Collar bolts are a type of clinger that is extensively used to hold the bundle in the exact place for removable bundle heat exchangers. This ensures the channel removal without breaking the seal between the shell and tube sheet. An extremely flexible and steady design is needed for this. They can be designed and manufactured to bear very high and very low temperatures.
What are Collar Bolts and where they used?
Collars bolts are used for removable bundle heat exchangers, to holding the bundle in place, if only the channel must be removed.
Approximately 25%, with a minimum of four, of the flange studs shall be Collar Bolts, in order to keep the bundle in place. All Collar Bolts shall have a square extension beyond the threads for use of a wrench to prevent turning of the bolt when the nuts are tightened or removed.
Example of Exchanger assembly with Collar Bolts
Example of Exchanger assembly where channel is removed
If only the channel must be removed, it's important that the bundle on the shell side remains fixed. If the bundle comes loose, it is very likely, that the gasket behind the tube sheet will be damaged, so that it is no longer possible, in order to achieve a good seal. As a result, the bundle must still be pulled out, in order to replace the gasket.
As an alternative to collar studs, every fourth bolt hole in the tube sheet may be drilled and tapped to the size of the stud bolt. The studs in the threaded holes shall be double nutted on the shell side or provided with machined flats to allow of the tube side nut without rotating the stud.
The main objective and advantage of the collar bolts is the better maintainability considering that each time the channel removed, the bundle shall be removed for replacing the gasket between the tube sheet and shell to avoid leakage after pressurizing the exchanger. Bundle gasket replacement is time-consuming and increases MTTR (Mean Time To Repair/Restore).
Disadvantages of the use of the collar bolts
Some filed experience showed that it is nonmandatory to remove the bundle if the channel removed. This opinion is built on some special experience in using cam profile gaskets and the application of initial proper stress to reach the desired gasket stress.
In case of using tapped holes and the bolts get stuck and the attendance for the holes machining and replacement of the bolts would be time-consuming and might be beyond the readiness of the maintenance crew for the task.
Marking or stamping of the collar bolts has to be adequate to avoid misleading the maintenance crew otherwise, they may remove collar bolts by mistake.
Relative higher cost due to the bigger tube sheet size and the aching required for the bolt holes in the tube sheet.
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