Flue Gas Recycle Fan: Introduction

The Flue Gas Recycle Fan is part of the COGA unit. The fan increases the efficiency of the COGA unit by recycling a proportion of the incinerated gases from the COGA stack. The fan is critical in sustaining high rates of operation on the plant, catastrophic failure of this fan would result in a COGA unit shut down. This would increase the emissions to the atmosphere and heavy fines could be imposed by the Environment Agency.

Flue Gas Online Vibrartion Points

The above Photo illustrates the position of the fixed online vibration sensors that are fitted to the fan and motor (i.e. MNV = Motor Non Drive End Bearing Vertical).

Flue Gas Fan Vibration Frequencies

The above diagram shows the configuration of the flue gas fan. The above configurations, speeds, bearing details etc have being entered into the online system to assist in fault diagnostics.

In Mid May 2002 the Flue gas fan sensor (Fan Non Drive End Fan Horizl) entered into an alarm condition, the system automatically started rapid data collection. Two parameters were initially in alarm PK-PK Waveform, (Peak to Peak Value of time waveform measured in G’s) and a Bearing energy band also measured in G’s. Below is the Online watch screen showing the status of monitored plant.

Flue Gas Online Watch Screen

Flue Gas Fan Trend Display Waveform

The above online trend is taken off the Fan Non Drive End Sensor;  note how quick the fault is deteriorating. Planning windows are rare on the COGA unit due to the environmental consequences if the plant is shut down.

The next step is to analyse the data to pinpoint why the vibration levels were increasing. Spectrum and Time Waveform analysis was used for this. Below is a spectrum taken from the online system. (Fan Non Drive end horizontal Bearing.)

Flue Gas Fan Bearing Vertical

The filtered spectrum above indicates a problem with the cage and rollers on the NDE fan bearing, a clear match is made with the cage ( 0.4 orders ). 2X Roller spin freq is also present due to defects on the rollers impacting the inner and outer races as they rotate. Cage defects are well known to deteriorate quickly.

Flue Gas Fan Bearing Vertical 2

The time waveform shows impacting to 78 G’s, severe bearing damage.

After discussions with production and maintenance, the reliability team decided to plan for the fan bearings to be changed at the earliest opportunity. New bearings were ordered and a scaffold erected ready for a quick bearing change. The opportunity came on the 21st May when the Dryer Plant shut down on an overload issue. The COGA was still running at this point, so a plan was formulated to shut the flue gas fan down in controlled manner so that the COGA unit could still be run, but at reduced rates, this was only possible because of the Dryer plant shutting down.

Flue Gas Fan Trend Display Waveform PK PK

Flue Gas Fan Bearing

Upon inspection of the bearing it was found that the cage had disintegrated and the bearing had entered the final failure mode. The fan would have failed catastrophically later that day, this would have caused major shaft and fan damage. The bearings were replaced and the fan was running within 6 Hours.

The cleaned up NDE cooper fan bearing showing remains of the cage and the damaged rollers.

Flue Gas Fan Bearing 2

Estimated Cost Savings

(Using Best Practice)

Actual Costs (Action Taken)

Parts : New Bearings  Coo 01400 x 2 = £ 500

Labour : 2 men , 6 hours @ £ 30/ hour = £ 360

Production Losses: None, job planned in with Boil Out

Total Cost = £  860

Costs (No Action Taken)

40% of ERV (Estimated replacement value)

0.40 x £ 55000 (Cost of New Fan) = £ 22,000

Labour : 2 men, 48 hours @ £30 /hour = £3500

Environmental Effect (Fines)

3 Days to remove/repair/replace flue gas fan

If no credits/ Fines Potential £ 2 Million a /day

Production Loss / running on lower rates.

3 Days on lower rates £ 26012

Total Cost  = £ 47978

Estimated Avoided Cost = £ 51512

Final Note

This case study highlights the fact that cage defects can rapidly deteriorate; in this case it only took 5 Days. This type of fault is often missed using conveniently walk round programs; continuous monitoring is sometimes the only way to pick these faults up.


The fan is critical in sustaining high rates of operation on the plant, catastrophic failure of this fan would result in a COGA unit shut down. This would increase the emissions to the atmosphere and heavy fines could be imposed by the Environment Agency.

This case study highlights the fact that cage defects can rapidly deteriorate; in this case it only took 5 Days. This type of fault is often missed using conveniently walk round programs; continuous monitoring is sometimes the only way to pick these faults up.

John Stubbs, Geoff Copeland, Stuart Walker, Chris Bennet, Dean Whittle, David Shevels


stuart walker bio rms 2020

 Stuart Walker has worked in the Condition Monitoring and Reliability sector for the last 23 years. He started his career working for Dupont as a Mechanical and Production Technician after completing a four year apprenticeship. It was here he was introduced to Condition Based Monitoring and setup a successful program at one of DuPont’s UK sites. He utilized technology’s such as Vibration analysis, Oil Analysis and IR Thermography moving the site from a time based maintenance strategy to a predictive and proactive one. Stuart then worked for a Reliability Consultant company for a number of years implementing and running a number of other successful CBM programs across the UK.

In 1999 he setup Reliability Maintenance Solutions Ltd with his colleague Dean Whittle. Together they have successfully grown the company over the last 20 years. RMS provides reliability consultancy, training, service, and products to a number blue chip companies across the UK, Europe and Middle East. Stuart has worked in many industry sectors including Oil & Gas, Petrochemical, Power and Paper. He is currently working on projects introducing and implementing the new Motion Amplification Technology within RMS and across a wide range of Industries at home and abroad.

23RPM Defect on a 4 Point Contact Bearing

This was to be the final of the five case studies on Enhanced System Reliability by James Sylvester. But, following on from feedback we will have one more case study next week, this will be an extra long project case study!

In this fifth case study from his book “Enhancing System Reliability Through Vibration Technology”, James Sylvester from JPS Reliability and an Reliability Training Institute Trainer, demonstrates vibration analysis of a slow rotational 4 Point Contact Bearing, with a 23RPM Defect. This is to remind us that correct database set up, and Time Waveform Analysis is so important in slow rotational bearings.

The other case studies in this series can be viewed on our Blog.

james sylvester

 James Sylvester is an RMS Trainer at the Reliability Training Institute.

RMS has been a key Mobius Training partner since 2004 and James’ addition to the team will help us in delivery of the growing number of course offerings available. James will support in the delivery of our Accredited training courses conforming with ISO 18436, that offer either MiBoc or BINDT certification options.

Being a time-served engineer and having 20 years’ experience in the implementation of Condition-Based Maintenance and Reliability-Based Maintenance programs, with practical experience and qualifications make him an ideal candidate for the role as an RMS Trainer.

Starting as an apprentice fitter turner (millwright) and after 10 years in the Mechanical Engineering and Maintenance he made the move into Condition Monitoring and Reliability Engineering. He has spent the last 20 years working with many of the Blue-Chip companies in the UK and Australia.

James is also the Author of “Enhanced System Reliability Through Vibration Technology” ISBN 978-1-5272-5386-5.

Book Content

ISBN: 978-1-5272-5386-5

Chapter 1 – Condition Monitoring
Chapter 2 – Mechanics of Failure
Chapter 3 – Condition Monitoring Technologies

Chapter 4 – Setting up and Reporting
Chapter 5 – Practical Application
Chapter 6 – ISO Standards

Chapter 7 – Vibration Units
Chapter 8 – Accelerometer Selection and Mounting
Chapter 9 – Signal Processing
Chapter 10 – Bearing Condition Units
Chapter 11 – System Dynamics
Chapter 12 – Resonance & Natural Frequencies
Chapter 13 – Operating Deflection Shape Analysis
Chapter 14 – Motion Amplification
Chapter 15 – Modal Analysis Chapter 16 – Orbit Plot
Chapter 17 – Bode Plots

Chapter 18 – General Vibration Analysis
Chapter 19 – Rolling Element Bearings
Chapter 20 – Fluid Film Bearings
Chapter 21 – Gears
Chapter 22 – Paper Machines
Chapter 23 – Jaw Crushers
Chapter 24 – Vibrating Screens
Chapter 25 – DC Drives
Chapter 26 – AC Drives
Chapter 27 – Electrical Discharge Machining (EDM)
Chapter 28 – Pumps
Chapter 29 – Compressors
Chapter 30 – Reciprocating Engine Vibration
Chapter 31 – Turbo Machinery Notes

Chapter 32 – Balancing

Chapter 33 – Case Studies


Throughout May and June you can purchase the book at a reduced price of £44.50 (RRP £75) +p&p.

  • Postage to the UK is £4.45
  • Postage to the EU is £9.45
  • Postage to the Rest of the World is £13.45

On completion of the form, James will send the invoice and Paypal link.

3 + 4 = ?

James has also created a series of five technical case studies, each of which is full of insights. The case studies are free to review and are created in an easy-to-follow video format. The case studies can be found here.

Bearing Defects

Identifying bearing defects using vibration analysis is not going to improve plant reliability. To do this we have to look at the root cause of the defect and take action to prevent the same problem re-occurring in the future. 

Here is a nice example from a large ID Fan, after a 3 month shut down of the plant. The fan was restarted, straight away an outer race defect was picked up on the Fan NDE bearing by an RMS engineer.


rms fan NDE bearing defect blog

rms fan NDE bearing defect data before shutdown blog
The damaged bearing was planned in for replacement. At RMS we are always trying to find the root cause so the bearing was cleaned and inspected. You can clearly see the bearing has suffered from corrosion whilst the fan was stationary. Spalling has started to occur in one area.

rms fan NDE bearing defect root cause spalling

In this case the root cause of the problem was that the fan was not rotated during its 3 month shutdown. This allowed corrosion to take place within the bearing leading to a defect upon start-up. Plans are now in place for future shutdowns to rotate the shafts to eliminate this type of defect.