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.
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).
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.
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.)
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.
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.
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.
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.
FAQs
How important is the Flue Gas Recycle Fan to site efficiency?
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.
What was a key learning in the Flue Gas Recycle Fan case study?
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.
Report Authors
John Stubbs, Geoff Copeland, Stuart Walker, Chris Bennet, Dean Whittle, David Shevels
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.
