Steam return-line corrosion

Obrecht, Malvern F. PhD. Steam and Condensate Return Line Corrosion. Factors That Accelerate It, Retard Tt, with Emphasis on Oxygen. Heating, Piping Air Conditioning, USA, August 1964. 

Since iron is not commonly used in strongly acid environments, the factors governing its corrosion in media of low pH are less important than those in natural waters and soils. Nevertheless, there are certain applications where such factors must be considered—for example, in steam-return lines containing carbonic acid, as well as in food cans where fruit and vegetable acids corrode the container with accompanying hydrogen evolution. 

Another approach to the prevention of steam condensate and return line corrosion is that of using film-forming chemicals to lay down a protective film on surfaces. This approach has come into widespread use with the development of suitable long-chain nitrogenous materials for this purpose. It is especially effective in systems where high concentrations of carbon dioxide make the use of neutralizing amines uneconomical. 

Condensate returns lines are often copper. Copper has good corrosion resistance to oxygen and carbon dioxide individually. When both gases are present in the condensate, copper is susceptible to corrosion. Copper picked up in the condensate system and returned to the boiler causes serious corrosion problems in the boiler and any steel feedwater and steam pipework. Boiler tubes should last for 25 years but can fail within one year in a mismanaged or ill-designed boiler system suffering from these faults. 

Where problems develop, there is always a cause-and-effect process. In this case, as oxygen infiltrates the CR system, enhanced condensate line corrosion results (i.e., corrosion over and above the level that may be caused by the carbonic acid formed during steam condensation). This enhanced corrosion, in turn, creates the potential for further downstream corrosion debris pickup by the returning condensate and transporting this material back to the FW system. 

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

Steam condensate and return systems. Corrosion of steam condensate and return systems presents a twofold problem to power-generating and steam-heating plants. Equipment damage and frequent replacement of lines, valves, and traps result in a serious maintenance problem. In addition, corrosion products frequently formed are carried back into the steam-generating equipment and deposited there. The result is plugging of lines, localized overheating, and promotion of corrosion in the boiler system itself. 

One approach involves fihn-forming materials, such as sodium silicate, oils, or polyphosphates. Sodium sihcate reduces corrosion but cannot prevent it entirely. A very successful approach is the use of long-chain nitrogenous compounds as film formers for condensate and return lines. They do not normally accumulate in the boiler because they are either eliminated at the vent of the deaerating heater or steam distil from the boiler water. 

The first underground railway in the world was opened on January 10,1863 in the City of London, operating with steam locomotives. The first line with electric traction and a three-rail system was built in 1890. The four-rail system, still in use today and consisting of two insulated conductor rails and two running rails, was introduced in 1903 in the course of electrification of the old steam tracks. The Metropolitan Company, responsible for a part of the track, used ac, while the District Railway preferred dc as a consequence of the connection with the American railways. This dispute came before a British arbitration tribunal in 1900. The problem of corrosion of public supply lines by the returning current fi om electric train... 

The pre-boiler cycle consisting of feedwater heaters, feed pumps and feed lines are liable to corrode by the condensate return. The main contributors to corrosion are carbon dioxide and oxygen. Corrosion must, therefore, be minimized. This is done by feedwater control, which reduces the ingress of harmful impurities and gases to the boiler water circuit and also to the steam. The best way to prevent corrosion is by controlling the pH, which should be maintained in the range of 8.5-9.2. 

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