SubscribeThe latest generation of condition monitoring technology, developed to detect contamination in jet fuel, is enabling the performance levels of aircraft to be optimised while minimising operating costs considerably, explains Matt Fielder, Hydrocarbon/Aviation Market Manager at Parker Hannifin’s Filter Division Europe.
The issue of fuel contamination demands attention across a diverse range of applications, from the automotive industry to power generation. However, the need to monitor dispersed contamination in aviation fuels is especially critical, due to the particularly serious safety and commercial repercussions of equipment failure at 30,000ft. While a number of simple tests have been used for some time to monitor fuel for contamination, these have often proven to be inaccurate and unreliable. Now, however, the latest generation of condition monitoring technology has the potential to offer a far more effective solution at realistic cost.
How does fuel become contaminated?
Jet fuel is at risk from contamination as soon as it leaves the refinery, flowing through pipes, being transferred into bulk storage tanks, shipped as marine cargo, and coming into contact with other potentially contaminated distribution systems. As it is typically transported in dedicated systems, jet fuel is better protected from particulate contamination than other types of fuels; however, when it comes to water, jet and other hydrocarbon fuels have a marginal solubility for water and can easily be contaminated.
As this solubility is temperature dependent, small fluctuations in temperature can result in the precipitation of free water droplets; as the solubility for water decreases, the free water drops to the bottom of the tank or pipe and forms a water bottom. As the temperature rises, more water can be absorbed, typically from the atmosphere, into the fuel, and the cycle continues, increasing the volume of the water bottom.
In addition, poorly maintained storage and transportation systems are likely to produce microbiological growth, which, depending on the type of bacteria, can rapidly corrode the tank coatings and the vessel floor or wall to such an extent that in extreme cases a breech may occur. If the system is uncoated, any free water will very quickly start to attack the steel, producing a never ending supply of solid particulates in the form of rust, or magnetite (black rust) in pipes where the oxygen content is low.
In the case of new pipelines, iron oxide debris is often already in the pipe as a result of component parts being left open to the elements prior to assembly. New pipelines are also typically dirty following the excavation process. In a recent example, it was estimated that one particular pipeline of approximately 50km in length had over 2.5 tonnes of solid particulate matter inside it that needed to be removed.
This proved to be an expensive and time consuming undertaking, which required the pipeline to be flushed, or pigged, numerous times. It is worth noting that flushing and increasing the velocity of the fluid through a pipeline will only remove so many of the finer particles. Larger, heavier particles often remain, breaking down into smaller particles over time, and entering fuel systems.
These problems have been known for some time, and filtration systems have been developed and installed in order to remove the solids, water and other contaminates which can cause serious problems to engines. It is estimated that on average fuel leaving the refinery will be filtered up to 14 times before being burnt. However, what have not been available are systems to measure accurately contamination levels in order correctly to specify these filtration systems and ensure that, once installed, they are working effectively.
Measuring contamination
The methods conventionally used in the aviation industry to measure contamination have been in place for many years. The simplest is the Clear and Bright (C&B) test, also known as the White Bucket test, which essentially involves an operator taking a sample of the fuel to be tested and observing it. If they are able to see any solid particles, or the fuel is hazy when held up to the light, the fuel fails the test. Although this test gives an indication of contamination, it suffers from a number of limitations.
The test is extremely subjective; the eyesight of the person looking at the sample, the light conditions at the time, and the type of container in which the sample is held, can all have an impact on the results. Furthermore, the human eye is only capable of seeing objects of 30 to 40 microns or larger; under normal circumstances these relatively large particles will drop out of suspension, and even if they are carried along with the flow of fuel, the filtration systems installed are more than capable of catching these particles.
Also, in many cases the sample bottle used has not been washed or cleaned, leading to leftover contaminated matter. During the process the bottle is held directly under the sample point tap, which is opened and then closed once the bottle has been filled. This means that any dirt from the tap itself is washed into the sample, once again confusing the results of the test.
Another method of testing, called the Gravimetric Test method, requires an operator to draw a sample of fuel through a filter pad consisting of 2 layers. The first is the working layer which captures the dirt, and the second is a control layer used as a datum when measuring the solids loading. This filter pad is then taken to a laboratory where it is dried and the two membranes separated. The difference between the two is the amount of contamination in the fuel, expressed as mg/ltr.
This test, while more scientific in its nature, is also capable of producing results with limited accuracy and repeatability. Additionally, the time taken to analysis the sample makes it impossible to delay or cancel a flight if contamination is found in fuel. Admittedly the gravimetric test is not used as a pre-fuelling check; these are taken once every six months (according to IATA Guidance) but still require laboratory time, typically between 24 and 48 hours, to produce a result.
Furthermore, Gravimetric analysis reports a mass of contamination per ml; it does not look at the number of particles collected, and as such provides little insight as to the actual condition of the fuel. For example, a sample membrane may have one large particle or 1,000s of small particles both with the same mass. The small particles are far more damaging to fuel systems, but with the current method there is no way of discerning the size of the particulate contamination.
No allowance is made for the type of contamination in the system. Even if the numbers of particles on the membrane are known, potentially through microscopy techniques, the results can vary greatly unless the type of contamination can be determined and therefore its density recorded. For example, 1,000 particles of 10mµ(c) of iron oxide will weight almost four times that of 1,000 particles of 10mµ(c) silica. In this respect the measurement of particulate as a mass expressed in mg/ml tells the operator little of use.
In addition to these points, there is also the issue of taking the sample itself. In some instances the amount of sample taken is not the correct volume, the sample membranes are not of a matched weight, or there is a leak path allowing the control membrane to capture contamination. If the fuel sample is too dirty, then the 0.8mµ layer can block up and provide a cake layer, which creates an even finer level of filtration, once again affecting the results of the test.
Particle counting
The latest condition monitoring technology provides a far more effective alternative to these limited testing methods. Leading manufacturers of fuel filtration systems, such as Parker Hannifin, have been developing portable Automatic Particle Counters (APCs) for nearly 20 years, with the latest generation of the technology capable of highly accurate and repeatable results in the field.
Particle counters use a process called light obscuration, light blockage or light extinction, which is essentially what occurs when an object passes in front of a light source and creates a shadow. In these devices, the shadow of a particle suspended in a fluid passing through a light source is measured by way of a voltage drop across a light sensitive diode. The signal generated as a result of the shadow is dependant on the size of the particle and the speed at which it passes through the light. There are other types of particle counting but the light obscuration method is a more common and accurate method and is well regulated through ISO standards and practices.
These latest units are robust and portable, suitable for the most demanding applications, including military operations around the world. Particle counting has been proven to be effective over many years in the hydraulics industry, and is ideally suited for use with fuels, specifically in the aviation industry, as it provides results in minutes, enabling action to be taken if contaminated fuel is found, and also allowing more tests to be carried out in a specific timeframe, reducing resource requirements.
The portable nature of the technology makes it suitable for both laboratory and field use, while its accuracy removes subjectivity from the testing process. Explosion-proof versions of the technology have also been developed allowing the counters to be used in areas where previously a hot work permit or gas detector has been required.
The particle counting units are also able to report on any free water in the fuel system, allowing the operator to look at the percentage volume distribution of the contamination present, and, by way of a simple graph displayed on the instrument's handset, qualify the presence of free water. In order to allow accurate trend analysis, the latest particle counters have a test memory capable of holding hundreds of results, which can be read from the handset, printed as hard copies, or downloaded to a PC for simplified analysis and distribution.
In addition to these handheld units, a small inline particle detector is being developed for use with fuels. This unit can be fitted to any pipeline or refuelling vehicle to monitor contamination levels continuously by a slip-stream sampling method. While unable to provide individual particle counts, the device can offer the user real time information given in ISO 4406 codes relating to the level of dispersed contamination within the process line.
Through simple utilities software designed to be supplied with this device, the operator can easily modify its performance characteristics in terms of duration of the test period, when the device reports its results, if it starts automatically when powered, and how long the alarm relay stays on once an alarm limit is reached.
Conclusion
The issue of fuel contamination is possibly more critical in the aviation industry than any other, the ability to monitor fuel accurately is essential. The latest particle counting technology is enabling fast, reliable, repeatable and reproducible data to be achieved in real time; data that was previously only attainable in the laboratory. This development is making it simpler than ever before for fuel suppliers to ensure that the fuel being delivered is free from contamination, and therefore able to offer safe and effective performance.
