7.3.3 Off-load condenser cleaning
Although improvements in the on-load techniques continue to be made, off-load cleaning is frequently required for various reasons.
Failure to cope (or breakdown) of the on-load equipment, may cause a fouling condition to develop which becomes irrecoverable when the plant is re-commissioned.
Late filling of the fine debris filters immediately upstream of condenser waterboxes may allow marine fouling to occur, necessitating tubeplate picking to remove debris.
Except in a few exceptional circumstances, off-load condenser cleaning is done during outages. The techniques used are now discussed.
This is the most extensively used method of cleaning the inside surface of condenser tubes, and involves forcing bullets down the tubes to brush off surface deposits. Depending on the form of deposit to be removed, the bullets vary from hollow and solid rubber balls, to nylon and steel bristle designs.
Compressed air or water pressure, or a combination of both, forces the bullets down the tubes from lightweight guns which are designed to develop a seal at the tubeplate without effort from the operator.
This method is restricted to condensers suffering from scaling problems, e.g., Midland stations which draw their make-up water from the River Trent. Calcium carbonate and phosphate scales form the deposits and have been cleaned using a variety of acids, including hydrochloric, citric, acetic, and EDTA (ethylene diamine tetra-acetate).
It is important when adopting this technique that the condenser steam space is subsequently flooded with demineralised water to prevent acid entering the condensate when the unit is recommissioned.
Some stations have experienced stress-corrosion cracking of expanded tube ends from acid cleaning.
An alternative to bulleting, this method is known as sinusoidal grit blasting; it involves blasting graded silica-free mineral grit through the condenser tubes. A special grit blasting nozzle incorporates a venturi which accelerates the grit and causes it to follow a sinusoidal path through the tube, stripping off deposits which have accumulated. This method has a 90% effectiveness in cleaning tubes, and requires a charge of grit at approximately 1 m/s.
This method uses either high or low water pressure, depending on the application.
To remove surface deposits, a high pressure lance with forward and side-facing jets is passed down each tube, blasting water at a pressure of 700 bar. This method has an advantage over acid or grit cleaning method, since its action does not have a detrimental effect on the tube surface. However, its operation is awkward within the confines of the water-boxes, and is therefore generally not used.
Low pressure water washing is effectively used to remove loose sludge and sediment deposits prior to leaving the condenser dry during overhaul. Often a combined water/compressed air gun (similar to the type used in bulleting) is employed, having a discharge pressure of 7 bar.
These have been used to control mussel growth in seawater station CW culverts, and to clean condenser tubes by drying internal deposits prior to flushing with water.
The former application controls marine fouling by raising the temperature to 40°C for one hour every six weeks. This restricts mussel growth to sizes which will not lodge in condenser tubes. The heating is achieved either by introducing an auxiliary steam source in the CW culvert, or by reversing the flow of CW through the condenser.
Drying the deposits in condenser tubes prior to flushing has also been successful: fan heaters are used to blow warm air through the condenser waterbox access doors.