Tech Briefs

The use of automatic particle counters is prevalent in the hydraulics/hydraulic fluid industry.

Fuel quality assurance is accomplished by conducting periodic fuel sampling for the condition monitoring of aviation fuel by detecting, measuring, and reporting the levels of contaminants in the fuel. Current methods have several drawbacks including operator subjectivity, lack of detailed analysis, limitations in providing reliable data, and the turnaround time needed to get the test results.

The AAFARS fuel sampling port for use with light obscuration particle counters.
The U.S. Army and U.S. Navy have conducted laboratory evaluations of particle counter technologies for fuel contamination monitoring. The particle counters tested were unable to adequately distinguish between free water and sediment contamination. Conclusions from the laboratory evaluation indicated that particle counters cannot replace current technology where quantification of both free water and sediment contamination is required. However, this technology showed significant promise for monitoring overall fuel quality.

The U.S. Army conducted tests that clearly demonstrated the online particle counters’ susceptibility to providing erroneous results in the presence of air bubbles in the fuel stream. The data had spikes corresponding to the fuel pump automatically shutting off every 10 minutes, and were theorized as being caused due to a “water hammer” effect in the fuel system that shook water free from pockets within the fuel system piping.

Light obscuration particle counters on the Advanced Aviation Forward Area Refueling System (AAFARS) were utilized for this testing. The AAFARS (see figure) was fed fuel from a tanker truck, and a fuel sample port was inserted into the recirculation loop downstream of the filter separator to simulate being in line with the fueling nozzle.

The AAFARS was run under the following conditions to simulate aircraft refueling operations while particle counts where obtained. Fuel was pumped from the tank, through the filter separator, and back into the fuel tank. A valve downstream of the filter separator and particle counter was rapidly opened and closed to create a water hammer effect on the hose line and filter separator. Fuel was again pumped from the tank, through the filter separator, and back into the fuel tank.

Two gallons of fuel were removed from the filter separator vessel, creating a pocket of air in the vessel. The particle counters were started and then the AAFARS fuel pump was initiated, pushing the air from the filter vessel back to the fuel tank. This test was to simulate failing to purge the filter vessel of air following filter replacement of water bottom removal. Air was purged from the filter separator vessel, and fuel was again pumped from the tank, through the filter separator, and back into the fuel tank.

Ideally, these instruments can be left in the field to monitor and collect data for fuel transfers. For this demonstration, the units were configured to pull fuel samples when manually initiated by the operators for each data set.

The data obtained during testing was consistent with the Army’s previous experiments utilizing light obscuration particle counters. Following startup of the AAFARS pump, the particle counter was started and took its first reading, falling out-side of the Army’s proposed particle counter limits due to air in the particle counter sampling line. The following six samples in recirculation mode saw the particle counts drop below the proposed limits and continue to drop as cleaner fuel was pumped through the filter separator.

To simulate the opening and closing of a fueling nozzle, a valve downstream of the pump, filter separator, and particle counter was rapidly opened and closed causing a water hammer effect on the AAFARS. Three particle counter samples were recorded under these conditions with readings higher than the recirculation reading taken just prior, potentially due to the pressure shock on the filter elements shaking dirt off the element and into the fuel stream.

After purging the filter vessel of air, of which there appeared to be none, the fueling simulation was resumed by opening and closing a valve downstream of the pump, filter separator, and particle counter causing a water hammer effect on the AAFARS. This caused the particle count readings to jump above the readings seen when the air was in the system. The increase was again perceived as being caused by the pressure shock on the filter elements pushing dirt off/through the element and into the fuel stream.

To conclude the testing, the AAFARS was returned to recirculation mode and two final particle count readings were taken with readings falling closely in line with the previous recirculation readings. The test results indicate that on-line particle counters, while susceptible to the presence of air found in fueling systems, appear to be compatible to the Army’s tactical fueling systems.

This work was done by Joel Schmitigal of the U.S. Army TARDEC. ARL-0167