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Machine-health monitoring — the application and payback
Machine-health monitoring the application and payback R.G. Monk
Vibration has long been regarded as an indicator of the condition of rotating machinery and today developments in instrumentation are rapidly turning the mechanic's hunch into an exact science. This paper deals with the application of vibration monitoring to industries employing large numbers of process machines engaged in expensive processes. The author details the advantages of using established techniques of vibration, or machine-health, monitoring to safeguard investment in machinery such as pumps, compressors and fans in industries like refining and petrochemical engineering.
Vibration monitoring has been practised ever since machines were first built for industrial use, and one feels almost instinctively that when a machine vibrates it is a symptom of some abnormality or defect. From the days when machine condition was assessed from how it felt to the hand or through a screwdriver to the ear, we have progressed through the stage of assessment from objective measurements of vibration level to the present time when machine-health monitoring has taken on more of a predictive role and defects can be screened out and kept under observation at an earlier stage of inception, with obvious advantages for process plant where continuous operation of machinery is required. It has been estimated that for industries with a high capital investment in plant and machinery per employee, and a high added-value output per employee, condition monitoring in general and vibration monitoring in particular can save on average about 1% of the total added value ouput per year. Of this saving, about 65% will be ouput related and 35% will be maintenance related. The paper is intended to deal essentially with the application of established vibration-monitoring techniques to industries, such as the refining and petrochemical industries, which have large numbers of process equipments, per location, including centrifugal pumps, fans and compressors, and also a high added-value ouput per employee.
Application to working equipment The main reasons for such applications, as we have already seen, are firstly to reduce production losses due to running equipment failure, secondly to reduce maintenance costs, and thirdly to reduce the probability of fire and other Dr Monk is a director of Acoustic Technology Ltd in Manchester, UK. This article is based on an address to the United Kingdom Mechanical Health Monitoring Group at Leicester, UK, last November.
NON-DESTRUCTIVE TESTING . JUNE 1976
hazards which might be caused by catastrophic failure of running equipment. Clearly, with many plant items to cover and the useful constraints of limited manpower and equipment, resources must be directed at strategic areas of plant operation for maximum benefit and cost effectiveness. Protection of productive capacity is usually considered to be the most important area of application for vibration monitoring, and running equipment has to be categorised according to its priority in maintaining plant throughput. A distinction between equipment priorities can be made. First-line critical equipment such as large high speed, high capital cost centrifugal compressors, and also generation equipment are machines which are not spared and whose breakdown would cause a total loss of plant throughput. Although second-line critical equipment such as pumps and fans may be spared, breakdown would cause a loss of plant throughput, not to mention embarrassment. All other equipment comes in the third category. Equipment in the first two categories is a candidate for regular vibration monitoring. The general approach is fundamentally similar for all equipment, the idea being to keep a watch on the vibration characteristic or frequency spectrum rather than on the overall vibration level. There are, however, differences in the detail approach, and for most first-line critical equipment such as turbo machinery, systematic narrow-band frequency analysis is essential. Some equipment will already be fitted with permanent monitoring instrumentation, but regular frequency analysis of the monitored signal, possibly supplemented by additional casing measurements, will add a predictive dimension to the basic alarm system. The frequency spectrum can be divided into two windows, one at the low frequencies which contains the primary orders and reflects the basic shaft dynamics. This can be obtained either from bearing house measurements or from proximity probes which measure shaft and bearing displace-
127
more bands may be indicative of mechanical defect and diagnosis will depend to a large extent on which bands are affected and in what way. In the case of the octave-band vibration levels taken from a 7.5 MW steam turbine, the increase in levels has occurred in the octave band containing the rotational speed of the turbine, and the machine was diagnosed as out of balance (Fig. 2). This was later confirmed, the out-of-balance found being 0.14 Nm (1400 gmf cm). In addition to the systematic measurements on critical equipment, there are other areas where diagnostic measurements are most useful. For example, measurements on equipment following repair or overhaul (inspection measurements) are an excellent way of screening out poor alignment particularly, but also such things as badly fitted or even faulty bearings, poor balancing, and the human element generally.
Fig. 1 Vibration-monitoring equipment usually needs to be portable and have an independent power supply
ments. The second window is at the middle and high frequencies and contains the gear-meshing and blade passage frequencies. Information in this region can usually only be obtained from bearing house or casing measurements, and is useful for the detection of blade and nozzle fouling, and gear problems. Where there are many items of first-line critical equipment to be monitored, a great deal of data can be generated and it is really for equipment in this category that rapid analysis methods, such as time-compression and FFT analysers, and also computer-aided analysis techniques can be used to advantage. For second-line critical equipment such as pumps and fans, systematic frequency analyses are equally important, but they need not be so complex as for first-line critical equipment. For most fixed speed equipment, an octave band or } octave band analysis is usually adequate. The measured parameter is usually the velocity level as recorded in three planes on each bearing, in line with most current standards on machinery vibration evaluation.
Measurements on all new equipment at manufacturers works and again on site before it goes into service are strongly advocated. To assume, without conducting confirmatory checks, that all new equipment will meet the appropriate vibration criteria as defined by a contractual specification, and also that it is free from defect, is inviting disaster. A bearing taken from a 1000 kW four-pole electric motor which had been run for about two hours prior to going into service is a classic example of damage due to electrical arcing, (Fig. 3). Frequency analyses of the vibration levels were taken first with the motor running light on its contract base, and secondly with the motor on load during commissioning (Fgi 4). Most of the octave-band levels have increased, particularly from 125 Hz upwards and more especially in the 2 kHz band. Taken together, these increases were indicative of bearing damage, even though the measured levels, on load, were within the generalised specification limit of 3 mm s -~ root mean square.
The rewards of detection Now to the question of what happens in process plant when, by taking detailed vibration measurements on large numbers of equipment on a regular basis, early warning can be given of incipient failure. What are the engineering and
June 1973 dotum levels
Z~
April 1974 irnbolonce evident
E
=o 1.5 eal {:~1 "6 o 1.0 !
128
~
n
g
if
Gathering the data Most vibration instrumentation for use in the field needs to be easily portable and battery operated, with facilities for measuring displacement and acceleration as well as velocity, and also an output for magnetic tape recording of the measured signal (Fig. 1). Measurements can be logged in tabular or graphical form and the individual band levels, corresponding to the rotational speed of the machine and harmonics thereof, compared with previous measurements for indications of change. Any significant change in one or
j
05 o >
el
I
I
I
I
65
125
250
500
_
I
I
I000
2000
0ctove bond centre frequency
~'~
4000
IHzl
Fig. 2 In the course of nine months vibration at the rotation frequency increased in two bearings on a 7.5 MW turbine
NON-DESTRUCTIVE
TESTING.
JUNE 1976
Table 1. Detection rates for different categories of equipment Type of Equipment and vibration check
Fault detection rate
Vital and essential e q u i p m e n t
3-4%
checked every 30 days Non-monitored e q u i p m e n t Checked prior to plant shut-down
Fig. 3
20-25%
Equipment of any type Checked f o l l o w i n g repair or overhaul
9-10%
New e q u i p m e n t of any type Checked during commissioning
3%
Electrical arcing has damaged these bearingsfrom an electric
motor
financial benefits, and how can they be quantified? How, for example, can the benefits be related to the estimated savings in production losses and maintenance costs forecast for British industry? If you relate the number of units of defective equipment found from the detailed measurements to the total number of equipments checked, this leads to the concept of a detection rate, which may simply be defined a s : - the number of defective equipments found divided by the total number of equipments checked, times 100. Assuming that the analysis and diagnostic procedures are sufficiently effective, then the detection rate is a very good indication of the incidence of mechanical defect, and it is this parameter which provides the link between the application of vibration monitoring techniques to process equipment and the anticipation output and maintenance related benefits.
atically monitored. Because of this, the number of catastrophic failures and consequent unscheduled downtime of monitored equipment is very much reduced, if not eliminated, which means that the reliability of monitored equipment is improved. Now, the reason for this improved reliability is worth stating even though it may be self-evident. What happens in prac-
579 BariowMoc,'RoaO Cn~on Manches~rM212AE TeJel~one061.8818807
VIBRATION MONITORING DATA SHEET PLANT
Works NO,
UNiT NO.
p202,~
It has been found that for typical refinery plant which is systematically monitored on an approximate 30 day schedule, one can expect to find something like a 3 to 4% detection rate; in other words, out of every 100 units of equipment checked every 30 days or so, between three and four, on average, will be found to be in need of attention. For nonmonitored equipment which is checked prior to a plant shut-down, about 20 to 25 out of every hundred are found to be in need of attention. Thus the detection rate is 2 0 25% depending on the type and age of the plant, and the location. For equipment which is checked following repair or overhaul, the detection rate is about 9%, again depending on the location. For new equipment, checked prior to going into service, the detection rate is about 3%. The usual defect for new equipment is misalignment, as one might expect, whereas for monitored equipment, the most common fault is deterioration of the drive end bearing of the driven side. Table 1 summarises the different detection rates discussed above. The interesting thing here is of course the difference in the detection rates, and in particular the low detection rate for monitored equipment. What this demonstrates is that at any given time equipment which is systematically monitored has a lower incidence of mechanical defect, and hence a lower probability of failure, than equipment which is not system-
N O N - D E S T R U C T I V E TESTING . JUNE 1976
Fig. 4 Records show that bearings on the electric motor are damaged; the top two rows for each bearing show vibrations when the motor runs light on its base and the third row shows running under load on commissioning
129
~o~ o
,oo;
1
8O 5.
8 = 60 = ~o,~ 40 ~ .~ 20 ~°® o ~°
~.
s 30
_,
-tlo ~e elo8 El ,~, o I
1970
I
1971
I
197;)
I
1973
L
a:
1974
Yeer Fig. 5 Pump repairs at a refinery have slumped since vibration monitoring came in while the value o f the plant has risen
tice is that the monitoring generates more immediate maintenance, narrowing the gap between the onset of a defect and its eventual repair. This permits a change in emphasis from maintenance based on schedules and intervening break-downs, with a consequent high content of workshop overhaul, to a system with a much higher proportion of remedial maintenance in the field. At one large refinery, there has been a 25% reduction in the number of pumps undergoing workshop overhaul since the introduction of vibration monitoring about 5 years ago (Fig. 5). There has been a similar drop reported in the number of electric motors undergoing workshop overhaul.
130
Now this does not mean that there has been a 25% reduction in the number of repairs carried out; this will have remained the same, and may have increased somewhat. Furthermore, there may be other unknown factors which could have contributed to the observed change in maintenance work patterns. But such a significant drop in overhaul work is clear evidence of a reduction in the number of catastrophic failures and supports the thesis that, because of vibration monitoring, more remedial maintenance work is being carried out in the field. Now clearly, it must be far less expensive to replace worn or damaged parts in a machine in situ than it is to carry out major repair work to a machine that has failed catastrophically. The difference in cost for a typical process centrifugal pump would be several hundred pounds. For an electric motor, the difference in cost could be several thousand pounds. Not many incidents are needed to show quite a significant saving in maintenance and repair costs, and the estimate for maintenance related savings of 0.35% of the total added value ouput per location seem quite realistic.
Conclusion To conclude, therefore, when vibration monitoring techniques are introduced into a process plant, there are immediate and obvious improvements in engineering standards. The techniques are used not to achieve engineering perfection, but to screen out mechanical faults and incipient failures. When equipment is systematically checked, the probability of catastrophic failure is reduced and this leads to an improvement in machine reliability and to a reduction in maintenance and overhaul work, and hence costs.
NON-DESTRUCTIVE
T E S T I N G . J U N E 1976
Vibration has long been regarded as an indicator of the condition of rotating machinery and today developments in instrumentation are rapidly turning the mechanic's hunch into an exact science. This paper deals with the application of vibration monitoring to industries employing large numbers of process machines engaged in expensive processes. The author details the advantages of using established techniques of vibration, or machine-health, monitoring to safeguard investment in machinery such as pumps, compressors and fans in industries like refining and petrochemical engineering.
Vibration monitoring has been practised ever since machines were first built for industrial use, and one feels almost instinctively that when a machine vibrates it is a symptom of some abnormality or defect. From the days when machine condition was assessed from how it felt to the hand or through a screwdriver to the ear, we have progressed through the stage of assessment from objective measurements of vibration level to the present time when machine-health monitoring has taken on more of a predictive role and defects can be screened out and kept under observation at an earlier stage of inception, with obvious advantages for process plant where continuous operation of machinery is required. It has been estimated that for industries with a high capital investment in plant and machinery per employee, and a high added-value output per employee, condition monitoring in general and vibration monitoring in particular can save on average about 1% of the total added value ouput per year. Of this saving, about 65% will be ouput related and 35% will be maintenance related. The paper is intended to deal essentially with the application of established vibration-monitoring techniques to industries, such as the refining and petrochemical industries, which have large numbers of process equipments, per location, including centrifugal pumps, fans and compressors, and also a high added-value ouput per employee.
Application to working equipment The main reasons for such applications, as we have already seen, are firstly to reduce production losses due to running equipment failure, secondly to reduce maintenance costs, and thirdly to reduce the probability of fire and other Dr Monk is a director of Acoustic Technology Ltd in Manchester, UK. This article is based on an address to the United Kingdom Mechanical Health Monitoring Group at Leicester, UK, last November.
NON-DESTRUCTIVE TESTING . JUNE 1976
hazards which might be caused by catastrophic failure of running equipment. Clearly, with many plant items to cover and the useful constraints of limited manpower and equipment, resources must be directed at strategic areas of plant operation for maximum benefit and cost effectiveness. Protection of productive capacity is usually considered to be the most important area of application for vibration monitoring, and running equipment has to be categorised according to its priority in maintaining plant throughput. A distinction between equipment priorities can be made. First-line critical equipment such as large high speed, high capital cost centrifugal compressors, and also generation equipment are machines which are not spared and whose breakdown would cause a total loss of plant throughput. Although second-line critical equipment such as pumps and fans may be spared, breakdown would cause a loss of plant throughput, not to mention embarrassment. All other equipment comes in the third category. Equipment in the first two categories is a candidate for regular vibration monitoring. The general approach is fundamentally similar for all equipment, the idea being to keep a watch on the vibration characteristic or frequency spectrum rather than on the overall vibration level. There are, however, differences in the detail approach, and for most first-line critical equipment such as turbo machinery, systematic narrow-band frequency analysis is essential. Some equipment will already be fitted with permanent monitoring instrumentation, but regular frequency analysis of the monitored signal, possibly supplemented by additional casing measurements, will add a predictive dimension to the basic alarm system. The frequency spectrum can be divided into two windows, one at the low frequencies which contains the primary orders and reflects the basic shaft dynamics. This can be obtained either from bearing house measurements or from proximity probes which measure shaft and bearing displace-
127
more bands may be indicative of mechanical defect and diagnosis will depend to a large extent on which bands are affected and in what way. In the case of the octave-band vibration levels taken from a 7.5 MW steam turbine, the increase in levels has occurred in the octave band containing the rotational speed of the turbine, and the machine was diagnosed as out of balance (Fig. 2). This was later confirmed, the out-of-balance found being 0.14 Nm (1400 gmf cm). In addition to the systematic measurements on critical equipment, there are other areas where diagnostic measurements are most useful. For example, measurements on equipment following repair or overhaul (inspection measurements) are an excellent way of screening out poor alignment particularly, but also such things as badly fitted or even faulty bearings, poor balancing, and the human element generally.
Fig. 1 Vibration-monitoring equipment usually needs to be portable and have an independent power supply
ments. The second window is at the middle and high frequencies and contains the gear-meshing and blade passage frequencies. Information in this region can usually only be obtained from bearing house or casing measurements, and is useful for the detection of blade and nozzle fouling, and gear problems. Where there are many items of first-line critical equipment to be monitored, a great deal of data can be generated and it is really for equipment in this category that rapid analysis methods, such as time-compression and FFT analysers, and also computer-aided analysis techniques can be used to advantage. For second-line critical equipment such as pumps and fans, systematic frequency analyses are equally important, but they need not be so complex as for first-line critical equipment. For most fixed speed equipment, an octave band or } octave band analysis is usually adequate. The measured parameter is usually the velocity level as recorded in three planes on each bearing, in line with most current standards on machinery vibration evaluation.
Measurements on all new equipment at manufacturers works and again on site before it goes into service are strongly advocated. To assume, without conducting confirmatory checks, that all new equipment will meet the appropriate vibration criteria as defined by a contractual specification, and also that it is free from defect, is inviting disaster. A bearing taken from a 1000 kW four-pole electric motor which had been run for about two hours prior to going into service is a classic example of damage due to electrical arcing, (Fig. 3). Frequency analyses of the vibration levels were taken first with the motor running light on its contract base, and secondly with the motor on load during commissioning (Fgi 4). Most of the octave-band levels have increased, particularly from 125 Hz upwards and more especially in the 2 kHz band. Taken together, these increases were indicative of bearing damage, even though the measured levels, on load, were within the generalised specification limit of 3 mm s -~ root mean square.
The rewards of detection Now to the question of what happens in process plant when, by taking detailed vibration measurements on large numbers of equipment on a regular basis, early warning can be given of incipient failure. What are the engineering and
June 1973 dotum levels
Z~
April 1974 irnbolonce evident
E
=o 1.5 eal {:~1 "6 o 1.0 !
128
~
n
g
if
Gathering the data Most vibration instrumentation for use in the field needs to be easily portable and battery operated, with facilities for measuring displacement and acceleration as well as velocity, and also an output for magnetic tape recording of the measured signal (Fig. 1). Measurements can be logged in tabular or graphical form and the individual band levels, corresponding to the rotational speed of the machine and harmonics thereof, compared with previous measurements for indications of change. Any significant change in one or
j
05 o >
el
I
I
I
I
65
125
250
500
_
I
I
I000
2000
0ctove bond centre frequency
~'~
4000
IHzl
Fig. 2 In the course of nine months vibration at the rotation frequency increased in two bearings on a 7.5 MW turbine
NON-DESTRUCTIVE
TESTING.
JUNE 1976
Table 1. Detection rates for different categories of equipment Type of Equipment and vibration check
Fault detection rate
Vital and essential e q u i p m e n t
3-4%
checked every 30 days Non-monitored e q u i p m e n t Checked prior to plant shut-down
Fig. 3
20-25%
Equipment of any type Checked f o l l o w i n g repair or overhaul
9-10%
New e q u i p m e n t of any type Checked during commissioning
3%
Electrical arcing has damaged these bearingsfrom an electric
motor
financial benefits, and how can they be quantified? How, for example, can the benefits be related to the estimated savings in production losses and maintenance costs forecast for British industry? If you relate the number of units of defective equipment found from the detailed measurements to the total number of equipments checked, this leads to the concept of a detection rate, which may simply be defined a s : - the number of defective equipments found divided by the total number of equipments checked, times 100. Assuming that the analysis and diagnostic procedures are sufficiently effective, then the detection rate is a very good indication of the incidence of mechanical defect, and it is this parameter which provides the link between the application of vibration monitoring techniques to process equipment and the anticipation output and maintenance related benefits.
atically monitored. Because of this, the number of catastrophic failures and consequent unscheduled downtime of monitored equipment is very much reduced, if not eliminated, which means that the reliability of monitored equipment is improved. Now, the reason for this improved reliability is worth stating even though it may be self-evident. What happens in prac-
579 BariowMoc,'RoaO Cn~on Manches~rM212AE TeJel~one061.8818807
VIBRATION MONITORING DATA SHEET PLANT
Works NO,
UNiT NO.
p202,~
It has been found that for typical refinery plant which is systematically monitored on an approximate 30 day schedule, one can expect to find something like a 3 to 4% detection rate; in other words, out of every 100 units of equipment checked every 30 days or so, between three and four, on average, will be found to be in need of attention. For nonmonitored equipment which is checked prior to a plant shut-down, about 20 to 25 out of every hundred are found to be in need of attention. Thus the detection rate is 2 0 25% depending on the type and age of the plant, and the location. For equipment which is checked following repair or overhaul, the detection rate is about 9%, again depending on the location. For new equipment, checked prior to going into service, the detection rate is about 3%. The usual defect for new equipment is misalignment, as one might expect, whereas for monitored equipment, the most common fault is deterioration of the drive end bearing of the driven side. Table 1 summarises the different detection rates discussed above. The interesting thing here is of course the difference in the detection rates, and in particular the low detection rate for monitored equipment. What this demonstrates is that at any given time equipment which is systematically monitored has a lower incidence of mechanical defect, and hence a lower probability of failure, than equipment which is not system-
N O N - D E S T R U C T I V E TESTING . JUNE 1976
Fig. 4 Records show that bearings on the electric motor are damaged; the top two rows for each bearing show vibrations when the motor runs light on its base and the third row shows running under load on commissioning
129
~o~ o
,oo;
1
8O 5.
8 = 60 = ~o,~ 40 ~ .~ 20 ~°® o ~°
~.
s 30
_,
-tlo ~e elo8 El ,~, o I
1970
I
1971
I
197;)
I
1973
L
a:
1974
Yeer Fig. 5 Pump repairs at a refinery have slumped since vibration monitoring came in while the value o f the plant has risen
tice is that the monitoring generates more immediate maintenance, narrowing the gap between the onset of a defect and its eventual repair. This permits a change in emphasis from maintenance based on schedules and intervening break-downs, with a consequent high content of workshop overhaul, to a system with a much higher proportion of remedial maintenance in the field. At one large refinery, there has been a 25% reduction in the number of pumps undergoing workshop overhaul since the introduction of vibration monitoring about 5 years ago (Fig. 5). There has been a similar drop reported in the number of electric motors undergoing workshop overhaul.
130
Now this does not mean that there has been a 25% reduction in the number of repairs carried out; this will have remained the same, and may have increased somewhat. Furthermore, there may be other unknown factors which could have contributed to the observed change in maintenance work patterns. But such a significant drop in overhaul work is clear evidence of a reduction in the number of catastrophic failures and supports the thesis that, because of vibration monitoring, more remedial maintenance work is being carried out in the field. Now clearly, it must be far less expensive to replace worn or damaged parts in a machine in situ than it is to carry out major repair work to a machine that has failed catastrophically. The difference in cost for a typical process centrifugal pump would be several hundred pounds. For an electric motor, the difference in cost could be several thousand pounds. Not many incidents are needed to show quite a significant saving in maintenance and repair costs, and the estimate for maintenance related savings of 0.35% of the total added value ouput per location seem quite realistic.
Conclusion To conclude, therefore, when vibration monitoring techniques are introduced into a process plant, there are immediate and obvious improvements in engineering standards. The techniques are used not to achieve engineering perfection, but to screen out mechanical faults and incipient failures. When equipment is systematically checked, the probability of catastrophic failure is reduced and this leads to an improvement in machine reliability and to a reduction in maintenance and overhaul work, and hence costs.
NON-DESTRUCTIVE
T E S T I N G . J U N E 1976