What is a fuel oil purifier?
A centrifugal separator, also known as a ‘centrifuge’, is a type of machine used for separating phases of a fluid based upon density. The name ‘centrifugal separator’ arises because the working principal of the machine relies upon centrifugal force. Purifiers and clarifiers are types of centrifugal separator.
This article focuses upon fuel oil purifiers used in the maritime world. Throughout this article, it is possible to replace the word ‘fuel’ with ‘fluid’ and apply the knowledge and working principles to other applications and industries e.g. lube oil purifier (lubrication oil purifier) instead of fuel oil purifier. If you want to learn more about fuel oil purifiers after reading this article, please see our Fuel Purifier and Clarifier Basics Online Video Course.
Note: The terms ‘centrifugal separator’ and ‘centrifuge’ are often used interchangeably; there is no difference between the two items other than the terminology.
Centrifugal Separator Applications
The separation of a fluid into its various phases is required in many engineering processes, some common examples include:
- Removing solids and water from fuel oil (water from diesel etc.).
- Removing fat from milk to make skimmed milk (the removed fat is sold as cream).
- Removing gums and lecithin from vegetable oil (improves quality of oil, gums sold as a by-product).
- Removing water from wet clothing (washing machines).
Marine Fuel System
This article focuses upon fuel oil purifiers used within a ship’s fuel system i.e. purifiers that remove water and impurities from fuel oil. A typical fuel oil system for a large commercial vessel (container ship, oil tanker etc.) is shown below. The ‘centrifuges’ could be either purifiers or clarifiers, depending upon the requirements of the system.
Commercial Vessel Fuel System
Commercial ships traditionally burnt heavy fuel oil (HFO), but this has changed over the past decade due to new environmental protection legislation. A ship burning diesel oil will not require heaters in the fuel system, as the fuel flows readily under most conditions (low viscosity). Heavy fuel oil requires heaters within the fuel tanks and any intermediate storage tank thereafter due to its high viscosity (high resistance to flow i.e. it is ‘thick’ at lower temperatures and does not flow readily).
What’s the difference between a purifier and a clarifier?
Purifiers and clarifiers are types of centrifugal separator. A purifier is designed to separate phases of a fluid and impurities. A clarifier is designed to separate impurities only. For example, a fuel oil purifier will separate water from fuel, which represent different phases of the fluid, and it will also separate solid impurities such as rust particles, iron particles, aluminium particles etc. from the fuel. A clarifier will only separate solid impurities from the fuel, it will not separate water from the fuel.
Fluid Flow into and Out of a Purifier
The above image shows a purifier being fed with dirty fuel oil. Clean oil and water are continuously discharged from the purifier (downward arrows). Any solids/impurities in the dirty fuel will accumulate within the purifier and must be removed periodically.
Purifier and Clarifier Compared
There are several important differences between a purifier and clarifier:
- Purifiers have a gravity disc, clarifiers do not.
- Purifiers require sealing water, clarifiers do not.
- Purifiers separate water and impurities, clarifiers separate impurities only.
Fuel Oil Purifier Internal Parts
The internal parts of a purifier can be roughly split between disposable and non-disposable. Disposable parts are seals and O-rings; non-disposable parts are all other parts. The common internal parts of an auto-desludging fuel oil purifier are shown below.
Fuel Oil Purifier Internal Parts
The internal parts of a fuel oil purifier consist of:
- Locking Ring (Lock Ring) – an upper and lower locking ring. The upper locking ring is referred to as the ‘big locking ring’ whilst the lower locking ring is referred to as the ‘small locking ring’.
IMPORTANT
Locking rings have a left-hand thread i.e. they must be rotated left to tighten them, and right to loosen them. Other internal parts also use a left-hand thread!
- Bowl Hood – attaches to the bowl body to form a sealed container.
- Top Disc – retains the inlet and outlet pipe, paring discs, and gravity disc, in position above the disc stack.
- Inlet and Outlet Pipe – fluids enter into the bowl, and are discharged from the bowl, through this connection.
- Gravity Disc – sets the interface between the fuel and water within the purifier. Clarifiers have no gravity disc because they do not separate water from fuel.
- Bowl Discs – bowl discs are pressed together on the distributer to form a disc stack.
- Auto-desludging – the discharge slide, operating slide, holder, and operating water ring, are all used to control the desludging operation of an auto-desludging purifier (discussed later in the article).
- Bowl Body – forms the lower part of the sealed container; the upper part of the container is the bowl hood.
All parts of a fuel oil purifier must be manufactured to exact tolerances due to the high operating speeds at which they operate. Any imperfections present on the internal parts will cause imbalances when the purifier is in operation, with corresponding detrimental effects to the purifier. Each of a purifier’s parts have a manufacturing code stamped/embossed onto their surface to ensure a suitable replacement part can be sourced from the manufacturer (the manufacturer balances each of the parts at the factory and records their characteristics). Purifier parts should never be shared between machines!
Fuel Oil Purifier External Parts
The external appearance of each fuel oil purifier differs considerably, there are however some common parts present on most purifiers.
Fuel Oil Purifier External Parts
Some of the most relevant parts are discussed in more detail below.
- Brake – for decelerating the machine. The brake should be applied when the purifier is no longer rotating, or when rotating at a very slow speed.
- Oil System – consisting of the drain plug, filler cap, and oil level sight glass. Lubrication oil is added through the filler cap and drained through the drain plug. An oil level sight glass is provided to visually check the oil level (oil level should be between the minimum and maximum markings on the sight glass).
- Speed Counter – displaying the rotational speed of the unit in revolutions per minute (rpm).
- Drive Motor – an electric induction motor connected via gears or a belt to the purifier drive shaft. The rotational speed of the motor is usually regulated by means of a variable speed drive (VSD)/variable frequency drive (VFD).
- Connections – for dirty fuel, water, and clean fuel.
Fuel Oil Purifier Drives
Fuel oil purifiers may be gear or belt driven. The arrangement of a gear driven purifier is below.
Gear Driven Fuel Oil Purifier
Belt driven fuel oil purifiers are also quite common; an arrangement is shown below.
Belt Driven Fuel Oil Purifier
The gears within a fuel oil purifier are manufactured from stainless steel and do not wear considerably over time, thus they are infrequently replaced. Belts must be replaced frequently as they wear over time and lose their tensile strength, which leads to them stretching (elongating).
Fuel Oil Purifier Theory
To understand how a purifier works -indeed how any centrifuge works-, it is necessary to understand the concept of density and centrifugal force.
Density
Density can be described as the weight of something with relation to its volume. A heavy object (large weight) with a small volume is considered to have a high density. A light object (low weight) with a large volume is considered to have a low density. For example, the metal lead is considered to be very dense because it has a high weight to volume ratio.
If two fluids of different densities are mixed together, they will separate by density if left unagitated. The diagram below shows a tank that is filled with dirty fuel oil i.e. fuel oil that contains water and impurities.
Fuel Oil Tank
The process above can be described as:
- The left image shows the fuel tank full of dirty fuel oil.
- The middle image shows the tank condition after waiting a period of time e.g. 24 hours.
- The right image shows the condition of the tank once the water has been drained from the tank.
When looking at the middle image, notice that the water in the fuel has gathered at the base of the tank because it is denser than the fuel. Lighter fluids will always be displaced by denser fluids if left unagitated i.e., denser fluids sink below lighter fluids. Water is generally considered ‘heavy’ and most fuel oils -and vegetable oils- will be displaced by it if mixed together in a container. The type of separation described is referred to as ‘gravity separation’, or ‘separation due to gravity’.
Vegetable Oil Displaced by Water
Centrifugal Force
Centrifugal force is not a true force, it is a fictitious (not true) force used to explain Newton’s second law of motion. Newton’s second law of motion is described as:
The change of motion of an object is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed.
For engineering purposes, it is best to imagine that centrifugal force is the outward force exerted on a rotating object. For example, the image below shows two forces acting upon the Earth, centripetal and centrifugal. If centripetal force is causing the Earth to be pulled towards the sun, another force is required to maintain the Earth in its orbit; this force is centrifugal force.
IMPORTANT – the explanation given above is not strictly correct in terms of Newtonian physics, but a more in-depth explanation is beyond the scope of this article.
Centrifugal and Centripetal Forces
It is possible to increase the centrifugal force exerted on an object by increasing its rotational speed. When an object is restrained to a central point and a force is applied, the object will rotate around that point, because the centripetal and centrifugal forces are balanced. If the object becomes unrestrained, it will travel on a path at a right-angle to the rotational point; this can be demonstrated in the below image of a man swinging a child around in circles.
Centrifugal Force Example
It can be seen that when an object is rotating at speed, forces act upon that object. When a fluid is rotated at speed, forces act upon the fluid causing the fluid to separate into its various phases. Denser phases move further away from the point of rotation and displace any lighter phases in the process. For example, if a sealed cylindrical container was full of dirty fuel oil and rotated very fast along its centre axis, water molecules would travel to the outside of the container and oil molecules would be forced towards the centre of the container; dense impurities in the dirty fuel would displace the water and accumulate on the outer most periphery of the container.
Fuel Oil Purifier Separation
How Fuel Oil Purifiers Work (Basic)
A fuel oil purifier is a sealed container that is rotated at high speed by an electric motor. A fluid is fed into the container and the rotational motion (kinetic energy) of the container is transferred to the fluid. As energy is imparted to the fluid, it begins to separate into its phases (water and oil) due their density differences; impurities accumulate on the outer periphery of the container as these are the heaviest particles suspended within the fluid. Water and clean fuel are continuously discharged from the purifier, whilst impurities are removed periodically.
Note - the ‘container’ is formed by clamping the bowl body and bowl hood together.
How Fuel Oil Purifer Works (Detailed)
Dirty fuel oil is fed into the purifier via the dirty fuel inlet. The dirty fuel oil enters the purifier cover and flows down through the distributor and out of its base. From the base of the distributor, the dirty fuel oil flows upwards through the disc stack. Flow through the disc stack occurs through small holes located in the outer periphery of each of the discs.
Fuel Oil Purifier Internal Components
The separation of water and impurities occurs between the discs of the disc stack. Water molecules coalesce on the underside of each disc and travel outwards towards the periphery of the bowl; foreign particles and other impurities also travel towards the periphery of the bowl. ‘Clean fuel oil’ (fuel without water and impurities) travels to the top of the disc stack and is extracted through the clean oil discharge connection.
Purifier Disc Stack
Water accumulates on the inside surface of the purifier bowl until it has sufficient volume to flow around the gravity disc and out through the water discharge connection. A trickle of water from the water discharge connection can often be seen when a purifier is in operation, but the amount of separated water seen depends heavily upon the type of fuel and its water content.
Water and Clean Fuel Outlets
Impurities accumulate on the inner surface of the purifier bowl and can be discharged automatically via auto-desludging, or, must be removed manually.
Purifier Maintenance
Purifiers and clarifiers require comparatively little maintenance. Maintenance interventions are classified as either ‘minor’ or ‘major’, and these occur at scheduled intervals based upon time (service hours) or date (days since last service, irrespective of service hours).
Minor Servicing
A minor service will only require the purifier to be cleaned and seals and/or O-rings to be replaced. Spare parts needed for a minor service are provided in a Minor Service Kit.
Major Servicing
A major service includes a minor service and other additional tasks. A major service requires the use of many more spare parts, such as bearings and shafts etc. Spare parts needed for a major service are provided in a Major Service Kit.
Why use centrifuges instead of gravity separation?
The process of gravity separation is too slow to be used for most industrial processes. Many industrial processes require large continuous volumes of fluids, and it is not possible to achieve this when relying only upon gravity separation. Aside from the low volumes, the quality of the output is also difficult to determine compared to when using a centrifuge.
Centrifugal separation occurs because the effects of gravity are simulated and magnified within the purifier. When a purifier rotates at high speed, the fluid phases within the purifier separate due to the increased forces they are subjected to. Astronauts experience a similar situation when they are subjected to increased gravitational force (g-force) as part of their training for entering space. They are placed within a cockpit and rotated at high speed to simulate increased gravitational force; the type of machine used for this is called a ‘human centrifuge’. The machine shown below is capable of delivering 20g (20 x the force of Earth’s gravity). This type of training has proved valuable because blood will flow away from the brain as g-force increases, which can cause tunnel vision and black outs; training for this eventuality increases the likelihood of an astronaut identifying and remedying the situation before it becomes critical.
20 g Human Centrifuge
How Fuel Oil Purifier Auto-Desludging Works
Larger fuel oil purifiers are fitted with an auto-desludging capability. As the bowl of a purifier becomes dirty, the working diameter of the bowl gradually decreases, and the efficiency of the unit decreases. In older purifiers, it was necessary to open the purifier frequently in order to clean impurities/sludge from the inside surface of the purifier bowl, but the auto-desludging feature allows a purifier to be flushed with water and de-sludged without needing to open it.
Auto-desludging Operation
The desludging operation is achieved using a hydraulic control system located under the purifier sliding bowl (a small bowl placed within the main bowl). Water is drained from the system, which causes the sliding bowl to move downwards, uncovering desludging ports. Sludge is discharged through the discharge ports in the purifier bowl, then water (operating water) refills the chamber under the sliding bowl, which causes the sliding bowl to rise and again cover the sludge discharge ports.