Reliable Plant, August 2010

Traditionally, plumbness measurements on a vertical hydro-turbine/generator shaft involved stringing a series of wires along the length of the shaft, attaching a weight to the end of the wires, and then measuring the space from the wire to the shaft using an electronic micrometer. Although this method was inexpensive and has been used for many years, it did require access to a long length of the shaft to achieve an accurate resolution. Also, measurements involve physically measuring the distance between the wire and the shaft at various elevations on the shaft, increasing the amount of time and personnel requirement for the measurement. Ludeca presents its experience with a laser-based system that replaces the time-consuming wire method. Measurements now can be performed in a fraction of the time it would otherwise take with the wire measurement method. Ludeca’s measurement system, known as the PERMAPLUMB, uses a self-adjusting mechanical mirror, always plumb to earth, that reflects a Class 1 laser beam into a detector. It requires only 14 inches of axial space along the shaft.

The mirror and transducer are attached by a bracket that uses magnets on the turbine shaft. From a single 270-degree shaft rotation, the system calculates and displays angularity and corrective moves and provides a statistical quality measurement of the data. A “move” function allows monitoring of corrections as they are being made. The resolution is better than 0.00002 inches per foot, which is more accurate than required by NEMA. Adjacent turbines also can continue to operate since the system is insensitive to vibration.

Read our article: How to achieve vertical shaft plumbness using laser alignment