Filters are an important part of the lubrication system because they remove foreign particles that may be in the oil. This is particularly important in gas turbines as very high engine speeds are attained; the antifriction types of ball and roller bearings would become damaged quite rapidly if lubricated with contaminated oil. Also, there are usually numerous drilled or core passages leading to various points of lubrication. Since these passages are usually rather small, they are easily clogged.
There are several types and locations of filters used for filtering the turbine lubricating oil. The filtering elements come in a variety of configurations and mesh sizes. Mesh sizes are measured in microns, which is a linear measurement equal to one millionth of a meter (a very small opening).
A main oil strainer filter element is shown in Figure 6-34. The filtering element interior is made of varying materials including paper and metal mesh. [Figure 6-35] Oil normally flows through the filter element from the outside into the filter body. One type of oil filter uses a replaceable laminated paper element, while others use a very fine stainless steel metal mesh of about 25–35 microns.
Most filters are located close to the pressure pump and consist of a filter body or housing, filter element, a bypass valve, and a check valve. The filter bypass valve prevents the oil flow from being stopped if the filter element becomes clogged. The bypass valve opens whenever a certain pressure is reached. If this occurs, the filtering action is lost, allowing unfiltered oil to be pumped to the bearings. However, this prevents the bearings from receiving no oil at all. In the bypass mode, many engines have a mechanical indicator that pops out to indicate the filter is in the bypass mode. This indication is visual and can only be seen by inspecting the engine directly. An antidrain check valve is incorporated into the assembly to prevent the oil in the tank from draining down into the engine sumps when the engine is not operating. This check valve is normally spring loaded closed with 4 to 6 psi needed to open it.
The filters generally discussed are used as main oil filters; that is, they strain the oil as it leaves the pump before being piped to the various points of lubrication. In addition to the main oil filters, there are also secondary filters located throughout the system for various purposes. For instance, there may be a finger screen filter that is sometimes used for straining scavenged oil. These screens tend to be large mesh screens that trap larger contaminants. Also, there are fine-mesh screens called last chance filters for straining the oil just before it passes from the spray nozzles onto the bearing surfaces. [Figure 6-36] These filters are located at each bearing and help screen out contaminants that could plug the oil spray nozzle.