Vibration Fundamentals Episode 1 doc

P REDICTIVE M AINTENANCE The fact that vibration profiles can be obtained for all machinery that has rotating or moving elements allows vibration-based analysis techniques to be used for

ROOT CAUSE FAILURE ANALYSIS

P LANT E NGINEERING M AINTENANCE S ERIES

ROOT CAUSE FAILURE ANALYSIS

R Keith Mobley

Boston Oxford Auckland Johannesburg Melbourne New Delhi

Newnes is an imprint of Butterworth–Heinemann

Copyright © 1999 by Butterworth–Heinemann

A member of the Reed Elsevier group

All rights reserved

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, with­ out the prior written permission of the publisher

Recognizing the importance of preserving what has been written, Butterworth–Heinemann prints its books on acid-free paper whenever possible

Library of Congress Cataloging-in-Publication Data

Mobley, R Keith, 1943­

Root cause failure analysis / by R Keith Mobley

p cm — (Plant engineering maintenance series)

1 Plant maintenance 2 System failures (Engineering)

658.2’02—dc21

CIP

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

The publisher offers special discounts on bulk orders of this book

For information, please contact:

Manager of Special Sales

Part I THEORY: INTRODUCTION

TO VIBRATION ANALYSIS

Chapter 1 INTRODUCTION

ANALYSIS APPLICATIONS

ANALYSIS OVERVIEW

SOURCES

THEORY

DYNAMICS

TYPES AND FORMATS

Chapter 8 DATA ACQUISITION

TECHNIQUES

VIBRATION ANALYSIS

Chapter 10 OVERVIEW

MONITORING PARAMETERS

DEVELOPMENT

4 Vibration Fundamentals

Table 2.1 Equipment and Processes Typically Monitored by Vibration Analysis

Centrifugal Machine-Trains Continuous Process

Transmissions Metal-working machines Petroleum production lines Turbines Rolling mills, and most Neoprene production lines Generators machining equipment Polyester production lines

Source: Integrated Systems, Inc

P REDICTIVE M AINTENANCE

The fact that vibration profiles can be obtained for all machinery that has rotating or moving elements allows vibration-based analysis techniques to be used for predic­tive maintenance Vibration analysis is one of several predictive maintenance tech­niques used to monitor and analyze critical machines, equipment, and systems in a typical plant However, as indicated before, the use of vibration analysis to monitor rotating machinery to detect budding problems and to head off catastrophic failure is the dominant predictive maintenance technique used with maintenance management programs

A CCEPTANCE T ESTING

Vibration analysis is a proven means of verifying the actual performance versus design parameters of new mechanical, process, and manufacturing equipment Preac­ceptance tests performed at the factory and immediately following installation can be used to ensure that new equipment performs at optimum efficiency and expected life-cycle cost Design problems as well as possible damage during shipment or installa­tion can be corrected before long-term damage and/or unexpected costs occur

Q UALITY C ONTROL

Production-line vibration checks are an effective method of ensuring product qual­ity where machine tools are involved Such checks can provide advanced warning that the surface finish on parts is nearing the rejection level On continuous pro­cess lines such as paper machines, steel-finishing lines, or rolling mills, vibration

5 Vibration Analysis Applications

analysis can prevent abnormal oscillation of components that result in loss of product quality

L OOSE OR F OREIGN P ARTS D ETECTION

Vibration analysis is useful as a diagnostic tool for locating loose or foreign objects in process lines or vessels This technique has been used with great success by the nuclear power industry and it offers the same benefits to non-nuclear industries

N OISE C ONTROL

Federal, state, and local regulations require serious attention be paid to noise levels within the plant Vibration analysis can be used to isolate the source of noise gener­ated by plant equipment as well as background noises such as those generated by fluorescent lights and other less obvious sources The ability to isolate the source of abnormal noises permits cost-effective corrective action

L EAK D ETECTION

Leaks in process vessels and devices such as valves are a serious problem in many industries A variation of vibration monitoring and analysis can be used to detect leak­age and isolate its source Leak-detection systems use an accelerometer attached to the exterior of a process pipe This allows the vibration profile to be monitored in order to detect the unique frequencies generated by flow or leakage

A IRCRAFT E NGINE A NALYZERS

Adaptations of vibration analysis techniques have been used for a variety of specialty instruments, in particular, portable and continuous aircraft engine analyzers Vibration monitoring and analysis techniques are the basis of these analyzers, which are used for detecting excessive vibration in turboprop and jet engines These instruments incorporate logic modules that use existing vibration data to evaluate the condition of the engine Portable units have diagnostic capabilities that allow a mechanic to deter­mine the source of the problem while continuous sensors alert the pilot to any devia­tion from optimum operating condition

M ACHINE D ESIGN AND E NGINEERING

Vibration data have become a critical part of the design and engineering of new machines and process systems Data derived from similar or existing machinery can

be extrapolated to form the basis of a preliminary design Prototype testing of new machinery and systems allows these preliminary designs to be finalized, and the vibration data from the testing adds to the design database

7 Vibration Analysis Overview

Figure 3.1 Periodic motion for bearing pedestal of a steam turbine

Figure 3.2 Small oscillations of a simple pendulum, harmonic function

8 Vibration Fundamentals

A CTUAL V IBRATION P ROFILES

The process of vibration analysis requires the gathering of complex machine data, which must then be deciphered As opposed to the simple theoretical vibration curves shown in Figures 3.1 and 3.2 above, the profile for a piece of equipment is extremely complex This is true because there are usually many sources of vibration Each source generates its own curve, but these are essentially added and displayed as a composite profile These profiles can be displayed in two formats: time domain and frequency domain

Time Domain

Vibration data plotted as amplitude versus time is referred to as a time-domain data profile Some simple examples are shown in Figures 3.1 and 3.2 An example of the complexity of these type of data for an actual piece of industrial machinery is shown

in Figure 3.3

Time-domain plots must be used for all linear and reciprocating motion machinery They are useful in the overall analysis of machine-trains to study changes in operating conditions However, time-domain data are difficult to use Because all of the vibra­tion data in this type of plot are added to represent the total displacement at any given time, it is difficult to determine the contribution of any particular vibration source The French physicist and mathematician Jean Fourier determined that nonharmonic data functions such as the time-domain vibration profile are the mathematical sum of

Figure 3.3 Example of a typical time-domain vibration profile for a piece of machinery

9 Vibration Analysis Overview

Figure 3.4 Discrete (harmonic) and total (nonharmonic) time-domain vibration curves

simple harmonic functions The dashed-line curves in Figure 3.4 represent discrete harmonic components of the total, or summed, nonharmonic curve represented by the solid line

These type of data, which are routinely taken during the life of a machine, are directly comparable to historical data taken at exactly the same running speed and load How­ever, this is not practical because of variations in day-to-day plant operations and changes in running speed This significantly affects the profile and makes it impossi­ble to compare historical data

Frequency Domain

From a practical standpoint, simple harmonic vibration functions are related to the circular frequencies of the rotating or moving components Therefore, these frequen­cies are some multiple of the basic running speed of the machine-train, which is expressed in revolutions per minute (rpm) or cycles per minute (cpm) Determining

10 Vibration Fundamentals

Figure 3.5 Typical frequency-domain vibration signature

these frequencies is the first basic step in analyzing the operating condition of the machine-train

Frequency-domain data are obtained by converting time-domain data using a mathe­matical technique referred to as a fast Fourier transform (FFT) FFT allows each vibration component of a complex machine-train spectrum to be shown as a discrete frequency peak The frequency-domain amplitude can be the displacement per unit

time related to a particular frequency, which is plotted as the Y-axis against frequency

as the X-axis This is opposed to the time-domain spectrum, which sums the velocities

of all frequencies and plots the sum as the Y-axis against time as the X-axis An exam­

ple of a frequency-domain plot or vibration signature is shown in Figure 3.5

Frequency-domain data are required for equipment operating at more than one run­

ning speed and all rotating applications Because the X-axis of the spectrum is fre­

quency normalized to the running speed, a change in running speed will not affect the plot A vibration component that is present at one running speed will still be found in the same location on the plot for another running speed after the normalization, although the amplitude may be different

11 Vibration Analysis Overview

Interpretation of Vibration Data

The key to using vibration signature analysis for predictive maintenance, diagnostic, and other applications is the ability to differentiate between normal and abnormal vibration profiles Many vibrations are normal for a piece of rotating or moving machinery Examples of these are normal rotations of shafts and other rotors, contact with bearings, gear-mesh, etc However, specific problems with machinery generate abnormal, yet identifiable, vibrations Examples of these are loose bolts, misaligned shafts, worn bearings, leaks, and incipient metal fatigue

Predictive maintenance utilizing vibration signature analysis is based on the following facts, which form the basis for the methods used to identify and quantify the root causes of failure:

1 All common machinery problems and failure modes have distinct vibra­tion frequency components that can be isolated and identified

2 A frequency-domain vibration signature is generally used for the analysis because it is comprised of discrete peaks, each representing a specific vibration source

3 There is a cause, referred to as a forcing function, for every frequency component in a machine-train’s vibration signature

4 When the signature of a machine is compared over time, it will repeat until some event changes the vibration pattern (i.e., the amplitude of each dis­tinct vibration component will remain constant until there is a change in the operating dynamics of the machine-train)

While an increase or a decrease in amplitude may indicate degradation of the machine-train, this is not always the case Variations in load, operating practices, and

a variety of other normal changes also generate a change in the amplitude of one or more frequency components within the vibration signature In addition, it is important

to note that a lower amplitude does not necessarily indicate an improvement in the mechanical condition of the machine-train Therefore, it is important that the source

of all amplitude variations be clearly understood

V IBRATION M EASURING E QUIPMENT

Vibration data are obtained by the following procedure: (1) Mount a transducer onto the machinery at various locations, typically machine housing and bearing caps, and (2) use a portable data-gathering device, referred to as a vibration monitor or analyzer,

to connect to the transducer to obtain vibration readings

Transducer

The transducer most commonly used to obtain vibration measurements is an acceler­ometer It incorporates piezoelectric (i.e., pressure-sensitive) films to convert mechan­ical energy into electrical signals The device generally incorporates a weight

12 Vibration Fundamentals

suspended between two piezoelectric films The weight moves in response to vibra­tion and squeezes the piezoelectric films, which sends an electrical signal each time the weight squeezes it

Portable Vibration Analyzer

The portable vibration analyzer incorporates a microprocessor that allows it to con­vert the electrical signal mathematically to acceleration per unit time, perform a FFT, and store the data It can be programmed to generate alarms and displays of the data The data stored by the analyzer can be downloaded to a personal or a more powerful computer to perform more sophisticated analyses, data storage and retrieval, and report generation

14 Vibration Fundamentals

Rotor Imbalance

While mechanical imbalance generates a unique vibration profile, it is not the only form of imbalance that affects rotating elements Mechanical imbalance is the condi­tion where more weight is on one side of a centerline of a rotor than on the other In many cases, rotor imbalance is the result of an imbalance between centripetal forces generated by the rotation The source of rotor vibration also can be an imbalance between the lift generated by the rotor and gravity

Machines with rotating elements are designed to generate vertical lift of the rotating element when operating within normal parameters This vertical lift must overcome gravity to properly center the rotating element in its bearing-support structure How­ever, because gravity and atmospheric pressure vary with altitude and barometric pressure, actual lift may not compensate for the downward forces of gravity in certain environments When the deviation of actual lift from designed lift is significant, a rotor might not rotate on its true centerline This offset rotation creates an imbalance and a measurable level of vibration

Flow Instability and Operating Conditions

Rotating machines subject to imbalance caused by turbulent or unbalanced media flow include pumps, fans, and compressors A good machine design for these units incorporates the dynamic forces of the gas or liquid in stabilizing the rotating ele­ment The combination of these forces and the stiffness of the rotor-support system (i.e., bearing and bearing pedestals) determine the vibration level Rotor-support stiff­ness is important because unbalanced forces resulting from flow instability can deflect rotating elements from their true centerline, and the stiffness resists the deflection Deviations from a machine’s designed operating envelope can affect flow stability, which directly affects the vibration profile For example, the vibration level of a cen­trifugal compressor is typically low when operating at 100% load with laminar air­flow through the compressor However, a radical change in vibration level can result from decreased load Vibration resulting from operation at 50% load may increase by

as much as 400% with no change in the mechanical condition of the compressor In addition, a radical change in vibration level can result from turbulent flow caused by restrictions in either the inlet or discharge piping

Turbulent or unbalanced media flow (i.e., aerodynamic or hydraulic instability) does not have the same quadratic impacts on the vibration profile as that of load change, but it increases the overall vibration energy This generates a unique profile that can

be used to quantify the level of instability present in the machine The profile gener­ated by unbalanced flow is visible at the vane or blade-pass frequency of the rotating element In addition, the profile shows a marked increase in the random noise gener­ated by the flow of gas or liquid through the machine