The latest condition-monitoring sensors keep industrial systems humming.
The latest condition-monitoring sensors keep industrial systems humming.
Edited by Kenneth Korane
John K. Duchowski
Hydac Technology Corp.
There has arguably never been more pressure on OEMs and machine users to improve maintenance and condition-monitoring practices. One factor is the increasingly complex nature of industrial machinery that, in turn, requires new and more-sophisticated maintenance techniques. And higher capital expenditures typically involved are an added incentive to keep equipment up and running.
Another factor is that today's selling and leasing approaches often include service contracts. Regardless of the equipment involved machine tools, excavators, or wind turbines deals are often based not only on initial selling price but also include equipment reliability, operating life, and life-cycle cost guarantees.
In many cases equipment operation depends on the quality and performance of hydraulic and lubricating oils. To handle more-stringent demands, fluids have gotten more specialized and are often tailored to work with specific hardware under specific operating and climatic conditions. There has also been a shift toward more complex formulations of both base stocks and additives.
As a result, there is an increasing trend toward more-sophisticated fluid-monitoring techniques that predominantly rely on in-line or online sensors. These diagnostic devices continuously assess fluid conditions in near-real time. In contrast, conventional practices generally involve sending oil samples to off-site labs for analysis. Thus, online measurements permit faster access to data and more-informed decisions on maintenance and scheduling fluid replacement.
Although online monitoring systems clearly have advantages, designers face a number of considerations prior to installing them. Sensors usually contain delicate electronic components that must withstand exposure to extreme temperatures and pressures, aggressive fluids, and contaminants. They must be economical enough to be used throughout a plant or vehicle fleet. And sensor outputs have to be collected and transmitted with minimal loss or interference from surrounding equipment.
Most present-day systems monitor properties that provide a reasonably good assessment of system performance and fluid conditions. For example, monitoring changes in particle counts, water content, acidity, and viscosity provides information on:
- Severity of wear.
- Contaminant ingression.
- Seal failure.
- Cross-contamination due to adding improper fluids or leakage.
- Water ingression from condensation.
- Fluid degradation caused by oxidation, aging, or excessive temperatures.
Here's a look at recent sensor developments and suggestions for hydraulic and lubricating systems.
One of the simplest, yet most informative, diagnostic techniques is the online particle count, for several reasons. First, particle counters work in a wide variety of applications and are relatively easy to install and operate. Second, particulate contamination is often a primary cause of accelerated wear, malfunction, and even failure of sensitive system components. Third, monitoring particulate levels lets technicians determine the rate of contaminant ingression from external sources and detect the onset of accelerated wear although finding the particle source usually requires additional investigative methods such as microscopy or ferrography. That is because particle counters cannot determine composition; they only assess particle size.
Nevertheless, in many cases, particle counts can warn of processes such as gear micro-pitting or accelerated bearing wear, as these often generate well-defined changes in particle size distribution. In fact, changes in particle size often show up before macroscale changes, such as those detected by vibration analysis.
Optical particle counters, regardless of the light source, operate by light extinction. Basically, when a particle in the fluid stream comes between the light (a light bulb, LED, or laser) and detecting element (usually a PIN diode), light intensity falls in proportion to the particle size (or somewhat more accurately, its surface area.) Combining this information with the fluid-flow rate yields the number of particles of given size per unit volume. Data can then be translated to an ISO 4409 Range Code or NAS 1638 Cleanliness Class. Internal diagnostics and calibration standards such as ISO 11943 for online measurements make this a well-accepted industrial practice.
Depending on the application, particle counters can be installed in pressure lines (preferably), reservoirs, or return lines, provided the latter are not subject to aeration and, hence, interference from air bubbles.
Online counters tend to be more accurate than bottle-sample analysis and results are usually lower by one to two ISO Code Ranges, especially in clean systems. That is because bottle samples often suffer from cross-contamination when technicians obtain and handle them. Advances in electronics and optics have improved accuracy, dynamic range, and speed of response. Future developments will likely include more-sophisticated manipulation of parameters such as light intensity and current draw to overcome present-day difficulties measuring opaque, translucent, and nonhomogeneous (water or air-containing) fluids.
EFFECTS OF WATER
Water is one of the most pervasive and damaging contaminants that threatens hydraulic and lubricating systems. It changes a fluid's physical properties, for instance by lowering viscosity and load-carrying capacity. Water also interacts chemically with fluids, leading to hydrogen embrittlement, corrosion of system components, and hydrolysis of base fluid and additives. It is also difficult to quantitatively detect water with currently available instruments. This is especially true for online monitoring in industrial applications.
Part of the problem is that water can exist in several different states, each of which requires different detection techniques. For instance, at low concentrations water remains completely dissolved. It forms a homogeneous solution in the oil and appears transparent. At higher concentrations, water droplets disperse throughout the fluid in an emulsion. Fluid appears cloudy or opaque although bulk-phase separation has not taken place. Finally, at still higher concentrations water droplets coalesce and completely separate from the oil. Depending on fluid densities, this so-called "free" water settles to the bottom of the tank with some synthetic and hydrocarbon-based fluids or migrates to the top with phosphate esters.
Despite the difficulties in sensing water, researchers have made significant progress in recent years. Most online water sensors for hydraulic and lubricating systems measure capacitance. Capacitive elements are inexpensive, robust, and suitable for a wide range of fluids. They have a dielectric polymer sandwiched between metal electrodes, encapsulated in a ceramic substrate for added durability. Water that migrates into the dielectric increases capacitance, and an RTD (resistancetemperature-detector) sensor in the probe compensates for temperature changes.
Measuring free-water content requires modifications to the sensing capacitor. In particular, a tube-shape capacitor replaces the polymer-based sensor. Fluid flows axially through the sensing zone and oil acts as the dielectric medium. Capacitance changes are correlated to free-water content in bulk oil.
Such systems are predominantly used in return lines where free water is a concern, such as on the wet-end of machine lubricating systems in paper mills or on Morgoil systems in steel plants.
Instrumentation is also getting smaller. For example, the Hydac Aqua Sensor 2000, developed about three years ago, includes a probe and separate console which houses signal-acquisition and data-processing electronics, a display panel, and control buttons. The unit can be permanently installed or serve as a portable device for checking water content in reservoirs.
The company's new Aqua Sensor 1000 is less than half the size and incorporates signal-processing electronics in a rugged aluminum housing. Both units measure dissolved water content, handle pressures to 725 psi, and install in oil supply and return lines in most hydraulic and lubricating systems. Although reservoir installation is also possible, response time greatly improves when the sensor lies in the oil flow path.
Engineers often overlook P transducers for condition monitoring, even though they can detect impending failures. This is because in well-behaved and stable systems, differential pressure across filter elements exhibits a well-defined and nearly exponential rise over time. However, should the fluid contamination level significantly change, P versus t curves provide early warning and help pinpoint sources of problems.
For instance, an earlier onset or steeper rise of the exponential curve usually indicates a significant increase in contamination. And changes in the curve's shape for example, from exponential to linear may indicate a different filter loading mechanism (usually caking) generally associated with a change in contaminant makeup.
A transducer with two electronic pressure sensors measures inlet and differential pressures across a filter. The latest versions can set the bypass cracking pressure with the help of analog and switching outputs. They also recognize whether a filter element has been installed in the housing and even if it is the proper one. In short, the sensors provide important information on system stability and internal processes that affect performance.
The latest sensors have significantly advanced the art of contamination control. But for condition monitoring to be truly useful, one must also accurately interpret the data. In many cases, combining and superimposing sensor data reveals trends that might otherwise be missed if each signal were interpreted individually. Combined sensor data usually presents a more-comprehensive picture of system and fluid conditions and helps users make better-informed decisions.
State-of-the-art data-acquisition controllers collect readings from distributed arrays of dis-similar sensors such as particle counters, water sensors, and differential-pressure transducers transmit it via industry standard communications protocols, and use sophisticated software to present it in a userfriendly manner. Ideally, the system should also trigger alarms and send messages via e-mail or cell phone, letting technicians respond to abnormal conditions.
A number of manufacturers produce controllers with varying levels of sophistication. For example, Hydac's handheld HMG 500 Data Acquisition unit simultaneously displays data from two sensors, regardless of sensor type. It accepts analog current (4 to 20-mA) and voltage (0 to 10-V) signals and displays data as individual values or a differential reading between the two.
The more-sophisticated HMG 3000 is a high-end data-acquisition unit about the size of a portable oscilloscope. It simultaneously handles up to 10 sensors, regardless of sensor or signal type. In addition to analog inputs, it also has two input ports for digital speed and frequency data. A high-resolution color dis-play shows data in graphs or tables. It stores up to 50 measurement curves, each with up to 500,000 individual values. Also, the unit can be programmed with machine and measurement-related information to facilitate setup. Data stored in the HMG 3000 can be downloaded via a built-in USB port.
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