The pressure rise curve of a boiler is generally slow at first and then faster – why?

During the pressure increase process in a drum boiler, the change in the temperature of the drum’s metal depends on the rate at which pressure is increased. The lower the pressure, the greater the increase in the saturated temperature for each unit increase in pressure; therefore, small changes in pressure during the low-pressure stage can result in large changes in the saturated temperature, leading to excessive temperature differences across the walls and thus high thermal stresses. As a result, the pressure increase is slow at the beginning. In later stages of pressure increase, the temperature differences between the upper and lower walls as well as between the inner and outer walls of the drum have decreased significantly, allowing for a faster rate of pressure increase compared to the low-pressure stage. Hence, the pressure increase curve is slow at first and then speeds up.

Mainly, the temperature differences between the upper and lower walls, as well as the inner and outer walls of the drum, are different in the pre-pressurization stage and the post-pressurization stage

I’ve learned it! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !