4.If an allowable pressure drop of 25% has been selected throughout the system, this may now be further divided between pressure pipes, return pipes and components. 5.The designer will eventually control the specifications for the components, and in this sense he can allocate any value he chooses for pressure drop across each component. 6.Once pipe lengths, flow rates and permissible pressure drops are known, pipe diameters can be calculated using the normal expression governing friction flow in pipes. 7.It is normal to assume a fluid temperature of 0 ° C for calculations , and in most cases flow in aircraft hydraulic systems is turbulent . 8.Pressure losses in the system piping can be significant and care should be taken to determine accurately pipe diameters. 9.Theoretical sizes will be modified by the need to use standard pipe ranges , and this must be taken into account.
Fluid Conditioning Under normal working conditions hydraulic fluid needs cooling and cleaning . Occasionally it is necessary to de-aerate by the connection of ground equipment, although increasingly modern systems are being produced with devices to bleed off any air accumulating in the reservoir . For cooling purposes a fuel/oil heat exchanger is used. This ensures that cooling on the ground is available. Further air/fluid cooling may be provided once the aircraft is in flight . Since heat exchangers are low pressure devices they are normally situated in the return line to the actuator/service. When a pump is running off load, all the heat generated during its idle running is carried away by the pump case drain line . The heat exchanger should therefore be positioned to cool this flow before its entry into the reservoir. Care must be taken to determine the maximum pressure experienced by the heat exchanger and to ensure that, not only is adequate strength present to prevent external burst, but in addition no failure occurs across the matrix between fuel and hydraulic fluid .
Hydraulic Oil Contamination Testing: Hydraulic Fluid Monitoring Particle contamination is the primary source of component wear and the cause of over half of all hydraulic system failures. One can minimize expensive, dangerous failures by regularly monitoring a hydraulic system’s contamination level by gravimetric, microscopic, or colorimetric analysis. Millipore test kits
Hydraulic Oil Contamination Testing: Hydraulic Fluid Monitoring
Hydraulic Oil Contamination Testing: Particle counting technique In this test the oil is drawn through a membrane of known pore size and the number of particles in a variety of size ranges is counted by viewing the membrane under a microscope . Although this technique is still used today, it is tedious, time-consuming and un-reproducible when compared to other techniques. Other contaminant analysis techniques exist, such as Patch Tests , Gravimetric Analysis and determination of silting indices . All these tests, while providing total contamination levels, provide no information on the distribution of particle size .
You've reached the end of your free preview.
Want to read all 79 pages?
- Fall '20
- Fluid Dynamics, Hydraulic machinery, Hydraulic Systems