Establishing Moisture Contamination Targets for Hydraulic Systems

Water contamination in hydraulic systems can devastate an organization’s reliability objectives. Fortunately, with a diligent effort, water contamination can be effectively controlled by setting goal-based target dryness levels, achieving the targets through effective exclusion and removal of water and periodic monitoring to ensure that target levels are maintained.

The critical first step is to establish target levels that reflect the organization’s reliability goals and take into account the mechanical sensitivity of the hydraulic system in question.

Rust and corrosion are the most obvious effects of water contamination. However, water also lies at the root of vaporous cavitation, hydrogen-induced embrittlement and blistering (Figure 1) and fatigue wear in rolling contacts.

Figure 1. Hydrogen-induced Embrittlement and
Blistering Caused by Water Contamination

Water’s destructive path extends beyond the machine to the lubricant itself. Water promotes base oil oxidation and hydrolysis, additive degradation and washing, microbial growth and formation of suspended ice crystals in cold application.

Water enters the hydraulic system at points where the system interfaces with its environment. Condensation is common in moisture-rich environments - particularly where the machine is frequently started and stopped. New oil is often contaminated with water due to poor handling practices. Cooler leaks, exposure to rain and direct water spray also result in contaminant ingestion.

Setting Moisture Contamination Targets

The first step in moisture contamination control is to establish appropriate target moisture content levels. The decision should be driven primarily by mechanical sensitivity and economics. The benefits associated with controlling moisture contamination should sufficiently outweigh the costs to provide a reasonable return on investment.

Research on rolling element bearings, which operate primarily under elastohydrodynamic lubrication, suggests that halving the fluid moisture level increases the life of the bearing by roughly half-again over the normal life when all other variables remain constant.

In other words, a bearing with a normal life of 1,000 hours at 500 ppm water would last about 1,500 hours by reducing the oil’s moisture level to about 250 ppm. Likewise, reducing water contamination to about 125 ppm would increase the same bearing’s life to about 2,300 hours of service (Figure 2).

Figure 2. Rolling-Element Bearing Life vs.
Water Contamination Level

Other research on journal bearings, which operate under hydrodynamic lubrication, suggests that halving the average water contamination level reduces wear rate by about 20 percent (Figure 3). For example, reducing the moisture level from 500 ppm to 250 ppm in a plain bearing would increase the component’s life from 1,000 hours to 1,200 hours, on average.

Likewise, decreasing moisture from 500 ppm to 125 ppm would yield an increase in component life from 1,000 hours to almost 1,500 hours (Figure 3).

Figure 3. Journal Bearing Wear Rate vs.
Water Contamination Level

Hydraulic systems operate under both hydrodynamic and elastohydrodynamic lubrication regimes, and are also typically at risk for cavitation-related wear. Research at the Nippon Mining Co., Ltd. in Japan revealed a substantial increase in hydraulic pump wear with the addition of just 500 ppm of water.

Two oils were tested in 22 gpm vane pumps at 112 bar of pressure. Wear generation more than doubled for oil X, and increased by orders of magnitude when oil Y was used (Table 1).

Table 1. Increased Pump Wear in
the Presence of Water is Evident

Water increases wear under both hydrodynamic and elastohydrodynamic lubrication regimes. Due to variations in hydraulic system design, fluid type and operating conditions (such as pressure), the relationship between water contamination levels and wear rates would likewise vary for the systems.

It seems reasonable to deduce however that the effect of halving water contamination levels could reduce wear rates by 20 to 50 percent for pumps and other hydraulic components subject to triboligcal wear. Given that mechanical reliability is related to moisture contamination levels, the question for the technologists becomes how much reliability do you want to buy through moisture contamination control?

A good starting point for assigning moisture target levels is the equipment supplier’s manual, if such a recommendation is available. You should then adjust this recommended level according to the following factors:

• Safety Requirements - If a failure or repair of a failure places people at risk, or if your organization’s safety assurance requirements are stricter than normal for a class of equipment, the moisture target should be adjusted downward.
• Reliability Goals - If production losses caused by a hydraulic system failure are unusually high, or the expected duration of lost productivity caused by a failure is abnormally high, adjust your target down.
• Application Severity - If your machine is operating at the outer limit of its design capability, adjust the target moisture level down.
• Environment Severity - If the risk of moisture contaminant ingestion is higher than normal, adjust the target down.
• Repair Costs - Systems with expensive or hard-to-get parts, or those that are difficult to repair due to lack of access or maintainability, require tighter-than-average moisture control.

One systematic approach for developing target cleanliness levels for moisture has its roots in reliability-centered maintenance (RCM). Start by evaluating the machine’s mission criticality using the reliability penalty factor (RPF) calculator (Figure 4).

Click Here to See Figure 4. Reliability Penalty Factor

The RPF method rates the machine as a function of mission criticality, cost to repair and effectiveness of any early warning systems, like oil analysis, that are used to detect failures. While the RPF is systematic and it produces a number as output, it is not truly quantitative.

Tools like the RPF calculator are most accurate when they reflect the consensus of a representative group of organizational stakeholders. So employ a Delphi-type method to produce RPF scores that represent the collective opinions and experience of the stakeholder group.

Click Here to See Table 2. Hydraulic Target Dryness

Once the RPF score is defined, refer to Table 2 to arrive at a recommended target moisture level. The recommendations are experience-based and have proven useful for plant-level engineers. This target level should serve as a starting point. Adjust these levels according to field conditions.

If 200 ppm is easily achieved, it might be reasonable to push the limit down to 100 ppm and so on. Conversely, if achieving 200 ppm seems unrealistic given available technology, adjust the goal upward accordingly. Ideally, the water contamination should be kept below the oil’s saturation point. Economics should drive your moisture contamination control efforts. Discontinue efforts when further control becomes economically unviable.

Water contamination adversely affects the health of hydraulic machines and fluids. Its control is central to reliability, dependability and low cost of equipment ownership. Water contamination control requires the establishment of a sensible goal-based target level, contaminant exclusion and removal initiatives to achieve the target. Getting the target moisture level set is the critical first step. It drives all other water contamination control decisions.

References

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13. Troyer, D. “Estimating Values in the Absence of Real Data - Deploying the Delphi Method.” Practicing Oil Analysis magazine, January 2002.