Blade tip clearance control is one of the main parameters governing turbine efficiency. Large clearances lead to rapid efficiency drop off, low clearances lead to the risk of rubbing, or even catastrophic failure. But the prospect of percentage point increases in fuel efficiency is tempting enough to interest major manufacturers and users. Tip clearance monitors have been successfully used by gas turbine manufacturers to verify blade tip to frame clearances in prototype engines for many years, while in recent years active clearance control mechanisms have been developed by the likes of Siemens and GE.

Since tip clearance varies with component temperatures, achieving full control and maximising the benefit in fuel efficiency requires continuous real time monitoring, and the use of that information as one signal in the adaptive control loop that controls running turbines. At least one company, Tyco Thermal Controls, has developed a sensor system, CapaciSense, capable of fulfilling that function at the high temperatures found in the hot section, and more recently has developed one capable of being retrofitted to turbo-generators in the field.

Such sensors are part of a wider picture. The aim of condition monitoring systems is ultimately to reduce costs, and by prolonging uptime, to increase revenue. Technical advances have progressively increased the amount of information available 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.

The capacitative sensors shown in the illustrations will survive in high temperatures to 1400ºC (2552ºF). Even the turbine stage blade clearance can be measured, an important factor as this means the whole engine can be monitored, not just the lower temperature compressor areas. This raises the possibility of controlling the various stages differentially and optimising the whole turbine, rather than just one zone of it.

Blade height condition monitoring is also beginning to be an essential tool for power plant operators and is starting to be installed on new turbines. Sensors measure the height of every blade in real time on the stages where they are installed. The electronics translate the capacitive signal from the sensor into a live blade height signal that can be remotely monitored.

Matching with modern electronics can produce a system capable of providing a completely new set of signals to diagnose eccentricity and fuel efficiency. If the development of high bandwidth electronics, which is underway now, is included in the picture one is looking at future systems that can help predict high cycle blade cracking.

The technologies

Formerly, rubbing strips or similar devices were used to give a crude measure of blade case interference but these do not yield sufficient information for control purposes. To give a measured distance, new sensor technologies were required.

The three technologies that have shown promise are capacitive, eddy current, and laser. Laser systems are accurate but are subject to lens fogging in turbine operating conditions, blinding the sensors. Fibre optic probes under development may solve that problem. Eddy current systems are able to ‘see’ through the casing, but they are severely limited by their operating temperature range and somewhat by being usable only with metal blades. Capacitive systems have a limited range but operate well at all the temperatures seen in the hot stages and the probe size is suitable for the small clearances typical of normal turbine operation. However they are said to have a poor frequency response, which in some cases can limit accuracy, and the turbine blade must be of ferrous metal.

In the field

Retrofitting new technology to older gas turbines is one way to maintain optimum operating efficiency. Capacitive tip clearance systems are beginning to play a part here and are breaking through to operational machines with probes capable of use even in the highest temperatures in the exhaust. They are being fitted in new engines to deliver accurate data on compressor and turbine blade-tip clearance measurements, rotor shaft and disc displacement. Work has begun to make these systems suitable for retrofit by designing small diameter probes that can use existing holes in the frame to monitor clearances. These may be installed as a retrofit item during periodic outages, or used for inspection checks during running conditions.

“Given the interest from operators, manufacturers and maintenance engineers it is only a matter of time before the first retrofit takes place” says Chris Mayes of Tyco Thermal Controls. “Whilst there is much discussion about the appropriate method for locating the probes in the engine and who is going to pay for the modification, there seems no doubt that this is a technology whose time has come”.

Capacitive blade tip to shroud clearance systems are already being used on some of the largest power generating gas turbines for long term condition monitoring. In one case, the lifetime of the probes has already exceeded 10 000 hours of continuous operation. “We have has probes returned after that time following routine refurbishment at outage and they are still in working order. We have recently secured European ATEX approvals for the system & are soon to add North American FM & Asian TIIS approvals.” says Mayes.

These approvals allow the systems to be used safely in operating power plants and are of particular interest for those considering retrofitting turbines with additional condition monitoring sensors for predictive maintenance, or to check blades in ‘near rubbing’ condition.

The product

The systems consists of probes, turbine mounted electronics and remote electronics. Getting ATEX approval for the Tyco turbine mounted electronics for use in hazardous areas means that blade height can be monitored live for the first time on running engines.

There are two variants in the family. ‘FM’ systems are suitable for non-contact capacitive proximity measurement of fan blade tip gaps for both power generation and aerospace turbines. ‘AM’ systems are suitable for measuring gaps on continuous surfaces such as rotors or shafts of turbo machinery.

Tyco Thermal Controls panel Relative position of probe and blade tip High temperature sensors
Mid-temperature sensors