Vane Pass F requency

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.

Vane pass frequency = (shaft speed) x (no. of blades or vanes)

Similar to GMF

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.

A 5-Vane Impeller of Centrifugal Pump

Ca

itation

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

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^{. I}t usually occurs when a liquid is subjected to rapid changes of pressure that cause the

formation of cavities where the pressure is relatively low^.

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

Trend Display

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^{. R}emarkable reduction in vibrations was observed^.

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

POINT POINT CURRENT PREVIOUS

ID DESCRIPTION VALUE 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

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

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^.

POH=^Pump Outboard Horizontal ^.115 ^.220^In/Sec

POV=^Pump Outboard Vertical ^.083 ^.294^In/Sec

POA=^Pump Outboard Axial ^.099 ^.243^In/Sec

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 (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; B^N3300 system is used for KGT-2501 online monitoring^. B^N3500 system is used for K-441 and other turbomachinery at^Plant-^II.

Advanced diagnostics tools such as AD^RE are connected to obtain and analyse real time data of such machines for effective problem

identification^.

Root cause analysis and acceptance testing to improve 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 reasonwhy the fault occurred in the first place^{. I}t 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^{. P}recision 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^.