Falling costs and greater simplicity in vibration monitoring systems enable plant operators to do predictive maintenance on high-value and basic equipment.
Water and wastewater plant maintenance has undergone a revolution. Operators have moved from routinely servicing machinery at predetermined intervals regardless of condition to applying predictive maintenance.
Until recently, costly and complex technology and low awareness of alternative solutions have limited predictive maintenance to large or high-value filtration and treatment systems. Now, the falling price of technology makes it easier than ever to monitor the operating condition of more basic yet still critical assets such as pumps, motors and fans.
Vibration is a classic indicator of wear, imbalance or misalignment in shafts, bearings and other rotating parts. There are two key advantages to vibration monitoring for detecting wear rates or changes in operating conditions over time. First, it provides information that helps engineers determine if and when essential maintenance will be required. Second, it lets them easily identify at an early stage the exact component where a fault is developing.
Work can then be planned at the most appropriate time, and replacement parts can be ordered only when actually needed. This helps reduce stocking of spare parts and, more important, eliminates unplanned downtime, minimizes maintenance costs, extends service intervals, and prolongs asset operating life.
Accelerometers are the most widely used tool for detecting changes in vibration levels. An accelerometer incorporates a piezoelectric crystal bonded to a mass. Under an accelerating force such as vibration, the mass compresses the crystal and generates an electrical signal directly proportional to the force applied. This signal, once amplified and conditioned, can then be used by data acquisition or control systems.
Accelerometers fall into two categories: AC and 4-20mA. While both work on the same principle, AC accelerometers are typically used with data collectors for routine or periodic monitoring of assets, while 4-20mA devices are normally used with PLCs to provide the option of continuous monitoring and remote data capture. Both can detect imbalance, bearing condition and misalignment, but AC accelerometers can also identify cavitation, looseness, gear defects and belt problems.
As maintenance and plant engineers increasingly favor prevention over cure, techniques such as vibration monitoring are being used more widely, and that trend is likely to continue. A recent report from MarketsandMarkets, for example, estimated that global demand for vibration monitoring systems will reach nearly $1.5 billion by 2020, a compound annual growth rate of more than 6.5 percent.
This growth is being driven in part by the relatively low cost of vibration sensors. With their compact size and simplicity of installation, sensors can now be fitted to almost every water and wastewater plant asset, no matter how small. Similarly, it is now much easier to fit multiple sensors to larger, more complex filtration or treatment systems. The challenge is how best to manage the large volumes of data generated and use it to run an efficient maintenance program.
Data can be gathered in two ways:
Offline, usually with a hand-held data collection instrument that interrogates a permanently fixed sensor or simply uses a hand-mounted accelerometer temporarily positioned on, for example, a motor or pump casing.
Online, where data is automatically transferred to a centralized control system for real-time monitoring. Though more expensive, this method is generally more reliable and efficient.
For offline monitoring, maintenance teams must develop the most efficient data collection route through the plant. This is normally determined by the criticality of equipment: The most essential assets are checked more frequently than those of lower importance. The maintenance route is programmed into the data collector to guide each operator through the correct routine.
Online and offline techniques can be combined. For example, it may be appropriate for high-value assets to be continuously monitored with a direct link back to a local or remote control center, while lower-value assets are monitored offline at set intervals.
Regardless how data is captured, it is critical to analyze the correct frequency spectra. These are dictated by the fault frequencies of the fastest-turning component or components in the machinery being monitored.
A slow-turning sludge scraper system, for example, will have a narrower and lower frequency spectrum than a high-speed blower fan. Once the range of frequencies is known, the measurement resolution is set within the vibration software for spectrum analysis so that the fault frequencies of rotating components are correctly isolated and not, for example, mistaken for natural background machine resonance.
Correctly recorded and analyzed data can play an important role in plant efficiency. Readings from an aeration blower fan, for instance, may reveal that the bearings were incorrectly aligned during installation, that an outboard bearing was never locked down correctly, or that grid couplers are incorrectly lubricated.
The results of an inspection can also throw up surprises. For example, a vibrating blower fan may suggest that the rotor needs balancing, while in reality the vibration could be caused by an unrelated issue, such as the buildup of debris on the fan blades. In each case, the simple process of monitoring vibration levels will allow a potential fault to be identified and rectified before it causes a larger problem.
Deploying with care
Finally, it’s important to recognize that an accelerometer will only produce accurate and repeatable measurements if mounted correctly. This is especially true of temporarily positioned or hand-held sensors, where users must take care to mount each device onto a flat, smooth, unpainted surface that is wider than the base of the accelerometer itself.
The surface should also be free of grease, oil or other contaminants, be close to the source of vibration, and be perpendicular to the axis of rotation. Standard accelerometers normally record vibration in one axis, although compact versions are available for measuring vibration in two or three axes simultaneously.
Although vibration monitoring may not always be seen as the most important maintenance option, the growing need to maximize uptime and extend the operating life of all plant assets makes this measurement technique an increasingly valuable weapon in the water and wastewater plant operator’s armory.
About the author
Chris Hansford is managing director of Hansford Sensors, a manufacturer of vibration and condition monitoring devices based in the U.K.