As the practice of oil analysis and oil sampling evolves from a “whatever works” attitude to a more defined trade skill, techniques and procedures must also evolve. The principles behind oil analysis as a proactive maintenance tool allow companies to use gathered information to perform root cause analysis and turn an oil analysis program into more than just an oil change indicator. Given the proper training and information, employees can use an oil analysis program as an effective condition-based monitoring tool.
When upgrading an outdated or ill-conceived oil analysis program to an effective condition-based monitoring tool, there are a few things that may need to be updated and improved; like sampling locations, the type of sample valves used and the procedure employed to draw the sample safely and accurately from equipment. After all, the analysis performed by the lab is only as good as the sample of oil it receives.
Figure 1
This is particularly true for industrial hydraulic applications, which typically include a multitude of different components, such as valves, pumps, motors, filters, etc. Historically, oil samples from hydraulic systems have been taken straight from the reservoir.
While this location is acceptable if the information desired pertains to the condition of oil’s physical and chemical properties, a truly effective oil analysis program should also include tests for wear and contamination like ISO particle counting, water concentration, and wear debris detection and analysis. All of these tests are extremely sensitive to sampling location, so as a program evolves, so too should the sampling strategy.
Hydraulic Sampling
High-pressure hydraulic systems use reservoirs to hold the lubricant at atmospheric pressure. This is where most oil analysis programs begin and where most analysts draw samples. Reservoirs are a good place to test for lubricant health. However, the data collected from this location is highly nonspecific in terms of indicating equipment health or contamination ingress.
If oil analysis is to become a truly effective condition-based monitoring tool, then the sampling location should be reassigned to isolate system components and to provide more representative samples of the entire machine. This may require taking more than one sample per system. However, once the components are identified and isolated, better decisions can be made regarding the reliability and the maintenance of the equipment based on the data collected.
For example, consider the simple hydraulic system shown in Figure 1. Failure of the thrust plate in the pump may generate a certain amount of wear debris, equivalent to a concentration of 50 ppm as measured by elemental spectroscopy, directly after the pump.
However, once the oil passes through the system and returns to the reservoir, the wear debris concentration will be significantly lower, due to a combination of both the return line filter, but more significantly the dilution factor, caused by suspending the same amount of wear debris in the large volume of oil in the reservoir. In fact, hydraulic pump failures are often missed altogether due to reservoir sampling. To the uneducated oil analysis user, the perception is that oil analysis is not an effective condition-monitoring tool, when in reality it is the sampling procedure that is at fault.
In order to more effectively monitor complex systems, a component isolation sampling strategy must be employed. Component isolation means exactly that; the component is isolated and sampled separately from the rest of the machine. In doing so, oil analysis provides a snapshot of the health of that part of the equipment at a specific moment in time separate from other locations. To do this, samples should be collected downstream of all critical components and the results compared to data collected upstream of the same component.
All the elements of the hydraulic system in Figure 1 are considered critical. If any one of these components fails, the system cannot operate within design specifications. It is thus essential that the condition of these components be trended in order to maintain reliability.
When employing a component isolation strategy, sample ports should be installed in turbulent zones where maximum data density is achieved with minimum data disturbance.
While sampling after each component in the simple system (such as the one shown in Figure 1) is easily achieved, once sample valves have been installed, in more complex systems, component isolation may require around 10-15 sample points. Since sampling each point on a routine basis is usually cost-prohibitive, a good strategy is to locate a primary sampling point for routine monitoring.
The primary is then backed up by appropriately located secondary sampling points used in the event of an exception for troubleshooting purposes. For most hydraulic systems, a good location for a single primary sampling port is on the return line, directly upstream of the return line filter, as shown in Figure 1. For systems with multiple return lines, multiple primary sampling locations may be required.
Depending on the specific components in the system, more than one sample port may need to be installed on a single component. For example, consider a variable speed radial piston motor. This type of motor has two separate internal chambers that are independent of each other. One chamber is the motor case, which houses all the mechanical components of the motor including bearings, retaining rings, bushings and seals.
The other chamber is the pressure side of the motor, which houses the pistons for the inlet and the outlet of fluid. The casing drain and motor outlet can provide independent, data rich samples that indicate the health of the motor and should both be considered as sampling points. In this instance, because the major working components of the motor are housed and lubricated in the casing of the pump, most of the wear debris will drain back to the tank without first going through the system.
A return line sample in this instance will have little chance of diagnosing a failure in the motor case side of this component, whereas a case drain sample will not provide a warning of piston problems. To effectively monitor this component, two samples must be taken.
Figure 2
Today’s market offers several options for high-pressure oil sampling ports. The most reliable type of valve for use in high-pressure applications is a check style valve that utilizes a normally closed check ball on a metal seat (Figure 2). These valves can be used in systems with pressures up to 9000 psig allowing probe-on sampling up to 5000 psig. To draw a sample, an adapter is used to unseat the check ball and allow fluid to flow into the sample bottle.
Sampling Hydraulics Safely
When mishandled, high-pressure hydraulic systems can cause serious injury, or even death. Even a small pinhole in a high-pressure hydraulic line can cause serious injury. Whenever a sample is drawn, every precaution necessary must be taken to ensure the safety not only of the sample collector, but also of those working around the system.
Due to these safety concerns, there are often restrictions to accessing equipment while it is running. Micro-bore hoses are widely used in conjunction with standard sample ports and bulkhead sample ports to bring the location of the sample valve to a panel or other location considered safe. Because of the 2 mm nominal bore within the micro-bore hose, only a relatively small amount of fluid needs to be flushed allowing a representative sample to be taken. Even for 10 feet of hose, only 120 ml of fluid needs to be flushed.
Taking a sample from a high-pressure hydraulic system requires the use of tools that ensure the safety of the technician drawing the sample. Pressure-reducing valves are widely used to reduce the system pressure to a safe level for drawing an oil sample. These valves are used in association with standard sampling accessories. Typically a short micro-bore hose (2 mm nominal bore size) connects the sample port to the handheld pressure-reducing valve.
Figure 3
The pressure-reducing valve then reduces the pressure from 5000 psig to an acceptable output pressure as low as 50 psig. A sample port adapter and clean sample tube are then attached to the outlet of the pressure-reducing valve. A similar effect can be achieved using a small helical coil of tubing as shown in Figure 3. Whichever method is used, it is important to flush the micro-bore hose, and any valves or additional tubing used at least five to ten times their total volume.
In many industries, hydraulic systems are critical to production and process control. When used correctly, oil analysis is an effective tool in highlighting not just the condition of the fluid, but of all critical components in these systems. By following a few simple rules, and installing primary and secondary sampling valves at appropriate locations, early warning signs of contamination ingress and component wear can be used as vital tools in the fight against unscheduled downtime.
