Torque tube instrumentation has been a common solution for level control applications. However, many processing plants are converting to a displacer level transmitter, which uses range spring technology, for more reliable level measurement and control. Displacer level transmitters, such as the Magnetrol® E3 Modulevel® linear variable differential transformer (LVDT) transmitter, feature greater output stability, structural integrity and ease of installation, compared to torque tubes. The following outlines why displacer level transmitter technology offers an excellent alternative to existing torque tube units.
Principle of Operation
E3 MODULEVEL electronic level transmitters are advanced, intrinsically safe, two-wire instruments that utilize a simple buoyancy principle to detect and convert liquid level changes into a stable 4-20 mA output signal. The linkage between the level sensing element and output electronics greatly simplifies mechanical design and construction.
Liquid level change acts upon the spring supported displacer causing vertical motion of a core within a linear variable differential transformer (LVDT).
The enclosing tube acts as a static isolation barrier between the LVDT and the process media. As the core position changes with liquid level, voltages are induced in the secondary winding of the LVDT. These signals are processed in the electronic circuitry and used to control the current in the 4-20 mA output current loop.
Benefits Over Torque Tubes
As noted above, reliable output, structural integrity and ease of use are primary reasons displacer level transmitter technology outperforms torque tubes.
- Range spring technology dampens the effects of vibration and features a longer travel zone, yielding an output signal that is four times more stable than torque tubes.
- Range spring movement is free of wear and friction, unlike torque tubes, whose twisting motion causes friction buildup and fatigue failure.
- Displacer level transmitters are easier to install than torque tubes, and don’t require shutdown of process lines.
Replacing Existing Torque Tubes
I&C technicians who want the benefits of a range spring controller can reduce installation costs by not having to field pipe new cages. The existing torque tube cage may be used, based on the following considerations:
Torque tube units should have side/bottom or side/side process piping connections. It is not possible to utilize the existing cage on a top-in/bottom-out torque tube cage. By replacing the entire unit with a complete MODULEVEL device equipped with top-in/bottom-out process piping cage connections, the process piping connection can be matched.
If the existing cage has a vent connection piped into the top of the torque tube cage, it will also have to be repiped to a tee. The tee will have to be added to the top process connection on the cage side of the isolation valve or to a vent connection.
The displacer length must be the same or shorter than the existing torque tube displacer.
Torque tube displacers are larger in diameter than the MODULEVEL and manufactured in the same standard spans. By matching the torque tube chamber parting flange size, pressure, material, bolt circle, and the displacer span, a MODULEVEL unit can be mounted on the existing chamber.