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

Basic Concept VI

Piezoelectric Accelerometers

 

•          Piezoelectric crystal is sandwiched between a seismic mass and outer case.

•          Preload screw ensures full contact between crystal & mass

•           When mounted on a vibrating surface seismic mass imposes a force equal to mass x acceleration

•           Charge output of piezo crystal is proportional to applied force

•           Since mass is constant, output charge is proportional to acceleration

Piezoelectric Accelerometers

Converting Charge to Voltage

•          The output of accelerometers is charge. Usually expressed as picocoulomb / g (pc/g)

•          Electronic charge amplifier is required to convert charge signal to voltage signal

–        Impedance of accelerometer is high. Cannot be connected directly to low impedance instruments

–        Charge amplifier has high input impedance and low output impedance so that long cables can be used.

•          Charge amplifier can be external or internal

–        In bigger accelerometers amplifier can be located inside

–        In small, high frequency units amplifier is located outside

–        Also located outside in high temperature accelerometers

Accelerometers Mounting

•          Mounted resonance of accelerometer drops with reduction in mounting stiffness.

–        This causes a reduction in the upper frequency range

•          Ideal mounting is by threaded stud on flat surface

–        Maximum stiffness, highest mounted resonance

–        Resonant frequency 32 KHz. Usable range 10 KHz.

•          Magnet mounting simpler but lower response

–        Resonant frequency drops to 7 Khz. Usable range 2 KHz

•          Handheld probe convenient but very low frequency response

–        Due to low stiffness of hand resonant frequency < 2 KHz

–        Frequency response < 1 KHz

 Accelerometers Resonance & Frequency Response

•          Frequency response depends on resonance frequency

•           Higher the resonance frequency, higher the useful range

•           Maximum useable frequency range is 1/3^rd of resonance

•           Resonance frequency, however, depends on mounting

Frequency Response - Screw Mount

•          Screw mount has the highest resonance and, therefore the highest frequency response

•           This film of silicon grease improves contact.

•           Make sure bottom of accelerometer contacts measured surface

 Frequency Response - Magnet Mount

•          Weight of magnet determines the mounted resonance 

•           Smaller the magnet higher the frequency response

 Use the smallest magnet that holds the accelerometer without slipping. Use a machined surface for the best grip

Frequency Response Hand Held

•          Poor high frequency response - < 1 KHz

•           Response may change with hand pressure

•           Repeatability is poor when high frequencies are present

•           Hand holding accelerometers should be avoided except for low frequency work

Filtering Necessary for Accelerometers

•          Any high frequency vibration in the resonant range will be highly amplified. 

–        Amplification can be up to 30 dB or almost 1,000 times

–        Filtered amplitudes will be highly distorted

•          Resonant frequency highly depends on mounting

–        By previous example – 32 KHz for screw mount. Only 2 KHz for handholding

•          Therefore, resonance range should be filtered out

–        For screw mount low pass filter should be set at 10 KHz

–        For hand holding filter should be set at 1 KHz.

–        Analyst must know frequency response of accelerometer used for different mounting conditions.

Filtering can be done in FFT Analyzer by setting maximum frequency correctly.

 Advantages of Accelerometers

•          Measures casing or structural absolute motion

•          Rugged and reliable construction

•          Easy to install on machinery, structures, pipelines

•          Small size, easiest to install in cramped locations

•          Good signal response from 600 to 600,000 CPM

•          Low frequency units can measure down to 6 CPM

•          High freq units can reach 30 KHz (1,800,000 CPM)

•          Operates below mounted resonance frequency

•          Flat phase response throughout operating range

•          Internal electronics can be used to convert acceleration to velocity – Bently Velometer

•          Units available from a cryogenic temperature of minus 200^oC to a high temperature of > 600^oC

 Disadvantages of Accelerometers

•          Sensitive to mounting and surface conditions

•          Unable to measure shaft vibration or position

•          Not self generating – Need external power source

•          Transducer cable sensitive to noise, motion and electrical interference

•          Low signal response below 600 CPM (10 Hz)

•          Temperature limitation of 120^oC for ICP Acceleroms

•          Double integration to displacement suffers from low frequency noise – should be avoided

•          Signal filtration required depending on mounting

•          Difficult calibration check

Machine With Both Shaft and Bearing Housing Vibration Monitoring

 Refferensi Book

1. Machinery Malfunction Diagnosis and Correction – Robert C Eisenmann – Prentice Hall
2. Fundamentals of Rotating Machinery Diagnostics – Donald E. Bently – Bently Pressurized Bearing Press