Oil Sampling on System Returns
There are several rules for properly locating oil sampling ports on circulating systems. These rules cannot always be precisely followed because of various constraints in the machine’s design, application and plant environment. However, the rules outlined below should be followed as closely as possible:
Turbulence. The best sampling locations are highly turbulent areas where the oil is not flowing in a straight line but is turning and rolling in the pipe. Sampling valves located at right angles to the flow path in long straight sections of pipe can result in particle fly-by, which basically leads to a substantial reduction of the particle concentration entering the sample bottle. This can be avoided by locating sampling valves at elbows and sharp bends in the flow line (Figure 1).
Figure 1. Highly Turbulent Area
Ingression Points. Where possible, sampling ports should be located downstream of the components that wear, and away from areas where particles and moisture ingress. Return lines and drain lines heading back to the tank offer the most representative levels of wear debris and contaminants. Once the fluid reaches the tank, the information becomes diluted.
Filtration. Filters and separators are contaminant removers, therefore they can remove valuable data from the oil sample. Sampling valves should be located upstream of filters, separators, dehydrators and settling tanks unless the performance of the filter is being specifically evaluated.
Drain Lines. In drain lines where fluids are mixed with air, sampling valves should be located where oil will travel and collect. On horizontal piping, this will be on the underside of the pipe. Sometimes oil traps, like a goose neck, must be installed to concentrate the oil in the area of the sampling port. Circulating systems where there are specific return lines or drain lines back to a reservoir are the best choice for sampling valves (Figure 2).
Figure 2. Return or Drain Line
They allow the sample to be taken before the oil returns to the tank and always before it goes through a filter. If the oil is permitted to return to the tank, then the information in the sample becomes diluted, potentially by thousands of gallons of fluid in large lubricating and hydraulic systems.
In addition, debris in the reservoir tends to accumulate over weeks and months and may not accurately represent the current condition of the machine.