Machinery's Handbook, 31st Edition
Lubricant Filtration 2521 A diagram and tabulation of the filtration requirements of a closed loop hydraulic sys tem are shown in Table 20. Fluid in the reservoir is sucked up by the pump through the suction filter (A) and pumped into the delivery header line under pressure. If a depth filter option is used, a small percentage, up to 15 percent, of the oil flow is diverted for deep cleaning through a depth filter (H) and sent back to the reservoir for recycling. The lubri - cant is then forced through the primary pressure filter and allowed to perform its work at the bearing point or hydraulic device before it eventually channels into the system return line under gravity to pass through a low pressure return line filter that takes out any wear materials gathered along the way. Once through the return filter the oil makes its way back into the reservoir. The reservoir is protected against airborne contaminants and conden- sation by the vent filter, and is protected against ingress of course solids by the fill neck screen filter. Because water is heavier than oil it will settle to the bottom of the tank where most of it can be drained off by opening the drain valve. Metallic debris also settle to the bottom and are captured by the magnetic drain plug at the bottom of the reservoir. As the lubricant oxidizes and breaks down, sludge will form on the bottom of the reservoir, which must be cleaned out periodically manually by removing the reservoir clean-out hatch. Filter Efficiency: Most surface filters are sold in either one-time-use, or cleanable-reus able forms. Depth filters are all one-time-use filters. All filters are performance rated according to the media’s particle removal efficiency, known as the filter’s filtration ratio, or beta ratio . Not all filters are made equal, and they are tested for dirt holding capacity, pressure differential capability, and filter efficiency, using an ISO 4572 Multipass Test Procedure in which fluid is circulated through a mock lube system in a controlled manner. Differential pressure across the test filter element is recorded as contamination is added into the lubricant upstream of the filter. Laser particle sensors determine contamination levels both upstream and downstream of the filter element and the beta ratio is determined using the following formula: (6) where B = filter filtration ratio x = specific particle size Example: If 100,000 particles, 10 microns and larger, are counted upstream of the test filter, and 1000 particles are counted after or downstream of the test filter element, the beta ratio would equal: , B 1000 100 000 10 = Efficiency is determined using the following equation: B # Downstream Particulate #Upstream Particulate x =
1 1 B x – --- 1 1 100 – -----
100 ×
Efficiency x
=
(7)
100 ×
=
=
99%
Efficiency 10
The higher the beta ratio, the better the capture efficiency of the filter, see Table 21. Table 21. Filter Efficiency Beta Ratio
Filter Efficiency at Same Specific Particle Size
Beta Ratio at a Specific Particle Size
Filter Efficiency at Same Specific Particle Size
Beta Ratio at a Specific Particle Size
Filter Efficiency at Same Specific Particle Size
at a Specific Particle Size
1.01 1.1 1.5
1% 9%
5
80% 90% 95%
100 200 1000
99% 99.5% 99.9%
10 20 75
33% 50%
2
98.7%
…
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