Start up of energy equipment is guided by some restrictions of acceptable temperature changes with the aim to avoid higher stress conditions in construction materials.
These restrictions lie in a range of several [K/minute] to avoid higher stresses in construction during start up processes.
If these loadings, temperature changes in working fluid or steam in steam boilers and turbines, are greater than the recommended values, we can expect to get higher stress conditions in construction materials which can override the acceptable values.
In cases with higher temperature changes during time, normal stress conditions can be overrided and creep conditions can be expected, as shown in fig. 1.
Analyzing the probability to override the mentioned recommendations, we need to get information about the temperature changes in two levels:
a) temperature changes in working fluid , with values
in [K/s] or [K/minute], and
b) temperature changes in construction material , with
values in [K/s] or [K/minute]
This information can be useful during start up processes and after failure to estimate the condition of the equipment in long term analysis.
In cases with low thickness of equipment vessels, speed of temperature changes in construction material can get nearly the same speed of temperature change of the fluid, while at higher thicknesses these changes can be lower, as shown in fig. 2 and 3 for cooling conditions.
Depending on thickness, mentioned speed of temperature change can be variable and it is necessary to estimate the position of measuring, because the loading side of equipment can be heated/cooled with different speeds of temperature change (fig. 3 , loading side – 1, other side – 2, middle thickness - m) .
To avoid excessive temperature changes in material it is necessary to get this information and to lead the processes according the mentioned restrictions of acceptable temperature changes.
Control systems can be mounted on equipment with higher thermal loadings and can present acceptable data to control the processes during the occurrence of variable conditions.