COVID-recovery Series: Eliminating The Risks Of Refuelling With Traditional Filter Monitors
At Shell Aviation, we understand that while the industry is facing various challenges as a result of the COVID-19 pandemic, enabling and maintaining efficient and safe operations will be crucial for Aviation's recovery. In the COVID-19 -Recovery webinar series, Technical Experts from the industry will share their views and guidance on how to anticipate certain operational risks.
Today, most aircraft refuellers are fitted with filter monitors that prevent water and dirt particles from getting into the fuel and engine. While water is rarely present in aviation fuel, when it does occur, Superabsorbent Polymer (SAP) has been used to prevent it from getting into the aircraft. The aviation industry has committed to phasing out SAP-based water filters used in the refuelling process due to ongoing safety concerns1. This is because under certain circumstances SAP particles can be released from the filter and into the fuel. In rare cases, these particles move into the aircraft during fuelling and can cause significant operational issues in engines.
On 23 July Rob Midgley, Global Quality and Technical Manager with Shell Aviation was joined in conversation by Andreas Schmidt, Manager Jet Fuel Quality at Lufthansa to discuss a vital issue facing aviation: our industry's need to replace water filters in refuellers that contain Superabsorbent Polymer (SAP).
Vincent Begon, Commercial Airlines Manager at Shell Aviation, moderated the webinar session.
This is a transcript of questions and answers of the webinar Eliminating the Risks of Refuelling with Traditional Filter Monitors. It has been lightly edited for clarity.
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Why did it take such a long time to identify the problem with Superabsorbent polymer and Filter Monitors?
Questions about the safety of SAP first emerged in 2010 when a passenger jet belonging to a major international airline experienced engine trouble, causing it to lose thrust control and land at twice the average speed. Though not involved with the fuelling of the flight, Shell Aviation was asked as an independent expert by an Aviation Regulator and the airline, to investigate the cause of the incident. The team found a possible correlation between the presence of SAP and issues with the fuel-control unit experienced during the flight.
Following a further sequence of incidents, the International Air Transport Association (IATA) convened a Special Interest Group in May 2014 to investigate whether SAP was a factor in these cases as well. Participants included representatives from airframe and engine manufacturers, fuel-filter manufacturers, airlines and airline associations, the Energy Institute, and Shell Aviation, which chaired the Group.
Investigations take time and must consider many alternative causes and complications. A causal link needed to be made if the industry was to trigger the wholesale removal of a commonly used filtration method. It is appealing to think that a conclusion is obvious from the start. However, that is rarely the case.
Many of the initial incidents seemed to be related to a single-engine and aircraft type, despite other aircraft operating out of the same incident locations around the same time. Similarly, one location had three separate incidents – suggesting it may have been a location issue rather than a filter issue. At some locations, it was not possible to visit and inspect, so we could not be sure whether SAP was involved in the prior fuelling. Lastly, the Special Interest group was able to find SAP in engine components, but at lower levels than the original 2010 incident. The presence of SAP wasn't enough, as correlation isn't necessarily causation. At the time, there was little or no supporting data of what level of SAP was normal within correctly functioning engine systems.
Once the investigation was completed, the Special Interest Group excluded the possibility of all other considered alternative causes which could have induced the operational engine issues. But the elimination of alternative possibilities and finding the cause takes time. After the conclusion was formed, this led to the Special Interest Group presenting the results of their work to the industry in late 2017. These results showed that under certain circumstances, SAP particles could be released from the filter and into the fuel. And when these SAP particles move into the aircraft during fuelling, it showed they could cause significant operational issues in engines. For this reason, the Special Interest Group recommended the industry to phase out SAP-based water filters used in the refuelling process in late 2017.
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Is the industry commitment to phase out filter monitors (EI1583) from December 2020 onwards delayed?
The Energy Institute (EI) will be withdrawing its support of EI1583 on 31 December 2020. This means the Standard will not be maintained and no new or modified elements will be subject to qualification testing by the EI.
The EI has allowed filter elements that have already been qualified to EI1583 Ed 7 to maintain their approval status after 31 December 2020, but only if there are no changes to design, materials, or construction.
Based on this, JIG and A4A are allowing elements that have been previously qualified to EI1583 Ed 7, and meet the above conditions, to continue to be used while operating to their JIG/A4A Standards. However, even when meeting these operating standards, it remains a user's responsibility to approve equipment and to ensure that the equipment is fit for purpose, or able to do the job it was designed to do.2
Problems arise if a filter manufacturer changes filter design, materials, or construction, as it is not clear how subsequent qualification of such changes can be performed. Ultimately, the acceptance and approval of these types of changes will be the responsibility of the purchaser/user after 31 December 2020. This means the individual user will be responsible for defining the qualification testing required and assuring that all aspects of performance, such as filtration, electrostatics, and SAP migration, remain fit for purpose.
Most filter manufacturers have committed to not change the design of their filter monitors after 31 December 2020, but this cannot be guaranteed. Elements such as SAP production are often outside of the control of the filter manufacturers. So, users planning to continue employing filter monitors need to be aware that they may be expected to approve changes, or potentially face issues of availability.3
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Who will monitor the removal of Filter Monitors?
The responsibility for compliance will sit with the fuelling company. It is anticipated that if the operating standards controlled by JIG, A4A, and IATA are changed to eliminate filter monitors, then inspections by these bodies, and individual airlines, will also provide some compliance oversight.
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What will happen to companies who currently have bowsers with traditional filters in their fleets?
There is no reason why mobile trucks, or bowsers, cannot be converted once suitable technologies are approved. In principle, the filtration controls are no different from a hydrant dispenser.4
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What size are the SAP particles?
Extruded SAP particles that have been linked to operability issues on aircraft are predominantly in the diameter range of 20 microns. This is too small to be visible to the naked eye but large enough to cause issues in various engine components.5 Dry SAP can take a range of forms and vary depending on the manufacturer. Additionally, SAP can change its shape, size and consistency when exposed to different levels of water. It can also extrude through fine filtration and reform on the downstream side, making it difficult to contain with particulate filters, especially if it has been exposed to water.
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How will the free water parts per million (ppm) level be reported to airlines? By a go or no go, an average or is the maximum pick observed during the refuelling process?
Any approach is technically possible. What is reported will need to be agreed between supplier and airline or agreed through the industry standards.
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Could we use Filter Water Separator (FWS) carts connected to dispensers as the solution?
It is possible, as Filter Water Separators (FWS) are an approved filtration technology within the standards, but this is a cumbersome solution and not one that is convenient for the long-term.6
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Is there any evidence to suggest that more frequent draining of water from aircraft fuel tanks reduces the amount of extruded SAP that could get into the main engine fuel supply system?
Shell have seen no evidence that this is the case. The problem lies with the release of SAP from the filter on the fuelling vehicle before the fuel even reaches the aircraft. This indicates that the frequency of tank draining on the aircraft is of secondary importance. We also need to recognize that drain points cannot access all low points in an aircraft tank so that water can accumulate in some aircraft even with frequent low-point draining.
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Will interchangeability be impacted by introducing financial impact for into-plane (ITP) companies due to the lack of drop-in solutions from all manufacturers?
All filter manufacturers are attempting to develop technology that will retrofit directly into the filter monitor vessel. So, if all potential technologies are approved, it should be possible to convert a vessel from one technology type to another.
The only potential modification needed is the addition of an electronic water sensor in the pipework downstream of the filter vessel. This sensor is required for the Dirt Defence option, but it has not yet been determined whether it is needed for any of the other technologies still in development.7 We are unable to comment on financial impact, other than that; increasing competition generally improves market pricing.
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What are some of the procedural hurdles, if any, that operators have had to overcome?
There are differences in operating procedures for operating Dirt Defence Filters (DDF) with an Electronic Water Sensor (EWS), but these are not thought to be onerous. While in the transition phase, JIG recommends pausing fuelling to conduct a chemical water check when the EWS indicates 15 ppm of free water, but this is an additional oversight check. Practically, this is not dissimilar to the practice of conducting a visual sample during fuelling that is part of today's JIG 1 requirements.
The system automatically closes the fuelling down without operator intervention when the EWS detects water levels above a pre-set level; in JIG operations this is 30 ppm.
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Why did Shell choose the Electronic Water Sensor (EWS) and the Dirt Defence Filters (DDF) as alternative solutions for the traditional Filter Monitor?
Shell perceives the Electronic Water Sensor (EWS) and the Dirt Defence Filter (DDF) to be the most expeditious way to remove SAP from our operations. Shell has also done considerable work on test rigs and in simulated failures to support the decision and followed this up with full-scale operations to ensure that erroneous shutdown events are not common – for example, understanding if cavitation from rapidly closing valves would trigger the sensor. For our operations, this combination appears to be robust and allows us to move away from SAP as quickly as possible.
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How is the safety of the alternative of the Filter Monitor proven?
The Energy Institute has tested the performance of both the Dirt Defence elements and the Electronic Water Sensors. In addition, Shell has conducted extensive rig testing where different water levels, different flow rates, different additives, combinations of dirt and water all were tested. We have also conducted full-scale vehicle testing on rigs, where we can simulate other challenges, such as bulk water from a hydrant.
Additionally, Shell has conducted field trials to demonstrate the robustness of the system. This work was confirmed by recently completed field trials at several locations as overseen by JIG, A4A and IATA.8 It is the intention of the industry that other emerging filtration technologies should go through a similar amount of rigor before being included in the industry standards.
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Can Shell commit to replacing a whole fleet with Shell Jet Protection by January 2021?
Shell is going above and beyond to protect aircraft engines by converting all of our refuellers at Shell-managed locations worldwide to Shell Jet Protection so that we can be SAP-free by the end of 2020. We already have a rolling program of Electronic Water Sensor installations in place to achieve this goal. We intend to encourage our joint-venture (JV) sites also to have a plan for conversion, but this will need the agreement of other JV partners.
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How do you know when the electronic sensor is out of calibration? What are the warning signals that indicate it is out of calibration? And could the sensor fail between testing?
The calibration of the sensor is conducted on a rolling program that has been agreed to within the industry. This is based on a conservative assessment of past experiences with similar types of applications, and it is important to note that no drift has been seen on any sensor during all the trial activities.
In addition, there is a more frequent field test that will be conducted to ensure that the functionality of the sensor and related systems, other than the light source, remain functional.
The light source itself is somewhat self-checking in that a known amount of light is emitted. It needs to be received by the sensor as part of the start-up process. This requires the light source to work, the mirror to reflect all of that light and the sensor to detect the light at the expected level. If the amount of sensed light is less than expected, then the system does not allow the vehicle to flow fuel. Added to all this, JIG standards will continue to require a visual test of the fuel to be conducted as part of each fuelling.
Finally, it should be noted that an independent company was employed by the standards bodies to consider the failure mechanisms of the Dirt Defence and Electronic Water Sensor. The resulting successful Failure Mode and Effects Analysis (FMEA) was part of the approval process.
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How is it guaranteed that the sensor’s alarm in not bypassed?
The alarm water level for water (30 ppm downstream of the filter in JIG operations) is connected to the deadman control mechanism, which closes the pit valve for hydrant dispensers, or stops the pump refuelling vehicles thereby stopping the fuelling. The system achieves this without any operator input. There are several processes and checks to assure this functionality, and a Failure Mode and Effects Analysis (FMEA) has been conducted to show that the failure modes do not result in unintended consequences.
Of course, the EWS is only one component in a series of barriers that are designed to keep the fuel clean and dry. Other controls include into-storage water removal, tank positive draining, tank outlet floating suction, vehicle/hydrant filter water separators, low hydrant point draining, post-loading sampling of fuelling trucks, etc.
Some work within JIG has suggested that the end-to-end failure rate of these upstream barriers -- before the fuel even reaches the underwing filtration system -- is several million to one. This is supported by how infrequently we see filter monitors being changed due to water exposure before the end of their 12-month life. The success of all these barriers is further supported from the industry field trials where, within 11,000 fuellings of 126 million litres, there has not been a single event where 30 ppm of water was present in the fuel.
If the question is about what prevents the operator from sabotaging the EWS from functioning, then we would be moving into an area of questioning about how we might prevent gross negligence and wilful misconduct of an operator. This is not something the standards have ever addressed or would be easy to address. We could ask a similar question about what prevents an operator today from conducting wilful misconduct. For example, how do we prevent an operator from consciously operating a filter containing no elements? It could be argued that this would be captured by independent oversight inspection programs, which are also normal within the industry.
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Are there any concerns regarding the use of electronic sensors? Is there a warning of water content in fuel transferred to aircraft?
Shell has conducted testing at a wide range of flow rates, with different levels of water contamination, with and without dirt. To my knowledge, the testing we have carried out is the most extensive of any test program outside the work conducted by the equipment developer.
Of course, every test method also has a precision level below which it cannot differentiate one result from another. Still, all of Shell's work suggests that the currently assessed EWS technology has the capability of controlling water to a limit of 30 ppm of free water as required by the standards. In fact, in Shell's testing, it would seem that if there is any inaccuracy, it tends to skew high, meaning the sensor reports more water than is actually present. This means that the sensor would stop the fuel flow earlier than necessary.
It might not be appreciated by many, but it is extremely difficult to hold very low parts per million levels of water in suspension in fuel. Water is partially soluble in fuel and even small changes in temperature, such as with pumping, can affect the free water level when conducting testing. So, special care is needed when designing test rigs to do this type of work.
For example, if using a single element filter test rig commonly used by filter manufacturers to qualify filters, then the maximum fuel flow rate will be 30 USG/min (113 L/min). If trying to interpret 10 ppm of water, then on such a rig, we would need to be injecting less than 0.2 mL of water every 10 seconds into this fuel flow. That's the equivalent of emptying a 5-mL syringe of the type used in the Shell Water Detector method in a period of a little over four minutes. And if the flow rate is changed to 50%, the problem doubles, at 25% flow, it is even more challenging, and it would take well over 15 minutes to empty the syringe.
Under such conditions, it is difficult not to inject single drops of water with big spaces between. To do this, we need to consider how accurate the Reynolds Number of the flow is in such small-scale rigs, as this too affects dispersion. Taken as a whole, we find ourselves proving very little about the sensor's operational capabilities under such conditions. The conclusion of this is that care is needed when interpreting results from such experimental arrangements, and knowledge of the test rig limitations is critical to interpreting the results from high-resolution sensors.
Shell is confident in our own testing, and the information and methodology that Shell has developed throughout this program are currently being incorporated into improving the industry standards for Electronic Water Sensor testing, EI1598. Of course, others are at liberty to conduct their own independent due diligence if they wish.
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How does the user/operator know that the system is working?
The system self-checks on start-up, and if the light source, mirror, or sensor are defective in any way, then the fuelling cannot be started. Again, the failure modes and effects have been assessed through a dedicated FMEA study.
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How is the single element refuelling for GA addressed?
In principle, single EI1583 qualified elements could be replaced with any of the retrofit technologies in development once they are approved. However, I am aware that there are many 5" filter monitors that have been in use at GA locations. These elements have never been qualified to EI1583, nor have they been included in JIG standards, so the approval for the use of these elements is currently the responsibility of the user. It is assumed that companies using such equipment will need to follow a similar internal risk assessment/qualification approach to approve a replacement technology.
It is interesting to note that JIG standards already allow Avgas applications to use a microfilter (EI1590) as into-wing filtration9 , and this may also extend to Dirt Defence (EI1599) in due course. Also, the Energy Institute is currently developing a standard for SAP-free single element filters, which will come under the designation EI1587. At the time of writing, this standard has not yet been published and it is also not clear how many filter manufacturers will produce elements that qualify as meeting this standard once it is published, but it is likely that some will.
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How is the issue of coalescer element disarming addressed?
Dirt Defence filters do not have coalescer elements, and the DD + EWS combination does not rely on polar surface properties within the filter for water management and therefore is not vulnerable to disarming.
When looking at other filtration technologies, coalescer disarming within EI1581 filter water separators is currently controlled by a regular calendar-based change out of the elements.
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How about the financial aspect of this filter transition? Considering the cost to implement the new type of filter?
Companies will have to make their own decisions about risk and cost. It is difficult to assure fuel meets the specifications and operating limitations of the aircraft while using filter monitors. As meeting the fuel specification is a contracted obligation for fuel providers and a regulatory one for airlines, the decision is not purely a financial one.