How to prevent pump failure using correct filtration?

The majority of hydraulic pump failures are not the result of malfunctions in the pump; they are caused by contamination of fluid. Around 70-80% of all problems with hydraulic systems can be traced back to contamination. And the pump, being the most precise and highest-pressure part of this circuit, is nearly always the first to suffer. Making sure that the pump is properly filtered involves matching the micron values and beta ratios to the tolerance limits of the pump by placing filters at the correct locations within the circuit, checking the cleanliness of the water with respect to ISO 4406 targets, and keeping a filtering strategy in place rather than a one-time installation.

Pumps fail silently for a long time before they show signs of failure. When the technician hears a cavitation sound or detects metal particles in the reservoir, the internal clearances have usually already been damaged beyond the point of being able to operate. The correct filtration strategy is the most efficient way to prolong pump life. Knowing how to create and sustain a filtration program is crucial for any person accountable for the reliability of hydraulic systems.

Why is filtration the first protection?

Hydraulic pumps have extremely precise internal clearances—typically within the range of 0.5-5 microns. These clearances are between components like pistons, vanes, gears, and the surfaces that they mat. This clearance is what allows the pump to produce pressure in an efficient manner. Anything that is greater in diameter than that clearance functions as an abrasive and is able to wedge between surfaces, creating three-body wear.

The wear is gradual and is accelerating. The pump that begins to wear due to contamination produces more wear debris within the pump, which in turn reduces the cleanliness of the pump, which increases damage to downstream equipment, such as valves and the cylinders. Filtration is a way to stop the cycle before it begins.

The three sources of contamination Pumps are exposed

1. Inbuilt contamination—remnants of manufacturing, welding slag, or assembly contamination found in tanks, hoses, and fittings prior to commissioning.
2. Ingressed contaminants—dust and moisture that enter through the breather caps or worn seals during top-offs of fluid and reservoir maintenance.
3. Produced contamination -- wear particles generated internally by the valves, pumps, and cylinders in normal operation. They then return to the system unless they are captured.

A proper filtration strategy takes care of all three of them, not just the third.

Microns of the filter that match to the tolerance of the pump

The selection of filters should be guided by the type of pump used rather than by preference or habit. Pumps with gears are typically more resilient to pollution than vane pumps and piston pumps—especially those used for axial pistons in high-pressure systems are among the least resistant because of their tinier clearances between the bore and the piston.

General filtration goals for pump type:

• Gear pumps A 25-micron (absolute) filtering is usually enough
• Vane pumps: 10 microns of absolute filtration is recommended
• Piston pumps 5-10 micron absolute filtration for high-pressure piston pumps, usually needing three micron

Micron ratings alone are an uncomplete metric. It is the beta ratio that the element is rated at, a measure of the efficiency of its capture of particles that are smaller than that. A filter that is rated at 10 microns, with an alpha ratio of 2. Beta10 (2) eliminates only 50% of particles 10 microns in size with a single pass. A filter that has Beta10 = 200 eliminates 99.5 percent of those particles. When it comes to piston pump parts, those that have large beta ratios (beta > 200 at the micron target dimensions) will be well worth the extra expense, as the repair costs downstream of pump failure are much greater than the price of filter media.

The best place to put filters is within the circuit

The placement of filters is as crucial as the filter rating. Many systems benefit from filtering across multiple points instead of relying solely on one filter.

Suction Filtration

Suction strainers guard the pump's inlet from debris of all sizes, but generally, they are very coarse (74-149 micron) as a restriction on suction flow could cause cavitation of the pump. Suction filtration is not enough to ensure the internal clearances of a pump—it is designed to protect against the catastrophic ingestion of large particles but not to reach the desired levels of cleanliness.

Pressure line filtration

Pressure filters, which are installed just downstream of the pump, provide the best filtration since they do not pose the risk of cavitation that is associated with suction-side restrictions. They're the best protection against the downstream components, such as proportional valves. They remove fluid after having been through the pump for a single time.

Return line filtration

Return line filters remove the contamination caused by valves, cylinders, and even the pump itself prior to the fluid's return to the reservoir. This is usually the most economical filtration option since it operates at a lower pressure, allowing finer media, without the structural expense of high-pressure housings. Furthermore, it stops the reservoir from becoming a reservoir for contaminants.

Offline (Kidney-loop) filtration

For large reservoirs or systems in which clean fluid is essential, such as mobile equipment, wind turbine hydraulics, or systems that use proportional and servo valves, an offline filtration loop constantly polishes the reservoir fluid, regardless of the system's operation. It is increasingly prevalent for systems built around IIoT-integrated valve monitoring in which the same level of fluid quality is regarded as an operational variable rather than as a maintenance niggling factor.

Monitoring and setting ISO clean codes

Filtration equipment on its own doesn't provide security; it only guarantees some. The real protection is checking that the fluid is at an acceptable level of cleanliness and remains there.

ISO 4406 cleanliness codes express contamination in three numbers that represent the number of particles per milliliter at 4 and 6 as well as 14 microns (e.g., 18/16/13). Every pump type has a suggested target:

• Pumps for gears: ISO 20/18/15
• Vane pumps: ISO 19/17/14
• Piston pumps (standard pressure): ISO 18/16/13
• Piston pumps (high pressure and servo/proportional systems): ISO 16/14/11

Regular sampling of oil and counting particles (ideally every quarter or as per OEM guidelines -- can confirm whether the filtration system is actually meeting its goal. A filter that appears to be operating (no bypass indicator activating) could nevertheless allow the cleanliness to slip off the mark when it's not sized correctly for the flow rate of the system or when bypass valves are opened in response to spikes in viscosity during cold start.

Common filtration errors that can cause pump failure

• Inattention to bypass indicators. A bypass filter is not filtering anything. Bypasses at cold start are not uncommon and are often overlooked; however, frequent bypass cycles mean that unfiltered fluid is circulating to the pump frequently.
• Oversizing the filter housings to make cost savings. Filter housings that are too small cause excessive pressure drop that can cause bypass valves to open early and decrease the efficiency of filtration.
• Do not forget to use breather caps. An inline filter of high-quality with a conventional, non-desiccant breather cap can create an entry point for contaminants that downstream filtration is unable to fully make up for.
• Filter changes are made on the basis of a calendar fixed instead of by conditions. Differential pressure gauges and clean monitoring provide a better idea of when replacement is required instead of a general 90-day period.
• Mixing the filtration strategy and the viscosity of the fluid. Fluid that's too viscous to be used in ambient conditions can cause pressure drop across the filter media, speeding up bypass events independent of the contamination load.

The process of creating a maintenance routine for your filtration system

A sustainable strategy for filtration combines the choice of hardware with a surveillance frequency:

1. Select the micron size of the filter and the beta ratio that is matched to the type of pump at the time of commissioning.
2. Install return-line filtration to serve as an example, and use the pressure-line filter for systems that protect vulnerable valves.
3. Create ISO 4406 cleanliness targets specific to the pump type that is in use.
4. Test fluid and sample for a specific time period and not only after obvious problems are apparent.
5. Replace filters based on the differential pressure readings and not only on the time.
6. Maintain and inspect the breather caps and seals in the same routine, because the control of ingress is linked to the performance of filtration.

Failure of the pump due to contamination is almost always preventable. The expense of proper filtering is always less than the expense of replacing the pump and the unplanned downtime and the subsequent damage worn-out pumps can result in to cylinders and valves. The idea of treating filtration as an engineered process instead of a piece of equipment that was purchased and then discarded is what differentiates hydraulic systems that have a long service life from those that have frequent, unproven pump failures.

What filter with the highest micron rating is the most effective in preventing failure of the hydraulic pump?

The right micron rating will depend on the pump type: 25 microns is the standard for gear pumps, 10 microns for vane pumps, and 5-10 microns (or larger in high-pressure applications) for piston pumps. Beta ratios of filters can be a factor just as the micron rating itself, as it affects the actual efficiency of capture at the particle size.

Does a suction filter by itself shield a hydraulic pump from failing?

No. Suction filters are generally large to keep flow from being restricted and causing cavitation. Therefore, they can only take in large pieces of debris. Fine filtering for protection of pumps is required from return-line or pressure line filters, usually paired with an offline loop of filtration for critical systems.

What exactly is ISO 4406, and why does it matter in pump filtering?

ISO 4406 is a three-number code (e.g., 18/16/13) that represents the number of particles per milliliter for 4, 6, and 14 microns. It offers a precise standard for cleanliness so that the filter performance can be confirmed by oil sampling, rather than assuming it based on the installation of filters on its own.

How often should filters for hydraulics be changed to avoid wear on the pump?

Replacement should be determined by different pressure readings as well as sampling of cleanliness rather than the fixed calendar. If a filter is in the process of bypassing as indicated by an increasing pressure drop, it needs to be replaced no matter the time since it was put in place.

Does a bypass valve's triggering indicate that the pump isn't protected?

Yes, temporarily. If a filter is bypassed, unfiltered fluid flows directly into the system, which includes the pump. The frequent bypasses, particularly when the pump is cold, suggest the filter is not sized correctly to handle the velocity or flow rate and must be reviewed.