Keeping Fluid Clean/Controlling Contamination

The first maintenance objective starts at the time the oil is received and stored and continues through the period of time it is in service in the system. Contamination, such as moisture, can enter ‘‘sealed’’ containers while in storage, through normal expansion and contraction of the fluid due to temperature changes. Moisture thus allowed to enter then condenses inside the containers. Contamination can also result while the oil is being transferred from storage (or containers) to the system. Dirty transfer containers might be at fault, or equip- ment that has been sitting in the open or has been used for other materials such as gear oils, engine oils, and coolants. Dirty reservoirs and debris around the fill location are also sources of contamination during filling or adding makeup oil to the system. In critical systems, sometimes quick-disconnect fittings are installed on reservoirs, or portable filter carts are used to facilitate adding oil to clean oil to these systems. Such measures minimize the potential for contamination.

The fluid in service must be clean, and the level of cleanliness depends on the system. Numerically controlled (NC) machine tools, for example, require high levels of cleanliness to accommodate the close-tolerance servovalves, whereas the hydraulics used to operate hydraulic lifts in automotive repair shops can run satisfactorily with minimal filtration. It should be noted that conventional filters will not remove water- or oil-soluble contaminants. Special coalescing-type filters are available for the removal of limited amounts of water.

B. Filtration

Full flow filtration is the most common type used on hydraulic systems to control the

levels of solid contaminants. These filters are generally installed in the supply (pressure) line but can also be installed in return (low pressure) lines to the reservoir. Full flow filter

Table 7.2 Typical Critical Clearance for Fluid System Components Clearance

Component micrometers inches

Antifriction bearings 0.5 0.000019

Vane pump: tip to vane 0.5–1.0 0.000019–0.000039

Piston pump: valve plate to cylinder 0.5–5.0 0.000019–0.000197

Gear pump: gear to side plate 0.5–5.0 0.000019–0.000197

Gear pump: gear tip to case 0.5–5.0 0.000019–0.000197

Servovalve spool (radial) 1.4 0.000055

Control valve spool (radial) 1.0–23.0 0.000039–0.000904

Hydrostatic bearings 1.0–25.0 0.000039–0.000984

Vane pump sides of vanes 5.0–13.0 0.000197–0.000511

Piston pump: piston to bore 5.0–40.0 0.000197–0.001575

Servovalves: flapper wall 18.0–63.0 0.000708–0.002363

Actuators 50.0–250.0 0.001969–0.009843

housings are generally equipped with bypass valves that open when the pressure drop across the filter exceeds a predetermined level. This assures that components will receive oil in the event of filter plugging or restriction of oil flow through the filter due to start- up or cold oil where viscosities are high. When filters go on bypass, unfiltered oil is supplied to components. Some filters are equipped with condition indicators or differential pressure gages to warn of restrictions or plugging.

Selection of appropriate filtration levels (fluid cleanliness) will be based on specific system components and operation. Table 7.2 shows some of the typical clearances in hydraulic system components. The vast majority of hydraulic systems function properly on 10␮m filtration with filter efficiencies of 98.7% or greater. Filter efficiencies are often referred to as beta ratios (␤). A beta ratio of 75 for a 10 ␮m filter would mean that 98.7% of the particles in the 10␮m and larger range will be removed. Tables 7.3and7.4give a brief explanation of beta ratios and filter efficiencies.

Bypass filters, sometimes referred to as polishing filters, generally are installed in

an independent system where from 5–15% of the system’s oil capacity (in gpm) is filtered to a finer degree. An auxiliary pump is used to take the oil from a low point in the reservoir, whereupon it is filtered and returned to the reservoir. With this type of system, oil purification can be continued whether the hydraulic system is in operation or shut down. An alternative to the independent system is to use a continuous bypass mode, in which a percentage of the oil flow from the pressure or return line is passed through suitable purification equipment and returned to the reservoir. Bypass purification equipment can be relatively small because only a portion of the total oil capacity is handled.

Portable filters, sometimes called filter carts or buggies, are also used to supplement

permanently installed system filters. These units consist of a motor-driven pump and filter arrangement, which circulates fluid from the reservoir, through a fine filter, and back to the reservoir. The suction and return hoses should be connected to opposite ends of the reservoir with quick-disconnect fittings. Generally, portable filters will operate for at least 24 h on each system to ensure that the full oil charge is filtered effectively. Portable filter units can be used in place of bypass filters if periodic or as-needed filtration is sufficient

Table 7.3 Filter Efficiency and Beta Ratios: The filtration ratio or beta is calculated by dividing the number of particles entering the filter by the number of particles exiting the filter. ␤5represents the filtration ratio at 5␮m or the ratio of the upstream to the downstream particles

larger than 5␮m. In Out Out In β ⫽ Particles⬎ 5 ␮m Filter In Out ␤5 Filter A 10,000 5,000 2 Filter B 10,000 100 100 Filter C 10,000 1 10,000

to maintain the desired levels of fluid cleanliness. Portable filtration units can be a simple arrangement, as just discussed, or may be reclamation units consisting of motor-driven pumps, oil heating elements, vacuum chambers, and fine filters. The advantage of reclama- tion units is that they can remove water and some volatile contaminants (such as some solvents) in addition to removal of particulates.

Batch filtration may be used where the fluid volume is very large or is heavily

contaminated. The large volume of oil may be removed and reclaimed by using batch- through settling processes, filtration, centrifuging, and/or reclamation units. The disadvan- tage of this process is that the machine must be shut down for removal of the fluid charge.

Table 7.4 Significance of Beta (␤) Values in Particle Count Control: Beta values can be directly related to efficiency. To determine the relative performance of two filters with different beta ratios, the downstream particle count from each filter can be calculated by using the beta ratio and an assumed upstream particle count (106_{in this example). The filter with the highest}

beta value will have the lowest downstream particle count.

Downstream count: ⬎ X ␮m when filter

␤ Value at Removal efficiency is challenged upstream

X␮m (␤x) (%): particles⬎ X ␮m with 106particles⬎ X ␮m

1.0 0 1,000,000 1.5 33 670,000 2.0 50 500,000 20 95 50,000 50 98.0 20,000 75 98.7 13,000 100 99.0 10,000 200 99.5 5,000 750 99.87 1,333 1,000 99.90 1,000 10,000 99.99 100