A proposal was put forward by LH Marthinusen (LHM) to Eskom two years ago to introduce a new procedure for balancing of large induced draught (ID) axial fan rotors, as used on the boilers, after undergoing repairs.
A test technician conducts a test on a large ID fan rotor from Majuba power station in the Large Rotating Machines test bay at LH Marthinusen’s Denver facility.
This was readily accepted by the utility due to the substantial advantages it offers over the previous system applied for this purpose.
As a result, the new procedure was brought into effect by LHM for the first time last year as part of the periodic inspections and overhauls it performs on the ID axial fan rotors of Majuba, Medupi and Kusile power stations.
The new procedure is a balancing test that is performed on each ID axial fan rotor after its condition has been comprehensively assessed and all wear and damage each unit has sustained during years of operation in the power plant have been repaired to “as new” condition at LHM’s main repair facility in Denver, Johannesburg.
“Previously the ID fan rotor balancing was done in situ after the repaired rotor was re-installed in the plant. This involved having to test run it repeatedly with corrective adjustments being made as imbalances were detected, resulting in delays on each occasion of up to 24 hours before full operation could be resumed,” said Craig Johnston, General Manager of the Fan Service division.
“Having balanced the ID fan rotors at our plant prior to delivery, they can go into full operation immediately after being re-installed. This means significant time- and cost-saving benefits for the power station, as it eliminates down-time on balancing before start-up, as well as saving production time lost as a result of the prolonged balancing procedure applied previously.”
Craig said putting the ID fan rotor testing system into effect was simply a matter of piggy-backing on LHM’s Large Rotating Machines division’s existing large test bay at Denver. “All that’s required to temporarily convert the large machines balancing bay into a test bay suitable for our purposes is the addition of a few items of ancillary equipment to the test bay,” he remarked.
To date the new balancing test procedure has been applied to a total of four large ID fan rotors from Majuba and Medupi over the past year.
“The test is carried out once the comprehensive inspection-and-repair procedure we perform on each ID fan rotor and its set of detachable axial fan blades, after having operated continuously for a period of years, has been completed,” Craig explained.
The function of the ID fan is extraction of flue gas from the boiler in the power plant. The design of the ID fan rotors and their axial fan blades for each of the three power stations differ substantially from each other and also have a different mass and power rating in each case.
“For example, a rotor at Majuba weighs 13.5t and is driven by a 7.2MW motor, whilst a rotor from Kusile weighs 19.5t and operates at 10.2MW,” Craig pointed out.
