Heavy Industry

Viscosity is a measure of fluids resistance to flow. Often defined as an ISO or SAE viscosity grade (such as VG 220 or SAE 15W/40). Typically measured at 40 or 100ºC. Each application will have a viscosity range suited to the task. Too high a viscosity can lead to lubricant starvation, wear in circulating pumps, increased temperatures and reduced efficiency. If viscosity is too low, the components will not be sufficiently separated resulting in excess friction and causing wear on the machinery. A change in viscosity could be due to:

• Contamination with water/fuel/solvents/very small particles
• Oxidation or ageing of the oil
• Incorrect oil
• Mechanical shearing of the oil

Oil oxidises over time and becomes more acidic indicating the age of the oil. If the oil is too acidic it can damage metal components and further accelerate the ageing process. A rapid increase in the Acid Number may be due to:

• Severe oxidation (often caused by overheating)
• Depletion of additive package
• Top up with a large volume of incorrect oil with a much higher base acidity e.g. hydraulic oil
• Contamination with process fluids; or in case of engine oils with combustion products

 

• A measure of total magnetic ferrous debris in the sample irrespective of particle size.
• Does not detect non-magnetic ferrous debris e.g. rust.
• Combine with Elemental Analysis and ISO Code for comprehensive assessment of the wear situation.

Induction Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is used to measure the concentration of over 20 different elements in the oil. These include wear metals, additives and contaminants. We have recently upgraded our instrument - you can read about some of the resulting improvements here.

By monitoring wear metal concentrations the wear rate and its origin can be established. Trending additive levels ensures that the right oil is used and that it remains suitable to the task, while measuring levels of contaminants helps prevent severe wear and loss of function.

For grease and debris samples a combination of a Rotating Disk Electrode Optical Emission Spectrometer (RDE) and an ICP-OES is used. The RDE eliminates a lot of cross-contamination issues and, as no dilution with solvents is required, allows for more accurate measurement of heavily contaminated samples which would settle at the bottom of the test tube if an ICP-OES was used. The ICP-OES is then used to cover elements not measured by the RDE (mostly additives, although the wear metals are also measured). The ratio of RDE to ICP wear metal levels gives an indication of wear particle sizes.

 

Excess water in the oil reduces the lubricating effectiveness by disrupting the oil film, accelerates corrosion (i.e. rusting of iron and steel surfaces), depletes and/or degrades additives and accelerates the aging (oxidation) of oil.  Where large quantities of water are present oil may become emulsified. The emulsions can combine with insoluble oxidation products to form sludge which impairs the operation and reliability of equipment. In addition excessive water if present as free water can promote bacteria growth or form hard deposits on bearing surfaces.

An increase in water content may be due to:

• Leaking covers on equipment
• Leaking oil coolers
• Excessive leaking turbine gland steam seals
• Condensation
• Using water contaminated fluid for topping up

 

Fluid cleanliness is particularly critical for hydraulic and turbine oils. High levels of particulates, especially if the particles are abrasive (e.g. silica), can increase wear of components and lead to reduced life and premature failure. It has been demonstrated that improving cleanliness by even a couple ISO Codes can lead to doubling of the expected life of a component. Conversely, contaminated lubricant will greatly reduce component lifespan and increase costs. Fluid cleanliness is quantified by counting particles in prescribed ranges of particle sizes. It is typically expressed using an ISO 4406, a NAS 1638 or an SAE AS4059 cleanliness code. There are several ways of obtaining the particle count with the most common being instrumental particle counting and the patch test method.

Instrumental Particle Count

Most instrumental particle counters relate a change in the amount of light (either visible or laser) transmitted through the fluid into a particle count using a stored calibration. When a particle flows between the light source and the sensor, the measured output drops and this is interpreted as a particle of a certain size. Some instruments scan over a particle and are able to capture its shape and outline. Other systems measure a pressure drop as the oil is passed through a series of sieves.

Patch Test

Another approach is to pass the fluid through a filter membrane and with the aid of microscopy to either count the deposited particles or perform a comparison with reference slides. The advantage of the latter approach is that, as well as obtaining the ISO Code, the types of wear and contamination particles can be examined and captured, giving further insight into the types of wear or contamination. This method is also insensitive to air bubbles and water droplets, which can interfere with the readings of the instrumental particle counters. An example of such patches can be seen in the slideshow. At STS we have invested in a new state of the art Leica motorised microscope and camera system with a range of illumination options. We are particularly excited about Darkfield Illumination - a way to light up the patch from all sides and get better definition of difficult slides and translucent contaminants in particular. We are also enjoying the Z-stack feature - it combines in-focus areas from multiple images to achieve a single fully focused composite as shown below. The overview/tile stitching facility lets us construct overviews of an entire membrane or ferrography slide. In the following overview we were able to use Dark Field illumination to successfully separate translucent particles from the woven 11µm membrane patch, which is also translucent. The following overviews show differences in large (inner ring) and fine (outer ring) particle densities in two ferrography samples.

Analytical Ferrography is a technique for depositing and analysing wear particles contained in an oil or grease sample. The sample is deposited onto a glass slide, with the particles trapped by strong magnets and the oil washed away with a suitable solvent. Both linear and rotary particle deposition systems exist. At STS preference is given to a rotary system, which has been developed at the company. It ensures good separation of particles over the three rings, with particles also being sorted by size with the larger particles settling out on the inner ring. Once deposited the particles are analysed by a metallurgist, who is able to report on the relative quantities, types and sizes of particles present. A Particle Quantifier Index of the slide is also recorded. All of this is taken into account to produce a comprehensive report on the wear rate and situation. You can find an example Ferrography Report available for download here.

Depending on the failure mode (e.g. wear related or catastrophic failure due to overloading), there may be wear metal evidence present in the lubricant or in the oil filters. We are able to extract and analyse this wear debris through a collection of different analytical techniques. Extracted particles can be deposited on a filter membrane during the Patch Test or deposited on a glass slide to enable Analytical Ferrography. Elemental data can be obtained from bulk fluid analysis as well as from Scanning Electron Microscopy. The latter allows for individual particles to be interrogated and for data to be compared against machine metallurgy.  

Oil filters are essential to maintaining oil cleanliness and removing wear metals and contaminants. As they perform their function, however, they also remove some of the information about wear and contamination levels from the oil stream. This is then stored in the filter itself. Fortunately Filter Debris Analysis grants us access to this store of information. By extracting and analysing the entrained particles we can learn a lot about the wear situation or the source of contamination. Oil filters come in many shapes and sizes and how we treat an individual filter will depend on its size and construction. Typically a section of the outer cage is cut out and removed to enable access to the filter media. A section of the filter media is then removed and placed into a beaker. The beaker is then filled with solvent and an ultrasonic bath is used to agitate and extract the particles and any residual oil present. The solvent is then evaporated leaving the particulates and any residual oil. A portion of the debris is then deposited onto a filter membrane for microscopic analysis. The remaining oil/debris mixture is then analysed using a Rotating Disc Electrode Optical Emission Spectrometer to determine the elemental composition of the debris mixture. This gives a breakdown of the different wear metals and contaminants present. Where more detail is needed, the membranes containing the particles are analysed using a Scanning Electron Microscope with Energy Dispersive X-Ray facility. This provides information on the exact elemental composition of individual particles, which can then be matched to specific component metallurgies.

In addition to wear debris analysis we are able to review failed components and engage a select group of partners to deliver a comprehensive analysis. Techniques available include the initial visual inspection, sectioning and metallographic analysis, hardness testing, elemental analysis via SEM-EDX, etc.

Varnish

Varnish is the sticky residue created by the decay of both mineral and synthetic lubricants. If left unchecked it can bring the entire operation to its knees. It can cause premature failure, erratic operation and can be the cause of a costly shut down. There are many signs of varnish build-up, including sticky valves, overheating bearings, decreased effectiveness of heat-exchangers, blocked filters and reduced lubricant life. A way of detecting build-up of varnish and its precursors early is through Membrane Patch Colorimetry.

The test is a reflection of both the solubility of the oil and the amount of varnish precursors in the lubricant. Different types of lubricant (e.g. Group I and Group II) have different solubility properties and are able to mobilise degradation products to a different degree. This means that some oils degrade more rapidly but are better able to maintain degradation products in solution preventing varnish build up. Others have better oxidation stability but at the cost of poor solubility - even relatively small quantities of varnish precursors may result in varnish deposits. Things get even trickier when a Group I lubricant is replaced with a Group II oil! This also means that a simple oil change is rarely sufficient to eradicate varnish problems.

It doesn't help that varnish does not deposit uniformly throughout the system. Regions with high local pressures are at risk as are the low flow areas, especially if the oil has cooled after passing through some exposed pipework or after a shutdown. This temperature sensitivity means that seasonal variations and interrupted operation make diagnosing varnish even trickier.

Whatever your situation, monitoring varnish potential through Membrane Patch Colorimetry will help you stay in control.

Membrane Patch Colorimetry

As the solubility of a lubricant varies with both temperature and time, the test procedure includes a heat and rest cycle. This is used to standardise each measurement and ensure consistent comparisons can be made with every sample. After the heat and rest cycle the lubricant is mixed with a solvent and filtered through a fine membrane filter. The shading of the membrane patch is then measured with a colorimeter to determine the varnish potential value.

If the measured MPC/VP value is high, varnish mitigation technologies can be employed to clean up the system. Varnish precursors are very fine sub-micron agglomerations of oxidation products and can pass through conventional filters, although when the oil is saturated they can cause filter blockages. Therefore special filtration technologies aimed at removing the fine sub-micron contaminants need to be utilised. Such technologies include electrostatic filtration, depth media filters and the electro-physical separation process to name but a few. Each comes with its own strengths and weaknesses and should be considered carefully depending on specific application.

Membrane Patch Colorimetry

A key property of the oil is its oxidation stability – that is, its ability to resist oxidation.

A standard measure of this property is the Rotating Pressure Vessel Oxidation Test (RPVOT, formerly RBOT, ASTM D 2272). The test presents a simulated worst case scenario, where an oil sample is subjected to the harsh conditions of high temperature and high pressure oxygen atmosphere in the presence of water and a copper catalyst. As oil oxidises oxygen is consumed and the pressure drops. Time taken to achieve a specific drop in pressure is then recorded.

By monitoring the RPVOT value of a lubricant its remaining useful life can be assessed. Note however, that while this test was very useful for assessing Group I oils, which age in a fairly predictable manner, with Group II oils things get a little trickier. Many of the modern synthetic lubricants achieve more than 1000 minutes in the RPVOT test. However, for some oils this is the result of the additives inhibiting the copper catalyst. As a result, the RPVOT value measured will stay very high until the additives have depleted, at which point it may reduce rapidly.

It is therefore prudent to augment the RPVOT test by also monitoring levels of the anti-oxidant additives. For that we can rely on the RULER test.

Remaining Useful Life Estimation Routine (RULER) uses Linear Sweep Voltammetry to measure the levels of Amine and Phenol anti-oxidant additives in the oil. These are typically compared with a baseline measurement obtained from a virgin oil sample (ideally from the same batch of oil). If anti-oxidants are allowed to deplete below a critical level the oxidation process accelerates, leading to reduced oil life and potential for varnish deposition. RULER monitoring enables management of the additive levels through top up or partial changes, significantly extending the life of the oil.

Fourier-transfer Infra Red Spectroscopy (FTIR) is a powerful tool for analysis of oil, grease and fuel samples. It is especially useful for monitoring and identifying certain types of contaminants.

The underlying principle is that infrared energy from the source is absorbed in the sample at  wave lengths which are characteristic of specific molecular bonds. Each scan generates an FTIR spectrum which can be analysed and interpreted.

At its most basic the technique can be used to measure predetermined parameters, such as oxidation, nitration, sulphation or presence of fuel and glycol in an engine oil. Some of those require prior calibration and a scan of a virgin sample to be used as reference.

More advanced analysis can help monitor degradation, identify and match unknown contaminants, verify lubricant formulation or help identify and source an unknown lubricant.

In some cases Gas Chromatography coupled to Mass Spectroscopy (GC-MS) is called upon to work in tandem with FTIR analysis on particularly tricky samples, where exacting information is nonetheless required.

You can download an Example FTIR Report here