In Figure 7-7, a schematic drawing of a hydraulic circuit shows filters in the standard locations, with typical filtration ratings listed next to them. Note that most circuits would not have all of these filters, but every circuit should have adequate filtration to protect the pump, valves, and actuators from contamination.
There are several sources of contamination in and around hydraulic units. Normal component wear, contamination in new oil, sloppy filling practices, poor plumbing installation, and dirt carried in on piston rods are the main ones. Some of these areas are simple to address ahead of time, while others can only be handled by filtration.
New oil from the supplier is not as clean as most hydraulic circuits require. At best about 25-µ cleanliness is all most suppliers will offer. One reason: the drums that new oil comes in probably were used before. Although they were cleaned, they are not contaminant-free. In addition, a standard drum pump draws fluid from the bottom of the drum where all the residue has settled.
Figure 7-8 shows a manually operated 3-way valve installed in the suction line of an offline filter pump. When this valve is shifted, all new fluid has to go through the offline filter before it enters the tank.
Another item in Figure 7-8 is a tank-fill filter with no bypass, which filters all new oil entering the tank. Another option is to use a purchased filter cart to fill the tank through its normal fill port.
Whatever the method, it is important to keep filling practices from introducing contamination. While the fill port offers one of the simplest places to address contamination problems, it often is overlooked.
Another way that contamination enters a hydraulic circuit – even before startup -- is through poor plumbing practices. All pipe, tube, and hoses should be inspected for contamination before installation and cleaned if necessary. It’s good practice to seal clean conductors with caps until installation time. Use care when cutting and preparing pipe or tube ends to make sure no metal chips or filings stay in the conduits. If the system includes servovalves, flush it with filtered oil through flushing covers for the time recommended by the valve manufacturer before startup. Use every possible precaution on a new or replacement plumbing system to make startup go smoothly.
In a running circuit, one of the most common sources of ingressed contamination is the cylinder piston rods. Every time a piston rod extends, it is damp with system oil. In a dusty atmosphere this oil-dampened piston rod attracts and holds fine particles. Many of these are dragged back into the cylinder and washed off. Most cylinders have a rod wiper to help keep out contamination but this wiper only catches large pieces. Everything smaller passes by. This type contamination must be filtered out continuously to protect system components. Some suggest flexible boots or bellows over the rod end of the cylinder to eliminate rod contamination ingression. A flexible bellows does a good job unless it gets a tear or other type hole. At this point it actually sucks in ambient air with its contamination and holds it closely to the rod.
A similar situation takes place at the tank breather. During each cycle, the fluid level in the reservoir changes -- either drawing dirty atmospheric air in or discharging air through the breather. The filler breather should be capable of trapping the inbound contaminants in this air flow.
The other main cause of contamination is normal component wear. Dynamic pumps, motors, and cylinders have constantly rubbing metal-to-metal contact areas. Even with good lubrication, small eroded particles get into the fluid. This contamination must be constantly captured by filters to eliminate a damaging buildup of residue.
Condensed water is another form of contaminant that should be addressed. Water in hydraulic fluid can cause corrosion, break down fluid additives, and make viscosity vary. Water vapor often enters the system through the breather and condenses to liquid form when the tank cools. Using a breather with a hygroscopic media can eliminate most water contamination. Most of these hygroscopic breathers must be changed when they become saturated. They often use silica gel that is blue tinted when dry and turns a pink tint as it gets wet, signaling that a change is needed.