Unlocking the Secrets to Compressor Health
Compressors are critical assets in our industry. Whether they are in an ammonia complex or another manufacturing facility, they are considered pillars in our industry. Hence, understanding how to assess the health of their lubricants or even anticipate future possible failures can save operators a lot of unplanned downtime and mitigate some risks. In this blog, we will discuss some of the strategies which can be used for compressors in monitoring the condition of their lubricants and overall health.
Given the evolution of equipment, there has been greater mechanical and thermal stress placed on compressor lubricants by the OEMs. Many critical compressors are centrifugal or axial in design as such, they require R&O inhibited (Rust & Oxidation) Oils typically within the range of an ISO VG 32 (although there may be some with an ISO VG 46 for different applications). These oils are now expected to perform at higher temperatures, in small capacity systems for longer periods all while being in the presence of contaminants.
These conditions have given rise to new degradation mechanisms where operators now see an increase in the presence of deposits in compressors and gearboxes. These deposits cause a range of issues as they adhere to the internals of the equipment and act as insulators causing the temperature to increase rapidly. This further accelerates the degradation of the oils.
Varnish challenges in compressors
Varnish is defined as a thin, lustrous deposit which adheres to the internals of equipment and is difficult to remove through wiping. Usually, it comprises of organic residue but can vary drastically depending on the chemical composition of the degradation mechanism.
Although many people believe that varnish is only caused by oxidation, it can also be because of thermal degradation mechanisms such as microdieseling or electrostatic spark discharge. In the cases of oxidation, varnish is formed after the reactive species react with oxygen and other metallic components in the presence of heat (or other catalysts).
Antioxidants are the defenders of oil and can help to prevent oxidation by sacrificing themselves as the preferred reactive species instead of oil. Therefore, by monitoring the health of the antioxidants, one can determine the resistance of the oil to degradation (via oxidation).
Typically, when varnish exists in compressors, they tend to experience the following:
- Increased vibration
- Bearing temperature excursions
- Control valve hysteresis or sticking
- Bearing deposits
- Gearbox deposits and accelerated wear
Essentially, these impacts can be seen in some of the monitoring equipment for compressors. For instance, the temperatures usually show up as sawtooth patterns and the vibration analysis also shows a similar pattern as in figure 1 below.
Figure 1: Axial bearing temperature increases (Red) correlated to increases in vibration signal (Green) in a compressor
There is also a correlation with rotor position and temperature increase as shown in figure 2 below.
Figure 2: Bearing temperature excursions (red) correlated to bearing position (black)
Typically, we would see a buildup of varnish on the metal surfaces as the polar degradation products are attracted to these surfaces. This follows an increase in temperature since varnish is an insulator. Additionally, this buildup also means that there is now a layer of deposit changing the position of the rotor slightly. When the force to move the rotor is greater than the film strength of the varnish, it will be wiped away accounting for the sudden decrease in temperature and change in position of the rotor (as shown in the sawtooth pattern in figure 1).
Oil condition monitoring strategies
Traditionally, oil analysis tests such as viscosity, elemental spectroscopy, acid number, particle count and the presence of water provided some value to the end users to assess the health of their lubricants. However, these tests no longer provide end users with the information they need to make assessments on the performance of their lubricants as lubricants have evolved over time.
Additionally, the RPVOT (Rotating Pressure Vessel Oxidation Test) was the standard to determine whether a lubricant had oxidized or not in the past. This test measures the length of time it takes for the pressure to decrease in response to the oil beginning to oxidize. However, this test is not highly repeatable in that if performed a second time, the results can vary significantly. Thus, it is not the most reliable method to measure or trend oxidation of an oil.
Instead, end users need to utilize more specialized tests which are repeatable and can give more directly relatable data. Such tests include the RULER® test (Remaining Useful Life Evaluation Routine) which quantifies the remaining antioxidants in the oil. This can relate to the rate of oxidation which has taken place in the oil.
During oxidation, sub-micron degradation particles typically form. These may consist of degraded base oil molecules or antioxidants. The MPC (Membrane Patch Colorimetry) test can ideally provide users with some insight into the presence of these particles or the oil’s potential to form them.
Some other helpful tests include:
- FTIR (Fourier Transform Infrared) Analysis – provides users with a molecular fingerprint of the oil allowing them to determine the presence and quantity of molecules in the oil. This can be helpful in indicating if there are any degradation products present.
- Particle count – can indicate the presence of contaminants falling within the range of 4-, 6- & 14-microns ranges.
- Air Release – is particularly helpful in measuring the ability of the fluid to dissipate air which can contribute to the efficiency of the oil (especially for the hydraulic applications where the oil transfers power).
- Elemental Spectroscopy by ICP – can identify the presence of wear metals to indicate if there is any wear occurring on the insides of the equipment.
For compressors, it is advised that these tests are performed quarterly, or this frequency can be adjusted depending on the criticality and environment of these machines.
The solution for compressor varnish deposits
One solution to prevent further development of deposits is the use of a solubility enhancer. Fluitec has developed the patented Solvancer™ technology which is incorporated into a range of its products; DECON™, DECON™ AO, DECON™ AW and DECON™ HD. This technology can be added to the machine while working, is compatible with the in-service lubricants and other system materials and does not affect the oil’s properties or give into oxidative stress.
One case study shows the immediate effect of adding Solvancer technology to a compressor system. Within one hour of the 3-5% treat rate to the compressor system, the temperature dropped dramatically as shown in figure 3 and was maintained.
Figure 3: Impact of Solvancer™ being added to a compressor system
It is therefore important to monitor the health of these compressor oils using tests which can accurately provide valuable insight into the degradation of the oil. However, in the advent of the presence of deposits or to help keep these away, the use of Solvancer technology is one solution which can be utilized.
Adapted from the Whitepaper, “Understanding and controlling bearing temperature excursions on rotating equipment” by Jo Ameye and Greg Livingstone available here.