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 and string-pots have difficulty with the temperature, vibration and intrinsic safety requirements.
Designed to address the limitations of other technologies, LVDT Sensors offer the following attributes that make them suitable as measurement devices in gas turbines:
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 (see Figure 1). Most other technologies do not meet the accuracy performances of an LVDT. While lasers are more accurate, they may will fail under harsh environments.
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Figure 1: 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 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 may not survive. Most other technologies can work in these environments; however what sets the LVDT apart is its life expectancy. LVDTs can last ten to twenty years without any performance degradation- a few cases show performance of up to forty years. The nuclear power industry is an example of this type of longevity.
Certifications/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 (see Figure 2). 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.
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Figure 2: LVDTs installed on turbines must meet intrinsically safe parameters
Turbine manufacturers also require exact replacement LVDT sensors that are certified to work with existing OEM control systems and updated retrofitted controls. No other technology is accepted.
Operating temperature
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. With electronics remotely located, an AC-operated LVDT can withstand high temperatures of 350° 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 costs, LVDTs Linear Position Sensors serve an important role in turbine operations as well as plant rehabilitation projects to provide optimum efficiency.
Typical application
As previously noted, controlling valve performance is essential for improved turbine efficiencies. As valves regulate the flow of fuel throughout the different components of a gas turbine, the accurate monitoring and controlling of flow from these valves enable turbines to operate more efficiently with minimal wasted energy.
Gas turbines incorporate a variety of control valves - inlet guide vanes, transfer valves, gate valve, bleed valve, etc. Each perform a different operation, for example, bleed valves “bleed” off small quantities of fuel while gate valves permit or prevent gas flow. The proper opening and closing of these valves depends on the plant control schemes.
LVDT Linear Position Sensors serve as the position feedback device that provides valve position measurements to confirm proper turbine operations. Constructed of stainless steel and rated (UL and ATEX) for operation in high temperatures to withstand the harsh and potentially explosive environments of gas turbine power plants, these displacement feedback devices convert the rectilinear motion of an object, to which it is coupled mechanically, into a corresponding electrical signal that is used by a programmable logic controller as part of a control system.
Mounted redundantly either in two or three sensor groups on actuators that move the gate of a gate valve or ball/butterfly of a rotating valve, the LVDT linear position sensors “tell” the turbine control system how far a valve is “open” or “closed” to a certain proportion. Using this position feedback, the control system determines if more or less fuel is required for proper turbine operations and signals actuators to move the gate, ball or butterfly to make the necessary valve adjustments.
When the actuator begins to move a valve, a corresponding LVDT Linear Position Sensor sends an analog electrical signal, proportional to the amount of valve displacement to the control system. With LVDT feedback provided in real-time, the actuator can continually be adjusted until the desired opening is reached (0 to 100% opened or closed), as indicated by the position feedback.
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