Common misunderstanding: the higher the steam pressure, the better
The choice of boiler steam pressure mainly depends on the needs of users. Low steam pressure, good safety of pressure components, low energy consumption, but poor steam quality, low heat, high humidity, even disinfection, sterilization, cultivation, heating and power generation needs are difficult to meet. Steam pressure is too high, energy consumption is high, the corresponding steam equipment, pipelines and valves also bear high pressure, maintenance requirements are also high, heat dissipation and leakage losses in the process of steam production and transportation are also great. Meanwhile, the salt carried by high pressure steam increases with the increase of steam pressure. These salts may cause scaling, overheating, bubbling, blockage or even rupture of heated surfaces of water wall pipes, pipes, drums, etc. Steam salt entering the steam turbine will reduce the flow area and internal efficiency of the turbine, which will seriously affect the heat transfer.
The Basic Reasons for the Change of Steam Pressure
External disturbance: The change of steam pressure caused by the change of external load is called external disturbance, which is called external disturbance. When the external load increases, the steam consumption of the unit increases, and the boiler is not adjusted to adapt to the new working conditions. When the boiler evaporation is less than the external steam demand, the material balance relationship will be broken and the steam pressure will decrease.
Internal disturbance: The change of steam pressure caused by the change of the working state of the boiler itself is called internal disturbance, which is called “ internal disturbance ”. The amount of steam required by the outside world will not change during operation. The vapor in the evaporation zone is caused by the change of combustion conditions of the boiler (such as unstable combustion or changes in fuel and air) and the operation conditions of the boiler (such as heat transfer). The material balance relationship between boiler evaporation and steam demand is destroyed and the steam pressure changes with the change of boiler evaporation.
What are the dangers of overpressure in boilers?
(1) Metal materials have specific yield strength of metal materials at specific temperatures. Overpressure operation can lead to thermal fatigue of metal materials. Operations beyond the fatigue limit will result in a reduction in yield strength. Breaking the yield limit will lead to rapid creep until bursting.
(2) The strength of metal material can be compensated by increasing the wall thickness of metal under certain conditions. Increasing wall thickness increases the temperature difference between the inner wall and the outer wall, and decreases the heat transfer rate. The wall temperature difference will lead to the increase and accumulation of thermal stress in metal materials.
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