The selection of steam pressure for gas boilers mainly depends on the needs of users.
Low steam pressure means low energy consumption, but the quality of steam is poor, the thermal enthalpy is low, and the humidity is high, which may even make it difficult to meet the needs of disinfection, sterilization, cultivation, heating and power generation.
If the steam pressure is too high, the energy consumption will be high, and the corresponding steam-using equipment, pipes and valves will also be under high pressure, and the maintenance requirements will also be higher. The heat dissipation and leakage losses during steam production and transportation will also be large.
At the same time, the salt content carried by high-pressure steam increases with the increase of steam pressure. These salts increase the possibility of scaling, overheating, bubbling, blockage and even bursting of heating surfaces such as water-cooled wall tubes, smoke tubes and boiler drums. When steam salt enters the turbine, the flow area of
Therefore, boiler operators must operate according to the specified working steam pressure, and the generally allowed fluctuation value of steam pressure is 0.05-0.15MPa.
Basic reasons for changes in gas steam pressure
External disturbance: The change in steam pressure caused by changes in external load is called external disturbance, referred to as “external disturbance”. When the external load increases, the steam consumption of the unit increases, and the boiler has not had time to adjust to the new operating conditions. The boiler evaporation will be less than the external demand for steam, the material balance relationship is broken, and the steam pressure drops.
Internal disturbance: The change in steam pressure caused by changes in the operating conditions of the boiler itself is called internal disturbance, referred to as “internal disturbance”. During operation, the external demand for steam has not changed, but due to changes in the boiler combustion conditions (such as unstable combustion or changes in fuel quantity and air volume) and changes in the boiler conditions (such as heat transfer conditions), the steam production in the evaporation zone changes, and the material balance relationship between the boiler evaporation and steam demand is destroyed, thereby changing the steam pressure.
What are the hazards of gas boiler overpressure?
(1) Metal materials have their own specific metal material yield strength at a specific temperature. Overpressure operation will lead to thermal fatigue of metal materials. Operation under conditions exceeding the fatigue limit will cause the yield strength to decrease. When the limit is exceeded, it will lead to rapid creep until bursting.
(2) The strength of metal materials can be compensated under certain conditions by increasing the wall thickness of the metal. The increase in wall thickness increases the temperature difference between the inner and outer walls, reduces the heat transfer rate, and the wall temperature difference will lead to thermal stress concentration in metal materials.
The boiler is designed with a rated pressure, which is also the allowable pressure during boiler operation. If it is normal, this pressure is controlled by the boiler’s safety valve, and there should be no sudden increase. This phenomenon is also not allowed. If it happens, it will exceed the design parameters of the boiler, which will affect the service life of the boiler. Excessive steam pressure and excessive mechanical stress will endanger the safety of the machine, furnace and steam pipeline.
When the safety door is activated, a large amount of exhaust steam loss will be caused. In addition, if the safety door is frequently activated, it will also cause wear or dirt to be deposited on the valve acne, which is prone to lax closure of the return seat, resulting in frequent steam leakage losses. Sometimes the boiler even needs to be shut down for maintenance. Excessive pressure can easily cause the heated pipe to swell and cause pipe explosion accidents. Although high parameter operation has improved certain economic benefits for boilers, the consequences are also huge.
