Answer / jwg

For steam turbines, usually "relative vibration"
means "shaft relative [us. against bearing shell]
vibration" and "absolute vibration" concern bearing
pedestal vibration against free space, measured by seismic
transducer.
By combination of signals measured in one common axis by
shaft relative vibration transducer and bearing absolute
vibration transducer, "shaft absolute vibration" can be
calculated by electronic devices. In the past, "shaft
absolute vibration" was measured directly by sensors with
seismic system being in contact with rotating shaft surface.

Answer / ahmed

In general, the shaft absolute transducer system (which is
shaft vibration relative to space)consists of a relative
displacement sensor and a case-mounted vibration sensor
located in the same external housing. The displacement
sensor is a non-contact sensor measuring shaft position and
movement relative to the case, and the case-mounted
vibration sensor measures case absolute relative to free
space. The two sensors are phase compensated for the
different units and mathematically subtracted to get
absolute displacement of the shaft relative to free space.

Answer / mst

is there any phase angle in between relative shaft vibration and pedastal vibration. because in same phase it will addition. in out of phase it will subtract

Answer / hemant bhati

Relative Vibration in case of steam
turbines,compressors,motors etc. means we are directly
measuring the vibration by fixing the vibration probes on
the casing of Steam Turbine(ST) neglecting vibration of
case itself.
absolute Vibration will account Vibration of casing as well.

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