The filter housing is the pressure vessel which contains the filter element. It usually consists of two or more subassemblies, such as a cover (or head) and a removable bowl that allows access to the element. The housing has inlet and outlet ports that enable fluid to enter and leave. Housing options may include bypass valves and element-condition indicators.

Primary concerns in the housing-selection process include mounting methods, porting options, indicator options, and pressure rating. Except for the pressure rating, all depend on the physical system arrangement and the preferences of the system designer. Pressure rating of the housing is far less arbitrary; it is determined by system needs before the housing style is selected.

Pressure ratings

Location of the filter in the circuit is the primary determinant of pressure rating of the component. Filter housings are generically designed for one of three locations in a circuit: suction, pressure, or return lines. One characteristic of these locations is their maximum operating pressures. Suction and return line filters are generally designed for lower pressures - 500 psi or less. Pressure filter locations may require ratings from 1,500 to 6,000 psi.

Note that it is essential to analyze the circuit for pressure-spike potential as well as steady-state conditions. Some housings have restrictive or lower fatigue pressure ratings. In circuits with frequent high-pressure spikes, another type housing may be necessary to prevent fatigue-related failures.

Bypass valves

Bypass valves open flow paths around filter elements to prevent their collapse or bursting when they become heavily loaded with contaminant. As contaminant builds up in the element, the differential pressure across the element increases. At a pressure well below the failure point of the filter element, the bypass valve opens, allowing flow to go around the element. Some bypass valve designs have a bypass-to-tank option. This directs the unfiltered bypass flow back to the tank through a third port, preventing unfiltered bypass fluid from entering the system. Bypass valves also prevent pump cavitation when used with suction line filters. When specifying a bypass-type filter, it generally can be assumed that the manufacturer has designed the element to withstand the bypass valve differential pressure when the bypass valve opens.

Note that some of the upstream contaminant particles also bypass the filter element with the fluid and enter the downstream system. When this happens, the effectiveness of the filter element is compromised and the attainable system fluid cleanliness degrades.

Other filters are designed specifically with no bypass valve (sometimes called a blocked bypass). They prevent any unfiltered flow from going downstream, thus protecting servovalves and other contaminant-sensitive components. In filters without bypass valves, higher collapse-strength elements may be required, especially if installed in high-pressure locations. When specifying a non-bypass filter design, make sure that the element has a differential-pressure rating close to the maximum operating pressure of the system, and that the filter has a condition indicator.

After a housing style and pressure rating are selected, the bypass valve setting needs to be chosen. This setting must be established before sizing the filter housing. Everything else being equal, the highest bypass cracking pressure available from the manufacturer should be specified. This will provide the longest element life for a given filter size. Occasionally, a lower setting may be selected to help minimize energy loss in a system, or to reduce backpressure on another component. In suction filters, either a 2- or 3-psi bypass valve is used to minimize the chance of potential pump cavitation.

Element-condition indicators

The element-condition indicator signals when the element is loaded to the point that it should be cleaned or replaced. The indicator usually has calibration marks which also indicate if the bypass valve has opened. The indicator may be linked mechanically to the bypass valve, or it may be an entirely independent differential-pressure sensing device. Indicators may give visual or electrical signals or both. Generally, indicators are set to trip at a differential pressure anywhere from 5% to 25% below that which opens the bypass valve.

Sizing housing and element

The filter housing size should be large enough to achieve at least a 2:1 ratio between the bypass valve setting and the pressure differential of the filter with a clean element installed. For longer element life, this ratio should be 3:1 or even higher.

Referring to typical flow/differential-pressure curves from a manufacturer's catalog, the filter specifier needs to know the operating viscosity of the fluid and the maximum (not the average) flow rate to assure that the filter does not spend a high portion of time in bypass due to flow surges. This is particularly important in return-line filters, where flow multiplication from large cylinders may increase the return flow compared to the pump's flow rate.

Always consider ambient temperature conditions when sizing filters. Low ambient temperatures may increase fluid viscosity to the point where pressure differential across the filter assembly also may increase considerably.

If a filter was fitted with a 50-psi bypass valve, the initial (clean) pressure differential should be no greater than 25 psi and preferably 16 2/3 psi or less. These pressures are calculated from the 3:1 and 2:1 ratios of the 50-psi bypass setting and initial pressure differential.

Standard filter assemblies normally are manufactured with a bypass-valve cracking pressure between 25 and 100 psi. The bypass valve in most of these assemblies actually limits the maximum pressure drop across the filter element. As the element becomes blocked with contaminant, the pressure differential increases until it reaches the bypass valve cracking pressure. At this point, part of the flow through the filter assembly begins to bypass the element through the valve. This action limits the maximum pressure differential across the filter element.

The relationship between the starting clean pressure differential across the filter element and the bypass valve pressure setting must be considered. A cellulose element has a narrow region of exponential pressure rise. For this reason, the relationship between the starting clean pressure differential and the bypass valve pressure setting is very important. This relationship in effect determines the element's useful life.

In contrast, the useful element life of single-layer and multi-layer fiberglass elements is established by the nearly horizontal, linear region of relatively low pressure drop increase, not the region of exponential pressure rise. Accordingly, the filter assembly's bypass valve cracking pressure, whether 25 or 75 psi, has relatively little impact on the useful life of the element. Thus, the initial pressure differential and bypass valve setting is less a sizing factor for fiberglass media.

Filter types and locations

The type of filter - suction, return, pressure, or off-line - and its physical location in the circuit are almost inseparable by definition.

Suction filters serve to protect the pump from fluid contamination. They are located upstream from the pump's inlet port. Some may be simple inlet strainers, submersed in fluid in the tank. Others may be mounted externally. In either case, suction filters have relatively coarse elements, due to cavitation limitations of pumps. (Some pump manufactures do not recommend the use of a suction filter. Always consult the pump manufacturer for inlet restrictions.) For this reason, suction filters are not used as a system's primary protection against contamination, and in fact, the use of suction strainers and filters has greatly decreased in modern hydraulic equipment.

Return filters may be the best choice if the pump is particularly sensitive to contamination. In most systems, the return filter is the last component through which fluid passes before entering the reservoir. Therefore, it captures wear debris from all of the system's working components and any particles that enter through worn cylinder rod seals before such contaminant can enter the reservoir and be pumped back into the system. Because this filter is located immediately upstream from the reservoir, its pressure rating and cost can be relatively low.

Note that retracting some cylinders with large diameter rods may result in flow multiplication. This high return-line flow rate may open the filter bypass valve, allowing unfiltered fluid to pass downstream. This probably is an undesirable condition and should be considered when specifying the filter.

Pressure filters are located downstream from the system pump. They are designed to handle the system pressure and are sized for the specific flow rate in the pressure line where they are located. Pressure filters are especially suited for protecting sensitive components, such as servovalves, directly downstream from the filter. Because pressure filters are located just downstream from the pump, they also help protect the entire system from any pump-generated contamination.

Duplex filters, a common special configuration, may include both pressure and return filters. Duplex filters provide continuous filtration. They have two or more filter chambers and include the necessary valving to allow for uninterrupted operation. When one filter element needs to be serviced, the duplex valve is shifted, diverting flow to the opposite filter chamber. The dirty element can then be changed, while flow continues to pass through the cleaner element. The duplex valve typically is an open cross-over type, which prevents any flow blockage.

Off-line filtration is an increasingly popular filtration arrangement. Also referred to as recirculating, kidney loop, or auxiliary filtration, it operates independent of a machine's main hydraulic system. This makes it attractive as a retrofit project for problem systems. An off-line filtration circuit includes its own pump and electric motor, a filter, and the appropriate connecting hardware. These components are installed off-line as a small subsystem separate from the working lines, or they may be included in a fluid-cooling loop. Fluid is pumped continuously out of the reservoir, through the off-line filter, and back to the reservoir. A rule of thumb: the off-line pump should be sized to flow a minimum of 10% of the main reservoir volume.

With its polishing effect, off-line filtration is able to maintain the system's fluid at a constant contamination level. As with a return line filter, the off-line loop is best suited to maintain overall system cleanliness; it does not provide protection for specific components. An off-line filtration loop has the added advantage that it is relatively easy to retrofit on an existing system that has inadequate filtration. Also, the off-line filter can be serviced without shutting down the main system.