Ultrasound PDM

1

Ultrasonic Condition Monitoring of Variable Speed Bearings John Herbert, Trevor Bell, Wayne Hutchinson

Fonterra, New Zealand

Dr James Neale, Hamish Wolstencroft Energy Research Group, University of Waikato, Hamilton, New Zealand

Abstract: Predictive maintenance (PdM) is an attempt to evaluate the condition of equipment by performing periodic or continuous (online) equipment monitoring. The ultimate goal of PdM is to perform maintenance at a scheduled point in time when the maintenance activity is most cost-effective, while maintaining the desired equipment performance threshold. The condition monitoring of rotating bearings is a well know application of structure borne ultrasound, however this is traditionally been limited primarily to fixed speed applications. With increasing focus on plant automation and energy efficiency many traditional fixed speed motor driven applications are being fitted with variable speed drives, resulting in variable machine loads. Traditional instantaneous condition monitoring techniques, such as structure borne ultrasound, rely on a fixed baseline reference frame to determine any abnormality for further investigation. Under variable load situations it is important to differentiate between a change in the load condition and the presence of a condition abnormality.

This paper and corresponding presentation will demonstrate the successful implementation of a structure borne ultrasound condition monitoring programme for variable speed applications in the New Zealand Dairy Industry. Initially basic equipment was isolated and tested under laboratory conditions to determine the correct monitoring methodology, followed by in-plant testing of live production equipment. The critical system parameters required to successfully monitor variable speed bearings will be discussed, including the characterisation of the required speed window, application load and recommended alarm levels. The primary benefits of this programme are the rapid testing of more equipment than would otherwise be possible, thereby increasing the level of plant coverage for the same maintenance budget spend.

.

1.0 Introduction: Todays production focus is on increased speed, accuracy, efficiency and economics. In the past ultrasound and vibration technologies have been used independently in the condition monitoring of bearings in rotating equipment. It is becoming more widespread to use ultrasonic inspection as a complementary device to support vibration analyses for bearing condition monitoring and failure prediction methods.

To date however this application has been limited to fixed speed applications, with ongoing condition referenced to a known good baseline. In a variable speed application the fixed baseline is no longer valid, leaving a degree of uncertainty in what the current refernce baseline should be.

With increasing energy costs and ongoing modernisation of plant equipment and control systems an increasing number of traditional fixed speed equiomewnt are being fitted with variable speed drives (variable frequency drivers). This eliminates the ability to use the traditional fixed speed baseline approach for both VA and ultasound conditon monitoring programmes.

This report explores the parameters and reliability of using Ultrasonic technology to detect faults in common industrial equipment, while operating under variable speed applications. As an initial test a series of laboratory tests were established to evaluate the effectiveness of the proposed methodology across a wide range of applications. These techniques were then applied to real world plant based systems for further ongoing evaluation and development.

The aim of the research enclosed in this report is to demonstrate the accuracy to which Ultrasonic technology can be used to monitor variable speed equipment in a real world industrial applications in an effective manner and at similar cost to traditional fixed speed applications.

The primary objective of the ultrasonic based condition monitoring programme is to identify the early affects of degraded bearing lubrication and also the early onset of bearing failure. As with any condition monitoring PdM programme the success or failure will be largely determined by developing the correct test procedure and corresponding acceptance standards (alarm levels).

2

2.0 Theory: The typical Ultrasonic sampling frequency recommended for bearing monitoring is 30Hz. The relative loudness of the bearing can give a good indication of the condition of the bearing if the baseline value is known. The baseline, if not known, can be taken as the lowest recorded noise that the bearing gives off whilst under normal operating conditions.

If the change in the noise emitted by a bearing is monitored and tracked over time, basic diagnostics can be carried out. Traditional fixed speed bearing condition monitoring programmes have identified that an 8-10dB increase in operating noise above baseline is a sign that the bearing needs lubrication. An increase of 16-20 dB is commonly referred to as the alarm level associated with the onset of initial failure of the bearing, and an increase that exceeds 35-40 dB will commonly signal pending/imminent (catastrophic) failure.

Under a variable speed application the baseline dB level for any given bearing becomes a floating reference that is directly related to the instantaneous speed. Under these conditions the instantaneous alarm levels based on the traditional fixed speed 2 level alarm approach outlined above will also float accordingly. The variation in the baseline dB with speed is application specific and also heavily dependent on the width of the speed window within which the equipment will operate.

A range of tests were devised to specifically identify appropriate acceptance standards and the associated test procedures for variable speed applications.

3.0 Laboratory Testing:

3.1 Impact of Speed on dB Level: The initial focus was to investigate the relationship between the noise emitted by a bearing (dB) and the speed of rotation. It was expected that the bearing noise would increase with increasing shaft speed; however the harmonics associated with specific applications were also expected to have some impact on this overall relationship.

Measurements were taken on variable speed driven (VSD) equipment in the large-scale laboratory at the University of Waikato. The following equipment was tested;

Smart Pump Pulp Screen Water Flume Pump (vertical Impellor) Air Heater Fan Motor

The equipment included a variety of motor sizes, drive arrangements and power transmission techniques. The drive arrangements and bearing placements ranged from cantilever to simply supported. Both belt driven and coupling driven

power transmission systems were employed. Ultrasonic sound readings and wave recordings were taken for all motor bearings and auxiliary bearings.)

The equipment was inspected and the position of the individual bearings located. The readings were taken at the position on the device, which had the most direct sound path to the bearing housing. This position was marked using a permanent marker for future reference. In many cases this was the grease nipple, but in cases where such was absent a position was marked using a permanent marker to insure readings were consistently taken from the same point.

The equipment used was an UE Systems Ultraprobe 10000 with the standard contact module and the magnetic mount contact module, as illustrated in Figure 1 below.

Figure 1 Ultraprobe 10'000 with accessories.

The contact module was used when a grease nipple was present due to its contact surface being a point, which could be inserted into the grease nipple insuring an identical point of contact each time. In cases of bare motor surfaces or enclosed bearing housings the magnetic transducer was used to obtain consistent results.

Each of the four test rigs were started at a fixed speed and the rpm was recorded. Noise levels of each bearing were recorded next to their identification number at a frequency of 30 kHz. The dB reading was taken after 3-4seconds once the value had stabilised. The speed of each piece of equipment was then varied through the respective full operational envelope, with dB readings taken for each bearing at regular speed intervals. The equipment was given adequate time to stabilise at each speed setting prior to measurements being recorded.

The speed ranges were not the same for each rig as motor size, transmission ratios and application dictated the operational envelope of each system.

3.2 Impact of Test Frequency: Once all the bearings had been tested at 30 kHz the test frequency was varied in 1kHz intervals across a range of 24 to 32kHz, to establish if there were any benefits or limitations to applying frequency tuning techniques to each of the various test rigs.

3

4.0 Results and Analysis:

4.1 Smart Pump The smart pump test rig is shown in Figure 2 below, with the four bearing locations highlighted. The corresponding dB readings across the available speed range for each of the four bearings is shown in Figure 3. Across the majority of the speed range the motor bearing dB level was surprisingly stable with a variation of less than 5 dB. The pump bearings in comparison had a slightly wider band, however if the low speed settings can be ignored (outside normal operational envelope) a similar dB band could potentially be identified.

Figure 2: The Smart Pump test rig shown with Motor bearings (3 and 4) and Auxiliary bearings (1 and 2).

Smart Pump - 30 kHz

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Motor OB

Motor IB

Pump IB

Pump OB

Figure 3: The Smart Pump dB level variation with speed (above), variation in dB with speed and test frequency for each bearing

(left), and overall variance in dB across the speed band for each test frequency (bottom left).

When the variation in dB level for each respective bearing is considered across the test frequency range of 24 to 32 kHz the underlying dB to speed relationship tends to be relatively uniform (with a few exceptions associated with natural resonances and competing ultrasonic sources). The overall variance in db level with test frequency was found to be best in the 30 to 32 kHz range; however care does need to be taken to ensure that sharp changes in db are not caused by a badly matched speed and test frequency, such as the 31 kHz test on bearing number 3 in the 660 to 780 rpm speed band, as indicated.

Bearing 1 dB vs. Shaft Speed

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Shaft Speed (Rpm)

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Shaft SPeed (Rpm)

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Shaft Speed (Rpm)

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332kHz


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Shaft Speed (Rpm)

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Variance vs.Frequency

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4.2 Pulp Screen The pulp screen test rig is shown in Figure 4 below, with the four bearing locations highlighted. The corresponding dB readings across the available speed range for each of the four bearings is shown in Figure 5. Across the majority of the speed range the motor bearing dB level was once again surprisingly stable with a variation of less than 5-6 dB. The pulp screen bearings in comparison had a much wider band, with the majority of this spread a result of the lowest speed setting (600 rpm) and the highest speed setting (2700 rpm). In practice the screen would not be operated at or near either one of these speeds, and if these two outliers are removed the variation across the normal operation band is considerably better with a total band width of only 8-9 dB.

In this type of application a sliding baseline would be required to ensure adequate detection of each respective bearing condition alarm level.

Figure 4: The Smart Pump test rig shown with Motor bearings (3 and 4) and Auxiliary bearings (1 and 2).

Pulp Screen - 30 kHz

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400 800 1200 1600 2000 2400 2800

RPM

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Motor OB

Motor IB

Pump IB

Pump OB

Figure 5: The Pulp Screen dB level variation with speed (above), variation in dB with speed and test frequency for each bearing


When the variation in dB level for each respective bearing is considered across the test frequency range of 24 to 32 kHz the underlying dB to speed relationship tends once again to be relatively consistent. The best performance is still at a test frequency of around 30 kHz for each bearing tested.


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594 1194 1487 1782 2390 2672

Shaft Speed (Rpm)

dB

24kHz

25kHz

26kHz

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30kHz

31kHz

32kHz

33kHz


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594 1194 1487 1782 2390 2672

Shaft Speed (Rpm)

dB

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29kHz

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32kHz


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594 1194 1487 1782 2390 2672

Shaft Speed (Rpm)

dB

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32kHz


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594 1194 1487 1782 2390 2672

Shaft Speed(Rpm)

dB

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dB Varience vs. Test Frequency

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4.3 Water Flume Pump The water flume test rig is shown in Figure 6 below, with the three bearing locations highlighted. The corresponding dB readings across the available speed range for each of the respective bearings is shown in Figure 7. Across the entire speed range the motor bearing dB level was very steady ( 2 dB). The corresponding bearing on the impellor shaft was also very stable with a variation in dB of less than ( 2 dB) across the full operational envelope.

In this type of application a standard fixed speed bearing base condition monitoring programme would work quite effectively, with no additional input variables required.

Figure 6: The Water Flume test rig shown with Motor bearings (3 and 4) and Auxiliary bearings (1 and 2).

Water Flume - 30 kHz

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Motor OB

Motor IB

Pump SB

Figure 7: The Water Flume dB level variation with speed (above), variation in dB with speed and test frequency for each bearing


When the variation in dB level for each respective bearing is considered across the test frequency range

of 24 to 32 kHz the underlying dB to speed relationship in this instance is very stable, with all test frequencies showing a variation of 2 dB or less. Interestingly the 30 kHz range does still produce one of the best overall results.


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Shaft Speed (Rpm)

dB

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Variance vs. Frequency

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4.4 Fan Motor The fan motor test rig is shown in Figure 8 below, with the three bearing locations highlighted. The corresponding dB readings across the available speed range for each of the respective bearings is shown in Figure 9. Across the entire speed range the motor bearing dB level was relatively steady ( 3 dB).

In this type of application, depending on the level of criticality of the equipment any variation in fan speed could be adequately addressed by either relaxing the alarm levels slightly to say 12 and 24 dB respectively, or maintain them at the standard fixed speed levels of 8-10 and 16-20 dB, with the understanding that a false alarm or early warning alarm may be triggered due to speed fluctuations.

Figure 8: The Fan Motor test rig shown with Motor bearings (3 and 4) and Auxiliary bearings (1 and 2).

Fan Motor - 30 kHz

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Motor OB

Motor IB

Figure 9: The Fan Motor dB level variation with speed (above), variation in dB with speed and test frequency for each bearing


When the variation in dB level for each respective bearing is considered across the test frequency range of 24 to 32 kHz the underlying dB to speed relationship in quite variable, with some clear indications of interference/resonance issues ate certain test frequencies. Fortunately there is also several test frequencies thats sowed a relatively stable variation in dB level across the speed range of interest, as shown in the 26-27 and 29-30 kHz bands.


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1075 1116 1143 1170 1212 1252 1281 1307

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1075 1116 1143 1170 1212 1252 1281 1307

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4.4 Laboratory Testing Summary The controlled laboratory testing provided several key insights into the viability of using traditional fixed speed ultrasonic bearing condition monitoring techniques on variable speed installations.

Firstly the recommended test frequency is the same as far fixed speed applications (30 kHz), however it is still very important to ensure that the optimum frequency is used (tuned) for each individual applications.

Secondly by establishing the application specific relationship between speed and dB levels a fixed baseline may be used in certain applications and where necessary additional compensation can be made for variations in instantaneous dB levels as a function of shaft speeds. The ultimate determining factor in how detailed/accurate these measures are can be directly tied to the degree of criticality of the equipment and the preference for having or avoiding early or false alarms.

2 1

7

5.0 In Plant Implementation: The above approach was applied to a range of identical fans configured like the fan and motor configuration shown in Figure 9. Ultrasonic measurements were taken over 3 successive periods, with the dB recordings for each Fan shown in Table 1. In the first instance there does not seam to be too large a variation in the dB reading for each bearing in time, however the comparison of like equipment shows several clear bearings that are of concern highlighted in red (level 2 alarm) and orange (level 1 alarm lubrication).

Figure 1: Fan arrangement for Fans 1, 2, 3 & 4

Table 1: In plant readings for 4 identical synchronised VSD fans

dB Readings

Machinery

Description

Bearing 2/09 2/10 2/11

Fan 1 1 71 68 66

Fan 1 2 54 53 52

Fan 1 3 38 40 42

Fan 1 4 40 39 38

Fan 2 1 53 48 47

Fan 2 2 45 40 38

Fan 2 3 37 35 50

Fan 2 4 35 37 40

Fan 3 1 67 62 63

Fan 3 2 57* 48 49

Fan 3 3 36 38 38

Fan 3 4 41 37 35

Fan 4 1 72 75 77

Fan 4 2 50 50 51

Fan 4 3 31 31 33

Fan 4 4 37 40 38

* Lubrication applied

Table 2 below compares the bearings within each individual fan arrangement against one another whist comparing the individual bearings against their equivalent counterparts.

Table 2: Like for like dB comparison

Bearing

D4 Exhaust

Fan No.

1 2 3 4

Fan 1 71 54 38 40

Fan 2 53 45 37 35

Fan 3 67 57 36 41

Fan 4 72 50 31 37

Initial inspection shows that bearing #1 has considerable dB variance across the four fans. The baseline for this bearing is taken to be 50dB due to it being the lowest noise emittance for that bearing type. Bearing #1 is the outboard shaft bearing closest to the fan housing, #2 is the fan inboard, with the motor inboard bearing # 3 and motor outboard #4.

The speed of the fans was not the same on each day that readings were taken, as illustrated by the variation in dB level seen for each bearing. The critical alarm levels can still be readily identified, and appropriate action planned and implemented.

7.0 Conclusions: The condition of variable speed bearings can effectively be monitored using structure borne ultrasound. The primary prerequisite for any such programme to be successful is the need to carefully establish the baseline dB levels for each bearing across the normal operational speed range for each piece of equipment. Each variable speed bearing can then be classified into one of three categories:

(a) Fixed speed bearing behaviour, no additional measures required.

(b) Small variation in dB baseline (10-15 dB), establish a speed dependent baseline and record speed as part of bearing PdM route. Modern digital ultrasonic guns have the capability to add this information as part of the normal operation.

The determination of which of the above three categories any given bearing may best fit can also then be weighted based on plant specific criticality. If a piece of plant is deemed critical a higher degree of monitoring can be implemented to provide the necessary cover. This increases marginally the work involved, but can still provide a useful time and cost effective PdM solution.

As part of the baseline establishment process it is also very important to ensure that the dB readings are associated with the bearing and not competing ultrasound form other sources linked to the process, i.e. in line turbulence (pumps, fans, etc.).

1Track 2: Manufacturing Process Reliability

Ultrasonic Condition Monitoring of Variable Speed Bearings

By,Dr James NealeUniversity of WaikatoMr Gary Mohr

UE Systems Inc.

Introduction to Ultrasound

Where is Hamilton?

Research into improving Industrial Energy Efficiency Compressed Air Steam Utility Loop Optimisation Heat Recovery and Heat Integration Pinch Analysis Industrial Fluid Flow Optimisation Renewable Energy Solutions Distributed Generation Energy Audit Methodology Development Energy Efficiency Policy Development Applications of Ultrasound

Airborne Structure Borne

Energy Research Group Overview

Energy Research Group Overview

Numerical Modelling Computational Fluid Dynamics Modelling Proprietary Software Development

Economic Modelling Capital Project Assessment Energy Future Scenario Modelling

Experimental Investigation & Analysis Laboratory Scale Plant Scale

Capital Project Implementation System Analysis System Design Verification

Presentation Overview

1. Project Rationalea. Site: Fonterra Te Rapab. Applications

2. Project Definitiona. Variable Speed Applicationsb. Test Variables

3. Laboratory Testinga. Applicationsb. Results

4. In-Plant Implementation5. Conclusions

a. Implementation in Your Plant?

Fonterra Overview

EDGECUMBE

REPOROA

LICHFIELDTIRAUTE AWAMUTU

HAUTAPU

TE RAPA

WAITOA

MORRINSVILLE

KAURI

MAUNGATUROTO

PAHIATUALONGBURN

TATUA

HAWERAKAPUNI

ELTHAM

EDENDALE

CLANDEBOYE

TAKAKA

KAIKOURA

STIRLING

CHRISTCHURCH

HOKITIKA

KARAMEA BRIGHTWATER

TUAMARINA

.

Process 14 billion litres of milk annually. At peak day 70 million litres / day. South Island is currently 27% of the total and

growing. Make 2 billion tonnes of Dairy products / year. We have 29 manufacturing sites. 18 in the North Island. 8 in the South Island. We have 4,800 staff. We are 22% of NZs total exports.

Earning in excess of $15USD Billion.

Fonterra our Unique Energy Features

NZs largest combined energy user? 29 main sites, nationwide (Fonterra), 198+

electricity connections, 24 gas sites, 4 coal mines

Biologically degradable product base, susceptible to supply interruption (inflexible)

Most electrical motors fitted with Variable Speed Drives (VSD).

Key Facts Fonterra Te Rapa

Te Rapa is located in the North Island of NZ Te Rapa in Maori means The Seeking Size of site: 12 ha Staff on site: 500 Tankers of milk handled each day: 304 Peak milk capacity: 7.5 million litres/day Site resources:

Product Technical Laboratory

Fonterra Te Rapa Site

Milk Powder Plant

(Driers 1& 2) Commissioned

Drier 1& 2 Upgraded

Drier 4 Commissioned

Cream Plant Commissioned

Drier 5 &Co-generation

Plant Commissioned

19901967 1991 1997 1998 2006

New Packing Lines

Commissioned

Te Rapa Site History

Key Products

Cream Products:ButterAnhydrous Milk FatCream CheeseHigh Fat Cream CheeseFrozen Cream

Milk Powders:Skim Milk PowderWhole Milk Powder

Project Definition1. Variable Speed Applications

a. Q: How does speed impact condition monitoring of bearings?

b. Q: What is our acceptance standard?2. Test Variables

a. Speedb. Test Frequencyc. Alarm levels?d. How does the acoustic response vary with speed

for different applications/equipment?

Laboratory TestingThe following equipment was used to test the effect of varying frequency and speed on the ultrasound level recorded.

Smart PumpPulp ScreenWater FlumeAir Heater Fan

Smart Pump

Smart Pump

Motor Specifications: 25 kW 1475 rpm max.

1

2

3

4

Bearing noise was measured in dB

at 4 points:

Motor

1 - Outboard Bearing

2 - Inboard Bearing

Pump

3 - Inboard Bearing


Speed v dB Results 30kHz

Smart Pump - 30 kHz

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Pump IB

Pump OB

Pulp Screen

Pulp Screen

Motor Specifications 15 kW 2935 rpm max.

4 3

21

Bearing noise was measured in dB

at 4 points:

Motor


2 - Inboard Bearing

Pump

3 - Inboard Bearing



Pulp Screen - 30 kHz

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Motor OB

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Pump IB

Pump OB

Water Flume

Water Flume

1

23

Bearing noise was measured in dB at 3 points: Motor

1 - Outboard Bearing 2 - Inboard Bearing

Pump 3 - Single Bearing

Motor Specifications

4 kW

960 rpm max.


Water Flume - 30 kHz

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Pump SB

Air Heater Fan

Air Heater Fan

2

1

Bearing noise was measured in dB at 2 points:

Motor 1 - Outboard Bearing 2 - Inboard Bearing

Motor Specifications

4 kW

1445 rpm max.


Fan Motor - 30 kHz

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Summary - Speed v dB @ 30 kHz

Variation in speed had little effect on dB level of motor bearings tested at this frequency (less than 5 dB).

Variation in speed had marginally greater effect on dB level of the application (driven end) bearings (5 to 25 dB). Very application Specific.

Potential for actual in-plant usage bands to be quite narrow, resulting in a lower dB band in practice.

Smart Pump Frequency Variation

Pump OB dB vs. Shaft Speed

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Pump IB dB vs. Shaft Speed

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Motor IB dB vs. Shaft Speed

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Motor OB dB vs. Shaft Speed

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Pulp Screen Frequency Variation

Screen IB dB vs. Shaft Speed

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Screen OB dB vs. Shaft Speed

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594 1194 1487 1782 2390 2672

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Water Flume Frequency Variation


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Prop dB vs. Shaft Speed

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Air Heater Frequency Variation


0

10

20

30

40

1075 1116 1143 1170 1212 1252 1281 1307

Shaft Speed (Rpm)

d

B

24kHz

26kHz

28kHz

30kHz

32kHz

33kHz


0

10

20

30

40

1075 1116 1143 1170 1212 1252 1281 1307

Shaft Speed (Rpm)

d

B

24kHz

26kHz

28kHz

30kHz

32kHz

33kHz

Test Frequency VariationVariance vs.Frequency

10

12

14

16

18

20

22

24

26

22 24 26 28 30 32 34

Frequency (kHz)

V

a

r

i

e

n

c

e

(

d

B

)

dB Varience vs. Test Frequency

0

20

40

60

22 24 26 28 30 32 34

Frequency (kHz)

V

a

r

i

e

n

c

e

(

d

B

)

Smart Pump

Pulp Screen

Test Frequency Variation

Water Flume

Fan Motor


0.0

0.5

1.0

1.5

2.0

2.5

22 24 26 28 30 32 34

Frequency (kHz)

V

a

r

i

e

n

c

e

(

d

B

)


0

5

10

15

22 24 26 28 30 32 34

Frequency (kHz)

V

a

r

i

e

n

c

e

(

d

B

)

Summary - Varying Test Frequency

Ultrasound level tends higher at lower test frequency for most speed ranges.

dB level relatively stable with varying motor speed at all frequencies on motor bearings.

dB level increased on some pump/prop bearings as speed increased, for all frequencies.

In-Plant ImplementationFonterra Te Rapa

Condition Monitoring Implemented across a range of critical variable speed equipment Water Pumps Main Inlet & Exhaust Fans to Drier MVR Fans: Drier Evaporator Chilled Water Compressors (Ammonia) Plate Freezer Compressors VSD Air Compressor

Will be carried out over an extended period to allow long term trending of results.

In-Plant Implementation Measurements

dB level motor speed (using displayed speed or a hand held strobe) Sound Files

Ultimate Objectives Traffic Light Assessment

Green good Red Bad Amber recheck etc

Avoid Measuring Speed Keep life simple

Sound File Primary Measure of Bad bearing Capture Lubrication Fault in timely manner

Implementationin Your Plant

Establish baselines What are the speed limits?

High Low

How does the dB Vary? Motor Driven End (Application)

What is your acceptance Standard? Degrees of criticality False alarms? Modified Alarm Levels

Lower level => dB driven Upper level => Sound File driven

Thank you

025_a025_b

Ultrasound PDM

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