Viscosity changes in oil can raise major red flags

Viscosity measurement is generally considered key to an effective oil monitoring and conditioning program. Changes in the viscosity of oil used in machinery can signal a number of problems -- oxidation, contamination and thermal degradation -- altering lubricant performance.

By Kerem Durdag

Editor's Note: There are four figures that accompany this story that can be downloaded via the "Download Now" button at the bottom of the page.

Any company manufacturing or using heavy equipment or machinery is concerned with lubrication. Viscosity measurement is generally considered key to an effective oil monitoring and conditioning program. Changes in the viscosity of oil used in machinery can signal a number of problems -- oxidation, contamination and thermal degradation -- altering lubricant performance.

Lubrication is essential to proper machinery maintenance. An effective oil conditioning and monitoring program can help extend equipment life, reduce scheduled and unscheduled downtime, reduce maintenance costs and extend lubricant change intervals. Having real-time, online viscosity data can eliminate the need to make decisions based on intermittent “snapshot” data acquired from periodic sampling. However, making accurate online viscosity measurements has been a challenge because it’s difficult to integrate conventional viscometers into online process flows. At the same time, viscosity can be affected by temperature, shear rate and other variables that can be very different off-line from what they may be in an online environment.

The critical need is the ability to detect changes from a baseline rather than simply measuring absolute values. Ideally, these changes would be identified online in real time. This suggests using viscometers with digital output that can integrate with other instrumentation via local area networking or WiFi. The goal is a continuous digital audit trail together with an early warning system that alerts maintenance to problems as soon as they develop.

Viscometer technology
Online viscosity instrumentation must be rugged and reliable enough to operate over extremes of temperature, pressure and vibration. In addition, many applications call for instrumentation that can operate reliably in harsh chemical environments. Users want “set it and forget it” online instrumentation that doesn’t require periodic recalibration or frequent maintenance while providing continuous data logging.

Most existing viscometer technologies originally were developed for off-line laboratory analysis. The simplest, and perhaps most common, method of measuring viscosity is the viscosity cup. A known volume of liquid is placed in the cup and allowed to drain through an orifice in the bottom. The design of the cup and orifice yield a cup factor and viscosity is measured in seconds. Another simple test is to time the fall of a metal ball or other object through a sample, again measuring viscosity in seconds.

The analytical choice in most laboratories is the rotational viscometer. These instruments measure viscous drag on a rotating disk of specific geometry. The relationship between torque and spindle speed is interpreted as intrinsic viscosity with units of milliPascal-seconds (mPa-s) or centiPoise (cP). While accurate and ideally suited for making static off-line measurements, rotational viscometers are highly susceptible to any motion not caused by the measurement device itself. Rotational viscometers can be adapted for online process use, however, size limits their application.

One approach to process viscosity measurement is to insert a capillary tube into the flow stream to measure pressure drop under a known flow rate. This provides a measure of kinematic viscosity with units of centistokes (mm^2/s or cS). The problem is that the capillary tube can clog and maintaining a constant flow rate, independent of pressure, is both difficult and expensive.

Mechanical and electromechanical methods have been tried. One approach uses a piston and cylinder to measure kinematic viscosity. Air or magnetic coils move the piston. Air raises the piston and the time it takes to fall provides a measure of viscosity. When magnetic coils are used, a constant magnetic force drives the piston up and down. The round-trip time provides a measure of the fluid’s viscosity.

While most mechanical viscometers can be used for online measurements if the sample isn’t affected by flow rate, they’re generally not suitable for high-flow-rate online applications.

Acoustic wave viscometers
Until the introduction of acoustic wave viscometers, no solid-state solution for measuring viscosity in real time has been commercially available. This technology combines solid-phase surface chemistries with ultra-sensitive acoustics. And, since there are no moving parts, acoustic sensors can be used in high-flow-rate, online applications. The viscosity sensors can be smaller than a matchbox. Designed to operate at pressure to 450 psi, these sensors have a range of 0 to more than 10,000 cP with 3% repeatability in operating environments of –20ºC to +135ºC. Because the sensor is hermetically sealed, it can be integrated into process piping or installed in a gearbox oil sump. At the same time, because viscosity is temperature-dependent, the sensor provides real-time temperature and viscosity measurement.

Acoustic sensors also are suitable for online sampling and for spot-check monitoring in process environments. When combined with a handheld reader, they can be used in laboratories where portability is important.

Viscosity measurements
Viscosity has a number of different measurements. The most familiar are kinematic viscosity (centistokes) and dynamic or absolute viscosity (centipoise). These two measurements are related as centistokes equal centipoise/specific gravity. Acoustic sensors measure viscosity in units of centipoises*specific gravity. This measurement is based on the transfer of acoustic shear wave energy from a quartz crystal or other solid waveguide having a characteristic impedance. The square of the power loss is proportional to the product of frequency, density and viscosity. Because the frequency is known, the sensor measures the product of viscosity and density.

Knowledge of specific gravity allows conversion from one measurement to another when shear rate and temperature are equal. Thus, the sensor’s digital output can be displayed in cup seconds or centipoise units if the fluid’s specific gravity is known.

The measurement is made by placing the wave resonator in contact with liquid. The liquid viscosity determines how thick a layer of fluid couples to the sensor surface hydrodynamically; and the energy dampening of the viscously-coupled film is determined by its thickness and density. Acoustic viscometer response is proportional to the product of the viscosity, the density and the radian frequency of the vibration in the limit of low frequencies.

The acoustic wave resonator supports a standing wave through its thickness. The wave pattern interacts with electrodes on the lower surface and interacts with the fluid on the upper surface (Figure 1). The bulk of the liquid is unaffected by the acoustic signal and the vibrating surface moves a thin liquid layer (Figure 3b).

Test results
Two tests were conducted to detect contamination by monitoring changes from a baseline. In the first test, a very small quantity of high-viscosity oil was added to five liters of pure oil (Figure 4). The sensor detected an immediate change in viscosity when the contaminant was added.

In a second test, an additional 250 mL of high-viscosity oil was added at 2,940 seconds, bringing the total of added oil to 364 mL.

Figure 5 shows the exact time when the contamination took place. The 1% to 2% fluctuations shown in the graph are related to minor temperature changes in the closed-loop system. In both tests, each sample was tested until viscosity and temperature had leveled off before added the high-viscosity oil.

As a solid-state device, acoustic sensors can incorporate on-board electronics and communications protocols. This enables manufacturers of compressors, gearboxes, turbines and other equipment to add viscosity monitoring and control to their products.

For example, the sensor can show the different, real-time behavior of new, used and contaminated synthetic oils in a gearbox. As illustrated in Figure 6, the new, used and contaminated oils each have a different viscosity value. The value of the new oil is lowest because it is most subject to shear thinning. The value for the contaminated oil is lower than that of the used sample because water had seeped into the gearbox. Note that the viscosity of the water-contaminated oil is lower because of the high shear rate of measurement by the sensor. Depending on the rheological curve and the behavior of the non-Newtonian mixture, shear rate viscosity values are of a different nature than those obtained by traditional mechanical viscometers.

Although these tests were simple and straightforward, the data clearly indicates:

  • Solid-state sensors detect changes in oil viscosity as they occur to provide an early warning of possible contamination.
  • A solid-state sensor detects viscosity changes at low levels of contamination.
  • Solid-state sensors can be easily integrated into online processes and, because they have no moving parts, they are virtually maintenance-free.

Kerem Durdag is chief operating officer of BiODE Inc., Westbrook, Me. Contact him at 207-856-6977.

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