Obtaining a representative oil sample from a closed loop lubrication system is essential to any good oil analysis program. As oil analysis programs become more standardized for routine monitoring, especially within equipment warranty support programs, equipment makers provide more sampling points on new equipment. These access sampling points make it easier for operators to quickly and easily take representative oil samples. This article outlines an overview of the theory behind obtaining consistent oil samples, current accessories for oil sampling, and offers guidance on when and where to sample (if your equipment does not have a OEM supplied point).
Understanding Dynamic Particle Equilibrium in Closed Loop Systems
Dynamic Equilibrium Condition (DEC) is defined as a steady state condition where the normal wear rate in a machine results in no net gain or loss of particles. Knowing what that level is in any lubrication system is necessary in order to detect departures from this level as a result of an abnormality. At equilibrium operating conditions, the rate of wear particle generation is constant. Under the same conditions, the particle separation and removal rates, although size dependent, are also constant, surprisingly, with or without a filter. Not surprisingly, the population of all but the smallest particles reaches an equilibrium level. Departure from that normal level is an indication of an abnormality in the system; perhaps the filter is bypassing or has failed, or a severe wear mode has begun. The smallest particles remain in the system, suspended and unaffected by the filter, so their population tends to increase during the life of the oil. Therefore, very fine particles are composed of the oldest particles, some of which may have been generated as larger particles, but have been reduced in size by various chemical and physical processes. The larger particles are relatively new and are more representative of the current condition of the wearing surfaces. The particles in an oil sample tend to settle out. If they are large and dense, they settle out rapidly. The particles of wear, corrosion, oil degradation, and contamination provide valuable diagnostic information about the condition of the oil as well as the condition of the wearing surfaces of the machine. Since particles exist as a separate phase in the oil, they are not evenly distributed in the system. All the freshly made wear particles will be present immediately after a wearing mechanism such as a roller bearing, gear, sliding surface, etc. The largest metal particles, which are of such critical interest to the analyst, are soon
removed by settling in the sections with slow moving oil, such as the sump, or they are filtered or otherwise separated. The very smallest particles tend to remain suspended and pass through all but the finest filters so they are generally distributed evenly throughout the oil piping system. Consequently, in order to capture a representative sample, the sampling location must be carefully considered (Figure 2).
Figure 1: Sampling points preinstalled on a CAT 320D excavator. Caterpillar SOS program relies on good oil analysis sampling to ensure warranty program is successful
Sampling in a Closed Loop System
The best primary location to monitor in a closed loop system is AFTER the area of greatest wear/stress, and BEFORE the filter. This may be before the return line filter housing, or in the reservoir at the return line zone. Secondary sampling points may be chosen also, such as AFTER the filter, and may be used to evaluate filtration efficiency (Figure 3). Another sample point is the sump or reservoir.
When sampling from lubrication lines, the sample should be taken from the “isokinetic line of flow” in the line. In other words the velocity of the oil at the point of sampling should be equal to the velocity of the oil at the midpoint of the line. A common sampling point to achieve this is on the elbow of the return line (Figure 4). Where and how to sample hydraulic lines is detailed in the ISO 4021 for hydraulic systems. A recent advance in sampling technology is to install “sample boxes” on equipment with multiple lubricant compartments and/or other fluids to be sampled, such as coolant, or DEF fluid. Sampling lines are installed and a junction box created so that an operator can sample all compartments with a dedicated point a mileage or hourmeter is installed for record keeping. This approach is gaining traction with larger mining equipment such as excavators and large trucks (Figures 5 and 6).
Sampling from Pipes
• The pipe ideally will contain oil soon after draining from the wearing part.
• The flow rate should ideally be turbulent to keep particles entrained in the oil.
• The oil should be hot; do not sample after cooling.
• If the pipe is large and the flow is laminar, take a sample from near the center of the pipe to avoid the lower wall where old debris may accumulate.
• If the sample valve is installed with a dead leg line, be sure to flush thoroughly before collecting a sample.
Sampling from Tanks
• Take the sample from near the center and well clear of the bottom or the sides. Use a pitot tube attached to a sampling port installed on the sidewall to ensure the sampling area of interest is addressed. Be mindful of the extra dead leg in the pitot tube line – this must be flushed before the bottle is filled.
• Sometimes a special spring steel sample stick can be useful in placing the suction tube just where you want it each time, thereby gaining repeatability. The narrow flat steel bar can be fitted with a standoff to prevent bottom
sampling, as well as clasps to hold the Tygon tubing.
Typical closed loop