Vibration detection in generators is of the utmost importance in monitoring their efficiency and safety. Severe vibration can cause excessive ware and damage to the generator, resulting in down time, and possible destruction of parts in the generator.
Current state-of-the art vibration detectors use fiber optic sensors mounted within the generator, usually on a stator coil end-turn. An example of a fiber optic sensor can be seen in FIG. 1. As vibrations occur, a sensor reed 14 mounted 12 within the sensor box 10 vibrates. Fixed at the end of the sensor reed is a grid 2 that physically passes through a gap 6 in a fiber optic cable 8. As the grid vibrates at the end of the sensor reed, the light passing through the grid is amplitude and frequency modulated in a measurable manner.
As is now practiced, the fiber optic cable providing light originates from a preamplifier unit outside the generator. This is passed into the generator shell through a seal, to the vibration sensor where it is modulated by the vibration, and then back out through the seal to the preamplifier. A main chassis unit, connected to the preamplifier unit, analyzes the signal from each sensor channel.
FIG. 2 shows a typical set up for passing fiber optic cables through the wall of a generator. The fiber optic cables 22 are gathered at a constructed hole 26, often a welded pipe or welded flanges, in the generator wall 24. Shown here is a simplified diagram where two sensors, resulting in just four cables, are used. Actual generators will have many more sensors in use. In order to maintain pressure inside of the generator, the fiber optic cables need to pass through a seal 28 that is secure enough to withstand the internal pressure from the generator. Unfortunately, fiber optic cables are inherently difficult to pass through a pressure seal, and therefore these seals are weak points in generators and notoriously problematic. Though the fiber optic cables may now be routed directly to a preamplifier, it is common practice to have them terminate 30 outside of the seal for mating with matching fiber optic cables 32. This provides a detachment point if the fiber optic cables, or other generators parts, need to be assembled, fixed or replaced.
Since the fiber optic cables are delicate, and since many generators are outside, the cables are secured within an armored sheath 34. At the terminus of the armored sheath, the fiber optic cables are separated 36 and hooked up with a preamplifier unit (not shown). The preamplifier unit itself needs to be in close proximity to the generator, which usually results in the need for a shed or similar structure to protect the preamplifier unit. At the preamplifier unit, signals from the fiber optic cables are converted to electrical signals for transmission to a main chassis unit.
The fiber optic seal, which essentially requires an individual seal around each fiber optic cable, and mating fiber optic trunk, are costly to make and install. The fiber optic seal is not hermetic and leakage from within the generator poses a major safety issue, especially considering that the generators typically contain 75 pounds per square inch (PSI) hydrogen. Since a single generator can have 12 to 16 vibration sensors, 24 to 32 fiber optic cables, the task of accommodating all of the fiber optic cables is onerous.
What is needed is a seal that reduces the weakness of the fiber optic penetration points through the generators wall, as well as being simpler and more cost effective.