Effects of water contamination
Water is one of the most common contaminants in a hydrocarbon fluid system and one of the most damaging. When water contaminates a system, it can cause serious problems such as:
- Causing engine stall
- Enhancing bacterial growth
- Disturbing production processes
- Removing additives
- Corroding by etching metal
- Reducing conductivity
- Enhances abrasive wear in hydraulic components
Free water is usually seen as a cloudy appearance of the fuel or oil, or as puddles of water at the bottom of the reservoir. Water, which is absorbed into the fuel or oil, is called dissolved water. At higher temperatures, fuel and oil have the ability to hold more water in the dissolved stage due to the expansion of the hydrocarbon molecules. As the temperature drops, this ability reverses and free water will appear where not visible before. The saturation point for your system is dependent on the type of fuel or oil and the temperature.
Typical sources of water contamination are the production process, transport (emptying of piping and cleaning of tanks), the changes in temperature due to the weather, tank breathing, rainwater intrusion and leakage from for example steam.
Diesel Coalescer cartridges are mechanical devices to filter solids and separate two immiscible liquids. The most common uses are the filtration and separation of water from aviation fuel, light fuel oil, diesel, gasoline and various types of oil. The free water droplets and solid contaminants in hydrocarbons are, in the vast majority of cases, no more than a haze of microscopic particles suspended in the fuel and invisible to the naked eye.
The measurement of these particles is usually quoted in microns or, more correctly, micrometers. As an indication of these measurements in comparative terms the thickness of a human hair is approximately 50 microns whilst the smallest particle visible to the naked eye is about 40 microns. The majority of water and solid matter particles in a contaminated hydrocarbons are less than 30 microns. It is the function of the coalescer cartridge of the filter/separator unit firstly to remove the solids and then to merge the particles of water into larger droplets of five to ten millimeters in diameter before they are ejected to fall, under gravity, to the sump from where the water accumulation can be drained.
Typical coalescer cartridge construction
The construction and design of a typical filter/coalescer cartridge is shown on the left. The fuel/water/solids mixture first flows through a pleated assembly of fine-grade filter media, the pleated configuration is necessary to obtain the optimum area consistent with maximum dirt-holding capacity and efficiency. Following the almost total removal of solid contaminant by this first filtration stage the fuel/water emulsion then passes through the coalescing media, graduating from a very fine grade material to a coarse grade material to effect the gradual coalescence of water particles from their original microscopic size to visible droplet size.
The coalescer core, which is an unique construction, provides adequate rigidity to the whole assembly after it is finally sealed within the reinforced nylon end caps using a high-quality fuel-resistant adhesive. Water droplets are finally entering the outer, specially treated, cotton sock. The weave pattern of the outer sock determines the ultimate size of water droplet that emerges from the coalescer stage.
The initial mechanisms of both filtration and coalescence in fibrous materials such as filter paper and fibreglass wraps are virtually identical in that they rely on the probability of collisions of particles with the fibres within the media. In both cases the media consists of numerous layers of fibres roughly perpendicular to the flow which forms a maze and the liquid passing through this maze is forced to follow tortuous paths around the fibres. If the flow through a section of the media is visualised as hundreds of tiny streams or jets, which are forced into many changes of direction and cross-sectional shape as they twist and turn through the maze of fibres, it will be easier to appreciate that the probability of solid or water particles colliding with a fibre is an extremely high one.
The droplets formed by the coalescing action can be removed from the main stream by using purely gravity or by employing separator cartridges. The majority of the droplets formed will settle within a short period of time. Smaller droplets will require a longer retention time to be separated from the main flow. In applications where due to the process conditions the reliability of separator cartridges is questionable the use of gravity separation is preferred. In other applications the benefit of smaller vessels is appreciated resulting in vessels which combine coalescer and separator cartridges.
Vessels usually contain more than one of each element type. Each element has a maximum recommended flow rate. This may dependent on the application. The length of these elements can vary up to 1420 mm or 56″. The filter water separator can be orientated either vertically or horizontally.
Mechanism of separation (stripper)
Inconsistencies can be caused by excessive additive content, certain chemical impurities or microbiological growths in the hydrocarbon, and through combinations of these possibilities there is a likelihood that some of the water droplets emitted by the coalescer cartridge will be too small to fall to the sump within the short available distance within the vessel. To prevent the carry-over of these smaller droplets into the filter/separator outlet a bank of stripper cartridges is interposed between coalescer cartridges and the outlet to act as a safety screen. This screen is, in reality, a hydrophobic (water-repellent) barrier, which allows the passage of fuel but prevents the penetration of water. The stripping mechanism is illustrated above and is known as ‘pore catchment’. By combining water resistance and a critical pore or mesh size the media repels any suspended water droplets and, while they are held against the surface of the media by the flow trying to force them through the holes, further droplets following behind will collide and merge with the initial droplets until they are enlarged to such a size that they will fall, under gravity, to the sump area below.
The stripper cartridges are, like the coalescer cartridges, cylindrical in shape but the media is a cylindrical wrap (horizontal and vertical units) or pleated element (vertical units only) of special material. The optional materials available are a high strength siliconised paper, Teflon-coated fine mesh, or a unique woven synthetic material. Stripper cartridges in the latter material simply require thorough cleaning at each service period.