Condition monitoring: Vital to meeting demand for power

HPI plants are one of the areas where condition monitoring systems can extend equipment lifetimes
Phil Burge, Communications Manager for SKF, looks at the issue of vibration in power generation plants and machinery, as well as highlighting how the latest condition monitoring technology can prolong the operating life of equipment.

Phil Burge, SKF, UK

The global power generation industry is often regarded as the foundation of modern civilization, with the world’s appetite for electricity rising year on year as standards of living continue to improve and populations further grow, particularly in developing nations such as India and China.

Indeed, global power consumption currently sits at around 17 109 665 TWh per year, placing the average power per capita at 297 W, with this figure expected to almost double by 2030, growing at a compound annual growth rate of 3.6 per cent.

Accordingly, efficient power plants, whether fossil fuelled, nuclear or renewable, are vital to meet the growing energy demands of today’s ever developing communities and industrial economies. Most critically, this means optimizing the performance, availability, uptime and service life of power generating equipment, while cutting the operating and maintenance costs of these facilities to ensure that they remain commercially competitive.

Above all, this calls for effective maintenance practices in order to identify and resolve the problems that come as a result of worn components, imbalances in shafts or the misalignment of rotating parts.

Vibration is a common problem in a wide range of equipment, including the pumps used in cooling towers, induced draft and gas recirculation fans, and gearbox and turbine shafts; it is a problem that must be eliminated to avoid increased energy consumption, high maintenance costs and, in extreme cases, the catastrophic failure that can come as a result of undetected wear or incorrect installation.

One proven method of achieving this is through the use of condition monitoring systems, which have been used for some time to detect vibration, typically using accelerometers. When used as part of a predictive maintenance programme, this technology can provide early warning of a change in operating conditions, indicating the presence of vibration and increasing temperatures, and allows remedial action to be taken before any substantial damage is caused.

Evolution of condition monitoring technology

The latest condition monitoring technology includes vibration detection and thermal imaging with a broad range of diagnostic tools and software. It offers extremely high accuracy from compact and often hand held devices that can be used in wireless networks for remote interrogation and control, helping engineers to manage the frequency and type of maintenance carried out in order to achieve optimum performance and long term cost control across power generation plants.

For example, the latest sensors or accelerometers harness piezoelectric or piezoceramic technology, which provides a reliable method of measuring high and low frequencies, with low hysteresis characteristics and excellent levels of accuracy over a wide temperature range. These can also be packaged within compact stainless steel sensor housings to protect them against moisture, dust, oils and other contaminants.

These devices are typically mounted in key positions on the equipment being monitored, with output data read periodically using handheld data collection devices, either for immediate analysis or subsequent downloading to PC. They can also be routed to a centralized or higher level system for continuous monitoring and analysis.

Latest devices

These handheld units are also helping engineers maintain a wide range of machinery with rotary or reciprocating motion by measuring vibration accurately, quickly and efficiently at the point of use. SKF’s Machine Condition Advisor (MCA), for example, measures velocity vibration signals and automatically compares them to preprogrammed ISO guidelines, while simultaneously applying the industry proven SKF Enveloped Acceleration technique to compare readings with established bearing vibration guidelines. If these measurements exceed guidelines, the unit sounds an alarm to indicate potential bearing damage. It also features a thermal sensor to monitor bearing temperatures for abnormal heat loads that could indicate lubrication problems.

Other handheld devices include portable monitoring and data collection instruments. Models such as Microlog have been developed to simplify the collection, analysis, use and sharing of machine condition data. They also include route-based instruments, which work with powerful predictive maintenance software systems, and stand-alone devices that offer on-the-spot advice and signal analysis capabilities.

Thermal monitoring

Thermal control and management is also a crucial element in condition monitoring and predictive maintenance strategies. Thermographic condition monitoring provides a reliable method of detecting heat and energy loss to identify the early wear of moving parts. As well as simple to use handheld instruments, such as SKF’s TKTL infrared thermometers, there is also a range of high tech digital cameras. These offer thermal and two-megapixel visual imaging, making them ideal for identifying hotspots, ranging from -10 à‚°C to +350 à‚°C, in a wide variety of power generating equipment. Designed to be simple to use, instruments such as SKF’s TKTI 10 look like digital cameras and operate in a similar way, while their unique ability to blend thermal and visual images makes for extremely simple image interpretation.

SKF’s TKTL 20 infrared thermometer with laser and contact adaptor

In contrast with low-end thermal imagers, two spot temperatures can be displayed on-screen and independently moved to select an area of interest. For experienced thermographers, these cameras offer features including automatic temperature ranging, selectable emissivity, reflected temperature compensation, hot and cold spot finding, temperature area measurement, and automatic and user selectable level and span.

As with vibration monitoring equipment, the latest thermal cameras can also be combined with associated software to deliver real time and historical analyses for effective predictive maintenance and process traceability, helping engineers boost uptime and productivity still further. By uploading the information gathered by such systems, it can be communicated throughout an entire power generation plant simply and rapidly to support plant wide asset management initiatives.

The use of these highly effective technologies as part of predictive plant maintenance techniques is already delivering significant benefits to pioneering power generation organizations. Indeed, the latest condition monitoring technology is helping engineers at one UK power station to realize significant reductions in downtime due to unexpected failures on the station’s induced draft cooling towers.

In particular, the integrated condition monitoring system is enabling engineers to assess the real-time performance and functionality of 12 large diameter cooling fans, together with associated drive motors and gearboxes, and to implement a long-term predictive maintenance strategy that has improved uptime by over 30 per cent and delivered tangible cost savings from these business-critical systems.

The station, which uses a 690 MW capacity combined-cycle gas turbine to power around 970 000 homes, has a single cooling tower, with 12 individual induced draft cooling cells. Each is fitted with a motor-driven 6 meter diameter fan with fibreglass blades, mounted in a cowl at the top of the cell, to enable ambient air to be drawn in from the base of the cell and across the heated turbine water as it falls through a series of baffles that maximize heat transfer. The drive motors are located on the outside of the cowls and provide power through gearbox units, which are positioned within the body of each cell.

These fans, motors and gearboxes are all subject to extreme conditions. Indeed, the site is in an exposed location on an estuary, meaning that high wind speeds and downdrafts can affect fan operation, while the concentration of salt in the atmosphere, combined with dust and particulates from a nearby aggregate plant, causes corrosion and damage to motor and fan units. Similarly, the position of the gearboxes beneath the fan units means that they are continuously exposed to the flow of water as it is blown down each cell.

Avoiding catastrophe

The nature of the application means that engineers have to pay particular attention to regular maintenance, as bearing and seals in drive shafts and fan spindles, in particular, can wear or fail with catastrophic consequences. Worn bearings on the drive end of the gearboxes can lead to excessive gear wear, reaching the point where the unit must be replaced or removed for overhaul. This is an expensive operation for the power station, requiring partial dismantling of a cell using heavy lifting gear.

SKF’s Microlog Field Balancer

A similar situation could also occur if fan spindle bearings ran out of alignment, as excessive vibration would eventually lead to delamination of the fibreglass blades. In each instance, it means taking a cooling cell off line, which can affect the power station’s generating capacity and thus its revenue-earning potential from the National Grid.

Until recently, engineers at the power station had used handheld vibration and thermal monitoring devices to check on the condition of the various systems used in each cooling tower cell. Although these instruments are reasonably accurate, this method can be a time consuming exercise, as motor, gearbox and fan units are difficult to access, and only provide historical data, rather than real-time information, meaning that engineers only knew about problems when they had already occurred or were about to occur.

To improve the operational efficiency of the power station, engineers embarked on a programme to install a dedicated condition monitoring system for the cooling tower, as part of a longer term predictive maintenance strategy.

The system is based around the SKF IMx-S online condition monitoring system, connected to a network of 60 accelerometers; five of them on each fan, motor and gearbox, plus an SKF Machine Condition Transmitter (MCT) fitted to each gearbox input shaft bearing to provide an alarm trip function. Outputs are relayed to a control room terminal running SKF @ptitude monitoring, analysis and reporting software, providing a real time display of all key operating parameters.

To ensure the success of the new condition monitoring suite, SKF provided a comprehensive training and support package throughout the planning and implementation phase, providing valuable information and practical advice to the power generation company.

With the new condition monitoring system in place, the engineers can identify wear in key components long before it becomes a potential problem, enabling them to plan essential maintenance in advance and at a time when demand for generating capacity will be low. Above all, there have been significant improvements in availability that are contributing directly to maximizing the revenue from the power generating plant.

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