It is a common saying that ‘oil and water don’t mix’, but this does not essentially apply in the case of lubrication oils. In lubrication oils, water can exist in a number of states, and if left unchecked, can cause considerable damage to valuable assets. This article explores the issues presented by water in lubrication oils and discusses the techniques that can be used by reliability professionals for determining water concentration in oils.
In industrial oils, contamination of water can create major problems with regard to machinery parts. This is because water will not only change a lubricant’s viscosity, but would also lead to chemical changes, resulting in a depletion of additives and formation of varnish, sludge, and acids. A part of any lubricant condition monitoring program is water testing. Traditionally, current methods make it difficult to determine the contamination of water in industrial oils having strong water separation properties.
Three phases of water can coexist in an oil sample - emulsion mix, dissolved, and free globules - all of which make it difficult to acquire a representative sample. In these types of oils, the additive formulations are designed to efficiently isolate water that is over the saturation limits of 50 to 250 ppm. Water above this saturation limit will form distinct water droplets, and if left to sit unattended, will ultimately separate from the oil (Figure 1). While dissolved water does not pose a major issue for industrial applications, emulsified or free water should be strictly regulated.
Figure 1. Sample of used Chevron GST 32, as received by the lab after shipment from a power generation plant.
What techniques are Available?
The Crackle Test is mostly qualitative and can be used to determine whether water is present or not in the lubrication oil. A hot plate can be used to carry out this test in the field. While the Crackle Test can show whether water is present or not in lubrication oil, it cannot determine the volume of water present. Although some semi-quantitative data regarding the amount of water present can be obtained through meticulous observation of the oil on the hot plate, it highly relies on the operator’s skill.
A temperature-controlled hot plate is required for the Crackle Test. Normally, the temperature of the hot plate is set to 320°F (160°C), and the sample is combined by shaking vigorously, producing a more uniform suspension of water in oil. Following sample preparation, a drop of oil is added to the hot plate and observed carefully.
If water is not present in the oil, “crackling” will not be seen, but if water is there in the oil, the heat will change it to the vapor stage, producing clear bubbles in the oil droplet. The size of the bubbles approximately corresponds to the volume of water present in the oil. In other words, if the bubbles are larger more water would be dissolved in the oil. Quantitative judgments about the concentration of water in the oil can be made by monitoring the size of the bubbles produced on the hot plate.
Below are the advantages and disadvantages of the Crackle test;
- Fast and easy
- Can be performed on-site
- Safety concerns
- Not quantitative
- Results vary between operators
Calcium Hydride Test Kit
Calcium hydride test kit is another way to determine the concentration of water in the field. In this technique, an approximate amount of oil is placed in a sealed container having an approximate amount of calcium hydride. When the container is vigorously shaken, the water in the oil reacts with the calcium hydride and creates hydrogen gas (H2).
A manometer is used to measure the H2 pressure, while the solid calcium hydroxide [Ca(OH)2] product precipitates out. This is a stoichiometric reaction, and a single mole of water in solution will create a single mole of H2 gas. The determination of the gas pressure ensuing from the reaction will provide data about the amount of water present in the oil sample. Using a syringe, an oil sample is taken to run the test. A test will normally require 20 to 30 ml of oil, which is eventually added to the reaction vessel featuring a screw cap to securely close the reaction vessel.
The oil sample would have to be diluted with a known amount of diluent if water levels are high. The relative amount of diluent and oil will rely on water concentration in the oil sample. Sometimes, the test has to be run more than once to measure the appropriate range of water contamination and also to choose the appropriate ratios of oil to diluent.
Usually, the calcium hydride is kept separate from the diluent and oil until the vessel is closed. This way, all of the H2 gas created due to the chemical reaction is retained in the sample cylinder. After the sample cylinder is closed, the calcium hydride and oil are mixed together and agitated to initiate the reaction. After several minutes, the internal pressure is read by the manometer mounted on the reaction vessel. Based on the relative mix of diluent and oil, this pressure reading is converted to a % water reading.
If the amount of water present in the oil is very high, then the vessel’s pressure will be higher than the range read by the manometer. In such situations, the pressure should be released the apparatus should be cleaned, and the oil to diluent ratio should be reduced to fall within the quantifiable range. Usually, fully assembled calcium hydride test kits are available with the pressure vessel, syringes, manometer, gloves, sealing rings, diluents, safety glasses, pre-determined packets of calcium hydride, and at times a magnetic stirring plate. If these test kits are used properly, they provide precise results down to 50 ppm emulsified or free water.
Below are the advantages and disadvantages of the calcium hydride test kit;
- Offers quantitative results
- Relatively easy
- Safety – vessel under pressure
- Needs solvents and chemicals
- More than a few runs may be needed based on water concentration
Karl Fischer (KF) coulometric titration (ASTM D6304) is the most popular technique used to detect water in oil. When performed by a skilled operator, Karl Fischer analysis for water can produce highly precise and repeatable results and serves as a comparative technique for other analytical methods for water measurement. The water can also be determined in any state, such as free, dissolved, or emulsified.
Karl Fischer titration is employed in many applications and industries where water measurement is important, such as in food, pharmaceuticals, and lubrication oils. A titration is a method where a known concentration of solution is utilized for measuring the unknown concentration of a solution. Typically, a burette is used to add the titrant, i.e., the known solution to a known quantity of the analyte (the unknown solution) until the reaction is complete.
For Karl Fischer titration, the amount of iodine used is comparable to the amount of water present in the sample. The water content of the oil can be measured accurately by determining the amount of iodine required to titrate the oil sample. A number of companies, including Metrohm, Mettler Toledo and Sigma Aldrich, develop commercial Karl Fischer titrators for lab applications. Due to the use of volatile chemicals and the sample preparation required, Karl Fischer titration is often carried out in labs as it is rather complicated to use at the site of the equipment being examined.
Below are the advantages and disadvantages of Karl Fischer;
- Can determine water in any state: free, dissolved, or emulsified
- Used by ASTM D6304 as the de facto standard
- Can determine different range of water concentration, from near zero to 100%
- Volatile chemicals needs a fume hood
- Must be carried out in a laboratory environment
- Costly equipment
Relative Humidity or Saturation Meter
Relative humidity (RH) sensors are employed in many industries where humidity has to be regulated such as pharmaceutical industries and food services. RH sensors are available in three types: resistive, capacitive, and thermal conductivity. Capacitive sensors are often used for determining the relative humidity in oil.
Relative humidity in air, especially in warm climates during the summer season, is a familiar concept. Air’s ability to hold water vapor is based on temperature, and the same is true with oil. If the oil is warmer, its ability to hold water becomes greater. Obviously, if the temperature of the oil is very high, such as in an internal combustion engine, the elevated temperature will make the water vapor boil out of the solution.
Humidity sensors are only capable of determining the dissolved water in oil and cannot determine the amount of emulsified or free oil and hence, their usage is rather restricted. Critical data can still be acquired by tracking the RH of oil. A capacitive RH sensor typically includes two conductive electrodes with a non-conductive layer between them. Usually, the non-conductive layer is either a polymer or a metal oxide. This layer’s capacitance alters with the water content and promotes a voltage change between the two conductive electrodes.
The dielectric constant of a capacitive humidity sensor experiences an increasing change which is proportional to the relative humidity of the surrounding environment. Capacitive humidity sensors can operate at high temperatures (up to 200°C), and are characterized by low temperature coefficient, reasonable resistance to chemical vapors, and full recovery from condensation. They exhibit a good response time and can offer precise readings within a matter of minutes.
The usual uncertainty of capacitive sensors is ±2% RH from 5% to 95% RH with two-point calibration. In addition, capacitive humidity sensors are restricted by the distance the detecting element can be situated from the signal conditioning circuitry. This is because of the capacitive influence of the connecting cable with regard to the small capacitance modifications of the sensor. <10 ft is the practical limit.
Below are the advantages and disadvantages of relative humidity;
- Simple measurements
- Easy to use
- Does not need special equipment or solvents
- Only determine relative humidity
- Cannot determine free or emulsified water in oil
- Must be employed in close proximity to the sample owing to capacitive effects from cabling; a long cable (>3m) between the readout and sensor will lead to errors
Infrared spectroscopy is considered to be the most promising method for determining water contamination. This technique is extensively utilized and provides a satisfactory chemical-free measurement. Overall, spectroscopy refers to the study between the interaction of matter and radiated energy. A spectrometer includes a computer, a detector, a radiative source, or other converter of the detector signal to useful data (Figure 2).
Figure 2. Schematic of typical spectrometer
The sample that needs to be tested is positioned between the detector and the radiative source. In the case of infrared absorption spectroscopy, the incident light beam is allowed to travel via a sample and the light that is transmitted is acquired by the detector and generally reported as a spectrum of the absorbed or transmitted light as a function of the wavelength λ of the incident beam.
Infrared light is absorbed by pure water and can be identified by a peak in the infrared spectrum at approximately 3400 cm-1. On dissolving water in another medium such as oil, infrared light will still be absorbed by a water molecule, though the peak may be slightly moved as a result of the varied environment around that specific water molecule.
Water measurement through the ASTM standard practice E2412 is done by associating the peak in the infrared spectrum at approximately 3350 cm-1 to the water concentration (Figure 3). This approach is indeed suitable for specific types of applications, like motor oils that include additives for dissolving water. Here, the analysis is simple, quick, and highly consistent and even novice users can obtain the results easily.
Figure 3. Measurement for water in ASTM E2412.
When emulsified or free water is present in oil, light traveling through the oil sample will be scattered, influencing the measurement considerably. Industrial lubricants contain demulsibility additives and as such most of the water exists as a free or emulsion water. Instead of absorbing light, water droplets that are suspended in oil will scatter it.
It is not easy to correlate a water absorption peak around 3400 cm-1 to the concentration of water over the saturation limit. The water stabilization method is one effective way to offset this scattering. In this technique, the sample is pretreated with an additive like a surfactant, which helps to dissolve the water in the oil. This way, the standard water peak can be identified with infrared spectroscopy.
While the water stabilization technique can be executed by a skilled operator, chemicals would still need to be added accurately to the oil sample before analysis, and this cannot be instantly accessed in the field when required. Due to this, the water stabilization method is not generally used by the end user in the field, or by condition monitoring laboratories utilizing FTIR as a screening tool.
Below are the advantages and disadvantages of infrared spectroscopy;
- Easy to use
- Correlates well to Karl Fischer
- Needs only a single drop of oil
- Results can be obtained within 2min
- Can determine other oil parameters, like TBN, TAN, sulfation, and oxidation
- More expensive than other test techniques
Another method to use a homogenizer to mechanically introduce the water as distinct droplets of water suspended in the oil. Homogenized samples showed good correlation to Karl Fischer coulometric titration. These were homogenized with a CAT 120X homogenizer and kept at room temperature for a minute. While the level of elastic light scattering induced by a water-in-oil mixture relies on water concentration, it is also considerably affected by the way the water is manually dispersed in the oil, that is the size and number of discrete water droplets present in the oil (Figure 4).
Figure 4. Graphical representation of light scattering in used turbine oil due to varied water droplets. Spectrum A is a used turbine oil with 29,000 ppm water contamination immediately analyzed after homogenization. Spectrum B is a used turbine oil with 9,500 ppm water contamination immediately analyzed after homogenization. Spectrum C is the same sample as in A (29,000 ppm) but has been allowed to sit for 45 minutes after homogenization. The change in concentration and water droplet size is apparent in the degree of baseline lift.
The homogenizer produces a repeatable distribution of water droplets that are evenly dispersed in the oil. Following this, an infrared spectrometer can be used to reliably determine the extent of light scattering.
The FluidScan oil analyzer is a robust handheld IR analyzer designed for measuring the chemistry and condition of oils. Using the homogenization method, a set of oil samples with different concentrations of water was prepared and determined on the FluidScan. The outcomes were compared to those acquired with the Karl Fischer coulometric titration method (Figure 5). The FluidScan readings displayed superior correlation to the titration technique.
Figure 5. Correlation of FluidScan IR readings to Karl Fischer
The FluidScan technique for studying water contamination in turbine oils is a powerful and reliable approach, and can provide instant results of major water contamination. Sampling is the largest contributor to the difference, and the homogenizer constitutes a main part of this technique.
Hand-shaking is not an adequate method to acquire a uniform sample and consistent results for water measurement on the FluidScan oil analyzer. For best results, sample preparation before analysis or instant analysis at-site with a commercial homogenizer can be followed. With best practice sampling methods, it is possible to achieve results correlating within 20% to the Karl Fischer coulometric titration method.
A number of techniques are available for reliability professionals to determine the presence of water in lubrication oils. However, the various methods are based on several factors like personnel training, budget, accuracy of data needed, etc. Reliable monitoring of the equipment for water contamination happens to be most critical aspect for preventing unexpected equipment failure and downtime.
This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
For more information on this source, please visit AMETEK Spectro Scientific.