So where do you put those hydraulic filters?
Published in: Hydraulics & Pneumatics
Date: 3/1/2004
By: Hitchcox, Alan L
Every hydraulic system benefits greatly from filtration, but which type of filter arrangement – suction, pressure, return, or off-line – is best for a specific system? Here are some recommendations from Bryan Chaka of Parker Hannifin’s Hydraulic Filter Div., Metamora, Ohio:
Suction filters protect the pump from fluid contamination. They should be located upstream from the inlet port of the pump. Some suction filters may be simple inlet strainers submerged in the fluid. Others may be externally mounted. In either case, they have relatively coarse elements to prevent pump cavitation. In fact, some pump manufacturers recommend not using suction filters. For this reason, they are not used as a system’s primary protection against contamination. Always consult the pump manufacturer for particulars about inlet restrictions.
Pressure filters are located downstream from the system pump. They are designed to handle the system’s operating pressure and sized for specific flow rate in the pressure line where they are installed. Pressure filters are especially suited for protecting sensitive components directly downstream from the filter. Because of their location, pressure filters also help protect the entire system from pump-generated contamination.
Return filters may be the best choice when the pump is the most contamination-sensitive component in a system. In a typical installation, the return filter is the last component through which fluid passes before entering the reservoir. Therefore, it captures wear debris from system components as well as particles entering through worn cylinder rod seals – before such contaminants can enter the reservoir and potentially be circulated by the pump. Because return filters are located immediately upstream from the reservoir, their pressure rating – and cost – can be relatively low.
Both pressure and return filters often are available in a duplex configuration that can provide continuous filtration. Duplex filters have two or more filter chambers with internal valving to direct inlet flow to a specific chamber. When an element becomes blocked, a duplex valve shifts, diverting flow to another filter chamber. The dirty element then can be changed, while flow continues to pass through the alternate filter chamber to the system.
Off-line filtration – also referred to as recirculating, kidney loop, or auxiliary filtration – is totally independent of a machine’s operating hydraulic system. Off-line filtration consists of a pump, electric motor, pressure filter, and appropriate hardware connections. These components are installed as a subsystem separate from the working hydraulic system. The pump moves fluid continuously from the reservoir, through the filter, and back to the reservoir. This continuous recycling helps maintain a constant cleanliness level of the fluid.
As with return line filters, this type of system is best suited to maintain overall cleanliness, but does not protect specific components. An off-line filtration loop is relatively easy to retrofit on an existing system that has inadequate filtration. Also, the filter can be serviced without shutting clown the main system.