A US power generation company had 36 similar high heat rate gas turbines in two of their plants. These units were used for peaking power in a large metropolitan area. On two of these units in the early 1990’s they tried switching from lubricated, gear-type couplings to single, machined diaphragm-type couplings for reduction of maintenance and improved flexibility. The gear couplings also caused a great deal of vibrations during startup, and many times units were shut down because of excessive vibrations.
Due to the extreme thermal growth of these applications, along with the stop-start nature of the application, the couplings were required to handle enormous amounts of misalignment. While the standard diaphragm couplings seemed like a good choice for the application because of their large misalignment capability, they proved to be unable to handle the excessive offset movements caused by thermal growth. The problem with the standard diaphragm was that it was too stiff or in other words, the spring rate was too high. In one case the coupling failed catastrophically very quickly under normal running conditions.
In the mid 1990’s the power company tried a different, unique style of diaphragm couplings that used multiple diaphragms with holes in them. The diaphragms were made of thin, uniform thickness steel instead of the more common machined ‘profile’ diaphragms. Because of the holes, these new diaphragms provided much more misalignment capability, and they also had far lower spring rates than the previous diaphragm couplings. The lower spring rates caused an enormous reduction in reactionary forces from the coupling, and they also eliminated many vibration problems caused by the original gear couplings.
The overall effect of this discovery was for the company to begin changing all similar units to this type of coupling. Now these 36 units which previously had severe vibration problems during startup, run very reliably. They have also eliminated the need for maintenance, which was an issue with the original gear couplings.
