A Brief Introduction to Vibration Analysis of Process Plant Machinery (IV)

Basic Concepts IV

Basic Rotor and Stator System

•          Forces generated in the rotor are transmitted through the bearings and supports to the foundation

•           Displacement probe is mounted on the bearing housing which itself is vibrating. Shaft vibration measured by such a probe is, therefore, relative to the bearing housing

•           Bearing housing vibration measured by accelerometer or velocity probe is an absolute measurement

Type of Rotor Vibration

•          Lateral motion involves displacement from its central position or flexural deformation. Rotation is about an axis intersecting and normal to the axis of rotation

•          Axial Motion occurs parallel to the rotor’s axis of rotation

•          Torsional Motion involves rotation of rotor’s transverse sections relative to one another about its axis of rotation

•          Vibrations that occur at frequency of rotation of rotor are called synchronous vibrations.

•          Vibrations at other frequencies are nonsynchronous vibrations

 The Relationship Between Forced and Vibration

•          Forces generated within the machine have may different frequencies

•           The mobility of the bearings and supports are also frequency dependent. Mobility = Vibration / Force

•           Resultant Vibration = Force x Mobility 

 Alternative Measurements on Journal Bearings

•          Relative shaft displacement has limited frequency range but has high amplitude at low frequencies – running speed, subsynchronous and low harmonic components

•           Accelerometer has high signal at high frequencies – rotor to stator interaction frequencies – blade passing, vane passing

 Types of Machine Vibration

•          Casing Absolute is measured relative to space by Seismic transducer mounted on casing

•           Shaft relative is measured by  displacement transducer mounted on casing

•           Shaft Absolute is the sum of Casing Absolute and Shaft Relative.

Shaft Versus Housing Vibration

Shaft Versus Housing Vibration
(Selecting the Right Parameter) 

•          Shaft vibration relative to bearing housing

–        Machines with high stator to rotor weight ratio ( For example in syngas comp the ratio may exceed 20)

–        Machines with hydrodynamic sleeve bearings

–        Almost all high speed compressor trains

•          Bearing housing vibration

–        Machines with rolling element bearings have no shaft motion relative to bearing housing.

–        Rolling Element bearings have zero clearance

–        Shaft vibration is directly transmitted to bearing housing

•          Shaft absolute displacement

–        Machines with lightweight casings or soft supports that have significant casing vibration

 Bearing Housing Vibration
 

•          Shaft-relative vibration provides

–        Machinery protection

–        Low frequency (up to 120,000 CPM) information for analysis

•          Many rotor- stator interactions generate high frequency vibrations that are transferred to the bearing housing

–        Vane passing frequency in compressors

–        Blade passing frequency in turbines

–        These frequencies provide useful information on the condition and cleanliness of blades and vanes

•          These vibrations are best measured on the bearing housing using high-frequency accelerometers.

–        Periodic measurements with a data collector.

 Shaft Rotation and Precession

  

•          Precession is the locus of the centerline of the shaft around the geometric centerline

•           Normally direction of precession will be same as direction of rotation

•           During rubbing shaft may have reverse precession

 

 IRD Severity Chart

  

•          Values are for filtered readings only – not overall

•           Velocity is expressed in peak units (not RMS units)

•           Severity lines are in velocity

•           Displacement severity can be found only with reference to frequency.

•           In metric units

•           Very rough > 16 mm/sec

•           Rough                     > 8 mm/sec

•           Slightly rough > 4 mm/sec

•           Fair              - 2 – 4 mm/sec  

•           Good           - 1 – 2 mm/sec