engineering Officer. Wishes to be this Generations' Yevgeny Zamyatin. Likes to dabble in Poetry, Literature, Art and Cinema. Lifelong Obsession with all things Nerdy.

2 thoughts on “Centrifugal Purifiers or Separators Theory, Working and ALCAP Principal

  • July 13, 2016 at 11:57 am
    Permalink

    We have purifier ALFA LAVAL S815 and we are using gravity disc 40mm in offshore rig when the oil density is 0.85 kg/m3, so please advise the recommended gravity disc which we should use to improve the oil cleaning quality?

    Reply
  • October 30, 2016 at 2:20 am
    Permalink

    Very explanatory article, which would definitely a guide to all juniors as well a refreshing note for all senior marine engineers. However, how to adjust the throughput (reducing the flow for better quality of oil) should be explained.

    Reply

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