When a large volume of fuel needs to be filtered in a short time or the fuel is contaminated with several unwanted substances the best type of filter to use is centrifugal.
Vessels of all sizes use something referred to as a centrifugal separator. These are filtering devices that allow a large volume of fuel to pass through while removing significant amounts of water and particulates in the untreated fuel.
Separators come in many sizes. They can be as small as a drinking glass or large enough to fill a small room. All of these systems operate independent of scale so the physics that make a separator the size of your hand work also work in the largest devices.
Although the devices work the same way there are differences in efficiency according to size and complexity.
Smaller filters can only remove the largest particles and traces of water. Their small diameter does not allow them to deliver as much force to the suspended contaminants.
To see this in action take a small weight on a string and hold it about one foot from the weight while swinging the weight in a circle. Note the amount of force produced. Now do the same thing but hold the string about three feet from the weight, you should feel a more forceful pull from the weight as you swing it in circles.
A larger circle allows more of the force to be directed outwards. The less curved path makes the centrifugal force dominant over the forces that are causing the weight to move in an arc.
Filtering Diverse Contaminants
The size of a separator depends on several factors. Are you filtering a distillate or residual grade of fuel? How pure is the fuel? Are there on vessel circumstances that will contaminate the fuel further? How much space and budget do you have?
If you are using a distillate maritime fuel grade you have the option of a smaller separator. These fuel oils are low viscosity and have much less contamination to be removed.
The most common type of distillate separator is simply a cylindrical chamber where a vortex is formed by a nozzle that sprays fuel at a low angle into the “cup” or cylinder which is full of fuel. This spinning vortex causes contaminants to the side of the cup where they make their way to the bottom for removal.
Purified fuel is drawn off at the center of the chamber and is then pumped to a pleated filter for final processing. The cleaning of the cup is done manually in the smallest systems but larger more complex fuel polishing systems are mostly automated. All types of systems require some maintenance so they should be mounted in an accessible location.
If you are burning a residual fuel in your engine a much more complex system is needed. The complexity has to do with residual fuel having a much higher proportion of contaminants but also a higher viscosity.
The lowest and cheapest grades of maritime fuel must be preheated before reaching the filters or engines because at room temperature they are nearly solid.
Even with heating the fuel has a greater viscosity than distillate fuels. This greater viscosity and often greater density requires a faster spin in the vortex chamber. In most cases this is accomplished with an electric or hydraulic motor spinning a shaft. High speed rotation of the fluid can exert thousands of times the force of gravity on the liquid which allows molecular level contaminants to be removed.
To help the heavier contaminants move to the side, and eventually the bottom, of the chamber some separators use buffer discs or cones mounted on the central shaft. These slow the passage of unwanted substances enough so the centrifugal force can move them to the outside of the vortex.
The larger systems employ sensors in the vortex chamber to measure particulate density in the collection area and automatically remove excess water.
One unusual aspect of these purifiers is that they often require a charge of starter water to be added before initial operation. This water remains at a constant level at the bottom of the vortex chamber which allows the lighter fuel to remain well above the waste outlet. Without a proper water level it is possible that useful fuel would be drawn into the waste outlet until enough water was removed from the fuel to fill the waste outlet.
Even if you have a great supplier of fuel and you buy a very clean grade it is always possible that some sediment can find its way into your fuel system.
The greater and more common danger is that atmospheric moisture will find its way into your tanks through condensation. A small dedicated system might be a ten thousand dollar investment but that is good cheap insurance when your engine cannot fail. Less expensive nozzle driven systems cost a few hundred dollars per engine and provide good piece of mind if they are suitable for your application.