The ability of heat exchangers and specifically shell and tube type heat exchangers, to efficiently perform their function has a direct impact on the overall efficiency of power plants. Fouling is an inherent problem that results in reductions in heat transfer and an increase in production costs. This requires an ongoing and annual heat exchanger cleaning and maintenance protocol.
While fouling of heat exchangers can result from a variety of mechanisms, it is usually one that predominates and increases over time. Each time the tube deposits, sedimentation, biofouling and obstructions are removed, the tube surfaces are returned to an almost bare metal state. This restarts the lifecycle of the heat exchanger as the protective oxides will quickly rebuild themselves to create a passive cleaned tube.
Heat exchanger cleaning as wells all heat exchanger maintenance procedures must be performed while the unit is offline. The predominant method of cleaning is via mechanical means. Great care must be taken to avoid damaging any tubesheet or tube coatings which may be present. If not performed properly by an experienced cleaning firm, it may lead to tube leaks or corrosion that is undetectable until the unit is brought back on-line.
For off-line mechanical cleaning, the tool selected has to be the most appropriate for removing a particular type of deposit. Moulded plastic cleaners (pigs) as well as brushes can be used to remove light silt soft deposits as well as some types of microbiological deposits.
Flexible cleaner apparatuses have been designed to traverse the U-shaped tube bundles of heat exchangers to remove deposits. Difficult deposits often require the use of compressed air driven devices that utilize brushes or blades to bore into and remove the deposits as they push through each tube.
These metal cleaners have various designs that are meant to tackle a particular type of deposit. Most metal cleaners are designed to have a controlled spring-loaded cutting edge with design parameters to match the dimensions of the tube’s internal diameter. Hard calcite deposit tools have also been developed by a number of manufacturers.
The removed material can be simultaneously collected in a plastic container for later drying. As part of the heat exchanger maintenance it is imperative to weigh the deposits in order to establish density for maintenance reports that can help establish ongoing maintenance timetables.
This process is followed by inspection of coatings and any necessary coating application, which should always be done by certified personnel. A final inspection of all aspects of the tubes, shell and covers is performed along with torque-certified closing of the unit to manufacturer specifications.