A pipe cleaning rig that flushes HFE™ solvents through pipes using a series of pressure vessels was developed and tested to determine whether the solvents and the equipment could clean aircraft oxygen system piping to the RAAF's specifications. The first aircraft pipes tested were 1/8 in. copper with either brass dome or cone-shaped end-fittings. To achieve 0.09 m2 (1 sq ft) of internal surface area, over 9 meters of pipe was required.
Results showed that HFE-7100™ was sufficient to remove most oils and greases, but one 600 mL flush with HFE-71DE™ was required to remove OM-15 hydraulic oil. However, initial particulate analysis showed the procedure did not meet the required particle specification. The reason was identified as particle contamination caused by the reconnection of pipes during the verification process. Once the verification method was modified to eliminate further connections, the particle specification was achieved.
This particle contamination was investigated further because this would occur during installation of these pipes in aircraft. The number of particles contaminating the wetted area was found to vary linearly with the number of reconnections, and between different fitting designs.