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It’s tough at the tip

Blade tip clearance monitoring systems for turbines can provide valuable data to enable better timing of maintenance, reductions in unplanned downtime and enhance turbine and fuel efficiency.

Paul Seccombe, Tyco Thermal Controls UK

There are tremendous advantages in having the means to “see inside” an operational turbine and gain early warning of failure or to allow management of the machine’s efficiency. Collaborative developments in this area by gas turbine manufacturers and equipment suppliers is now delivering precisely that, with state of the art equipment capable of recording at least one measure of significance – blade clearances.

Systems capable of monitoring blade tip clearances are now coming to market, following many years of use for development purposes, and they have already started to be employed in modern turbines. In an operating environment, the benefits from effective use of these new systems are potentially very large and may be divided into two primary areas – maintenance/uptime benefits and fuel efficiency.

Business issues

Two of the key issues driving profitability for power plant operators are uptime, or load factor, and efficiency. This has always been the case, but as fuel costs spiral ever higher, it is becoming more important to manage the turbine in such a way that both factors are maintained at high levels.

As a power plant is spun up or slowed, turbine blade clearances change and in extreme cases blade rubbing can occur due to differential thermal expansions of key components. At best this can lead to larger gaps due to erosion of the tip, and at worst can cause catastrophic failure.

Capacitive probe at >1000à‚°C capable of operating in gas turbine hot stages
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Using capacitive sensors and associated electronics, power plant operators can now monitor these clearances in real time to identify any issues. This is particularly valuable when the turbines are used for peaking supply and are thus being spun up and down regularly.

By monitoring blade tip to casing clearances specifically, a key operating variable in a turbine moves from the unknown into factual knowledge. And, changing from a traditional time-based maintenance cycle to those based on actual operating condition can drive turbines towards longer maintenance cycles. In general, monitoring allows for deferral of planned maintenance based on accurate knowledge of the current condition and by preventing unplanned and financially expensive outages.

As these systems provide additional information on the mechanisms and can be a valuable tool for the power plant operator, gas turbine manufacturers are also coming under pressure from operators to provide such systems and they are already being installed on some new turbines. In certain systems the height of every blade is measured – in real time – on the stages where they are installed, and blade height condition monitoring is becoming an essential tool for the power plant operator.

Data from blade tip sensors is fed to the control room monitoring console
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Turbine fuel efficiency is also critical for plant operators and active clearance control is rapidly becoming a valuable method of maintaining high efficiency levels, leading to lower operating costs per MW. This is highly significant to power plant operators, often swinging the decision on which turbine to buy and run and one measure of the value of efficiency for the largest turbines shows that for each additional one per cent of fuel efficiency, financial savings on fuel are roughly equivalent to $1 million a year.

Furthermore, theoretical work shows that fuel efficiency is inversely proportional to the blade tip to casing gap: the smaller the gap, the better the efficiency. No wonder then that many of the new turbines have these systems built in to monitor that gap.

The technologies

A decade or more ago, any company developing turbines and wanting to know the distance from blade to case would probably have used degradable rubbing strips or break pins. These provided only a very crude assessment of the clearances and it was increasingly felt that new sensor technologies were required to give a measured distance, rather than a “rub/break or not” switch.

Subsequently, the four technologies that have shown promise are capacitive, eddy current, laser and ‘radar’ measuring three different physical properties. Each of the technologies has both limitations and good points.

  • Laser systems are seen as having high accuracy but are generally agreed to be a useful development tool rather than useable in the field. The main issue tends to be lens fogging in turbine operating conditions that progressively blind the sensors.
  • Eddy current systems have the potential advantage of being able to work through the casing, but they are severely limited by their operating temperature range.
  • ‘Radar’ based systems are an interesting emerging technology, but like so many leading edge solutions, it is too early to say whether they will be useable and there are still issues that may never be overcome.
  • Capacitive systems have a limited range but operate well at all the temperatures seen in the hot stages and the probe sizes are not prohibitive for the typical small clearances seen in operation. Triaxial cable systems perform better, whilst coaxial-based systems have smaller cables.

Of these differing technologies, capacitive systems appear to be the most suitable for long-term operating engine use and are already in commercial service. The latest designs feature probes capable of withstanding tip temperatures up to 2550à‚ºF (1400à‚ºC), an engine-mounted electronics package carrying full ATEX approval for use in hazardous areas and remote monitoring electronics.

Using the data

With fuel costs spiralling, it is no surprise that fuel efficiency in cost per MW generated can be one of the key purchasing points for the buyer – and hence a key selling tool in the turbine manufacturers’ sales pitch. As small clearances increase efficiency it follows that more effective measurement allows the optimization of the clearances without compromising safety and risking rubbing during start-up or cool down cycles.

Turbine developers and manufacturers need to be able to verify initial calculations and assumptions about how those clearances will change during start / run / stop cycles. Clearance systems also allow the blade clearances on prototype engines to be closely monitored and provide a valuable stream of individual blade heights for each stage monitored. This type of use has been dominant over the decade that these systems have been employed by designers.

Turbine probes used in commercial systems
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The aim of condition monitoring systems is ultimately to reduce cost and technical advances have progressively increased the amount of information flowing from sensors on turbomachinery for analysis of vibration, exhaust gas temperature and other data. The immediate benefit is better informed decisions on the maintenance and operational strategies that allows assets to be better managed, deliver higher plant availability and reduced maintenance and life cycle costs.

In some cases engineers in operating plants are already asking for retrofit of capacitive-based systems to generate information over and above that which is supplied by the gas turbine vendors.

Furthermore, a system combining a sensor that can measure the tip clearance with controls that actively control those clearances allows direct management of fuel efficiency, a key driver for the commercialization of these active blade control systems.

View from the tip

Turbine blade clearance measurement systems are available now and are already operating on several modern turbines. The extreme environment in the turbine stages and the need for accuracy of the data is driving most companies towards triaxial capacitive systems.

CAD picture of blades and sensor
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In an operating environment, the benefits from effective use of these new systems are potentially very significant, both in terms of maintenance/uptime and fuel to electrical power conversion efficiency. Operators, keen to ‘see’ inside the casing under operation are beginning to beat a path to the door of capacitive system manufacturers to acquire these as retrofit options.

The future developments of these systems will be in adding tip-timing outputs to replace strain gauging for development systems and allow the monitoring of blade stress in real time for running engines. Companies at the forefront in capacitive blade tip-clearance system technology are now working to extend both probes and electronic signal processing to provide probes capable of supplying both tip-clearance and tip-timing information at the high temperatures encountered in operating gas turbines.