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[ramsingh_mech]

Basic Notes on Steam Quality :

 

It is important that the steam leaving the boiler drum should be free from impurities which may deposit in the super heater , reheater or turbine. Such impurities may arise from the dissolution of boiler water impurities into the steam phase, notably phase Silica and Sodium hydroxide, and from mechanical carryover. Mechanical carryover occurs when there is incomplete separation of the two phases in the drum and droplets of boiler are carried over with the steam.

 

It is accepted that silica in steam will not cause problems in the turbine if the concentration does not exceed 0.02mg/kg. The distribution ratio between water and steam at about 180bar drum pressure is approximately 10 , thus the boiler water silica level must be maintained below 0.2 mg/kg. Boiler water silica is controlled by limiting the makeup water silica level to less than 0.02mg/kg and by the operation of boiler blowdown.

The acceptable limit for sodium in steam is 0.01 mg/kg where austenitic superheaters are used because of the stress of corrosion. The partition coefficient for sodium species in steam is less than for silica and the limits are usually achieved by operating within the guidelines for boiler water chloride and sodium hydroxide.

Boiler chemical conditions are maintained by high pressure dosing pumps which injects slug doses of dilutes sodium hydroxide into the boiler drum, and by low pressure dosing pumps which inject dilute ammonia and hydrazine solution into the feed system. Extensive water and steam sampling and monitoring equipment is provided. It is normal practice to locate the final sample cooling systems and continuous analyser in centralised  monitoring rooms.

 

Regards

 

RAM