This is a widespread misconception across many industries that if there are more than 4 bolts around the circumference of a flange connection, hot bolting somehow becomes 'safe' without using any flange clamp.
There are two main areas of concern with this approach that are described below:
1. The reason you are changing bolts is that you are questioning the integrity of the bolts that are currently installed. Since their installation, there has likely been a loss of tension in the bolts, and the bolt lengths have increased over time. Removing one bolt loads the neighbouring bolts even further, causing a loss of localised gasket compression at best or potentially overstress of neighbouring bolts to failure at worst. Either way, the condition of the bolts is unknown and cannot be relied upon to hold the flange connection together reliably whilst personnel carry out bolt exchange. The use of a BoltEx® flange clamp mitigates this risk.
2. Not all flanges are designed the same. It is known that 150# flanges (even when newly bolted) have insufficient gasket compression provided by neighbouring bolts when a single stud is removed. See below extract from an SRJ finite element analysis showing an 8" 150# flange with a bolt removed (with no BoltEx® flange clamp fitted). The loss of gasket compression in the area is evident, leading to the loss of process fluid containment in the real world. A BoltEx® flange clamp removes this risk and assures flange connection containment during bolt exchange.
If you have seen this practice of hot bolting 8-bolt flanges without a flange clamp on your asset we would strongly recommend that you review and revise the approach.
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