| The centrifugal compressor has been the reasoned choice for refining plants and the oil & gas industry for more than 50 years. These compressors have capabilities ranging from below 1 MW up to around 50 MW and are preferred because of their high efficiency, good economy, low maintenance and high degree of reliability. Centrifugal compressors are as standard equipped with displacement sensors measuring either an overall shaft vibration level or the Smax level, at the bearing locations, depending on vender monitoring recommendations for judging the vibration state. Today’s centrifugal compressor is required to meet wider and wider performance conditions (wide operational speed range combined with large flow variations resulting in large variation in head or delivered pressure). In time with superior demands for high performing flexible compressors it is often seen that designs are stretched to their limit. In recent years one of the main problems when considering high performing compressors applications, has been control of the design, especially when considering aerodynamic stability (e.g. surge prediction, flow instabilities, like rotating stall in the stationary or rotational flow path etc.). Rotordynamic stability, which was the main issue from the mid-fifties up to early-nineties, is today one of many areas of concern, when designing high performance applications. Consequently, when choosing monitoring solutions for centrifugal compressors it is important to consider not only shaft vibrations originating form the rotational speed, but also any special characteristics of various machine elements, aerodynamics, etc. | 
