E-Vibration Consultancy Services · Machinery Vibration Analysis and Diagnostic Studies Report...

Machinery Vibration Analysis

and

Diagnostic Studies

Report

E-Vibration Consultancy Services

B-22, Aspen Block, Citilights Meadows, Nolambur, Chennai – 600 095, India

Ph: +91-9840372209, (India), +974-66279565 (Middle East and International)

email: [email protected], www.evibrationconsultancy.com

B-22, Aspen Block, Citilights Meadows, No.82, Andal Ammal Street, Nolambur,Chennai – 600 095, India,

Phone: +91-9840372209 (India), +974-66279565 (Middle East & International)

email:[email protected], www.evibrationconsultancy.com

of 12

Table of Contents

Table of Contents………………………………………………………………………………………………….………….…02

1. Executive Summary……………………………………………………………………….……….…………………………03

2. Introduction…………………………………………………………..…………………………………………….…………..04

3. Equipment Details………………………………………….……………………………………………………………..….04

4. Equipment Sketch…………………………………………..………………………………………………………………..05

5. Allowable Vibration Limits…………….……………………………….…….………………………………………..…05

6. Vibration Analysis………………………………………………………………..…………………………………..….05-10

7. Interference Diagram Analysis…………………………………………………….………………………..….………11

8. Solutions……………………………………………………....................................................................11-12

9. Recommendation.……………………………………………………………………………………………………….……12

10. Conclusion………………………………………………………………………………………………………………………..12




of 12

Executive Summary

The machinery vibration analysis and diagnostics studies for 7.0 MW XXXXXXX Pump have been

requested by XXXXXXX Company Ltd – China on 20-Jan-2010. The following details were informed

during the discussion with Technical Services Department Engineer.

• The pump was installed newly and commissioned on 03 Oct 2010. During the start-up; the

pump is tripping at high vibration of 100 microns. The trip set value is 100 microns.

• The pump is 9 stage high pressures type and make is XXXXXXXX. The driver is a 2-pole motor

and coupling the pump with speed increasing gearbox.

• The speed of the motor is 3000 rpm and pump is 7220 rpm. The rated power is 7.0 MW.

• The bearing type is pressure dam and inspected found normal. The bearing radial clearance is

0.1 mm.

• The pump is installed with Bently machinery monitoring system and have an eddy current

displacement probes to measure vibration amplitudes.

• The lubrication is forced type and lubrication oil is ISO grade 32. The lub oil pressure is 2.0

kg/cm2

Through remote access of machinery monitoring system desktop, the vibration data were reviewed

along the Technical Engineer.

Based on review of submitted data and remote analysis of the machine, the reason for high vibration is

identified as FLUID INDUCED INSTABILITY of the pump system. The fluid instability can happen by the

bearings, wear rings, impellers, balance pistons or seals of the pump system.

To resolve the fluid induced stability, the solutions are

1. Change or tune the rotor-bearing system natural frequency such way that fluid whirl frequency

(0.3 x order < fw< 0.6 x order) will not interfere or re-excite the 1st

natural frequency.

2. Eliminate the sub synchronous fluid whirl frequency by modification of the bearing design,

increase the eccentric ratio and reduce the attitude angle towards zero degree.

3. Reduce the operating speed to 6900 rpm.

Out of the above solutions the best option and recommended is re-design or change the existing bearing

design to tilting pad type. The modification of tilting pad type bearing will eliminate the fluid whirl

frequency. The bearing will create large eccentric ratio and keep the attitude angle towards zero

degree. The more details of the analysis please refer the report below.




of 12

Introduction

The machinery vibration analysis and diagnostics studies for 7.0 MW XXXXXXX Pump was requested by

XXXXXXX Company Ltd – China on 20-Jan-2010. The following data have been received and analysed.

• Polar and Bode Plot of start-up and coast down

• Waterfall and cascade of pump start-up/coast down

• Vibration Full Spectrum at operating speed

• Orbit and Time Waveform at operating speed

• Equipment Data Sheet

• Pump P & ID

• Lub Oil P & ID

• Pump Cross Section Drawing

• Pump Operation and Maintenance History/Background

Equipment Details

Pump Description : Multistage XXXXXXX Pump

Pump Entity Number : XXXXXXXXXXX

Rated Power : 7.0 MW

Rated Speed : 7220 rpm

Rated Capacity : 400 m3/hr

Suction Pressure : 74 Kg/cm2

Discharge Pressure : 300 Kg/cm2

Lub oil Pressure : 2.0 Kg/cm2

Lub Oil Flow : N/A

Lub Oil Temperature : N/A

Bearing Clearance : 200 microns - diametrical




of 12

Equipment Sketch

Allowable Vibration Limits

Acceptable limit : < 50 microns Pk-Pk

1st

Alarm limit : 75 microns Pk-Pk

Trip limit : 100 microns Pk-Pk

Vibration Analysis

Pump Inboard Bearing data Interpretation

The start-up and coast down bode plot

indicates the first natural frequency is at

4047 CPM




of 12

The start-up and coast down polar plot


4025 CPM

The orbit and time base shows 2:1 ratio

(1/2 X Order) forward whirl with maximum

amplitude of 90 microns Pk-Pk

The waterfall shows the dominant

frequency is at 72.5 Hz (4350 CPM) at

7223 rpm with vibration amplitude of 52

microns PK-PK




of 12

The Full Spectrum shows the dominant

frequency is 72.5 Hz (4350 CPM) at 7217

rpm with vibration amplitude of 67

microns PK-PK

The cascade shows the dominant



microns PK-PK

The average shaft centreline of start-up of

the pump shows very small eccentric ratio

and attitude angle is approached 90

degree at 7200 rpm




of 12

Pump Outboard Bearing Interpretation

The start-up and coast down bode plot


4047 CPM

The start-up and coast down polar plot


4001 CPM

The orbit and time base shows 2:1 ratio

(1/2 X Order) forward whirl with maximum

amplitude of 100 microns Pk-Pk




of 12

The waterfall shows the dominant



microns PK-PK

The Full Spectrum shows the dominant

frequency is 72.5 Hz (4350 CPM) at 7240

rpm with vibration amplitude of 86

microns PK-PK

The cascade shows the dominant



microns PK-PK




of 12

The following are the observations of the vibration data of inboard and outboard bearing of the pump

• The maximum overall amplitude of 82 microns in inboard and 101 microns in outboard bearings

at dominant sub synchronous frequency of 72.5 Hz (4350 CPM).

• The orbit, time base, full spectrum, waterfall and cascade are clearly shows forward precision ½

order (forward whirl) sub synchronous vibration.

• The polar and bode plot of start-up and coast down data are confirms the first natural frequency

of the pump system is 4000 CPM (Range 2400 – 5200 CPM)

• The amplification factor (AF) is calculated from the bode plot of inboard and outboard bearings.

The values are 3.2 & 2.5 respectively. The separation margins of critical speed for this

amplification factors are well away.

• The critical damping ratios are 0.15625 (inboard) and 0.2 (outboard). The damping is greater

than recommended damping value (> 0.0625)

• The frequency ratio of operating speed to natural frequency is 1.8. The ratio is well above and

the system is in isolation.

• The average shaft centreline of the both bearings are indicates shaft, bearing and fluid are in

concentric. The eccentric ratio of shaft vibration to bearing clearance is very small. The attitude

angle of both bearing is approached 90 degree at operating speed.

The above analysis on pump is clearly shows that the pump is under fluid induced instability. The fluid

instability can be happen by the bearings, wear rings, impellers, balance pistons or seals of the pump

system.

The average shaft centreline of coast

down of the pump shows very small

eccentric ratio and attitude angle is 90

degree at 7227 rpm




of 12

Interference Diagram Analysis

The interference diagram was plotted and it clearly shows that the first natural frequency is re-excited at

about 7200 rpm by whirl frequency. This is the reason vibration was amplifying while approaching the

operating speed of 7220 rpm.

Solutions

Based on the interpretation of the vibration data it is clear fluid induced instability is the cause for the

high vibration while approaching operating speed. The following are solutions to resolve this vibration

issue

1. Change or tune the rotor-bearing system natural frequency such way that fluid whirl frequency

(0.3 x order < fw< 0.6 x order) will not interfere or re-excite the 1st

natural frequency.

2. Eliminate the sub synchronous fluid whirl frequency by modification of the bearing design,

increase the eccentric ratio and reduce the attitude angle towards zero degree.

3. Reduce the operating speed to 6900 rpm.




of 12

• Increasing the natural frequency of the system

Tune the frequency to 5220 CPM by increasing rotor and bearing stiffness however this is not advisable

due to separation margin of natural frequency will be very close to operating speed and it may affect

balance response.

• Decreasing the natural frequency of the system

Tune the natural frequency of the pump lower than 2166 CPM (< 0.3 x order) by reducing stiffness or

increasing of the rotor-bearing system. However this may be difficult task and involve more

modification of the pump shaft, impeller or diffusers or bearing. Therefore reduction of the natural

frequency is also not advised. By small modification (impeller wear rings or balance piston or seal or

bearing) if natural frequency of the pump system is reduces below 2166 CPM means this option can be

considered; however while redesigning should not lower the second critical speed and close to

operating speed. The separation margin of 2nd critical speed should maintain 20% well above the

operating speed. Practically reduction of existing natural frequency from 4000 CPM to less than 2166

CPM will be difficult task and may not achieve.

• Reduce the operating speed to 6900 rpm

The lower the speed of the pump needs change of gearbox design; the speed reduction will lower the

pump operating performance. So this modification is not recommended. If customer prefers and lower

the performance of the pump is not a concern then this modification can be considered.

Recommendation

From the analysis of the above it is recommend re-design or change the bearing to tilting pad type. The

modification of tilting pad type bearing will eliminate the fluid whirl frequency. The bearing will create

large eccentric ratio and keep the attitude angle towards zero degree. The bearing design change will

be the best option to resolve this fluid induced vibration issue for this pump.

Conclusion

Our above recommendation is required design change and in-house modification is not practically

applicable, so it is suggested consult the pump OEM or bearing manufacturer for the bearing design

change. If the design data of the pump and bearings are provided; the rotor dynamic calculation shall

be reviewed and provide you the best solutions to re-design rotor-bearing system.

E-Vibration Consultancy Services · Machinery Vibration Analysis and Diagnostic Studies Report...

Documents

Transcript of E-Vibration Consultancy Services · Machinery Vibration Analysis and Diagnostic Studies Report...