One of the most important parameters to determine when implementing a fluid sampling program is deciding the proper sampling frequency. There are many variables to consider when determining a sampling frequency, such as the equipment value, system criticality, operating environment equipment condition, and the system usage. Determining a close approximation to an ideal sampling frequency right from the beginning will eliminate costs from over sampling with minimal gains and will ensure sampling is frequent enough to eliminate unexpected breakdowns.
Equipment Value
When determining the appropriate sampling frequency for individual machines it is important to consider the value of the equipment. Custom fabricated machines designed for a single task can be very expensive to purchase and owners typically want their equipment to retain its high value. Considering the value of the equipment will help determine if a fluid sampling program is beneficial, or whether the current sampling frequency can be adjusted appropriately.
System Criticality
Deciding the criticality of the system and other important factors such as the effects of breakdowns, operator safety, productivity, maintenance costs, and the down time for repair will reflect the appropriate sample frequency. Systems that are critical to equipment operation and performance should be sampled more frequently to monitor the system and components more closely. A greater sampling frequency will increase the operating cost of the sampling program but can significantly reduce the severity of breakdowns and costs will be recouped over a longer period of time. Determining system criticality can be difficult at times but the main concern is to determine how reliable the system must be for your application. Sampling frequency can vary anywhere between continuous to monthly or to annually all depending on the reliability required.
Operating Environment and Equipment Condition
The sampling frequency should reflect the cleanliness of the operating environment. Equipment operating in dusty and dirty off-road environments as well as humid and wet conditions will require sampling on a more frequent basis due to the increased amounts of airborne particles that can ingress into the fluid system and to monitor moisture contamination. Fluids operating at higher than recommended temperatures tend to significantly reduce in viscosity as temperature increases which can affect the lubricating properties and lead to increased component wear and heat generation. Fluid systems working with harsh chemicals can react more seriously to moisture and debris contamination and therefore should have tighter limits on fluid contamination, which can be monitored through more frequent sampling. Equipment typically goes through two phases where failure is more probable which occurs when the equipment is new and in the wear-in phase and then again in the wear-out phase. To offer the greatest protection in the early and late stages of equipment use, sampling should be performed in greater frequency to more closely monitor wear-in and wear-out conditions. Fluid also has a wear-out phase where the fluid experience deterioration due to the continuous loading and unloading, heating and cooling, and from the accumulation of contaminants and debris.
System Usage
Systems used for heavy loading, rapid cycling, and high operating pressures will require sampling on a more regular basis due to higher stress imposed on the fluid to perform work, higher operating temperatures, and contamination risks due to worn seals and wipers. Examples of where hydraulic rams and cylinders experiencing this kind of loading can be found in construction, forestry, agricultural, and mining applications. Lubricating oils found in low speed gearboxes for example can operate much longer between fluid samples due to lighter duty work, slower speeds, and minimal pressure. Systems that experience only periodic loading or non-continuous work will produce less wear metals but the fluid is more susceptible to moisture contamination since the system rarely reaches high enough temperatures to evaporate moisture.
Conclusion
Determining a proper sampling frequency will increase the efficiency of the sampling program by sampling only as required for the level of reliability. Sample frequencies can also be adjusted to suit the failure probability of the equipment throughout its production lifecycle.
Table 1: Approximate Sample Frequencies
System
|
Sample Frequency (Hours)
|
Diesel Engine
|
250-350
|
Transmissions, Final Drives
|
400-500
|
Differentials
|
400-500
|
Gearbox, Low speed
|
1000-1200
|
Gearbox, High speed
|
300-400
|
Turbines, Gas and Steam
|
500-600
|
Bearings and Compressors
|
500-600
|
Industrial Hydraulics
|
650-800
|
|