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| | | These techniques require a larger measurement range than can be offered through standard proximity probe equipment, the necessary probe target is also not easy to achieve. This is where LVDTs are used to provide the expansion measurements required. A total range of 50mm usually suffices and the various mounting options available with LVDTs makes installation straightforward. The movement of the turbine pedestals on the cylinder sole plates is a relatively easy measurement to make requiring an LVDT mounted on the turbine and the extension rod fixed or sprung onto the slides. The environment is not hostile although care must be taken to prevent mechanical damage to the transducer. Turbine cylinder crabbing, movement of the cylinder in the horizontal plane on the cylinder sole plates, is monitored at the cylinder front and rear by two pairs of LVDT transducers mounted so that one pair is to the cylinder front outer corners and one pair to the rear corners. The outputs of a pair of LVDT transducers are summated and conditioned by an arithmetic unit to determine movement and to give a single output, which is displayed in the control room as cylinder crabbing. | | |
| | | Rotor to casing expansion It is also possible by a combination of two different techniques to monitor the rotor to casing expansion by measuring the rotor and casing movement separately with respect to the bearing pedestal and adding or subtracting to achieve the rotor to casing displacement. This is particularly relevant when the rotor differential expansion, carried out typically by a proximity probe, cannot be referenced to the casing through fixed mechanical connection. Sensonics Sentry range provides a solution through the use of an LVDT module for the casing expansion and a displacement module for the rotor expansion. An additional process module is implemented to calculate the relative displacement. This configuration is illustrated below. | | |
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| | | DT monitoring cylinder casing expansion | | |
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| | | The inner to outer cylinder measurement requires a much higher degree of sealing within the transducer against moisture as the measurement is made inside each outer cylinder. The transducer body is mounted on a bracket.This is adjustable for initial setting up and calibration against the inside structure of the outer cylinder in such a position that the spring loaded core is held at approximately mid travel against a point on the inner cylinder. Expansion of either inner or outer cylinder with respect to the other, changes the relative position of the LVDT core, giving a change in output to the position monitoring instruments. | | |
| | | Understanding the relative positions of the LP rotor and casing is extremely important, as contact of these parts can be catastrophic. This is most critical on run-up and run-down, where the expansion between the shaft and casing is occurring at different rates. | | |
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