By Vincent A. Carucci
Hot bolting refers to tightening a flange while it is in service. It is used to stop small stable leaks or as a preventive measure in high temperature or cyclic services. "High temperature" in this case may be defined as over 800°F (427°C). A cyclic service is one where the operating temperature changes by more than 300°F (149°C) in less than 30 minutes.
Most flanges that are assembled and bolted using appropriate procedures should never need to be hot bolted. Most locations do not routinely hot bolt flanges that are not leaking. However, scheduled hot bolting can be used to improve the reliability of certain troublesome joints if experience has shown that leaks occur with time or process upsets.
On many occasions, the writer has recommended and successfully used hot bolting as a preventive measure during the startup and initial operation of new process units. In these cases, construction scaffolding was installed before startup in order to provide ready access to the flanges of interest (i.e., the ones in high temperature or cyclic services).
Hot bolting should only be considered if it can be done safely. Each case must be reviewed by local safety and engineering personnel before attempting to retighten a flange on-line. The following are some considerations that must be addressed:
Ideally, hot bolting should be done using the same method as was used for the initial flange boltup (i.e., hammer and wrench, torque wrench, stud tensioner, etc.). However, in the case of tensioning, this cannot be done above 250°F (121°C) due to the typical temperature limits of the seal materials used in the tensioning head construction. Since most hot bolting is required on flanges that operate at well over that temperature, it will normally be done with torque wrenches.