Gas Turbines generally have heavy rotors and light casings, and some of them exhibit significant motion between bearing housing and free space. Hence, rotor forces would have higher transmissibility when measured on the bearing housing. This makes casing absolute measurement as bare sufficient for vibration monitoring and protection in Gas Turbines. Most of the Industrial Gas Turbines in field have their acceptable vibration limits defined in velocity units. Typically 1/2 inch per sec 0-pk is referred to as alert set point where as 1 inch per sec 0-pk as danger set point as per ISO-10816-1.
However, such casing based measurements many a times doesn’t yield necessary information on what might be going on in the journal bearings, which are having larger clearances with rotor. This also happens when the Gas Turbine does not exhibit significant motion between bearing housing and free space as said by Maalouf . Some typical situations arise in Gas Turbines which might also need shaft relative vibration measurements using proximity probes. This paper discusses some of the experiences of the author in the field of Gas Turbine vibration diagnosis using shaft relative and casing absolute methodology. It explains how shaft relative measurement adds more value to find out the actual malfunction diagnosis when casing measurement data becomes insufficient in such cases.