Proper Sampling Techniques
EP Editorial Staff | March 1, 2007
Oil analysis is a very powerful technique in a Reliability Centered Maintenance Program. Previous articles in this publication have discussed the use of oil analysis in a condition-based program being both predictive on equipment condition and proactive on lubricant condition. Sampling is the first step—and a key element—in determining oil and equipment conditions. Since bad data may lead to the wrong conclusions and cause either an unnecessary or wrong action, it is critical to sample properly. As an example, let’s say you visit a physician for a blood test to determine your cholesterol level. Clearly, it is going to be very important for the doctor to get a good “representative sample” to make the correct diagnosis. One hour before the exam, however, you eat a double cheeseburger with fries. In all likelihood, your results will come back positive for high cholesterol and the doctor may put you on medication and a strict dietary program. If you’ve not had problems in the past, the correct course of action would be to take another sample. If it is a good representative sample and comes back positive, you can rest assured that your doctor took the right course of action from the outset. On the other hand, if the second blood test comes back negative, there is a possibility that the first sample was not truly “representative”— and that your doctor took the wrong course of action. Our cholesterol test example is very similar to what can happen with oil analysis. If a sample is not taken properly, it may lead to the wrong conclusions. Therefore, you never shut down a piece of equipment based on the results of just one sample. You should always resample when unusual results come back on an oil analysis report.
The main objectives of a sampling program are to:
- maximize data concentration;
- minimize outside interferences;
- sample at proper intervals.
Important considerations in an effective sampling program are:
- sampling location
- sampling hardware
- sample bottles
- sample procedure
Oil analysis provides the following basic information:
- oil condition
- equipment condition
- contaminant type and amount
Table I summarizes available oil analysis tests and the information they can provide.
Keep in mind that the sampling process is dependent on the objectives for the data. For example, if we only are interested in oil condition, such as viscosity, FTIR and acid #, the sample location and the technique are not as critical as they are when we are analyzing for large wear debris indicating possible catastrophic failure. Hydraulic systems with precise controls—such as servo valves—require very clean oil. Therefore, the particle counts need to be very accurate, requiring a good representative sample of the system.
Live zone sampling
The preferred method is to collect a sample as it is flowing through the system with as much turbulence as possible. The best place to do this is at an elbow or T for good mixing, as illustrated in Fig. 1. The primary location should be within the return line of a circulating system, as shown in Fig. 2.
Many times, if there are a number of components, more than one sample point is needed. The primary sample point identifies any problems in the whole system and is the main return line to the system, which is fed by secondary lines to the main components. If, for instance, a high Fe content is found in the primary line, each of the secondary lines needs to be sampled to identify the component(s) experiencing the problem(s).
Live zone sampling, the preferred sampling method, gives the most representative data in a circulating system. Assume you have a 10,000 gallon turbine oil reservoir and you are trying to identify initial wear of journal bearings that are tin based Babbitt. With emission spectroscopy, you would look for tin and correct the problem before the bearing is wiped. Sampling from the reservoir would dilute the amount of tin to the extent that it could not be identified. The goal is to maximize data concentration, which can be achieved through live zone sampling as close to the bearing as possible, using primary and secondary sample points.
Utilize oil analysis laboratories to assist in installing sample points that should be on all new equipment with circulation systems. You should plan on installing the necessary valves on existing equipment when an outage is scheduled.
Finally, be sure to adhere to the following sampling guidelines:
- Sample from live fluid zones.
- Sample from turbulent zones such as elbows.
- Sample downstream of bearings, gears, pumps, cylinders and actuators.
- Sample machines during typical working conditions and at normal operating temperatures.
Avoid the following missteps in sampling:
- Don’t sample from dead pipe legs or hoses.
- Don’t sample from laminar zones.
- Don’t sample after filters or from sumps.
- Don’t sample when machine is cold or not operating.
Static (reservoir) sampling
Many samples are taken in static systems by sampling from an oil reservoir. Here, again, it is important to sample from the correct location with the proper procedures. The most common technique is to use a vacuum gun and plastic tubing as illustrated is Fig. 3.
The sample needs to be drawn from the same point every time—as close to the middle of the reservoir near the return line. A good way to do this is to wrap a plastic collecting tube around a steel rod, as shown by the layouts in Fig. 4. To avoid contamination problems, use a new tube for every sample.
A better technique for static (reservoir) sampling is to permantly install a sample tube. Pitot tubes, as shown in Fig. 5, are the best alternative when static sampling. The ideal location is near the return line. In a hydraulic reservoir with baffles, the pitot tube should never be placed after the baffle toward the suction line.
Whenever a sample is taken, the lines carrying the fluid must be flushed properly. A minimum of five times the volume line is recommended. As a general rule of thumb, with a 4-ounce sample bottle, use the following procedure, which is courtesy MRT Laboratories.
1/4″ SS tubing = 3/4 bottle/ft of sample line 3/8″ SS tubing = 1 bottle/ft of sample line 1/2″ SS tubing = 2 bottles/ft of sample line
When using carbon steel double the above volumes.
Bottle cleanliness is very important when collecting samples for particle counts. Two types of bottles are used for oil analysis as illustrated in Fig. 6.
The frosted bottle will handle hotter fluids, but the clear PET allows you to see the fluid more clearly. Neither bottle is clean enough when low ISO cleanliness numbers are being measured. Specialized bottles need to be purchased depending on degree of cleanliness required. The following are the bottle cleanliness requirements:
- Clean maximum 100 particles/ml > 10 micron
- Super Clean maximum 10 particles/ml > 10 micron
- Ultra Clean maximum 1 particle/ml > 10 micron
The most important factor in setting sampling intervals is the criticality of the equipment. For example, there actually are situations where equipment is sampled weekly. Most critical equipment is sampled monthly. The more data points that are available, the better the trend analysis in determining equipment condition. Table II should serve as general guidelines in selecting the correct sampling frequency.
Oil analysis is a critical part of any predictive maintenance program and sampling is a critical component of any oil analysis program.
Consistency in collecting samples is critical to obtaining good data. The same person should collect the sample from the same location every time.
Samples should be labeled properly and sent as soon as possible to the oil analysis laboratory. Training in collecting oil samples is available from oil analysis laboratories and should be utilized.
Never forget that your data is only as good as the person who collects it and your reliability program is only as good as your oil analysis program. It would be better to have no data than to have bad data. That’s because bad data could encourage you to take incorrect action with your equipment.