At first look, the practice of insulating flanges seems to have several clear advantages, including:
- It cuts back on radiated heat, thus providing more thermal efficiency and cost reductions to most thermally intensive industrial processes.
- It lowers the risk of a worker accidentally coming into contact with the hot, exposed flange.
- It provides an initial barrier of protection in the event of a high-temperature steam or other hazardous substance leak.
Although these reasons seem compelling, they miss the central point. The most important function of a gasketed joint is to prevent leakage. When viewed in terms of this primary function, it becomes clear the negative effects of insulating flanges outweigh the apparent benefits. These negative effects could include:
- Insulating flanges boosts the temperature of the studs. In the hottest of flange applications, this increases the chance the studs could thermally yield.
- Insulating flanges can increase the chance for leakage when a flange is cooled rapidly from the inside (such as when a wash-oil runs through a system). Under these conditions, the bolts cool slower than the flanges, resulting in a measurable (6,000-10,000 psi) drop in the stud load. The precipitous drop in stud stress is responsible for a high percentage of the leaks that occur with thermal upsets. Even though this is a transient effect, it is only made worse by the use of insulation, which slows the rate at which the bolt can radiate its heat.
If the primary purpose of the proposed insulation is personnel protection, that goal can be achieved through other methods that don't impact the thermal balance of the flange. For example, a refinery could use expanded metal guards that prevent accidental contact with the flange, while still allowing free airflow.
Insulating flanges is not only going to make it more likely that you will suffer leaks but will also prevent one from noticing these leaks when they first occur, which is when they can most easily be abated.
One refinery has taken steps to prevent thermal yielding of the stud and conducted a study to review the effect of a thermal upset to the heat exchanger.
For more information and to see the full study, including a chart on stud load versus temperature, visit http://info.lewis-goetz.com/ flange-insulation-lp
