Monitoring oil performance in operationally critical and economically significant locations enables effective interference with emerging risk factors. Today, factors affecting oil performance can be effectively monitored by remote solutions. But for the most lubricant oil users, monitoring is still based on time-stamped oil samples.
Sampling is the most critical phase when an oil sample is traveling from the system to the laboratory, to finally create an analysis. The oil sample should of course be representative of the oil batch and contain material only from the desired object. External materials in the sample produce extra work.
ASTM – the oil sampling standard D8112-17 – was updated in the end of 2017. The updated standard deals with making sampling standardized in energy industry’s more demanding targets like turbines and oil systems. The standard can be applied as a good foundation for proper sampling in different operating environments.
Applying the ASTM D8112-17 standard
I go briefly through the most interesting sections of this standard about sampling. Naturally, a short blog post is not intended as a complete manual but rather as a wake-up call. You can also ask help from us in order to do more accurate sampling and self-sampling. It is also worth noting that the standard and the writing do not take into account the security issues that must always be addressed when applying best practices.
The objectives of sampling
Based on the oil sample, the aim is to be able to verify the usefulness and performance of the oil in its planned function. The sample must be well representative of the oil generally circulating in the system and also comparable to other samples taken from the system. If samples are used to determine oil performance trends, ensuring the representativeness and timeliness of samples is even more important.
The aim of sampling is often as simple as detecting already occurring but not yet escalated damage in the system, determing the operating condition of the oil at the sampling time or determining need for oil change. Today, oil samples can also be used to predict oil and system condition factors and oil life cycle factors.
Equally, sampling objectives should be defined and their implementation should be monitored. Without monitoring, sampling will never really be useful and costs will be in vain.
The sample complying with the above requirements is taken into pure sample flasks. In the D8112-17 standard ISO 4406 9/7/4 is definied as the minimum sample cleanliness level. A good sample is never taken into a used bottle.
It is recommended to use a clear polyethylene terephthalate (PET) bottle for mineral oils. High density polyethylene (HDPE) or polypropylene bottles (PP) can also be used. The sample size is recommended to be at least 125ml and its multiples.
Borosilicate glass is recommended for phosphate ester oils. If the sample is not analyzed for its cleanliness and the specimen is not particularly hot, low density polyethylene (LDPE) or polypropylene bottles (PP) can also be used.
If the oil is analyzed for gas concentration, it is recommended that the sample is taken with a sampling syringe that has a 3-way valve. This way the sample is recovered without direct contact with the atmosphere.
A vacuum pump capable of yielding at least 0.34 (34 kPa) absolute vacuum for at least 30 seconds can be used for sampling. It is essential that the structure of the vacuum pump used for sampling is not able to touch the sample oil.
Samples must be marked individually so, that the sample, target, time and sampler can be verified.
• The sample bottles are always new and clean, not reused
• PET, HDPE or PP bottles for mineral oils
• Borosil glass bottles for phosphorus ester oils
• Samples should be individually marked
Structure of the sampling site
Sampling sites must be selected so that sampling objectives can be achieved. Often the sampling point has already been installed as a part of the system, but sometimes adjustments need to be made in order to get samples that serve a purpose.
In separate sampling points it is recommended to use materials that are not oxidized or react with the environment or the liquid in use. Of course, the sampling point must withstand the stress of use without damage. The sampling valve should close properly and the sampling tube head must be protected by cap.
The sampling point in the non-pressurized state shall be designed so that standard 125 ml sampling from the oil at a temperature of 40 °C through a 1200 mm long sampling tube should not take more than 4 minutes. In practice the sampling must be arranged in such a way that the sampling can be done representatively and the oil left in the piping from the previous samples can be drained in a separate container. The pressure must not be too high and the flow may not be too hard. These objects often use a mini-MESH type valve, from which the sample can be taken with the above conditions.
• Sufficiency (size)
Extensive systems may include several different sampling points, which representativeness to the entity may vary. The primary sampling point selection depends on the chosen level of representativeness, the desired sample quality and purpose, and, of course, access to the sampling point. Secondary sampling points are used for specific troubleshooting purposes or for other purposes. The variation should be taken into consideration when comparing the results of the different sampling points.
Return oil line - The return oil line provides a representative sample of the oil entering the system if all the oil comes through the return line. A sample from return oil may give early hints about the potential damage to the system.
After the pump / Main oil filter - The sample taken after the main pump or main line filter can detect the condition of the oil entering the system. For example the functioning of filters can be seen by comparing the samples taken between those sampling points.
Oil tank bottom valve - A sample from the bottom valve can be represententative of the oil in the tank. This may be useful, for example, to test the presence of microbial growth or free water in the oil.
Oil reservoir sampling valve - A sample valve specifically designed for taking a representative sample from the tank.
Sample taken from off-line circulation - You may also have to take a sample from a off-line circulation, for example from a sample check valve after the pump. In this case the representativity of the sample shall be assessed on the basis of the position of the suction line of the off-line circulation.
• The sampling point corresponds to the purpose of sampling
• The special features of the sampling point are taken into account
It is advisable to document the sampling method in each organization. At least the sampling points for the different sampling purposes, the method and equipment to be used and the sampling interval, should be documented.
Sampling methods must be applied so that sampling serves the objectives of maintenance and objectives of the organization. They also must allow the analysis of the oil sample to the desired extent. It is advisable to consult the sample analyzer for the application of sampling methods.
Sampling is useful if it serves the organization's maintenance or operational objectives and reduces the overall costs. The measures taken on the basis of the samples must be defined and the utilization of the results should be systematic.
Representative and purposeful sampling methods and locations are the basic requirement of the above organization's objectives.
Written by Mikko Oksanen