Many utility power plants are approaching the end of their design life, with engineers conducting life extension studies for boilers and related auxiliary equipment. To accommodate a changing operating environment, redesigning components and equipment to replace those which have reached the end of their useful life is increasingly becoming common practice, says engineering, manufacturing, construction and maintenance service provider Steinmüller Africa.
Steinmüller Africa process engineer Keyur Patel says heat exchangers, which form part of the auxiliary equipment that heats the feedwater entering the boiler, are vital components that need to be maintained to ensure optimal functioning of the power station. “If the temperature of the feedwater going into the boiler is lower than the boiler’s design condition, the overall efficiency of the plant will decrease.”
Therefore, it is desirable to maintain the performance of high-pressure (HP) heaters and replace them when required to not only improve cycle efficiency but also prevent abnormal operating conditions for the boiler and thermal shock in the boiler thick-walled components.
Patel notes that the company started the HP heater design development process in 2009. The ongoing development initiative entailed training a team of engineers from the process, mechanical and drafting departments so that they could produce a high-quality design in a short time. The team comprises six technical specialists – two engineers each from the process and mechanical departments, and two three- dimension draughtsmen from the drafting department.
The focus of development for the technical team was on specifying outcome requirements for the HP heaters, listing and familiarising themselves with the design codes to be used, generating process flow diagrams, allocating responsibilities to team members and closing information divides between departments.
The development initiative also included a long-term skills transfer programme conducted by an experienced German engineer, who was previously involved in the design of most of the HP heaters currently in operation at the local power stations.
Patel comments that an important aspect was to ensure that the design complemented Steinmüller Africa’s internal manufacturing capabilities without excessive additional capital expenditure. Consequently, the company, which is involved in the manufacture and maintenance of utility-scale pulverised coal steam generators, now offers turnkey solutions for header-type, tube-sheet-type, shell and tube heat exchangers.
“The header-type heat exchanger consists essentially of two thick-walled headers inside a pressure vessel shell, distributing feedwater through numerous ‘snake tubes’ connecting the two headers to absorb heat from the bled steam on the shell side of the heater,” explains Steinmüller Africa boiler process group leader Warwick Ham.
He notes that the headers are very similar to those used in HP water-tube boilers, components in which the company has extensive experience. Further, Steinmüller Africa also has extensive experience in modelling dynamic heat and mass transfer, component sizing, mechanical design and vessel manufacturing.
Ham notes that, based on the company’s current capabilities and its level of power-sector exposure, it is a natural fit for Steinmüller Africa to provide the design, manufacturing and supply of the complete range of HP heaters and also low-pressure heaters in future.
Steinmüller Africa recently supplied five HP heaters of the tube sheet type to the Arnot power station, in Mpumalanga.
Further, the company is also looking forward to receiving new work from the renewable- energy sector, especially concentrated solar power (CSP) plants. The CSP plants use the HP heat exchangers extensively to transfer the heat between the molten salt and the steam plant.
Explosive Tube Plugging
Ham notes the need for the maintenance of heat exchangers installed on utility power plants, with the plugging of leaking tubes routinely required. The company is using an innovative tube plugging process that relies on explosive expansion. The process requires minimum downtime, compared with existing alternative tube plugging processes.
During the conventional tube plugging process, an HP heater is taken off line and is either removed from the plant or is worked on on site. This involves gaining access to the tube bundle, identifying leaking tubes, cleaning the tubes, preparing a plug that is then either mechanically, hydraulically or pneumatically inserted and then fixed into the tube. Ham notes: “This method has higher quality assurance risks and takes much longer to execute.”
The new explosive plugging method is faster and provides a “cleaner” plug than the old procedure, says Ham. The new method requires that a custom-made plug be inserted into the leaking tube and fixed into position by detonating an explosive charge, Ham says, adding that the explosion is “very strictly controlled”, with the force of the explosion used to expand the plug into the leaking tube. The plug is specifically selected from a range of custom-designed shapes and sizes to suit the tube diameter and wall thickness, he notes.
Further, the amount of explosive substance used is selected from a predetermined quantity and this is based on the materials of the tube and header or tubesheet, Ham adds. Refinement of the entire explosive plugging process has been a very expensive development exercise and has led to the development of substantial intellectual property.
He concludes that the main benefit of this method is that the repair has a shorter turnaround time and greater consistency in the quality of the repair. The method requires fewer, but more specialised, resources and therefore enables rapid mobilisation to site and execution of the works.