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Precise Control for Better Efficiency
Learn how the construction and performance of LVDT position sensors from TE Connectivity (TE) align with the harsh environmental conditions of gas turbine requirements. Karmjit Sidhu, Business Development Director, Sensors for Oil and Gas
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 do not have the capability to measure long strokes. Potentiometers, magnetorestrictive position sensors and string-pots have difficulty with the temperature, vibration, and intrinsic safety requirements.
Figure 1: Basic construction of a LVDT position sensor.
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. Designed to address the limitations of other sensor technologies, LVDTs offer the following attributes that make them suitable as measurement devices in gas turbines.
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.
Measurement Range
LVDT linear position sensors can measure movements as small as a few millionths of an inch, which is important to confirm 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 confirming 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.
Environmental Resistance
While once only available in stainless steel, LVDT linear position sensors can be repackaged with modern materials such as Alloy 400, Alloy C276, 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.
Certifications and Approvals
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.
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 +175°C to +232°C (approx. +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 provide 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
Written by:
Karmjit Sidhu, Business Development Director, Sensors for Oil and Gas
