Martin Armson / Vice President / Hansford Sensors
Martin Armson
The wind industry is booming. Recent estimates put its global capacity at over 340,000 wind turbines with a generation capacity of more than 430 GW, a figure that may quadruple by 2030.
But it’s also a highly demanding sector. While wind turbines perform under punishing conditions, operators are under pressure to run as efficiently as possible by raising output and controlling costs. One key to that goal will be preventing unscheduled downtime from component failures. Recent instances tell of wind turbines exploding from gear failures, typically from high winds. After operators pay the repair bills, they could be slapped with fines and demand for compensation.
An enhanced maintenance and monitoring regime can help to cut these risks and costs. One of the most reliable techniques: vibration monitoring. Here, the vibration ‘signature’ of bearings and other moving parts is monitored using vibration sensors or accelerometers. Any variation from a norm can indicate early signs of failure, which allows fixing small problems before they turn into big ones.
For wind-turbine applications like this one, low-frequency accelerometers are the ideal choice for detecting anomalies. In general, the models used are 100 mV/g, or the higher sensitivity 250 or even 500 mV/g.
Downtime costs
Unscheduled downtime affects more than just the wind industry. According to a joint survey from MRO magazine Plant Services.com and ARC Advisory Group, downtime costs global process industries around $20 billion every year. The survey reported that nearly 90% of all companies use predictive maintenance to increase uptime. In addition, more than half wanted to use predictive maintenance techniques and processes to cut maintenance and overall operational expenses.
Among its conclusions, the survey recommends integrating predictive maintenance with plant-wide control systems, and linking the operation and outcomes of such systems to financial incentives for plant operators. It makes a strong case for using such systems, particularly in wind energy where maintenance is challenging and costly, and operators are under pressure to maximize efficiency and productivity.
While predictive maintenance has advantages, there are historical factors to overcome. Many engineers have genuinely considered it cheaper to continue running with worn equipment rather than invest in expensive replacements. However, when considering the costs of catastrophic failure, this run-to-break theory is significantly flawed.
Machines that begin to exhibit defects are at greater risk of failure than those without defects, and are more likely to generate unwelcome downtime costs. In contrast, a condition-monitoring system helps plan maintenance and replace defective components before problems occur.
Sensor installation
Vibration sensors like the Hansford Sensors HS-100 are used in many wind projects. Most required the use of a local junction box to house the accelerometer cabling at the top of the turbine. This is usually fed back down to the ground using multi-core screened twisted pair cable to connect it to an online monitoring system. This lets operators monitor turbine conditions in real time, using a handheld device with internet access.
There are two main types of industrial accelerometers: AC accelerometers and 4 to 20-mA accelerometers. AC accelerometers are typically used with data collectors for the vibration monitoring of more critical or complex machines, such as gearboxes and turbines. This makes the sensors well suited for wind turbines. In general, 4 to 20-mA sensors are used with PLCs to measure lower value assets such as pumps and motors.
The latest vibration-monitoring sensors operate over a wide temperature range, measuring high and low frequencies – with low-hysteresis characteristics and high levels of accuracy. Because of the punishing conditions they must withstand, the sensors offer reliable service in part due to stainless steel housings that prevent the ingress of moisture, dust, oils, and other contaminants.
Accelerometers can be mounted on casings to measure the vibrations of the casing or to measure the radial and axial vibration of rotating shafts, or both. A typical approach is to examine the individual frequencies in a signal that correspond to certain mechanical components or types of malfunction – such as shaft imbalance or misalignment – so that data analysis can identify the location and nature of a given problem. A typical example would be a rolling-element bearing that exhibits increasing vibration signals at specific frequencies as wear increases.
The typical vibration monitoring signature shows acceleration over time, from an AC output accelerometer.
Vibration specification
To specify a vibration accelerometer correctly, consider the vibration level and frequency range to be measured, weight or fitting restrictions, and environmental conditions. It’s best to work closely with a supplier that has appropriate industry experience and knowledge.
For wind-turbine applications, low-frequency accelerometers are the ideal choice for detecting anomalies. In general, the models used on wind turbines are 100 mV/g, or the higher sensitivity 250 or even 500 mV/g. These might be used to monitor the low-speed aspects of the generator such as output shafts. Such sensors will identify faults and predict failures before they get out of control.
Most turbines failures are caused by gearbox problems, such as bearing wear, shaft misalignment, and gear fatigue. Accelerometers are normally used on the main shaft that connects the rotor to the gearbox, and on intermediate speed gearbox shafts to monitor low-speed vibration. Frequencies there are low, typically between 0.1 and 10 kHz, with small acceleration amplitudes.
Using a predictive-maintenance regime based on vibration monitoring, operators can reduce catastrophic breakdowns, boost turbine availability, and increase the economic viability of wind energy. And they can start to make a dent in that $20 billion annual maintenance bill. 
Filed Under: Sensors
