The impact of thermal swing on flange’s bolt loads.
Thermal swing or thermal shock usually defined as the sudden heating or sudden cooling +/- 10 per min, which cause tension stresses in the material of static equipment such as the pressure vessel and heat exchanger.
For metal material, the resistance against thermal swing is good, either carbon steel material, or alloy steel material as indicated in the table below.
The temperature limitations for the metal material which are mentioned in ASME code is that for the heavy service life based on ASME section II, carbon steel material are usually limited to a maximum operating temperature of?750°F (399°C), while the low alloy steel within ?% molybdenum alloy steels to approximately 850°F (454°C) while the stainless steels to considerably higher temperatures depending upon the type used usually around 1400 °F (760 °C). (see the curve below for both carbon steel material and SS material temperature limitations).
It is understood that there is no impact on the material properties unless if the material is subjected to the temperature range like the temperature ranges in heat treatment processes which mentioned in section VIII -1 part UCS. The minimum temperature that impact on the material is 540 °C. the temperature rate range shall be maintained as per this section. In most common application it is not recommended to increase the raising in temperature rate more than 222 °C per hour, and the variation in temperature shall not drop below 83 °C during the mentioned hold period start from recorded temperature 595 °C.
?For the thermal swing or temperature shock, it is mentioned in TEMA standard that “E3-23 : Exchangers normally should not be subjected to abrupt temperature fluctuations. Hot fluid should not be suddenly introduced when the unit is cold, nor cold fluid suddenly introduced when the unit is hot.”
?Also, it is mentioned in ASME PCC-1 that the leakages between the flanges could be occurred due to loss of the bolt load due to the process thermal. (see extracted part from the code below).
The code is not specifying the exact temperature for the thermal swing phenomena; as it extended to include the operating conditions, not only beyond the design limitation.
?For the design of the heat exchanger, for such application which suffering from rapid fluctuating in temperature, it is recommended to use AES. It is not recommended to use CEM type; as the thermal swing phenomena will impact directly of the bolts load of the shell’s flanges as indicated in the extracted fig from TEMA below.
In some actual cases, the flange management is applied well with the recommended torque, however after start the operation after one month only, an external leakage was detected from the shell flanges.
The results from the investigation as following
“Thermal swing phenomena as the temperature could change within 50 C within mins at startup, and 30 C each min at normal operations. ASME code here is not specified if this fluctuating is within, or out of the design battery limit, but combination between increasing the pressure over the design pressure and the thermal swing could lead to the loss in bolts”