The single most important activity with regard to oil analysis is the practice of obtaining a representative oil sample. An oil sample that is not representative of the condition of the oil in the reservoir, or the quantities of wear particles or contaminants, will give false and misleading information to oil analysts and those charged with implementing recommendations from the oil analysis results.
The total equipment needed for good sampling varies according to the type of sampling required, the machinery and the lubrication systems, designs of the oil sumps, reservoirs, housing, line arrangements, and so on. Because there are many different sampling procedures, they are not addressed in detail in this section; however, detailed sampling procedures can be obtained from various suppliers. Included in this section are detailed presentations of the various types of sample points.
Sample Points
The key to obtaining adequate and representative oil samples is the requirement that the
machines are in service, in operation, and at or near typical loading conditions. The selection of the oil sampling points should not be driven by ease of sample procurement, but rather should be optimized to that location in the system that is most representative of the data desired. Typically, a single primary sample point that ensures maximum data density for analysis will be designated. This means that the critical monitored parameters for a system should be at their peak within the confines of the normal lubricant cycle so that the analysis trends can be significant. As an
example, systems for which wear particle concentration is an important monitored parameter will have a specific sample point selection strategy. The primary sample point will be located at a point in the oil flow where the oil has passed through the lubricated contacts, but before any point at which the concentration of wear will be diluted or diminished through filtration, mixing, or settling in the reservoir. Often, machines, as supplied by the manufacturer, do not have easily accessible sampling points for obtaining this required information. Therefore, it could be
necessary to retrofit this equipment with sampling valves or fittings that enable personnel to obtain the needed representative sample.
In other instances, operating conditions may prevent obtaining an optimal sample. For instance, in a splash bath arrangement where there is no defined oil return, it might be necessary to take the oil sample from the reservoir itself, to establish the first accessible point in the lifecycle of oil
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given piece of equipment. As mentioned previously, a primary sampling point might be defined by the need to obtain information about wear particle concentration. Secondary sample points may be defined to provide other information, such as post-filter condition, condition after an oil supply pump, condition of the tank bottom, and so on. These secondary oil-sampling points are typically defined for the purpose of follow-up samples. They are performed on an infrequent basis or to better define the condition of a system’s lubricant after initial indications are provided by the primary sample point analysis.
In a complex circulating oil system, it is not uncommon to have a single primary sample point, and as many as a dozen secondary sample points. In main turbines, for example, the primary sample point may be defined as the sample taken from the common bearing return header prior to its mixing with the reservoir contents. The secondary points may be defined as the drain valve at each of the bearings, the tank bottom condition, the condition on the discharge of the oil supply pump, and a sample taken from the discharge of the on-line lube oil conditioner. There are a number of commercially available oil sampling valves or fittings that can be used to retrofit equipment with inadequate sampling options. The cost associated with such retrofit projects can be minimized through a carefully orchestrated engineering plan that seeks to define a limited number of installation options (which are generically evaluated for impact on equipment and systems). Once approved for installation, the individual equipment can then be modified through the use of approved installation scenarios by using defined plant procedures to ensure proper installation and consideration of all engineering criteria.
Sampling Port Location—General Discussion
In general, the best sampling methods involve some sort of sampling hardware retrofit. This can be both time consuming and costly depending on the exact hardware chosen and the number of machines involved. However, this investment is a wise choice if a serious oil analysis program is envisioned.
For many utilities, the traditional methods of sampling large lubrication and hydraulic systems consist mostly of reservoir sampling. In the case of reservoir sampling, samples are drawn either by drop-tube vacuum sampling from a fill port, or off the bottom of the reservoir from the drain port. However, advanced oil analysis programs locate all sampling taps, where possible, on the drain line of circulating lubrication systems and on the return line of hydraulic systems. This permits access to wear debris and ingested contaminants before these materials are removed by filters, separators, or by settling action. Likewise, moisture levels are precisely represented from entry points such as steam impingement, process fluids, and coolant leaks.
Many of the recommended sampling points follow a consistent pattern. Presented here are six general cases for installing sampling fittings. These different cases describe the general procedure for installing the sampling hardware. Refer to this information when instructing maintenance technicians on how to install sampling valves and fittings. An example of sampling port installation is found in Appendix A.
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Case A—Dry Sump, Horizontal Drain Line
Figure 4-1 illustrates how a single sample is to be taken to assess wear metals and fluid condition coming down a large return header to a fluid reservoir. Sampling ports on this vented drain line should be on the underside of horizontal tubing where there is reasonable certainty that there will be a uniform body of fluid. To ensure turbulence and good mixing of the fluid prior to sampling, it is recommended that the sample location be near or just after an elbow.
The use of a mini-mess type sampling installed at this point can be interfaced with a vacuum pump sampler in order to draw fluid from the unpressurized zone (drain line). Alternately, a ball valve can be attached to the underside of a horizontal drain line allowing fluid to flow by gravity into a sample bottle. A male quick-connect valve is yet another option, allowing the mini-mess valve or ball valve to be carried from point to point. The connection, regardless of the port hardware, can be drilled and tapped or hot-tapped, depending on various factors. Figure 4-1 shows examples of various options for drain-line sampling taps.
Figure 4-1
Drain Line Sample Points
Case B—Dry Sump, Vertical Drain-Line
Figure 4-2 shows how diagnostic sampling ports can be installed on each of the drain lines coming from the individual bearings toward the header. This will enable troubleshooting of problem bearings in the event of non-conforming readings of wear metals from the main header sample. There is considerable added expense and effort to add these additional bearing drain-line
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If horizontal sections of these dry sump-bearing drains are available, then the ports can be
installed in any of the ways briefly described in Case A. However, in the event of the typical case that these lines are vertical down-comers, a trap will need to be installed to collect the oil in front of the sample valve.
Figure 4-2
Vertical Drain Line with Sample Trap
The most convenient way to access the fluid adjacent to the trap is to use a mini-mess valve as shown in Figure 4-2. Because the fluid will not be under pressure, the use of a common vacuum pump will be required to pull the oil into the sample bottle. Another alternative is to use a ball valve or a quick-connect coupling as mentioned in Case A. Naturally, the mini-mess with the vacuum pump has its advantages because the pump facilitates the flow of oil into the bottle and the bottle can be protected from atmospheric contamination using a simple zip-lock type of sandwich bag.
Case C—Pressurized Feed Line
It is often desirable to sample the oil as it is being supplied to the bearings and other lubricated components. This should always be done after filters or other contaminant removal
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devices/separators. It should also be done after heat exchangers, if used. The use of this sampling location is best combined with a drain or return line sample. The drain/return line sample is used for predictive purposes for the detection of abnormal wear conditions. However, the pressure line sample serves to ensure that oil being delivered to bearings and lubricated components meets important criteria of cleanliness, dryness, and a host of other important physical properties. Because the fluid is under pressure at this location, sampling can be more easily accomplished. The use of a mini-mess type valve enables simple probe-on sampling to be accomplished. Alternatively, ball valves and quick-connect couplings can be employed as well. Figure 4-3 shows the common options for installing a sample valve on a pressurized line. Figure 4-4 shows the use of a mini-mess on a high-pressure line.
Figure 4-3
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Figure 4-4
High-Pressure Line with Mini-Mess Valve Case D—Pressurized Return Line
Most hydraulic systems, as shown in Figure 4-5, return fluid to the tank under pressure, as opposed to gravity drain flow. These pressurized return flows are a continuous body of fluid without air zones occupying the lines (as in the case of vented bearing drains). It is also common for the fluid to pass through a filter on its way back to the tank). Here, a pressurized return line exists; therefore, the oil should be sampled just prior to the filter. If an upstream filter pressure- gage port is available, then this is an ideal location for installing the sample valve; simply remove the port plug and install the valve at that point (point P in Figure in 4-5). If a pressure gage is already occupying the gage port, then a T-fitting can be installed to enable the port to be shared. Because this is a low-pressure zone, the ideal sample valve is a standard mini-mess.
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Figure 4-5
Pressurized Hydraulic System
Fluid flowing straight back to the tank (no filter) should be sampled on elbows, if possible. They can often by drilled and tapped or hot-tapped. If a pipe or header leads straight back to the tank vertically, sufficient turbulence for a uniform (well-blended) sample may not exist. In this case, it is recommended that the line be followed back to the machine to see if an elbow can be found, or an otherwise more turbulent location (such as where fluid is just exiting the machine). If process debris on the return line is found to be excessively high and interferes with the monitoring and management of oil cleanliness (cleanliness targets are routinely not achieved), then another sampling port needs to be installed on the pressure line downstream of the pressure- line filter, leading to bearings or actuators (refer to Case C for sample ports on pressure lines). Here, the cleanliness, dryness, and other physical properties of the oil can be monitored, leaving the return line sample for wear debris analysis only.
Case E—Wet Sump, Splash, or Bath Lubrication
There are numerous instances where bearings or gear units are lubricated without the benefit of oil circulation. Too often these systems are either not sampled or they are sampled improperly, for example, using the drop-tube vacuum pump method or directly from a drain port. With the
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is threaded into the port. A mini-mess valve would be installed into the bushing, on the outside of the casing, to access the fluid for sampling.
In many systems, two drain ports exist. If an available drain port exists that is not otherwise needed for purging oil from the bearing or gear compartment, it can be fitted with oil sampling hardware similar to that shown in Figure 4-6.
Figure 4-6
Wet Sump Sampling Options
If there is not an available port for sampling because a level gage is occupying one of the ports, this port can also serve as a sample port. There are various possible configurations, one such configuration is a common two-way ball valve (preferably with a Teflon seat), as shown in Figure 4-7. The valve connects the fluid to the gage during normal operation.
When a sample is to be taken, the valve is rotated, thus restricting access to the level gage (which is a dead zone) and allowing fluid to flow freely into the sample bottle after an adequate flush. It is highly desirable to weld an inward static sampling (SS) tube, as shown, allowing the fluid to not be pulled off the bottom of the casing but instead from an active (moving) zone of the system away from the wall or floor of the casing.
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Figure 4-7
Two-Way Valve for Sight-Glass Sampling
In situations where there is only one drain port, an SS tube should be welded to the end of a bushing, which threads into the port cavity. A ball valve or mini-mess valve can then be threaded into the bushing for sampling. If a mini-mess valve is to be used, a vacuum pump will be needed during the sampling process. To use the port for the purpose of draining the gear case or bearing housing, the bushing is completely threaded out.
If desired, these systems can be fitted with male quick-connects on the drain and fill ports to facilitate attachment of an off-line circulating filtration cart (Figure 4-8). The circulating flow enables sampling from the filter cart, from the pressurized flow line, as shown in Figure 4-8. Upon completion of sampling, the filters can be valved-in to clean the oil. This method achieves sampling objectives, improves contamination control, and eliminates the need for scheduled oil changes designed to remove contaminants. Likewise, all of this can be accomplished while the machine is running, avoiding the need to schedule machine downtime for this PM service.
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Here are some general guidelines for applying this procedure:
• Select a flow rate that is appropriate for the fluid’s viscosity, considering the temperature at which the oil will be sampled.
• Outfit the filtration cart with dirt and water removal capabilities.
• Outfit the cart with a bypass around the filters so that oil can be sampled before engaging the filters.
• Run the cart in the bypass mode for several minutes to homogenize the fluid.
• It is sensible to dedicate a cart to a particular fluid type to avoid the burden of constant flushing.
• Avoid changing sump volume upon engagement of flow by leaving the plumbing of the cart and filters full of oil of the type used in the system.
Once a sample is drawn, engage the filters for a period of time sufficient to turn the sump volume over seven times for single-pass filtration. Turning the volume over seven times equals the equivalent of a single pass from one container to another. If two-pass filtration is desired, turn the volume over 14 times.
If desired, a second sample can be taken to ensure that target cleanliness objectives are met, and to provide a reference of comparison for the next sampling and filtering process.
Case F—Wet Sump, Circulating Lubrication
In those instances where the wet sump has on-board circulation to feed components, and the lubricant drains back to the sump, it is preferred to sample from the circulating system after the pump and before the filter (if applicable). During operation, the oil in the sump and the
circulating oil are homogenous before the filter. The pressure provided by the pump improves the convenience of sampling. Install the sampling valve or mini-mess on an elbow opposite the direction of the flow to avoid particle flyby and so that the particles don’t need to change direction to exit the valve.
If this sampling location serves as the primary sampling location for assessing the lubricant’s condition and contamination level and the machine’s condition, then install the sampling valve or mini-mess before the filter, if one exists (Figure 4-9 at arrow). When a filter is present, sample after the filter to assess its performance or to ensure that the components receive clean, dry oil. The after-the-filter location is typically reserved for troubleshooting when an over-limit particle count is observed at the primary location.
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Figure 4-9
Circulating System – Wet Sump
Power Generation Specific Sampling Locations
After having given a number of general sampling scenarios, Table 4-1 illustrates the proper approach for a number of common types of power generation facility equipment. These
examples can serve to illustrate the appropriate installation method through the use of drawings and by noting the specific locations on actual machinery pictures. In every case, purge volumes must be calculated or measured and the needed purge amount included in the sampling
procedures. In some cases, the needed purge volume can be determined by monitoring the temperature of the oil as it exits the sampling fitting or valve. As the oil is first drained, deadleg oil will be at or near room temperature. As the deadleg volume is eliminated, the temperature will gradually rise until it reaches the reservoir internal bulk oil temperature. Internal reservoir temperatures can be high, so be sure to take precautions to prevent injury while sampling. In some cases, the required hardware installation can be a significant cost or work scheduling issue. Where the installation of sampling fittings is so sufficiently slow as to hamper the progress of the program, temporary measures should be taken to obtain the best possible sample in the given configuration. Oftentimes, this involves using a standoff rod and inserting it through the fill cap. A metal rod with 1/4-20 washers tack-welded to it and cut on an angle can make an adequate standoff rod. Whenever placing anything inside the reservoir, be sure that there is no risk of leaving any foreign materials behind.
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Table 4-1
Power Plant Equipment/Lubrication Systems
Equipment Procedure/Notes Picture Drawing
Main Turbine Reservoir Oil Return Line
Indicated by the letter “E” in the drawing, the return line to the main turbine is the primary sample point for obtaining a composite of the turbine bearing drains. By using a long-handled scoop with a restraining lanyard, oil can be scooped from the area below the pipe where it is free-falling back to the reservoir. Getting the oil at