AS an OEM of hundreds of steam boilers in Africa, Babcock participated in the build of one of South Africa’s largest power stations, mainly focused on the HP piping systems construction. An event occurred that damaged one of the steam air heaters, for which the Babcock crew were awarded the contract to undertake repairs involving specialised rigging and welding expertise. The steam air heater Babcock repaired is a multi-row fin tube heat exchanger consisting of three tube bundles and two bypass dampers installed in the air ducting of the boiler.
Tommy Scheepers, senior piping engineer at Babcock, explains the significance of the rotary air preheater, which in combination with the steam air preheater, works sequentially to control both primary and secondary air temperatures before it enters the boiler furnace through the burners. Precise control of the combustion air temperature is crucial for regulation of the combustion process, ensuring efficient burning to minimise environmental emissions.
The damaged steam air heater could not be removed from the plant as it would have required extensive and costly rigging, together with additional structural support to maintain the integrity of the boiler ducting. This meant the replacement of the fragile 9-metre-long finned tubes had to be performed individually in situ. Specific rigging methodology, routes and lifting aids were identified and developed prior to the replacement.
Before commencing refurbishment work, Babcock first had to remove the existing damaged finned tubes from the steam air heater frames inside the ducting. This involved cutting out and removing the connecting auxiliary steam and condensate drain piping to provide access for rigging activities on the outside of the ducting. The inlet and outlet air chambers were also removed, followed by the removal of the steam headers and tube sheets.
The air heater finned tubes are made from carbon steel with embedded aluminium spiral fins. These finned tubes are sourced offshore with consequent delays in supply and are very expensive. This style of tube is necessary for efficient heat transfer between the internal steam-heated tubes and the secondary air flowing over the fins.
Scheepers explains that special precautions were taken to prevent any damage to the fragile finned tubes during handling, rigging and installation, bearing in mind the consequences for the team in terms of time, cost and quality of the overall repair. “The handling of these delicate materials in a confined area required meticulous planning, sequencing and control of all actions by the staff involved during the replacement process, making this a complex and detailed undertaking,” says Scheepers.
Babcock construction manager, Luis Ferraz De Almeida, emphasises the company’s commitment to providing safe and effective solutions throughout the entire lifecycle of power plants. Babcock is renowned for its expertise in welding special creep-resistant alloy steels and the front-end design of steam generation plants and pressure equipment.
“Our customer had confidence in Babcock’s boiler knowledge and in-house expertise, which assured them that we could successfully perform this specialised steam air heater replacement project within the specified timeline,” adds Ferraz De Almeida.
