Vibration Diagnostics

Vibration Diagnostics Arslan Hamid (AHm) MTS Machinery 19 October 2011

What hat

does MTS do??

What hat

does MTS do??

AGENDA      

What is Vibration

     

Vibration Characteristics

     

Vibration Instruments Instru ments

     

Vibration Measurement

     

Vibration Analysis Steps S teps

     

Imbalance

     

Misalignment

     

Looseness

     

Antifriction Bearings

     

Gear Problems

     

Electrical Defects

     

Fluid Vibrations

     

Case Studies

Vibration is the response of a system to an a n internal or external stimulus causing it to oscillate or pulsate.

Vibration has three measurable characteristics      Amplitude      Frequency      Phase AMPLITUDE

Amplitude tells us the magnitude of vibration. Amplitude can be measured in displacement d isplacement (mils or  µm), velocity (in/sec or mm/sec) mm/sec) or acceleration (g) (g ). Frequency less than 600 CPM

=

Use Displacement

Frequency is 600 to 120, 120 ,000 CPM

=

Use Velocity

Frequency over 120, 120 ,000 CPM

=

Use Acceleration

Plant-I

Displacement (proximity probes) is measured in mils or thou (= thousandth of inch) Velocity (seismic probes) is measured in in/sec Acceleration (accelerometers) is measured in g¶s or in/sec^2 Plant-II

Displacement is measured in µm or micron (= millionth of meter ) Velocity is measured in mm/sec Acceleration is measured in g¶s The average thickness of a human hair is about 5 mils, which is the same as TRIP set point of all radial probes installed at K-2502 Syn Compressor!!

FR EQUENCY

Frequency tells us how many times the machine is moving/vibrating per  unit of time. Frequency is the indicator of the vibration stimulating problem. Units are CPM or RPM. PHASE

Phase angle tells us in which direction a specific section of the machine is vibrating relative to some point. Phase is the angular measurement  between a reference point and the vibration peak in a time waveform. Phase angle is used to distinguish between several problems as indicated  by the frequency.

Vibration data collection can be collected using:  Mounted instruments / probes       Proximity       Seismic

Probes

Probes

 Portable instruments       Overall vibration meters       Vibration Analyzers

(Vibrapen)

(CSI 2130)

Seismic Sensors The seismic or velocity probe/transducer will measure total vibration at whatever point it is attached to usually the bearing housing of a running machine. Proximity Sensors The proximity probe measures actual shaft movement, usually within its bearing housing, onto or through which it is usually mounted. Overall Vibration

Meters (e.g vibrapen)

These are used for surveillance purpose only. These cannot be used for analyzing or  identifying problems. Vibration Analyzers (e.g CSI 2130)

These are used for analyzing and identifying  problems that cause vibration issues

Data Collection

Actual Bearing Movement: Elliptical

A Transducer Mounted Vertically "Sees" Only Vertical Movement

A Transducer Mounted Horizontally "Sees" Only Horizontal Movement

Vibration Analyzers can be used to collect vibration data in the following manner       

Vibration Time Waveform

     

Vibration Spectrum

     

Vibration Phase Angle

Each of the above methods of vibration data helps in a unique way to pinpoint the vibration stimulating problem.

Time Wave Form (amplitude vs. time)

Time wave form gives the amplitude and direction of vibrating motion of  a specific part of the machine in time domain.

This is usually over a small period of time. The same for a longer  duration is called trend.

Vibration Spectrum (amplitude vs. frequency)

Vibration spectrum is a calculated graph plotted through Fast Fourier  Transformation of a Time Wave Form. This vibration graph shows the amplitude of vibration in frequency domain.

Vibration problems can be diagnosed in relation to the frequencies being synchronous, sub synchronous or non synchronous.       Synchronous Frequencies are

those that are intergral multiples of  shaft speed (1X). Imbalance, Misalignment, Looseness, Vane pass freq, etc are examples of Synchronous Frequencies

      Non Synchronous Frequencies are

those that are not integral multiples of the shaft speed. Bearing Defects except for Fundamental Train Freq. FTF, Cavitation and Resonance are examples of   Nonsynchronous frequencies.

      Sub Synchronous Frequencies are

those that are below shaft speed. Cage frequency of roller element bearings, oil whirl are examples of  Subsynchronous frequencies.

This was all about Vibration Basics.

Let¶s now proceed towards How to Analyze a Vibration Signature and How to Diagnose a Problem through Vibration Spectrum«

Before analyzing a vibration problem, the following should be known to the analyst. Without this data, Vibration Analysis is like groping in the darkness.      Machine

configuration. Identify all major  components and sketch on a paper , like motor , pump, gearbox, etc.

     Determine

as many as possible forcing frequencies before taking vibration data      

Shaft speeds in the machine train

     

Bearing fault frequencies

     

Belt frequencies

     

Gear Mesh Frequencies

     

Blade pass frequencies

 Now collect the vibration data and analyze it for possible problems. 10 - Ammonia Solution Pump 10-MP-07 B-MIA Motor Inboard Axial

1.6   c   e    S    /   m   m   n    i   y    t    i   c   o    l   e    V    K    P

   3    7  .    3    3

1.2    3    7  .    9    2

   1    0  .    0    1

0.8

Route Spectrum 09-May-11 08:59:33 OVERALL= 1.90 V-DG PK = 1.89 LOAD = 100.0 RPM = 3000. (50.00 Hz)

0.4

0 0

10

20

30

40

50

60

70

80

90

Frequency in Orders 5 4   c   e    S    /   m   m   n    i   y    t    i   c   o    l   e    V

Route Waveform 09-May-11 08:59:33 PK = 1.73 PK(+/-) = 3.87/2.90 CRESTF= 3.16

3 2 1 0 -1 -2 -3 -4 0

1

2

3 Revolution Number 

4

5

Ordr: 33.72 Freq: 1686.2 Spec: 1.184

Since we don¶t live in an ideal world, almost always there is one problem or the other which is brewing up in the machine. It is not possible to attend the machine on the onset of slightest of   problems diagnosed by vibration analysis as it results in machine down time, man hours and spares cost. As a rule of thumb, allowable limit of Overall Vibration in the Spectrum of Centrifugal machines is 0.3 in/sec for machines at Plant-I, and 7.6 mm/sec for machines at Plant-II. P.T.O

What is Imbalance Imbalance is the result of a shaft¶s center of mass not rotating at the center of rotation. This is because of a heavy spot on the rotor. This heavy spot produces a centrifugal force that forces the rotor to rotate offcenter and causes a high vibration amplitude at 1 x Turning Speed (TS) Frequency in the spectrum and sinusoidal waveform in the time domain data. Imbalance can be      Static

Imbalance

     Dynamic

Imbalance

Static Imbalance Freq Domain : 1 x TS Time Domain : Sinusoidal Phase Difference :

0 Deg H to H or V to V across the rotor  90 Deg H to V on same bearing

Couple Imbalance Freq Domain : 1 x TS Time Domain : Sinusoidal Phase Difference :

180 Deg H to H or V to V across the rotor  90 Deg H to V on same bearing

 High level radial vibration: Steady 1X component ± waveform and frequency Amplitude at 1X increases steadily with speed Low level at 2X, 3X, etc.  Low level axial vibration.  Notes:  Strong 2X, 3X, « indicate misalignment, looseness, bent shaft, or  cocked bearings which must be corrected before checking for imbalance.  If 1X µbeats¶, check for broken rotor bars or cocked bearings in motors.  If small change in speed causes drastic changes in 1X component, Resonance is suspected.  On horizontally mounted machine, check H, V and A vibration. If V is higher , suspect base looseness. If A is higher , suspect misalignment. Correct these problems before balancing the machine.

What is Misalignment Misalignment occurs because of poor alignment between mating pieces such as coupling halves, bearings, shafts and pulleys. Misalignment Characteristics      Misalignment

results in high axial and radial vibrations. The highest radial vibration usually occurs in the direction of the misalignment.

     The

axial vibrations can be as much as 0.5 to 2 times the amplitude of the radial readings.

     The

radial readings can appear at 1X, 2X, 3X and even higher  multiples of shaft turning speed. However in certain cases, the  predominant vibration occurs at 1X TS and can be confused with unbalance. In such cases, phase data is used to distinguish between unbalance and misalignment.

Misalignment Freq Domain :

High1x, 2x, 3x. Low 4x-10x harmonics (if high, suspect looseness).

Time Domain :

Repeatable periodic time waveform with 1, 2, 3, or  4 clear peaks per revolution. No high µg¶ impulses.

Phase Difference :

180 Deg in radial or axial direction across the coupling

Other

High axial vibration, excessive bearing temperatures

Types of Misalignment Misalignment can be      

Angular Misalignment

     

Parallel / Offset Misalignment

     

Combination of the above two

Angular Misalignment Freq Domain :

1x TS in axial (high) and radial direction

Time Domain :

Sinusoidal with one or two clear cycles per  revolution

Phase Difference :

180o in radial, vertical or axial direction across the bearings of the same machine

Other

2x TS in radial direction if  Offset Misalignment is also present.

:

Parallel Misalignment Freq Domain :

Dominant peak in 2x TS in radial direction

Time Domain :

Sinusoidal with one or two clear cycles per  revolution

Phase Difference :

180o in horizontal or vertical direction across the coupling.

Other

1x TS peak in radial & axial direction.

:

Other

Types of Misalignment

Misalignment can also be :      

Coupling Misalignment

     

Bearing Misalignment / Cocked Bearing

     

Bent Shaft

Coupling Misalignment Coupling misalignment occurs when coupling is worn or the coupling is not installed properly. The characteristics of a misaligned coupling are a combination of  Rubber dust due to rubbing Angular and Parallel misalignment.

Bearing

Misalignment

Bearing misalignment occurs when the bearings are not installed in the same plane, they are cocked relative to the shaft or if the machine distorts due to thermal growth or soft foot. Freq. Domain :

Normally highest peak at 1x A. Harmonics at 2x, 3x or number of balls x TS in the axial direction

Time Domain :

Waveform often shows truncated or flattened  pattern indicating a rub. It may also appear   periodic or sinusoidal with low amplitude.

Phase Difference:

180o phase difference in axial direction from top to bottom or side to side on the same bearing

Bent

Shaft

Freq. Domain :

Dominant peak at 1x TS in radial and axial direction if the shaft is bent near the center. Dominant peak at 2x TS if the shaft is bent near  the coupling.

Time Domain :

The waveform is a mixture of misalignment and imbalance

Phase Difference:

180o in axial direction across the bent shaft. 90o or  270o from horizontal to vertical direction on a  bearing. 180 o from top to bottom or side to side in axial direction

Center bent shaft inboard bearing housing

Coupling end bent shaft inboard bearing housing

What is Looseness Mechanical looseness occurs when structural or rolling element components do not fit properly. Freq. Domain: A large number of TS harmonics characterizes looseness in the spectrum. In some cases and stages looseness exhibits subharmonics of 1/2 x TS. The highest amplitude typically appears radially in the vertical direction. Time Domain: Waveform shows a great deal of energy and impacting in a random, high frequency pattern. Phase:

The phase has no set relationship and is unsteady.

Base looseness of horizontal machines will often appear as high 1X level in V; greater than H component.

Loose Machine Foot Foot looseness may emerge due to weakness in the base plate or  foundation, deterioration of the grouting, loose hold-down bolts, or a cracked or broken foot. Freq. Domain: Dominant frequency is at 1xTS radially. Time Domain: Waveform is periodic. Phase:

180o phase diff. between machine foot vertically and concrete base.

Loose Machine Foot

To help determine a soft foot issue, loosen each foot one at a time, always keeping the others tight, and then retighten while the spectrum is still measuring. Any notable reduction in the one-times (1x) energy during the loosening process can be a strong indication of the relief of machine frame distortion. 1X

1X

Before the loosening sequen ce: All four feet are tight ened; the 1X amplitude is 0.127 inches/second. Radial measurements were taken in the vertical direction.

After the loosening sequence: One of the four feet was loose; the 1X amplitude is now 0.048 inches/second.

Antifriction bearing faults can be identified by bearing defect frequencies in a spectrum. Following are the bearing defect frequencies: FTF

=

Fundamental Train Frequency or Cage Frequency

BSF

=

Ball Spin Frequency

BPFO =

Ball Pass Frequency Outer race

BPFI =

Ball Pass Frequency inner race

These frequencies are specific for each bearing type. Although complex formulae are available for calculation of these frequencies for each  bearing, yet it is convenient to use bearing software that are designed for  the specific purpose of bearing defect frequency determination.

These bearing defect frequencies only appear in a spectrum if there is a fault in the bearing. Following is an example of these bearing frequencies Bearing : 6214 (P-2504 O/B bearing) Pump RPM

=

3570

# of Balls

=

10

BPFO

=

14655

BPFI =

21045

BSF

=

9653

FTF

1465

=

 Note that FTF is sub synchronous and the rest are non synchronous.

Gear problems are usually a bit tricky to identify. Before analyzing a signature for gear problems, the Gear Mesh Frequency (GMF) of the gear should be known GMF = (# Gear teeth) x (RPM of the same gear ) GMF Side Bands

It should be noted that the gear mesh frequency will always be present in the spectrum whether or not there is a problem with gears. Gear   problems are detected with the help of side bands of the GMF GMF Sidebands = GMF



TS

The appearance of the sidebands of the GMF in the spectrum indicate that a problem exists with the gear. As the amplitude of these side bands increases, it means that the gear problem is worsening.

Broken

Tooth

The problem of a broken tooth cannot be detected by the spectrum alone. The time waveform needs to be looked at for this. If one tooth is broken then a pulse will be generated once per revolution of the gear with the  broken tooth.

AC Induction Motor Defects Usually in order to find out if the vibration problem is because of motor  or because of the driven machine, the motor is discoupled from the driven equipment and the vibration is checked. However , it should be kept in mind that electrical defects disappear from vibration spectrum if  the motor is operated in discoupled state. In discoupled state, only the mechanical defects of the motor can be picked up in the vibration spectrum like bearing defects, rotor unbalance, etc.

Similar to GMF

Vane Pass Frequency Usually people think of Vane Pass as the pump¶s primary vibration frequency. Vane pass is not always a defect frequency but is an operational frequency. It is normal to see this frequency in pump vibration data.

A 5-Vane Impeller of Centrifugal Pump

Vane pass frequency = (shaft speed) x (no. of blades or vanes) A problem with impeller¶s vane appears as high vane-pass frequency harmonics with sidebands ± radial or axial. Blade damage may cause imbalance. Impeller looseness may look like mechanical looseness.

Cavitation

Cavitation is the formation and then immediate implosion of cavities in a liquid  i .e. small liquid-free zones ("bubbles")  that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low .

Vibration due to Cavitation is typically seen as harmonics of turning speed upto the impeller vane pass frequency. Higher frequency broad  band noise can also be found. This broad band vibration is usually  present above the impeller blade pass frequency. µRaised floor¶ is an indication of cavitation. If there is a raised noise floor - look in the time waveform. The waveform will show you the impacts, rubs, bursts of energy from cavitation, and so on. Cavitation is a common problem during startups. It can be rectified by increasing the fluid head (pressure) at suction. P.T.O

Here is an interesting example. The first spectrum is 800 lines. Notice the broad bases of the  peaks. Could it be resonance? The next spectrum is 3200 lines - you can see there is more to it. And the third spectrum is zoomed in to the base of the peaks.

Time waveform of cavitation showing bursts

P-08 B (Urea Melt Pumps @ Plant-II)

Data Collected on 6 Sep 2011:

P-08 B (Urea Melt Pumps @ Plant-II)

Data Collected on 13 Sep 2011:

P-08 B (Urea Melt Pumps @ Plant-II)

Bearing damage was diagnosed. After opening the pump, the following was observed:

Thrust bearings are badly worn, wear marks can be easily seen on the balls. Outer race had become very loose.

 As a result, mechanical seal stationary face was found with considerable damage.

MP-43 A (Seal Water Flush Pump of P-01 @ Plant-II) Initial

data showed high 2X with presence of other harmonics. Misalignment was suspected. 10 - Seal flush water pump 43A 10-MP-43 A-MIH Motor Inboard Horizontal

  c   e    S    /   m   m    K    P

9 8 7 6 5 4 3 2

Trend Display Overall Value

0

80

12

  c   e    S    /   m   m    K    P

160 240 Days: 25-Sep-10 To 15-Sep-11

320

   0    0  .    2

9

Route Spectrum 15-Sep-11 11:03:48 OVERALL= 9.11 V-DG PK = 9.05 LOAD = 100.0 RPM = 2999. (49.99 Hz)

6 3 0

   0    0  .    1

   0    1  .    4

0

3

   5    8  .    4

   4    8  .    6

6 Frequency in Orders

9

12

1.5 0.9

  s      G   n    i   c   c    A

400

Route Waveform 15-Sep-11 11:03:48 P-P = 1.42 PK(+/-) = 1.21/1.12 CRESTF= 2.41

0.3 -0.3 -0.9 -1.5 0

20

40

60 Time in mSecs

80

100

MP-43 A (Seal Water Flush Pump of P-01 @ Plant-II)

After uncoupling the motor and pump, motor solo run data was taken which still showed high vibrations. Additionally, phase difference on axial side on either motor bearings was 180 degrees. This indicated cocked bearings problem . Problem in pump was thus ruled out .

After the motor bearings were attended at I&E workshop, the motor resulted in minimal vibrations. After coupling the motor back with the pump, data was again taken. Remarkable reduction in vibrations was observed.

MP-43 A (Seal Water Flush Pump of P-01 @ Plant-II)

A comparison of vibration data after attending the issue is given below:

POINT POINT ID DESCRIPTION ----- -----------------------------

CURRENT VALUE -------

PREVIOUS VALUE UNITS ----- -------

10-MP-43 A - Seal flush water pump 43A(17-Sep-11)   MOH=Motor Outboard Horizontal 1.684 6.952 mm/Sec   MOV=Motor Outboard Vertical .682 1.239 mm/Sec   MOA=Motor Outboard Axial .248 1.442 mm/Sec   MIH=Motor Inboard Horizontal 2.252 9.106 mm/Sec   MIV=Motor Inboard Vertical .683 .727 mm/Sec   MIA=Motor Inboard Axial .772 1.982 mm/Sec

MP-2504 (Catacarb Lean Pump @ Plant-I)

On 4 October 2011, Ops asked MTS to check the subject pump due to abnormal sound. Vibration spectrum collected by MTS showed that both bearings of the pump had undergone damage and V PF sidebands were also present. Motor was in healthy condition. POINT

CURRENT REF ID DESCRIPTION VALUE VALUE UNITS ----- ----------------------------- ------- ----- ------- ------- ---- ---

Based on MTS recommendation, FM overhauled the pump and replaced its bearings . Data collected later on 6 October showed that the problem was resolved.

MP-2504

POINT

- Catacarb Lean Pump

6/10/11 PIH=Pump Inboard Horizontal .125 PIV=Pump Inboard Vertical .093 PIA=Pump Inboard Axial .091 POH=Pump Outboard Horizontal .115 POV=Pump Outboard Vertical .083 POA=Pump Outboard Axial .099

5/10/11 .380 In/Sec .432 In/Sec .181 In/Sec .220 In/Sec .294 In/Sec .243 In/Sec

MP-2504 (Catacarb Lean Pump @ Plant-I)

High 3X with side bands are a result of V PF

Vibration data collection methods that we have discussed so far are used for time based condition monitoring of the machines. However, for business or safety critical machines, online vibration monitoring system is used to avert undesirable situations . E.g; BN3300 system is used for KGT-2501 online monitoring. BN3500 system is used for K-441 and other turbomachinery at Plant-II.

Advanced diagnostics tools such as ADRE are connected to obtain and analyse real time data of such machines for effective problem identification .

Root

cause analysis and acceptance testing to i mprove reliability

Some vibration analysts believe that their sole job is to detect and report fault conditions. But that is not true. Locked inside your vibration data (and other condition monitoring data) may be the reason why the fault occurred in the first place . It is great if  you can report that a bearing may fail, but even better if you can say what caused the bearing to develop the fault (or why the machine is misaligned, out of balance, and so on). You

can report the fault and wait for the fault to develop again, or you can do your best to ensure that it does not happen again. Precision balancing and alignment, correct lubrication, acceptance testing (to ensure that machines are in good health when they are put into service) and other practices combine to improve the reliability of the equipment. Vibration analysis and the other condition monitoring technologies are still essential, but improved maintenance, purchasing and repair practices will ensure that machine life will approach the design life.