Gas turbines incorporate a variety of control valves, each performing a different function such as regulating or preventing gas flow. The precise monitoring and controlling of flow from these valves enables turbines to operate efficiently with minimal wasted energy.
Though actuators move the valves, feedback devices translate valve position back to turbine control systems, indicating "if" and "how much" a valve is opened or closed. While other technologies have been considered as feedback devices, the power generation industry has standardized on LVDT linear position technology.
As simple devices with only a few coils of wire and a ferrous core, LVDT position sensors can withstand high levels of shock and vibration, get extremely dirty, and still operate according to their specifications. Lasers become blind due to dirt and grime while capacitive and eddy current sensors don't have the capability to measure long strokes. Potentiometers, magnetostrictive position sensors and string-pots have difficulty with the temperature, vibration, and intrinsic safety requirements.
Magnetorestrictive position sensors have temperature limitations to +80°C due to the limitations of the wave guide and the integral electronics. The sensing element also cannot be partitioned from the electronics for operation in harsh environments. In continuous high-vibration environments, devices will not perform satisfactorily. Potentiometric devices with contact also do not offer the reliability and high temperature performances associated with LVDT linear position sensors.
Fig. 1: Basic construction of a LVDT position sensor.
Designed to address the limitations of other sensor technologies, LVDTs offer the following attributes that make them ideal as measurement devices in gas turbines.
LVDT linear position sensors can measure movements as small as a few millionths of an inch, which is important to ensure the movement of some valves to very minute degrees. Though many valve users may only need to know if a valve is open or closed, some operators need to know the exact position in their application. For instance, as bleed valves are modulating valves, they should be opened a specified amount, depending on how much power is being generated. By ensuring valves are properly opened or closed to the right degree, plants operate more efficiently. For a medium-sized plant, a 2% efficiency improvement could translate into a million dollars in fuel savings.
While once only available in stainless steel, LVDT linear position sensors can be repackaged with modern materials such as Monel, Hastelloy, and titanium to address the often extreme temperatures and highly volatile environments of gas turbines. Exotic alloys such as cobalt and nickel can deliver even higher performance from LVDTs where comparable technologies will not survive.
As gaseous vapors are present in gas turbines, LVDTs installed on turbines must meet intrinsically safe parameters and be approved by certified agencies such as UL, FM, CSA, and ATEX. Even if a sensor is designed for operation under hazardous conditions, certification provides reassurance to the end user that units are pre-approved for use in many hazardous environments of gas turbines.
Fig. 2: Designed specifically for use in gas turbines, Macro Sensors' HLR 750 Series LVDT linear position sensors are UL/ULC listed for Class 1, Division 1 & Class 1, Zone 2 hazards (operating temperatures of -20°F to +212°F), and Class 1, Division 2, Class 1, Zone 2 hazards (operating temperatures of -20°F to +300°F when installed per company standards). They are also ATEX approved for use in potentially explosive environments.
Operating Temperature Range
In environments where sensors are used to monitor turbine health or regulate valves, temperatures can reach +250°C (+475°F). The ability to separate LVDT electronics, which are located in the turbine control system, from the LVDT coils, is one of the main reasons that these sensors can survive so well in power plants. LVDT linear position sensors can be separated from signal conditioning electronics up to 100 meters. With electronics remotely located, an ac-operated LVDT can withstand high temperatures of +350°F to +400°F on the turbine, while sensors with built-in electronics would have to be specially configured (at a much higher price) to offer electrical components that can withstand this level of heat.
Compact Design to Stroke
Strokes of traditional LVDT linear position sensors were once too long for applications with limited space. New computerized winding techniques and smaller embedded microprocessors have considerably reduced the length of the linear position sensor body compared to its measurable stroke length. Today's modern LVDTs are up to 80% more compact for any given stroke. With a sweet spot between 0.5" and 12" of stroke length, LVDT position sensors offer high accuracy and durability.
Based on their reliable performance, repeatability, accuracy, and cost, LVDT linear position sensors serve an important role in gas turbine operations as well as plant rehabilitation projects to ensure optimum efficiency. When choosing an LVDT for operation in a gas turbine, consider the following:
- Operating environment such as temperature, vibration and shock
- Optimum operating frequency for accuracy and length of cable between LVDT and signal conditioning
- Circuitry against noise rejection
- Space envelope of the LVDT for critical mounting conditions
- Ease of connectivity to the electronics
About the Author
Karmjit Sidhu is vice president of Business Development for Macro Sensors. His previous work history includes working was Chief Transducer Engineer for Measurement Specialties. In addition to holding a BS in Electrical & Electronics Engineering and a MS in Industrial Measurement Systems from Brunel University, England, Karmjit is also a Doctoral candidate at New Jersey Institute of Technology (NJIT). Karmjit can be reached at [email protected].